Chapter 115. Lumbar Puncture

Meningitis and subarachnoid hemorrhage (SAH) are serious life-threatening conditions. They require prompt and accurate diagnosis in the Emergency Department due to their significant morbidity and mortality. There are many diagnostic modalities available to the Emergency Physician to assist in the diagnosis. However, the lumbar puncture (LP) is still considered the gold standard. The LP is a procedure that is often performed in the Emergency Department to obtain information about the cerebrospinal fluid (CSF) to aid in the diagnosis of a variety of medical conditions. Knowledge about the proper indications, contraindications, various techniques, equipment, and recognition and treatment of its complications is vital to any Emergency Physician who performs this procedure. An LP should be performed after a thorough neurological exam. Significant morbidity and mortality can result if the procedure is performed on the wrong patient.

While the entire cavity of the brain and spinal cord has a volume of approximately 1650 mL, CSF occupies approximately 150 mL of this volume. The brain literally floats in the CSF because the specific gravity of the CSF and brain are approximately the same. Approximately 500 mL (0.35 mL/min) of CSF is produced each day. Most (over two-thirds) of the CSF is produced by the choroid plexus within the lateral ventricles. Small amounts of choroid plexus can also be found in the third and fourth ventricles. Small amounts of CSF are secreted by the ependymal surfaces of the ventricles. A minimal volume of CSF is produced by the brain through the small perivascular spaces that surround the blood vessels entering the brain substance.

The flow of CSF through the ventricular system is rather simple (Figure 115-1). CSF produced in the lateral ventricles flows through the foramina of Monro into the midline third ventricle. It then passes through the Aqueduct of Sylvius into the fourth ventricle. From the fourth ventricle, the CSF flows into the cisterna magna via two lateral openings (foramina of Luschka) and one midline opening (foramen of Magendie). The cisterna magna is located beneath the medulla and cerebellum and is continuous with the subarachnoid space that surrounds the brain and spinal cord. The CSF then flows through the subarachnoid space to bathe the brain and spinal cord. The CSF is absorbed back into the venous system by way of arachnoid villi.

CSF pressure should average 130 mmH2O when measured in the lateral decubitus position. It can range from 70 to 180 mmH2O in a normal person. Since the CSF production rate is constant, the pressure is regulated by the rate of CSF absorption by the arachnoid villi that act as one-way valves into the venous blood of the dural sinuses. Certain disease states may impede reabsorption and lead to increased intracranial pressure (ICP).1

Familiarity with the anatomy of the spinal column is important when performing an LP. The anatomy will be briefly reviewed from superficial to deep as the spinal needle traverses the midline structures. The skin and subcutaneous tissue are the first layers encountered. These are followed by the supraspinous and intraspinous ligaments, located between the spinous processes of adjacent vertebrae. Deep to these ligaments is the thick ligamentum flavum that accounts for the characteristic “pop” that is described when performing an LP. The next layers encountered are the epidural fat, in the epidural space, followed by the dura mater, and finally the subarachnoid space.

When considering the lateral approach, there are subtle anatomic differences. The layers include skin and subcutaneous tissue followed by the paraspinal ligaments. The intraspinous ligament is less likely to be encountered with an extreme lateral approach, as this is a midline structure. The ligamentum flavum and deeper structures should be encountered in the same fashion regardless of the approach.

There are many indications for performing an LP. The primary indications to perform an LP are the suspicion for a central nervous system infection, such as meningitis, or for a SAH. It may also be performed for the evaluation of new-onset seizures, to obtain CSF biomarkers, to relieve CSF pressure, and confirm the diagnosis of pseudotumor cerebri. Other indications include CSF evaluation for central nervous system diseases (e.g., Guillain–Barré, multiple sclerosis, and systemic lupus erythematosus), to confirm demyelinating or inflammatory diseases, to administer antibiotics or chemotherapeutic agents, to aid in radiologic imaging procedures (e.g., cysternography or myelography), and to diagnose meningeal carcinomatosis.

Suspicion of Meningitis in Adults

An LP should be performed in adults when there is a clinical suspicion of a central nervous system (CNS) infection. While a fever is often present (most sources consider a fever >100.4°F or >38°C), it is not a dependable sign. Meningeal signs include nuchal rigidity, Kernig's sign, and Brudzinski's sign. Other signs of a possible CNS infection include a severe headache, photophobia, or a petechial rash. Unfortunately, these signs may or may not be present. This is especially true in the elderly, the young, or the immunocompromised patient. An LP should be a routine procedure in febrile adults with an altered mental status and no source of fever.

The most commonly looked-for signs of meningitis include the Kernig's sign and the Brudzinski's sign (Figure 115-2). Place the patient supine to test for these physical examination signs. Passively flex the patient's head until their chin touches the sternum. Flexion of the patient's hips and knees in response to the head flexion is known as the Brudzinski's sign (Figure 115-2A). The patient may also experience neck pain and resistance to flexion if meningitis is present. Passively flex one of the patient's legs to 90° at the hip and to 90° at the knee (Figure 115-2B). Passively extend the knee. Pain in the lower back or resistance to knee extension is known as the Kernig's sign.2

Suspicion of Meningitis in Children

When evaluating the febrile infant, the decision of whether to perform an LP will be based on the clinical suspicion of meningitis, the age and appearance of the child, and whether an identifiable source of fever is present. The Emergency Physician will often be faced with a well-appearing febrile infant with no obvious source of fever. Until recently, many institutions managed all febrile infants less than 3 months old with a full sepsis workup (including LP) and admission to the hospital.

Guidelines based on an extensive literature review and a meta-analysis by an expert panel identify patients at low risk for serious bacterial infection (SBI), where LP and hospitalization may or may not be indicated.3 Examples of serious bacterial infections include meningitis, sepsis, osteomyelitis, septic arthritis, urinary tract infections, pneumonia, and enteritis. Since clinical evaluation alone is inadequate to exclude serious bacterial infections in infants, it must be combined with laboratory studies that can define low-risk criteria. These criteria apply to nontoxic-appearing infants with reliable parents and prearranged follow-up. A more detailed discussion regarding the evaluation of the child with a fever is beyond the scope of this chapter.

It should be noted that any febrile child, regardless of age, who appears “toxic” should have an LP as an integral part of the sepsis workup. Certain bacterial infections have a high propensity for dissemination and systemic bacteremia. Examples include epiglottitis, buccal cellulitis, periorbital cellulitis, and septic arthritis. These children should also be considered for a LP in their evaluation and work-up.

Is an LP Indicated for the First Febrile Seizure?

The Emergency Physician will be faced with the decision of whether to perform an LP when evaluating infants and children with a first febrile seizure. The American Academy of Pediatrics recommends that a LP be “strongly considered in infants less than 12 months of age” and “considered in children 12 to 18 months of age.”4 This is based on the lack of clinical signs and symptoms often associated with meningitis in this age group. For children over 18 months of age, an LP should only be performed if there is a clinical suggestion of meningitis. Clinical signs and symptoms that have been shown to correlate with the presence of meningitis include petechiae, nuchal rigidity, coma, persistent drowsiness, Kernig's or Brudzinski's sign, status epilepticus, and paralysis.5

Benign febrile seizures are those that occur in children 3 months to 5 years of age, are associated with a fever at the onset of an illness, have a single generalized seizure lasting less than 15 minutes in a child with normal psychomotor development, have no history of prior febrile seizures with this current illness, and have no evidence of an intracranial infection or acute neurological illness. These children generally appear well except for the fever and generally do not require a LP. This is based upon the level of Emergency Physician comfort in a nontoxic-appearing child.

Complex febrile seizures are those that do not meet the criteria of a benign febrile seizure. These children require a complete septic work-up including an LP. Criteria include seizures that begin focally, seizures lasting over 15 minutes, children with a prolonged postictal period, suspicious findings on physical examination, children less than 12 months of age, children already receiving antibiotics, children who have seen a Physician for an illness in the preceding 24 hours, or those who have had multiple seizures during a single period of illness. Seizures not associated with the onset of an illness have an increased risk of being due to meningitis or bacteremia. Children seizing upon presentation to the Emergency Department are considered to be seizing over 15 minutes or to have recurrent seizures. Suspicious findings on physical examination that suggest a complex seizure include rashes, petechiae, cyanosis, hypotension, abnormal respirations, increased or floppy tone, stiff neck, difficult to console, deviated eyes, doll's eyes, nystagmus, ataxia, photophobia, bulging or tense fontanelles, unable to fix and follow, or children that do not respond to voice or painful stimuli.

Subarachnoid Hemorrhage

A suspected SAH is the other common indication for an LP. The classic description of a SAH is the sudden onset of an excruciating headache (“thunderclap”) during exertion that may or may not be associated with syncope, nausea, vomiting, diaphoresis, or meningeal signs. Physical examination findings may include nuchal rigidity, an altered level of consciousness, papilledema, retinal hemorrhage, third-nerve palsy, sixth-nerve palsy, bilateral lower leg weakness, nystagmus, ataxia, aphasia, or hemiparesis. Additional risk factors for a SAH include cigarette smoking, hypertension, alcohol abuse, a family history of SAH, polycystic kidney disease, connective tissue disorders, or sickle cell anemia.

It is estimated that 20% to 50% of patients with a SAH may have sentinel bleeds or small leaks that precede the major bleeding event. It is important to diagnose a sentinel bleed because early management and intervention can improve the overall outcome for the patient. A sentinel bleed may precede a major SAH by hours, days, weeks, or months.6,7

A CT scan of the head is often the first study used to investigate a patient complaining of the sudden-onset headache. The sensitivity of CT scan for SAH can range from 92% to 98% when performed within 24 hours of the onset of symptoms.8–10 The sensitivity decreases markedly (about 76%) when performed 48 to 72 hours after onset of symptoms.11–12 The head CT scan can be negative in the patient with a sentinel bleed.

An LP must be performed if the CT scan is negative and a SAH is still suspected.10 The presence of xanthochromia or red blood cells in the CSF will confirm the presence of bleeding. While the CSF will usually confirm a SAH, it is possible that the LP may be negative despite a recent SAH if there has not been sufficient time for the red blood cells to migrate to the lumbar spine area. In this case, despite a negative head CT scan or a negative LP, a cerebral angiogram or follow-up LP in 12 to 18 hours should be performed.

Knowledge of the contraindications to performing an LP is important. The Emergency Physician will often have to weigh the potential risks of performing the procedure with the benefits of obtaining the CSF. The decision must be made whether the procedure should be performed immediately or can be delayed until further studies are completed. Absolute contraindications to performing an LP include a cellulitis or abscess at the skin puncture site or signs and symptoms of increased ICP except for idiopathic intracranial hypertension (IIH, previously known as pseudotumor cerebri). The LP should be delayed in patients with an unstable airway, hypotension, shock, or status epilepticus until the patient has been stabilized. Hypoxemia, clinical deterioration, apnea, and cardiopulmonary arrest are reported complications of LP in unstable patients. Relative contraindications to performing an LP include the presence of a brain abscess, epidural or subdural fluid collection, brain tumors, and spinal cord tumors.

Note that antibiotics should not be delayed if meningitis is suspected and the LP must be delayed. If meningitis is highly suspected but the patient is unstable, treatment should be initiated with parenteral antibiotics and the LP delayed until the patient's condition is stabilized.

Increased ICP

An LP is relatively contraindicated in the presence of increased ICP.13–15 This includes patients with space-occupying lesions (e.g., tumor or abscess), lateralizing signs such as hemiparesis on the physical examination, or when signs of uncal herniation are present (unilateral third-nerve palsy). Brain herniation or coning has also been reported in patients with meningitis and increased ICP. The sudden drop in ICP induced by an LP may precipitate a pressure cone or herniation.

Pseudotumor cerebri, now known as IIH, has been treated with a combination of medicines and repeated LPs to decrease ICP. Removal of CSF for IIH is a common diagnostic and therapeutic procedure in the Emergency Department. The use of LP in IIH is sometimes questioned. Many of these patients are obese, making an LP more difficult to perform, often requiring multiple attempts, and painful for the patient. The CSF removed reforms within a few hours, thus the LP provides only temporary relief of the patient's symptoms. Consult a Neurologist before performing an LP in the patient with IIH. An LP may be required to make the initial diagnosis of IIH, but may be unnecessary in the patient when the diagnosis has been made previously.

Brain Abscess

Patients with brain abscesses are at high risk for herniation.16,17 Brain abscesses may present with a progressively worsening headache, low-grade fever, and the development of focal neurological signs (e.g., hemiparesis, papilledema, visual field deficits, and mild obtundation). Suspect a brain abscess in patients with a history of otic or paranasal sinus infection, orbital cellulitis, chronic pulmonary or abdominal infection, endocarditis, congenital heart disease, recent dental procedures, dental abscesses, recent neurosurgery, craniofacial trauma, open skull fractures, or recent meningitis.

When Is a CT Scan Indicated before an LP?

It should be emphasized that increased ICP by itself is not necessarily a contraindication to an LP. ICP is usually mildly elevated in patients with IIH or meningitis.18–20 A CT scan does not need to be routinely performed in straightforward cases of suspected meningitis in the patient with a normal neurological examination.21–25 The inability to visualize the optic discs does not constitute a focal finding and, by itself, is not an indication for a head CT scan prior to an LP.23 Despite this, there are occasional cases of herniation in patients with a normal CT scan.13,14,19

Most literature suggests that CT scans be performed prior to LP when patients are comatose or altered, have focal neurological signs, are HIV positive, have a progressively worsening headache, or have papilledema.23–25 The lack of papilledema is not always a reliable sign of normal ICP, as it often takes greater than 48 hours to develop papilledema.21 Papilledema may be absent in up to 15% of adults and up to 50% of children with early increased ICP.

The CT scan should be evaluated for mass lesions, shift of midline structures, or hydrocephalus due to obstructing masses, cisternal obstruction, and cerebral edema.25 There are three findings that may predispose a patient to herniation if an LP is performed.26 The first finding is midline shift. This suggests unequal pressures across the midline. The second finding is a loss of the suprachiasmatic and basilar cisterns. This suggests unequal pressures between the supratentorial and infratentorial compartments. The third finding is any evidence of a posterior fossa mass, obliteration of the superior cerebellar cistern, or obliteration of the quadrigeminal plate cistern caudal to the midbrain. These findings all suggest the presence of increased infratentorial pressure.

Do not delay the initiation of antibiotics if meningitis is suspected and a CT scan is indicated before performing an LP. Administer the antibiotics before the patient undergoes CT scanning. Several studies have shown that delays in the initiation of antibiotics are common in the Emergency Department. These delays are often Emergency Physician generated and result from the need for a CT scan prior to LP, waiting for LP results before administering antibiotics, and not giving antibiotics before a patient is transferred to the ward.27–30

Coagulation Defects

An LP is relatively contraindicated in patients with a coagulopathy. This includes hemophiliacs, those on anticoagulants, and patients with thrombocytopenia. An LP can result in a spinal epidural or subdural hematoma with subsequent spinal cord compression. Appropriate replacement of platelets and/or clotting factors should be undertaken prior to attempting an LP if the procedure can be delayed.31 The most experienced Emergency Physician should perform the procedure with a small gauge needle when an immediate LP is indicated in these patients.

Bacteremia

Some sources list bacteremia as a contraindication to an LP, especially in children. While there is some suggestion that there may be an association between performing an LP in a bacteremic patient and the later development of meningitis, the risk of this is low. An LP should not be withheld for fear of inducing meningitis. The risk of delaying the diagnosis of meningitis clearly outweighs the small chance of causing meningitis with an LP.32

• Sterile gloves and gown
• Face mask and cap
• 1% lidocaine solution
• Povidone iodine or chlorhexidine solution
• LP needles, various gauges and lengths
• Topical anesthetic agent (e.g., EMLA), optional
• LP kit

Most of the equipment necessary for performing an LP is available in prepackaged, sterile, disposable, and single-patient use commercial kits (Figure 115-3). These kits usually contain a 20 gauge Quincke spinal needle, syringes, needles (25 gauge and 22 gauge) for local anesthesia, a manometer with a stopcock, sterile drapes, specimen tubes, 1% lidocaine, gauze, brushes for prepping the skin, and bandages. Some kits provide povidone iodine or chlorhexidine swab sticks whereas other kits have a small basin that needs to be filled. The Emergency Physician should become familiar with the kit used at their institution.

Additional supplies may be needed to perform the procedure without interruption. For example, some Emergency Physicians prefer a smaller gauge Quincke needle or a nontraumatic needle such as a Sprotte or Whitacre that are not often provided in the commercial kits (Figure 115-4). The use of a 25 gauge spinal needle is recommended as it causes a smaller puncture hole and less postprocedural headaches.33–37 It has been suggested that atraumatic needles should be the standard when performing an LP.38,39 In general, it is a good idea to have extra spinal needles, lidocaine, gauze, and skin antiseptic (e.g., povidone iodine or chlorhexidine solution) when performing an LP. There are numerous formulae to determine the LP depth and the length of the spinal needle required.40,41 A reliable formula for estimating the required LP needle length is LP depth (cm) = 1 + [17 × (weight (kg)/height (cm))].41

There are subtle differences among the spinal needles commonly available (Figure 115-4). The standard Quincke needle has a sharp tip with a broad bevel at the end. The Whitaker and Sprotte needles have smaller tips with smaller diameter bevels. The bevel of the Sprotte needle is broader with a rounded tip so as to separate fibers of the dura as opposed to cutting through them. Always remember to keep the bevel oriented parallel to the fibers of the dura when performing an LP regardless of needle style.

Explain the risks, benefits, and complications of the procedure to the patient and/or their representative. Obtain a signed informed consent for the procedure. If the patient is a child or minor, ask the parents or caregivers whether or not they would like to be present for the LP.

There are a variety of different patient positions that can be used to perform the LP (Figure 115-5).33,42,43 Knowledge and proficiency in more than one approach will be useful for the Emergency Physician, especially when encountering a difficult tap. The position of the patient will be chosen based upon the patient's body habitus, their ability to assume a position, their level of cooperativeness, and Emergency Physician preference.

The sitting position is more commonly used in adults than the lateral decubitus position. It is easier to identify the midline and palpate the spinous processes with the patient sitting (Figure 115-5A). The sitting position is particularly useful when patients are obese. While the LP can still be performed in the lateral decubitus position for obese patients, palpating the spinous processes and identifying the midline can be difficult. The measurement of hydrostatic pressure in the CSF is not accurate with the patient sitting. It gives a falsely elevated pressure reading.

The lateral decubitus position is often the most comfortable position for the patient (Figure 115-5B). Place the patient with their knees flexed, the upper back arched to spread the interlaminar spaces, and the neck slightly flexed. Ensure that the patient's shoulders, back, and hips are exactly perpendicular to the stretcher and floor. This will increase the chances of keeping the needle in the midline as it is introduced parallel to the surface of the stretcher. This is particularly important in infants and children where an assistant may be called upon to hold the patient in position. Severe neck flexion is not necessary and can lead to airway obstruction or lack of CSF flow.

Children can be placed in the sitting or lateral decubitus position. An assistant can easily maintain neonates and infants in the sitting position (Figure 115-5C). Toddlers and school-aged children can also often be maintained in the sitting position (Figure 115-5D). The lateral decubitus position can be used for any child (Figure 115-5E). It is important to assess the child visually and with pulse oximetry during the procedure. These positions may cause improper neck flexion that can result in respiratory compromise, oxygen desaturation, hypoxemia, and anoxic encephalopathy.44,45 The largest interspinous space is achieved with the patient sitting upright with their hips flexed, leaning forward, and with slight neck flexion based on ultrasonographic studies.45–48

The subarachnoid space must be entered below the termination of the spinal cord that is situated at the lower level of L1 or the body of L2 (Figure 115-6). Identify by palpation the vertebral spinous processes in the midline and the posterior superior iliac spines. The patient can help to determine the midline.179,180 Just ask them if the needle is in the midline. Another option is to draw a line between the spinous process of C7 and the gluteal cleft.49 An imaginary line connecting the posterior superior iliac spines should intersect the midline at approximately the L4 spinous process or the L3-L4 interspace. One can select any of the spaces between L2-L3 and L5-S1 to perform an LP. Palpate the intended interspace before prepping the area. Some Emergency Physicians mark the site lightly with a pen or make a small indentation with the hub of a needle. Adjust the bed height so that you can sit in a comfortable position while performing the procedure.

Prepare the LP kit. Open the kit using sterile technique and place it on a bedside table. Place povidone iodine or chlorhexidine solution into the basin provided with the kit. Place any additional needles or supplies onto the sterile field. This procedure requires strict aseptic technique. The Emergency Physician should don full sterile and personal protective equipment at this point.50–52 This should include sterile gloves, a sterile gown, a face mask, and a cap. Prepare the stopcock and manometer. The manometer is usually in two pieces that slide together. Insert the manometer into the vertical port of the stopcock. Turn the handle toward the outflow side of the stopcock. In general, the stopcock handle will occlude the port that it points to.

Prepare the patient's back. Clean the skin of any dirt and debris. Apply povidone iodine or chlorhexidine solution using a circular motion from the intended site of entry outward. Allow the solution to completely dry prior to inserting the spinal needle.53 Chlorhexidine solution introduced into the spinal canal can result in arachnoiditis and neurotoxicity. Prepare an area of at least 10 cm in diameter. Most kits include a solid drape and a fenestrated drape. Place the solid drape between the patient's hips and the bed. Place the fenestrated drape with the adhesive side toward the patient's back and the opening centered at the desired level for the procedure.

Reidentify the anatomic landmarks. Place a finger over the desired interspace to use for the procedure. Place a skin wheal of local anesthetic solution subcutaneously over the desired interspace using a 25 gauge needle. Infiltrate and anesthetize the deeper tissue of the interspace along the projected needle track using the 22 gauge needle. The infiltration of local anesthetic solution, unless contraindicated, should be used in all patients, including neonates and young children. Local anesthetic makes it more likely the LP will be successful.46,47,54

Alternatively, a field block can easily be performed to produce anesthesia of the skin, interspinous ligaments and muscles, and the periosteum. The interspinous ligament and the periosteum are supplied by the recurrent spinal nerves branching off the nerve roots exiting the spinal canal at the same level. Inject local anesthetic solution into the interspinous ligaments, between the spinous processes superior and inferior to the intended puncture site, and on either side of the interspinous space (Figure 115-7).33 A topical anesthetic (e.g., EMLA cream) may be applied over the interspace for 30 to 60 minutes prior to performing the LP if the patient is awaiting a CT scan of the head and antibiotics have been administered. Unfortunately, this only anesthetizes the skin and superficial subcutaneous structures. The patient will still require a local anesthetic injection.

It may occasionally be difficult for some patients to remain still and cooperative with the procedure. This can include anxious patients, those with an altered mental status, and small children. These patients may require an intravenous anxiolytic, nitrous oxide administration, or procedural sedation.

Lateral Decubitus Position and Midline Approach

Palpate the intended interspace. Introduce the needle in the middle of the interspace and parallel to the bed. Orient the bevel of the spinal needle parallel to the longitudinal dural fibers to increase the chances that the fibers will be separated rather than cut by the tip of the needle.55 This has been shown to decrease the incidence of postdural puncture headache.56–58 The bevel should point up or down with the patient in the lateral decubitus position. Angle the needle 10° cephalad, or toward the umbilicus, and advance it slowly. The needle can be held between both index fingers and advanced with the thumbs (Figure 115-8A). Alternatively, it can be guided with a thumb and forefinger near the puncture site while the other hand holds the hub of the needle and advances it (Figure 115-8B).

Resistance will usually be felt as the needle penetrates the interspinous ligaments. Stop advancing the needle and remove the stylet frequently to check for the presence of CSF.59 Many describe a characteristic “pop” that is felt when the needle enters the subarachnoid space. The commonly used Quincke needles often decrease or eliminate this sensation. It may also not be felt using atraumatic needles.59 If bone is encountered, withdraw the needle to the subcutaneous tissue, confirm your landmarks, and readvance the needle in the midline. If bone is still encountered, redirect the needle slightly more cephalad and readvance it. Perform the procedure at a different level if still unsuccessful.

CSF should flow freely once the subarachnoid space is entered. The rate of CSF flow from the hub of the spinal needle may be slow. The flow rate can be increased by one of the following: ask the patient to Valsalva by gently coughing or bearing down, rotate the spinal needle 90°, or repeat the procedure with a larger bore spinal needle. Attach the stopcock and manometer directly to the needle. Alternatively, use the short extension tubing provided in most kits to connect the needle to the manometer. Hold the hub of the needle firmly between the thumb and index finger and brace your hand against the patient's back when attaching or removing anything from the spinal needle. This will prevent the needle from advancing or withdrawing. The stopcock handle should point posteriorly and CSF will begin to fill the manometer.

Ensure that the manometer hub remains at the level of the needle in order to get an accurate reading if using the extension tubing. Phasic changes with respirations should be noted as the manometer fills. Instruct the patient, or an assistant, to gently extend the patient's legs to decrease intraabdominal pressure and lower the reading. Normal opening pressure is 70 to 180 mmH2O. Obtain the pressure reading once the CSF flow stops. Turn the stopcock handle toward the needle hub (or patient) to empty the contents of the manometer into the first tube for collection. To continue collection, remove the stopcock or simply remove the manometer and continue collection through the stopcock by pointing the handle toward the manometer port.

In general, 1.0 mL of CSF in each of the four tubes should be adequate to perform the CSF analysis. Collect 2 mL in each tube if cytology or antigen testing is necessary. When the samples have been collected, carefully replace the stylet and remove the needle.

There is disagreement regarding the use of the spinal needle stylet.181 The literature notes an increased incidence of intraspinal epidermoid tumors after an LP if a stylet is not used. This led to the habit of leaving the stylet in place while inserting the spinal needle to prevent transferring epidermal cells into the spinal canal. Some now recommend removing the stylet after the tip of the spinal needle has been inserted past the epidermis.46,60 This method has shown to result in a greater success rate when performing an LP.

Reinserting the stylet into the spinal needle is potentially problematic. Many will hold and stabilize the spinal needle with one hand and reinsert the stylet with the other hand. This risks a potential needlestick injury. The author inserts the stylet with one hand without using the other hand to stabilize the spinal needle. Reinserting the stylet prior to removal of the spinal needle has been shown to decrease the incidence of the post-LP symptoms of dizziness, headache, nausea, and tinnitus.61 The method of using the stylet is left to Emergency Physician preference.

Sitting Position and Midline Approach

Place the patient sitting on the edge of the bed. Ask the patient to flex their lower back and lean forward onto some support, such as an assistant or bedside stand, in order to open the interlaminar spaces in the lumbar area. Orient the bevel of the needle laterally (to the left or right). The remainder of the procedure is the same as previously described.

Lateral Approach

The lateral approach may be useful in avoiding the calcified supraspinous and interspinous ligaments often encountered in elderly patients. This approach may be performed with the patient in the lateral decubitus position or the sitting position. Though it is less commonly used than the midline approach, it is a good idea for the Emergency Physician to become familiar with this technique as an alternate approach if the midline approach has failed. This may prove easier in the patient who has had multiple previous midline LPs.

Position the patient and select an appropriate interspace. Cleanse, drape, and anesthetize the area as previously described. Insert the spinal needle 1.5 to 2.0 cm lateral to the midline. The needle can approach from either side (left or right) if the procedure is being performed in the sitting position. Approach from the inferior side if performing the LP in the lateral decubitus position (Figure 115-9A). Direct the needle 10° cephalad and approximately 20° to the midline. This angle will direct the needle through the erector spinae muscles and lateral to the supraspinous and interspinous ligaments. The needle will penetrate the ligamentum flavum, the dura, and then the subarachnoid space (Figure 115-9B). If bone is encountered, partially withdraw the needle and redirect it at the same angle toward the midline but slightly more cephalad. The remainder of the procedure is as described previously.

LP in Infants and Children

Performing an LP in an infant or child is similar to that of an adult. Place the patient in the lateral decubitus position or the sitting position. Place the neck in midflexion in the lateral decubitus position. Severe flexion of the neck does not facilitate the procedure and can result in the lack of CSF flow or airway obstruction.

Hypoxemia has been reported during LP in infants. The increased intraabdominal pressure caused by flexing the knees into the abdomen may lead to compression of the diaphragm, ventilation-perfusion mismatch, and hypoxemia.62 For this reason, the sitting position or modified lateral decubitus position (hips only flexed to 90°) is preferred.63 Preoxygenation with 100% oxygen via face mask for 2 to 5 minutes may prevent hypoxemia.64 Consider the use of continuous pulse oximetry in infants and young children undergoing an LP.

Some authors advocate the use of a butterfly needle and using the tubing as a manometer to get a general idea of the opening pressure. The use of nonstyletted needles, however, may occasionally result in the implantation of cells and a subsequent epidermoid tumor. In some low ICP syndromes, CSF may fail to flow during the procedure and gentle suction with a 1 mL syringe can be used.

Local anesthesia should be used in all patients, even in neonates. There is evidence that pain perception is present even in premature neonates.65 The use of local anesthetic is often omitted in the neonate and young infant, possibly in fear of obscuring anatomical landmarks. Pinheiro et al. studied the success rate of LP in neonates given local anesthetic, the amount of struggling during lidocaine injection, and the amount of struggling during spinal needle insertion.66 They found that local anesthesia did not alter the success rate of the procedure and led to a decreased amount of struggling during spinal needle insertion.

The use of a eutectic mixture of local anesthetics (EMLA) cream or similar topical anesthetic is common and effective for venipuncture in children. It also can be used successfully before LP in children and adults.67,68 EMLA has been shown in some studies on adults to be more effective than lidocaine infiltration.69,70 The major disadvantage of EMLA cream is that it requires application for a minimum of 30 minutes before the procedure is performed. In general, it is more effective if it stays on longer. It also does not anesthetize the deeper tissues and local infiltration is still required.

Procedural sedation is usually reserved for those children getting routine LPs for intrathecal chemotherapy. If absolutely necessary, procedural sedation can be used while performing a diagnostic LP. Procedural sedation should not be used unless the child has a normal mental status and is hemodynamically stable. The decision regarding procedural sedation must be considered on a case-by-case basis.

For the past several decades, Anesthesiologists have been using ultrasound (US) to assist in various spinal procedures. More recently, Emergency Physicians have reported on the use of US to assist in performing an LP or to evaluate spinal anatomy.71–75 Peterson and Abele described two patients in whom multiple attempts at landmark-guided LP were unsuccessful. US-guided LP was performed easily and was successful in each patient. The authors concluded that the technique has great potential as a time-saving tool.73 Nomura and colleagues performed a randomized, double-blind study comparing LP using landmarks identified by palpation versus those identified by US.75 The use of US significantly reduced the number of failures in all patients and improved the perceived ease of the procedure in obese patients. A group of Radiologists studied US guidance for LPs in children.76 They found US superior to fluoroscopy as it allows three-dimensional guidance in real time and provides visualization of soft-tissue structures.

With the increasing availability of US machines in Emergency Departments, ultrasonography is more readily available to assist the Emergency Physician in performing many procedures including an LP.71,72 US can visualize both superficial and deep tissue structures. Emergency Physicians who consider using US in assisting with an LP should be properly trained in the use and theory behind US before attempting to use it in the clinical setting.

There are three possible approaches to establishing the indications for the use of US to guide an LP. US guidance can be used for an LP when the traditional landmark-guided approach has failed. This approach may result in more patients having multiple attempts at an LP. US guidance can be used as the initial method in patients predicted to have a higher failure rate with landmark-guided LP. This includes patients with spinal landmarks that are difficult or impossible to palpate.72,77,78 The landmarks are increasingly difficult to palpate in the body mass index (BMI) categories of overweight (BMI = 25 to 29.9) and obese (BMI ≥ 30) patients.71 Under this approach, patients in whom an LP is not predicted to be difficult would have it attempted initially with the landmark palpation method, reserving US guidance for failures. The final approach uses US guidance initially for all patients, regardless of their BMI or the predicted ease of an LP.

US can be used to visualize structures and mark the point of needle insertion. It can also be used to visualize the deep soft tissue structures, allowing the Emergency Physician to estimate the depth that the needle needs to be inserted to enter the subarachnoid space.79

A high-frequency linear US probe is the transducer of choice. In patients with a higher BMI, the linear probe may not allow visualization to the depth required. In these cases, use a lower-frequency US probe, generally a curvilinear array. A pen or surgical skin marker is needed to mark the skin at the point of needle insertion. Surgical skin markers have the advantage of using scrub-resistant, non-smearing ink.

Both transverse and longitudinal midline images centered over the lumbar spine are utilized.71,72 Ferre and Sweeney indicated a preference for the paramedian longitudinal view.72 This view was felt to allow better visualization of deep soft tissue structures such as the ligamentum flavum and dura mater. In the transverse view, the spinous process causes a shadow that extends from near the top to the bottom on the monitor screen (Figure 115-10).73,77 In the longitudinal view, the distal tips of the spinous processes are seen as a series of echogenic convexities (Figure 115-11).

Place the patient into the position they will be in during the LP, whether the lateral decubitus position or sitting upright. It is important that the patient be in the exact position, such as with spinal flexion and their knees drawn up, that will be used during the LP. Any change in patient position between the time of the US and the LP could change the location of the landmarks and decrease the likelihood of success.

Use US to locate the entry site of the spinal needle. Using the transverse view at the level of the iliac crests, move the US probe until the shadow caused by the spinous process is centered on the monitor screen (Figure 115-10). Use the skin marker to place a mark on each side of the US probe, exactly at its midpoint (Figure 115-12). When these two lines are connected, they will form a single line that marks the midline of the spine. Rotate the US probe 90° to a midline longitudinal view. Move the US probe until the tops of two adjacent spinous processes are seen, with the gap between them located in the center of the screen (Figure 115-11). Again, make two marks on the patient's skin, one on each side of the US probe at its midpoint (Figure 115-13). These two marks, when connected, will form a single transverse line that indicates the center of the gap between adjacent spinous processes.

Set aside the US probe. Connect the two pairs of skin marks to form two lines that intersect at a right angle (Figure 115-14). Their intersection marks the site of needle entry (Figure 115-14). Clean and prep the patient's skin and perform the LP as described previously.

There are numerous risk factors for obtaining a traumatic or unsuccessful LP.60,80 It is important to recognize these, as traumatic or unsuccessful attempts may cause diagnostic ambiguity and lead to unnecessary antibiotic use, hospitalization, and patient discomfort. In the setting of a high likelihood of a difficult LP, steps may be taken to minimize patient discomfort and maximize the chance of a successful LP. Have the most experienced Emergency Physician perform the procedure. If there is a high likelihood of a traumatic LP and there is a high suspicion of a SAH, consider the use of US or fluoroscopic guidance. Emergency Physicians should never delay administering antibiotics, or other potentially lifesaving treatment, because an LP may appear difficult.

In adults, the inability to either visualize or palpate the spinous processes is predictive of a difficult LP. The inability to visualize the spinous processes is associated with an increased risk of a traumatic LP.80 As described previously, the use of bedside US can assist in the identification of the spinous processes.

There are several risk factors in children associated with an unsuccessful or traumatic LP.60 These include patient-related factors such as young age and inability to visualize the spinous processes, physician factors such as less experience performing LP, and procedural factors such as no local anesthetic use, patient movement, and advancement of the spinal needle with the stylet in place.60

Physicians have traditionally been taught to keep the stylet in place as the spinal needle is advanced into the subarachnoid space as this will avoid the introduction of epidermal cells into the subarachnoid space and the subsequent development of an epidermoid spinal canal tumor. However, the stylet-out technique has not been linked to this complication as long as the stylet is used as the needle penetrates the skin.60 The greatest benefit of stylet removal appears to occur in young infants when the Emergency Physician is less likely to feel the spinal needle penetrate the dura. The stylet-out technique allows for continuous monitoring of penetration into the subarachnoid space by direct visualization for CSF return.

Clean the excess povidone iodine or chlorhexidine from the patient's back and apply a dressing or bandage to the puncture site. Immediately place the patient supine to decrease the potential for a postdural puncture headache (PDPH) and decrease the risk of local bleeding. Recumbent positioning will decrease the postural headache that sometimes follows LP, but it has not been shown to decrease the incidence of PDPH.

The use of proper technique is essential when performing an LP. It is also important that the Emergency Physician is aware of potential complications, how to recognize them, and how to manage the complications that can result from an LP. Refer to the article by Evans for a complete review of LP complications.81 The PDPH is the most common complication and cerebral herniation is the most immediately life-threatening complication. Localized cellulitis, dural abscesses, discitis, and localized bleeding are also potential complications.

Cerebral Herniation

The most serious complication that may result from an LP is brain herniation or coning.14 Theoretically, if a large pressure gradient exists between the cranial and lumbar compartments, herniation across the tentorial incisura or foramen magnum may occur after removal of CSF from the lumbar area. Patients with increased ICP secondary to intracranial mass lesions, cerebral edema, and acute hydrocephalus are at greater risk for cerebral herniation or coning. Herniation has also been known to occur in patients with meningitis. For many years, the role of LP in precipitating brain herniation has been the subject of debate. Several studies suggest that an LP is relatively safe in the patient with increased ICP.82–84 However, each individual patient's risks and benefits must be considered before proceeding.

Herniation has resulted from an LP in patients with meningitis and SAH.85–87 The actual role that an LP has in precipitating or facilitating the process of herniation is not known.85 Patients with decorticate or decerebrate posture, focal neurologic signs, or no response to pain should receive antibiotics but not a LP; this is true even in the face of a normal CT in suspected cases of meningitis.85,87 Deterioration after an LP has been reported in patients with a SAH. Fortunately, it is a rare outcome as a result of an LP. A CT scan should be obtained before performing an LP if there is a suspicion for a SAH.88,89

Postdural Puncture Headache

The PDPH is the most common complication of an LP. It is thought to be the result of continued CSF leakage at the puncture site. The reason why this causes a headache is unclear.90 It is thought that the lower CSF pressure induced by the leakage causes the brain to “sag.” This leads to traction on pain-sensitive structures in the brain such as the dura, nerves, and bridging veins. Intracranial venous dilation and increased brain volume may lead to a neurohumoral response identified as pain.

The headache begins within 24 hours of the procedure in 65% of cases and within 48 hours in 90% of cases.91 Delayed development of a PDPH 5 to 14 days after the procedure has been reported. The headache typically resolves within 7 days. It has been reported to last several months in rare individuals. The headache is usually located in the frontal or occipital area. It may vary in intensity. The PDPH is usually described as bilateral pressure that is throbbing or achy and improves with supine positioning. Associated symptoms may include nausea, vomiting, neck stiffness, auditory symptoms, and vestibular symptoms.91

The incidence of PDPH has been reported to be anywhere from 1% to 70%. The wide range is most likely due to the fact there are several identifiable risk factors that influence its development. Age and gender play a significant role.91–93 The highest incidence occurs in 18 to 30 year olds. There seems to be a decreased incidence after the age of 60, the reason for which is unknown.

The type of needle and its diameter also influence the development of a PDPH.34–38 This is based upon the amount of trauma and the size of the rent it makes in the dura. Smaller diameter needles and atraumatic needles lower the incidence of PDPH. However, a 22 or larger gauge needle must be used to determine the opening pressure and to collect samples in a timely fashion when performing a diagnostic LP.94 The bevel orientation should be parallel to the longitudinal dural fibers when using a Quincke needle. This significantly reduces the incidence of PDPH.58,95–97 The use of an atraumatic needle may also decrease the incidence of PDPH.98,99 These needles are designed to separate rather than shear the dural fibers. Replacement of the stylet before removing the spinal needle has been shown to decrease the incidence of PDPH.100 Repeated dural punctures have been associated with an increase in the incidence of PDPH.101

Other nonproven risk factors for a PDPH include psychogenic factors, the rapidity of CSF withdrawal, race, patient positioning, and hydration status. Bed rest for 24 hours was often widely recommended. However, it has not been shown to decrease the incidence of PDPH.102–104 Other studies have shown bed rest to increase the risk of PDPH.105,106 Early ambulation does not increase the incidence of PDPH.107 Dehydration was once felt to impair the patient's ability to produce CSF to compensate for the leaking CSF. Dieterich and Brandt found that the incidence of PDPH was independent of daily fluid intake.108 They postulated that it is the closure of the dural defect and not the CSF loss that is the critical factor in the termination of the PDPH.

Supine positioning can provide some symptomatic relief for initial or mild PDPHs. A single 300 mg dose of oral caffeine may provide transient relief.109 While offering no advantage over caffeine, an oral dose of theophylline can also be given. Administer 500 mg of intravenous caffeine, or 5 to 6 mg/kg of intravenous aminophylline, for more severe headaches. In the only study to date describing the use of intravenous caffeine, Sechzer and Abel reported an approximately 70% success rate in treating PDPH.110 A more recent study suggests that intravenous caffeine administered prophylactically may minimize the incidence of PDPH.111 There is some promise in the use of triptans.112,113 The use of cosyntropin, an ACTH analog, has shown promise in the treatment of PDPH.114,115 An occipital nerve block may resolve a PDPH.116,117

A more definitive but invasive treatment for the PDPH is the epidural blood patch.118 This procedure is to be performed by an Anesthesiologist. It involves injecting 10 to 20 mL of autologous blood into the epidural space at the level of the previous LP. The blood acts to tamponade any further CSF leakage and allows healing of the dural rent. Epidural blood patching is successful in 85% of patients after one injection and about 98% of patients if a second blood patch is required.119–121 Epidural blood patching should be performed no sooner than 24 hours after the LP. Complications of the blood patch include back pain, paresthesias, radiculopathies, and weakness; all of which are transient. A rare complication is the spinal subdural hematoma.122 Other modalities for PDPH relief that have been used but not widely studied are epidural saline injections, dextrose injections, gelatin injections, and epidural morphine.

Infections

Local infections including cellulitis, abscesses (lumbar epidural or spinal cord), and discitis can result from an LP. Performing an LP through an area with a local infection, such as a cellulitis or an abscess, can introduce bacteria into the CSF and lead to meningitis. Contamination of the needle by airborne pathogens can also occur. Always wear a sterile gown and gloves, a mask, and a cap while performing an LP.50 It is possible for the Physician's oral flora to contaminate the field and equipment, resulting in an iatrogenic meningitis.50–52

It was once thought that an LP could induce meningitis in a bacteremic patient. Further studies have shown this idea to be unfounded.123–125 Bacteremia is not a contraindication to performing an LP. Proper cleaning and disinfecting of the skin, avoiding infected areas with the spinal needle, and using aseptic technique will minimize but not eliminate any risk of infection.

Hemorrhage

A traumatic LP is a common occurrence. Up to 72% of LPs have anywhere from 1 to over 50 red blood cells.126 This is a common and usually uncomplicated occurrence in patients with a normal coagulation profile. Traumatic LPs can result in a spinal epidural or spinal subdural hematoma in patients with or without coagulation abnormalities.127–131 Epidural hematomas most likely result from needle trauma to the internal vertebral plexus or radicular vessels (Figure 115-15).126 The radicular vessels course down the length of each nerve root. It has been suggested that the bevel of the spinal needle can induce trauma and bleeding to these vessels much like they produce paresthesias when touching the nerve roots. Spinal epidural hematomas, if large enough, can result in a cauda equina syndrome.132

Subdural or SAH is a rare but catastrophic complication in patients with or without a coagulopathy.130 Edelson et al. recommend that the procedure be performed only if absolutely necessary in patients with a thrombocytopenia.133 Platelets should be transfused prior to an LP in patients with platelet counts less than 20,000 or if platelet counts are dropping rapidly. The most skilled Emergency Physician should perform the LP using a 22 gauge (or smaller) needle.133 Patients should be observed after the procedure for the development of neurological signs suggesting a hematoma. Such signs include paraplegia, lower extremity weakness, sensory deficits, or incontinence.

Another rare bleeding complication is an intracranial subdural hematoma.134,135 This may result from the same mechanism causing a PDPH, namely the downward displacement of the brain from decreased CSF volume and persistent leakage after an LP. This may occasionally cause tearing of the bridging veins and lead to a unilateral or bilateral subdural hematoma. Suspect this diagnosis when a headache sounding like a PDPH lasts for more than a week, is no longer postural in nature, or returns after initially improving.136

Miscellaneous Complications

Neuropathies involving cranial nerves III, IV, V, VI, VII, and VIII have been reported. They most likely result from traction on the nerves caused by low ICP after the LP. Typical complaints may include visual and auditory symptoms. Epidural fluid collections of CSF after an LP can be significant.137 They usually resolve spontaneously with time but can compromise the thecal sac if large enough.

Mild low back pain is a common complaint that results from the local trauma of the needle tract. Transient dysesthesias are fairly common, resulting from spinal needle contact with the nerve roots. A spinal needle that passes beyond the subarachnoid space into the annulus fibrosis can cause disc herniation. This can also result in a discitis and vertebral collapse.138

Intraspinal epidermoid tumors are composed of well-differentiated stratified squamous epithelium surrounding a mass of caseous substance formed by the desquamation of epidermal tissue. They are often congenital but may result from the introduction of epidermal fragments into the spinal canal. This may occur if the stylet of the needle is not used. Spinal epidermoid tumors may present months to years after an LP.139–142

A dry tap is often the result of lateral displacement of the spinal needle. Maintain the spinal needle in the midline while it is being advanced. Not penetrating deep enough with the spinal needle can also result in a dry tap. This is especially true in obese patients that may require long, 7 to 10 in., spinal needles to gain access to the subarachnoid space. It has been suggested that the standard 3.5 in. spinal needle is adequate for 97% of patients.143

Prevent secondary injury by always inserting the stylet into the spinal needle before removing it.61 Withdrawing the spinal needle without the stylet can result in the aspiration of a lumbar nerve root or arachnoid tissue into the epidural space. If this occurs, the patient may require a laminectomy to replace the nerve root or arachnoid tissue. The reinsertion of the stylet will also decrease the incidence of post-LP symptoms (e.g., dizziness, headache, nausea, and tinnitus).61

It is now common for adolescents and adults to have tattoos on their lower back. It is recommended to perform an LP in an area void of tattoo ink, at a higher or lower interspace, or nicking the skin with a needle or scalpel prior to inserting the spinal needle.144,145 All these techniques avoid the spinal needle penetrating through the tattoo ink. The tattoo ink may contain substances that can be irritating or toxic if introduced into the spinal canal.

Proper interpretation of the CSF is an important skill for the Emergency Physician who performs the LP. Tables 115-1 and 115-2 list the normal CSF values and the CSF values in a variety of different medical conditions.146,147

CSF Pressure

Normal CSF pressure ranges from 70 to 180 mmH2O in adults and from 50 to 80 mmH2O in infants and children. Note that many manometer kits use cmH2O on the demarcations whereas results are commonly interpreted in mmH2O. Elevated CSF pressure may be seen in bacterial meningitis, viral meningitis, brain abscesses, tuberculous meningitis, fungal meningitis, encephalitis, meningeal carcinomatosis, SAH, pseudotumor cerebri, and Guillain–Barré syndrome. It may be falsely elevated when the patient is tense or creating a lot of intraabdominal pressure by flexing their knees into the abdomen. The pressure will also be falsely elevated if the patient is in a sitting position. Although CSF pressure is not routinely recorded in infants and children (most likely because they are often crying, struggling, or difficult to hold), it should be recorded whenever possible. Low CSF pressure may be the result of a spinal root obstructing the flow of CSF into the needle or obstruction of flow from a spinal mass.

A novel device to measure pressure is the Compass LP (Mirador Biomed, Seattle, WA). This device is a single patient use and disposable unit that attaches to the spinal needle. It provides a digital readout of the CSF pressure. Further research is required before this device replaces the standard manometer.

Cell Counts and Differential

A variable amount of white blood cells (WBCs) may be normally present in the CSF depending upon the age of the patient.148 Neonates may have up to 32 WBCs/mm3 with 60% polymorphonuclear leukocytes (PMNs). Infants 4 to 8 weeks of age may have up to 22 WBCs/mm3. However, most sources consider anything greater than 8 to 10 WBCs/mm3 to be abnormal.149,150 Normal adult CSF should contain no more than 5 WBCs/mm3 with a differential of mononuclear cells or lymphocytes. The presence of more than one PMN should be considered abnormal.

In cases of bacterial meningitis, cell counts are usually greater than 500 WBCs/mm3 with a predominance of PMNs; though lymphocytosis can uncommonly occur.151 The CSF will usually contain less than 1000 WBCs/mm3 in patients with viral meningitis and have a differential of 100% lymphocytes. Early in the course of viral meningitis (the first 48 hours), 20% to 75% of patients will have a predominance of PMNs in the CSF, making it difficult to distinguish it from bacterial meningitis.152,153 Within 8 to 12 hours, approximately 90% of patients will show a switch to a mononuclear pleocytosis on repeat LP.154

Normal cell counts and differentials do not always exclude meningitis. Approximately 95% of the population does not normally have any PMNs in their CSF. The presence of one PMN could represent an abnormality. One PMN may be seen in approximately 5% of normal children. Bonadio and colleagues reviewed 424 LPs of which 106 had PMNs but no pleocytosis.155 All 106 patients had negative Gram's stains and cultures. The authors concluded that the older child without pleocytosis or abnormal CSF chemistries can be considered at very low risk for meningitis. If meningitis is suspected and the CSF is normal or has PMNs, close clinical observation and hospitalization for treatment and repeat LP should be considered until CSF culture results are negative.

A traumatic LP can often make the interpretation of the CSF difficult as peripheral WBCs can be introduced into the CSF. Clearing of the red hue of the CSF from the first to last tube suggests that the tap was traumatic. However, this is not always a reliable sign. Traditionally, the ratio of the RBCs to the WBCs in the blood is compared to the ratio in the CSF. This is based upon the assumption that when blood is introduced into the CSF, the ratio of RBCs to WBCs should stay the same. Some use a set RBC to WBC ratio of 750:1 or 500:1. This is not always accurate as a peripheral leukocytosis may often be present. By comparing the ratios, a predicted WBC count for the CSF can be obtained (predicted CSF WBC = CSF RBC × blood WBC ÷ blood RBC). The actual WBC count will then be the predicted WBC subtracted from the observed or measured WBC (actual CSF WBC = observed CSF WBC — predicted CSF WBC).

Most studies on patients with traumatic taps that did not have meningitis have shown that these formulae are often inaccurate.156 Oftentimes, the observed CSF WBC count is less than the predicted CSF WBC count.157 This raises concerns that in the presence of meningitis, the diagnosis could be missed. Mayefsky and Roghmann studied the use of the formula in patients that had meningitis and found that it led to many false-positive and false-negative results.158 They investigated the value of an O:P ratio (observed CSF WBC/predicted CSF WBC) in predicting the presence of meningitis. They found that a ratio greater than 10 had a sensitivity of 88% and a specificity of 90% in predicting culture positive meningitis. They concluded, along with others, that pleocytosis in bacterial meningitis is rarely masked by a traumatic tap.158,159 The O:P ratio of ≤0.01 can be used to identify patients with traumatic LPs that do not have meningitis.160

Glucose

Normal values for CSF glucose are listed in Table 115-1. Compare the ratio of CSF glucose to a simultaneously determined blood glucose level to determine if low CSF glucose (hypoglycorrhachia) exists. The ratio is abnormal in preterm infants if it is lower than 0.5 to 0.6. A ratio of less than 0.4 to 0.5 in children and adults is abnormal. Approximately 58% of patients with bacterial meningitis will have a glucose of <40 mg/dL. The sensitivity for detecting bacterial meningitis increases to about 70% if a CSF-to-serum glucose ratio of <0.31 is used.161 The normal steady state of 0.6 tends to decrease as serum glucose increases. Ratios of less than 0.3 should be considered abnormal in cases of severe hyperglycemia. The CSF-to-serum glucose ratio is less accurate when there are rapid changes in the serum glucose. A low CSF-to-serum glucose ratio should always raise the concern of bacterial or fungal meningitis. Other conditions such as tuberculous or syphilitic meningitis, meningeal carcinomatosis, or SAH can also be the etiology. Approximately 15% to 20% of patients with a SAH will have hypoglycorrhachia.162,163 Normal CSF-to-serum glucose ratios are usually seen with aseptic meningitis, encephalitis, brain abscesses, and subdural empyemas.

Protein

The normal CSF protein levels are listed in Table 115-1. Elevated CSF protein levels, often greater than 150 mg/dL, are seen in acute bacterial meningitis. Others causes of increased CSF protein include any type of meningitis, encephalitis, CNS tumors, SAH, demyelinating syndromes, and a traumatic LP.164 Correct the CSF protein by subtracting 1 mg/dL of protein for each 1000 RBCs in traumatic LPs.

Gram's Stain

The Gram's stain is a very reliable test when performed by properly trained individuals. In general, it is positive in identifying approximately 80% of bacterial CNS infections.165 The probability of detecting bacteria on a Gram's stain depends upon the number of bacteria present in the CSF.166 Approximately 25% of smears are positive with ≤103 colony-forming units (CFU)/mL, 60% with 103 to 105 CFU/mL, and 97% with >105 CFU/mL. False negatives can result from partially treated meningitis where the sensitivity decreases to about 60%.167 False positives can result from the use of contaminated LP trays or reagents, or the use of an unoccluded spinal needle.168

CSF Cultures

CSF cultures should be obtained in all patients suspected of having meningitis. Positive cultures are assumed to be 100% specific but may only occur in 80% of patients thought to have bacterial meningitis.169 Transport the CSF specimens to the laboratory promptly, as H. influenza and meningococcus will not survive storage or variations in temperature. In general, antibiotics given prior to LP can sterilize the CSF. However, depending upon the amount of bacteria in the CSF and the elapsed time from initiation of antibiotics, there is probably a window of about 2 to 3 hours where antibiotics do not affect the culture results. The percentage of positive CSF cultures decreases from 33% to 4% and Gram's stains from 41% to 7% when antibiotics are given prior to LP.170 Blazer et al. studied the effect of full intravenous antibiotic treatment on CSF cultures by performing an initial LP and then a repeat LP in 44 to 66 hours.171 All but one of the cultures became negative whereas the cytology and biochemistry were not affected. They concluded that partial treatment with antibiotics may alter the culture results but does not distort the other characteristics of a “bacterial” CSF.

Subarachnoid Hemorrhage

It is imperative to interpret the CSF results correctly when an LP is performed after a negative head CT to rule out the possibility of a SAH. Most sources agree that the presence of xanthochromia, which results from lysis of red blood cells, confirms the presence of intracranial bleeding.172,173 Xanthochromia, when measured by spectrophotometry, has a sensitivity that approaches 100% when performed between 12 hours and 2 weeks from the initial SAH. Do not accept gross xanthochromia as a positive finding.174–176 Gross xanthochromia can be a false positive. Always rely on a spectrophotometric determination of xanthochromia. Xanthochromia can develop in specimens within a few hours if the tap was traumatic.177 One cannot rely on the finding of xanthochromia if the WBC count is >10,000 or if the analysis of CSF samples is delayed.177

So how should patients that present within 12 hours of their symptom onset be managed? Delaying a LP for 12 hours would require holding patients in the Emergency Department or admitting everyone who required an LP for evaluation. This presents a legitimate logistical problem. Edlow et al. suggest that patients that have a negative CT should undergo immediate LP.178 If the CSF is persistently bloody without xanthochromia, and clinical suspicion is high, vascular imaging should be the next step.

Most LPs will be performed in suspected cases of meningitis or SAH after a negative head CT. The risks of performing an LP need to be weighed against the potential benefits of diagnosing these two potentially life-threatening illnesses promptly. Knowledge of the proper indications, contraindications, technique, and interpretation of the CSF findings will undoubtedly help the Emergency Physician to minimize the complications that can be associated with the procedure. Although most complications are rare, awareness of their existence, presentation, and proper treatment is imperative.