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 circulation around the brain and upper spinal cord.
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