Chapter 156. Intraocular Pressure Measurement (Tonometry)

This chapter is designed to help the Emergency Physician know when it is necessary to measure intraocular pressure (IOP) and reviews several reliable methods of tonometry to measure IOP. There are multiple traumatic, pathologic, and postsurgical causes of altered IOP. The clinical signs and symptoms of elevated IOP are similar regardless of the etiology. Digital palpation is the oldest and simplest form of tonometry and remains useful in select situations. Schiøtz indentation tonometry is discussed for historical purposes but is still an accurate method to measure IOP. The nonportable Goldmann or applanation tonometer serves as the standard for measuring accurate IOP. It requires the use of a slit lamp and can be difficult to master. The handheld Perkins and Kowa tonometers are based on the same principle as the Goldmann and require experience to use effectively. The electronic Tono-Pen is best known to most Emergency Physicians and is discussed at length. It is useful to become comfortable with one or more of these techniques because early detection of abnormal IOP can prevent irreversible vision loss.

Aqueous humor is produced by the ciliary body in the posterior chamber of the eye, directly behind the iris (Figure 156-1). Most of the aqueous humor flows forward, through the pupil, into the anterior chamber. It drains out of the eye through the trabecular meshwork located at the angle where the cornea and iris meet. This is the area referred to in open angle, narrow angle, and angle closure glaucoma. In healthy eyes, aqueous humor production is equivalent to outflow. IOP reflects the pressure of the ocular contents and by convention is expressed in millimeters of mercury or mmHg.1 The mean IOP in the general population is 16 mmHg with a standard deviation of 3 mmHg.2 Therefore, normal pressure is considered to range from 10 to 22 mmHg.

Aqueous humor production and outflow can be dramatically affected by disease or injury of the eye. Even small changes in IOP over long time periods can be vision threatening. However, significant increases in pressure can cause rapid and irreversible damage to vision in just a few hours. Nontraumatic conditions that result in an elevation of IOP include primary angle-closure glaucoma and secondary angle-closure glaucoma. Traumatic conditions associated with elevated IOP include retrobulbar hemorrhage, hyphema, and traumatic iritis. Conditions associated with low IOP that threaten vision include penetrating trauma and postsurgical complications.

Patients with primary or secondary acute angle-closure glaucoma often present with ocular pain and decreased vision, usually in one eye. They may describe a headache in the brow region, with or without associated nausea and vomiting. External examination frequently reveals that the conjunctiva is erythematous, the cornea appears milky or hazy, and the pupil is slightly dilated with a sluggish response to light (Figure 156-2).

Traumatic retrobulbar hemorrhage can result in markedly elevated IOP. Patients will present with a painful proptosis and fullness of the periorbital tissues. There is usually restricted movement of the eye in one or more fields of gaze. Acute onset of retrobulbar inflammation will present the same way, but there is no history of trauma. A spontaneous nontraumatic retrobulbar hemorrhage can occur in patients with a coagulopathy. Refer to Chapter 162 regarding the complete details and management of a retrobulbar hemorrhage.

Traumatic iritis may result in an elevated or a reduced IOP. Traumatic iritis presents following blunt, nonpenetrating trauma to the eye. Inflammatory cells circulating in the anterior chamber may impede aqueous outflow resulting in high IOP. Iritis may also affect the ciliary body causing a decrease in the production of aqueous humor and a lower IOP. These patients present with eye pain and photophobia. They usually describe their vision as slightly blurred and give a history of trauma within the last 48 hours. The history of recent trauma and light sensitivity is important given that the external examination of the eye may appear normal. The cornea will be clear and a slit lamp examination is usually necessary to identify the inflammatory cells in the anterior chamber.

A hyphema is visible blood that layers out in the anterior chamber between the iris and the cornea (Figure 156-3). A microhyphema is diffuse blood circulating in the anterior chamber. These patients present with decreased vision, photophobia, and blood in the anterior chamber. A common cause of blood in the anterior chamber is blunt, nonpenetrating trauma to the eye. A patient with blood in the anterior chamber and a history of sickle cell disease or trait is at a higher risk of elevated IOP.3 Consult an Ophthalmologist if a sickle cell patient presents with blood in the anterior chamber.

Patients presenting with a painful eye and/or decreased vision, and no history of ocular trauma, should have their IOP measured and documented. Do not measure IOP if there is a history of blunt trauma associated with these symptoms. Some of the common presentations that require documentation of the IOP are reviewed in the previous section. This includes confirming the clinical diagnosis of acute angle-closure glaucoma, determining IOP after blunt ocular trauma, and determining IOP in a patient with iritis.

It is essential to rule out a ruptured globe before tonometry is performed when a patient relays a history of trauma. Tonometry should be strictly avoided if there is evidence of a ruptured globe or the suspicion of a ruptured globe. A ruptured globe from penetrating trauma to the anterior segment of the eye, including corneal or scleral lacerations, is usually obvious upon clinical examination. However, blunt trauma resulting in a ruptured globe may be very difficult to see on examination. Any blunt force (e.g., closed fist) causing an anteroposterior compression of the globe can result in a scleral rupture posterior to the attachment of the extraocular muscles. This is rarely obvious on external examination or a nondilated slit lamp examination. The clue here is that the patient has dramatically reduced vision and prominent swelling of the periorbital tissues.

Viral conjunctivitis is one of the most common ocular conditions seen in the Emergency Department. Conjunctivitis is so highly infectious that it is a relative contraindication to performing tonometry. These patients may present with ocular pain and decreased vision just like patients with angle-closure glaucoma. Conjunctivitis symptoms are usually binocular and the pain is not as severe. Acute angle-closure glaucoma almost always presents in one eye. It is reasonable to proceed with tonometry to rule out increased IOP as an etiology of a painful red eye if the diagnosis of conjunctivitis is uncertain. Clean the instrumentation carefully to prevent spreading the infection to the contralateral eye. The instrument of choice in cases of suspected infection is the Tono-Pen as it uses disposable, single-use, latex covers for the instrument.

Topical Ophthalmic Anesthetic Agents

• Proparacaine hydrochloride 0.5% (Alcaine, Ocu-Caine, Ophthaine, Ophthetic)
• Tetracaine hydrochloride 0.5% (AK-T-Caine, Altacaine, Opticaine, Protocaine)

Topical Stain for Tear Film

• Fluorescein sodium 1 mg strips (Fluorets, Fluor-I-Strip-AT, Ful-Glo)
• Fluorescein ophthalmic solution (Fluress, Fluorescein Sodium)

Tonometers

• Goldmann applanation tonometer
• Handheld applanation tonometer (e.g., Tono-Pen)
• Schiøtz indentation tonometer

Antiseptic

• >70% isopropyl alcohol solution/swabs
• 3% hydrogen peroxide solution

Inform the patient of the need to measure the pressure in their eye. Explain that an instrument will contact their tear film directly over the cornea. Reassure the patient that tonometry is not painful. It is not necessary to have the patient sign a consent form, as tonometry is part of a routine eye examination. One may choose to document the discussion with the patient for completeness.

Remove all contact lenses before the instillation of any topical ocular anesthetic agent or fluorescein. Measuring IOP through a contact lens is unreliable. The fluorescein dye will permanently stain contact lenses and clothing.

Instill one drop of a topical ophthalmic anesthetic agent in both of the patient's eyes. The contralateral eye is used as the control, even if only one eye is of concern. Topical ophthalmic anesthetic agents begin working within 30 seconds and remain effective for 10 to 20 minutes. The drops may burn when applied and may cause a transient drying of the cornea. Instruct the patient not to touch or rub their eyes until the anesthesia wears off.

Instill fluorescein when using any prism applanation tip, such as in Goldmann, Perkins, or Kowa tonometers. Fluorescein is not needed when using the Tono-Pen or the Schiøtz tonometer. Fluorescein is available on strips or in solution. Fluorescein strips are applied to the tear film on the inner layer of each lower eyelid. Dampen the strip with a drop of topical anesthetic before application if the eye is extremely dry. If using fluorescein solution, instill a drop of fluorescein in the cul-de-sac of each lower eyelid.

All plastic prism applanation tonometer heads and the metal plunger of the Schiøtz need to be sterilized to prevent cross contamination. Do not use sterilization solution on electronic tonometer tips. Replace the disposable cover over the tip of the tonometer after each use. To sterilize the tip of a tonometer, use a prepackaged 70% alcohol swab or gauze soaked in 70% alcohol. Apply the alcohol to the tip of the prism for 10 seconds and allow it to air dry. Gauze soaked in 3% hydrogen peroxide may also be used. Rinse the tip with sterile saline to wash off the 3% hydrogen peroxide before application to prevent iatrogenic corneal abrasions. These described methods effectively disinfect for common bacteria and viruses, adenovirus, herpes simplex, and HIV.4,5

Digital Palpation

Digital palpation is not as accurate as applanation tonometry. Digital palpation has been an informal means of judging IOP, as there is a built-in comparison with the contralateral eye. Studies examining this technique have shown inexperienced examiners can improve their accuracy with a short training session to within 5 mmHg of the actual IOP 88% of the time.6 Digital palpation should not be attempted if there is any potential for a ruptured globe.

Place the patient supine or in a reclined position. Ask the patient to close their eyes and look straight ahead through the closed eyelids. Stand next to the patient's torso and facing them. Place the right thumb over the patient's left eye and left thumb over the patient's right eye. Place the remaining fingers of both hands on the patients' temples for stability. Apply gentle alternating pressure to each globe. The Emergency Physician should be able to ascertain any difference in pressure between the two eyes using the patient's contralateral eye as a control. The Emergency Physician may also gently palpate their own globe as a normal control. This qualitative measurement is often sufficient to determine a significantly elevated IOP.

Schiøtz Tonometry

The Schiøtz tonometer estimates IOP by measuring the indentation of the globe caused by a known weight.7 The Schiøtz tonometer is a sturdy, low maintenance, and affordable instrument. It is gravity dependent and requires the patient to be supine or have their head fully extended to get an accurate reading. The Schiøtz tonometer case contains the Schiøtz tonometer, a calibration scale, weights, and a calibration platform (Figure 156-4).

The tonometer consists of a plunger and a hammer device connected to a needle (Figure 156-5). The needle is calibrated to a scale, with each unit measuring 0.05 mm on the scale. The plunger, the hammer, and the needle weigh 5.5 g. This can be increased to 7.5, 10, or 15 g by adding known weights to the tonometer. The weighted plunger is heavy and has a large contact area that causes significant indentation of the cornea with each measurement. The more weight it takes to move the needle on the scale or indent the cornea, the higher the IOP. The scale reading is converted to IOP in mmHg by a conversion chart supplied with each Schiøtz tonometer (Figure 156-4).

Place the patient supine or sitting with their head fully extended. Ensure that the patient is comfortable. A tight collar, flexed neck, breath-holding, squeezed eyelids, looking toward the nose, or accommodation results in a falsely elevated reading. Place two drops of a topical ophthalmic anesthetic agent onto each eye.

Calibrate the tonometer by placing the footplate of the plunger on the platform provided in the case (Figures 156-4 & 156-5). The scale reading must be “0” while the footplate is on the platform. The instrument requires repair if it does not read zero while on the platform.

Instruct the patient to look at a fixation target directly overhead and to open both eyes as wide as possible. Spread the eyelids with the nondominant index finger and thumb if necessary. Grasp the Schiøtz tonometer by its handle. Ensure that the fingers holding the eyelids open are pushing on the orbital rim and not the globe, falsely elevating the IOP. Place the footplate of the Schiøtz tonometer directly over the pupil and gently lower it onto the tear film (Figure 156-6). Note the reading on the scale. Add more weight if the scale reading is less than three units. Do not push down on the cornea with the Schiøtz tonometer. This will cause a false elevation in the IOP reading. Repeat the measurement several times or until three readings are within 0.5 scale units. Convert the scale reading to IOP with the conversion chart (Table 156-1).8

Clean the tonometer immediately after each use. Clean the barrel with two pipe cleaners, the first soaked in 70% isopropyl alcohol and the second one dry. Clean the plunger with an alcohol swab. Allow the instrument to air dry 1 to 2 minutes before being used in the contralateral eye so that the alcohol evaporates and is not transferred to the corneal surface.10

Goldmann (Applanation) Tonometry

The Goldmann tonometer is considered the clinical standard for measuring IOP. This method of tonometry is based on the Imbert-Fick principle.4 It states that the pressure inside an ideal dry, thin-walled sphere equals the force necessary to flatten its surface divided by the area of the flattening (P = F/A; where P = pressure, F = force, and A = area). The Goldmann tonometer determines the force necessary to flatten or applanate an area of the cornea 3.06 mm in diameter. The degree of flattening is determined while viewing the cornea through a split prism device in the tonometer head. Fluorescein is used to stain the tear film in order to distinguish the cornea from the tear film. The eye is viewed through the cobalt blue filter causing the fluorescein-stained tear film to appear yellow-green. The technique is described below.5

The Goldmann tonometer is not portable. It requires the patient to be cooperative, the patient to sit upright, and a working knowledge of the slit lamp. The Goldmann tonometer in combination with a slit lamp examination provides the clinician with the most detailed information about the patient's ocular condition.

Place two drops of a topical ophthalmic anesthetic agent onto each eye. Place fluorescein dye in the eye. Position the tonometer head and cobalt filter on the slit lamp. Set the tension knob at 10 mmHg. It is more accurate to measure IOP by increasing rather than decreasing the force of applanation. Seat the patient in front of the slit lamp. Support the patient's chin in the chin rest and place their forehead firmly against the strap (Figure 156-7). Instruct the patient to look straight ahead and open both eyes as wide as possible. Spread their eyelids with a thumb and forefinger if necessary. Do not put pressure on the globe while holding the eyelids open.

Move the tonometer head within one-half inch of the cornea. Use the control stick on the slit lamp to center the applanation head directly over the pupil. Look for two semicircles on the ocular surface as the tonometer head is advanced onto the corneal tear film (Figure 156-8). Ensure that the tension knob is set at 10 mmHg. The semicircles will not be touching if the patient's pressure is above 10 mmHg (Figure 156-8B). Turn the tension knob clockwise to raise the reading and counterclockwise to lower the reading. Turn the tension knob up until the inside borders of the fluorescein rings are touching (Figure 156-8A). Read the IOP on the tension knob. The semicircles will overlap if the pressure is below 10 mmHg (Figure 156-8C). Turn the tension knob down until the inside borders of the fluorescein rings are touching (Figure 156-8A). Read the IOP on the tension knob.

Clean the tonometer head with an alcohol swab immediately after use. Let it air dry 1 to 2 minutes so that the alcohol evaporates and is not transferred to the corneal surface. Repeat the procedure on the contralateral eye.

Handheld Tonometry

The Tono-Pen XL and the Tono-Pen Avia (Figure 156-9) are lightweight, simple to operate, and can record IOP with the patient in any position. The small contact area is useful in patients with eyelid swelling and corneal surface irregularities. The Tono-Pen calculates IOP using a strain gauge that creates an electronic signal as the 1.5 mm footplate flattens the cornea.10 A single microprocessor chip analyzes each application of the footplate and averages 4 to 10 valid measurements to form one reading. The instrument should be stored in its case when not in use. An Ocu-Film cover should remain over the tip when it is stored and changed with each new patient (Figure 156-9). The Ocu-Film covers are made of latex. Ask the patient about latex allergies prior to using this device. The tip of the Tono-Pen is very sensitive, is easily damaged, and should never be touched.

The Tono-Pen XL unit is internally calibrated, thus the instrument calibration should be checked before the first use each day or in the event of unanticipated readings. Calibration should also be performed whenever it is indicated by the LCD display, when batteries are replaced, or after unsuccessful calibration. If the word “bAd” appears in the LCD window, it needs to be calibrated. The Tono-Pen is not used or calibrated daily in the Emergency Department. Thus, it is necessary to check the calibration prior to each use.

The display of “Good” in the LCD window indicates that the Tono-Pen is functioning properly. Press the operation button once. The Tono-Pen will display [8.8.8.8] in the LCD window, followed by a single row of dashes [- - - -], and then a double row of dashes [= = = =]. This indicates that the Tono-Pen is ready to measure IOP.

To calibrate the Tono-Pen aim the transducer end straight down. Depress the operation button twice within 1.5 seconds. The Tono-Pen will beep and display “CAL” in the LCD window. Wait for the Tono-Pen to beep and display “UP” in the LCD window. This can take as long as 15 seconds. Immediately and quickly invert the Tono-Pen so that the transducer end is pointing straight up. The Tono-Pen will beep and display “Good” in the LCD window if it is functioning properly. Repeat the calibration procedure if “bAd” is displayed in the LCD window.

Place the patient in a seated or supine position. Instruct the patient to look straight ahead. Place two drops of a topical ophthalmic anesthetic agent onto each eye. Fluorescein is not necessary when using the Tono-Pen. Grasp the Tono-Pen like a pencil and hold it perpendicular to the corneal surface (Figure 156-10). Gently contact the cornea directly over the pupil or central cornea (Figure 156-10). Make contact with the cornea in a series of light taps. The Tono-Pen should not indent the cornea if used properly, reducing the risk if there is an unidentified ruptured globe or a hyphema. The Tono-Pen chirps each time a valid IOP measurement is obtained. The microprocessor sounds a final beep after it receives four valid readings. The mean IOP will be displayed in the LCD window. A single row of dashes [- - - -] indicates that an insufficient number of valid readings were collected. Obtain additional measurements after pressing the operation button.

There are many additional handheld tonometers of varying cost, ease of use, and availability. The Perkins MK-2 (Haag-Streit, Essex, UK) and the Kowa HA-2 (Kowa Optimed, Inc., Torrance, CA) are two handheld tonometers utilizing the Goldmann prism technique (i.e., aligning the semicircular images). They are portable, but require a familiarity with Goldmann applanation. The Accupen (Accutome, Malvern, PA) is similar to the Tono-Pen with a few differences. These include ease of use, longer battery life, and not requiring calibration before every use. The Diaton transpalpebral handheld tonometer (Bicom Inc., Long Beach, NY) is designed to measure IOP through a patient's closed eyelids.11 The Pulsair IntelliPuff (Keeler, Brownhill, PA) is the only portable, handheld, noncontact tonometer. The iCare Rebound (Tiolat Oy, Helsinki, Finland) uses a smaller zone to indent the cornea and claims not to require topical anesthesia.12 This would be particularly useful for the Emergency Department should it prove to be reliable.

The normal range for IOP is 10 to 22 mmHg. A baseline IOP is specific to each patient and the patient's contralateral eye can serve as a control. We can make general assumptions about certain ranges of IOP in order to make rapid clinical assessments and facilitate patient care. Readings of 0 to 9 mmHg should be discussed with an Ophthalmologist, especially if there is a history of recent eye surgery or recent eye trauma. Readings of 10 to 21 mmHg are normal. Readings of 22 to 25 mmHg should be followed up with an Ophthalmologist within 2 to 3 days. IOP readings greater than 26 mmHg require an emergent consultation with an Ophthalmologist.

Infectious agents can be transferred via tonometer heads.9 It is essential to properly clean each instrument before use, before using it on the contralateral eye, and after use. The use of a 70% isopropyl alcohol swab is an effective disinfectant for the Goldmann and the Schiøtz tonometers. The Tono-Pen has single-use disposable latex covers.

Incorrect IOP measurements can occur for a variety of reasons. An improperly working plunger on the Schiøtz tonometer can result in falsely low measurements. IOP will be elevated if measured while the examiner's hand is holding the eyelids open and inadvertently pressing on the underlying globe. Measurement during eye movements, blinking, or eyelid movements will elevate IOP. Maneuvers that increase intracranial pressure (e.g., valsalva or breath holding) also increase IOP.

Corneal abrasions can occur while using contact tonometers. It is very important to have a cohesive tear film to avoid cornea abrasions. Instruct the patient to blink several times just prior to tonometry to spread the tear film. Apply artificial tears in patients with extremely dry eyes. The Schiøtz tonometer puts significant weight on the cornea and must be applied very gently. Warn the patient that their eye may be uncomfortable when the anesthetic wears off if a small abrasion is suspected after measuring the IOP. Instruct the patient to instill artificial tears every 4 to 6 hours. Treat larger abrasions with a topical ocular antibiotic of choice and schedule a follow-up visit.

The practice of tonometry is essential in guiding appropriate eye care. The Goldmann, Tono-Pen, or Schiøtz contact tonometers are readily available in most Emergency Departments. The literature frequently debates the comparative accuracy of each instrument. The Goldmann applanation tonometer is generally considered the clinical standard.11,12 However, all three instruments are useful for screening IOP in the Emergency Department. Factors such as the patient's ability to ambulate and the presence of periorbital swelling will influence the choice of an instrument. The Emergency Physician should select a tonometer that feels comfortable and use it routinely. The Emergency Physician will be able to measure IOP rapidly and reliably with repeated use of the tonometer.