Monday, August 28, 2006

Section 6-B: Intraocular pressure and corneal pachymetry

The normal intraocular pressure (IOP) is between 10 to 21 mmHg. Any intraocular pressure lower than 5 mmHg is considered abnormally low and is called hypotony. An intraocular pressure greater than 21 mmHg is considered abnormally high and is often referred to as ocular hypertension. High intraocular pressure does not necessarily mean that you have glaucoma. It simply increases the risk for the development of glaucoma. The higher the IOP, the greater the risk of developing glaucoma (see Chapter 5, Glaucoma Risk factors).

Intraocular pressure is not constant and can vary throughout the day. A diurnal variation is IOP fluctuation during the day, while a nocturnal variation is fluctuation of IOP during the night . Sometimes when the eye doctor is suspicious that there is a great fluctuation of IOP during the day, he may recommend measurement of diurnal IOPs. At the University of Iowa this is performed at 3-hour intervals starting at 7:00AM until 10:00PM as an outpatient. It is not uncommon to see intraocular pressures slightly higher in the morning than in the afternoon or evening. However, this is not necessarily true for all patients. While there is small diurnal fluctuation of IOP even in normal subjects, the glaucoma patient tends to have higher IOP and greater fluctuation of IOP throughout the day compared to normals.

There are a number of ways to measure the intraocular pressure (the measurement of IOP is referred to as tonometry). The most common measurement technique is the Goldmann applanation tonometry. With an anesthetic eyedrop, this device measures the IOP by putting a biprism plastic tip against the cornea and applanating (or flattening) the cornea. The intraocular pressure is read by dialing in an appropriate amount of force to flatten the surface of the cornea. The Goldmann applanation technique is based on the principle that the force required to flatten a certain defined area of the cornea (which has a curved surface) is proportional to the IOP. (Figure 6-1)

Figure 6-1. Checking intraocular pressure (IOP) using a Goldmann applanation tip at the slit lamp

Besides the Goldmann applanation tonometry, there are other devices that can measure the intraocular pressure. Other commonly used tonometers are Tonopen (Reichert, Buffalo, NY) and Perkins (Clement Clarke Inc., Columbus, OH) applanation tonometers; both of them are hand-held and portable (Figure 6-2 and 6-3). These devices come in handy when the patient is unable to easily place his chin at the slit lamp device, as in children or elderly in a wheelchair. Another commonly used device is a non-contact applanation tonometer (commonly referred to as “air puff” tonometer). It is based on a similar principle as the Goldmann applanation tonometry, except that it uses an air puff to flatten the cornea, rather than a direct contact with the tonometer tip. There are do-it-yourself devices such as Proview which is available for self-measurement of intraocular pressure at home by the patient. However, the reliability of the home tonometer is considered to be less than the ones commonly used in the doctor’s office. Because the home tonometer can sometimes give erroneous readings, it is not commonly utilized by eye doctors.

Figure 6-2. Tonopen XL. A hand-held digital device to measure the intraocular pressure. It is useful in patients who are not able to get into the slit lamp for Goldmann applanation tonometry.

Figure 6-3: Perkins tonometer. It is hand-held device used to measure the intraocular pressure. It is often used in infants or elderly in a wheelchair. It works on a similar principle as the Goldmann applanation tonometer.

As we have discussed in Chapter 4, accurate intraocular pressure measurement depends on the thickness of the cornea. Corneal thickness is measured using a pachymeter (Figure 6-4) and the normal corneal thickness is in the range of 0.53 – 0.55 mm. After an anesthetic eye drop is applied, corneal thickness is measured by gently touching the smooth tip of the pachymeter probe to the surface of the cornea (Figure 6-5). Intraocular measurements are underestimated in patients with thin corneas. In other words, with thin corneas the true IOP is higher than what is measured using an applanation tonometer. Conversely, in the thicker cornea the true IOP is lower than what is measured by applanation tonometry. If you have corneal thickness less than 0.50 or greater than 0.60 mm, the applanation tonometry will significantly underestimate or overestimate the intraocular pressure readings respectively.
Figure 6-4. Pachymeter. A pachymeter (Pocket Pachymeter, Quantel Medical, Bozeman, MT) is an ultrasound device that measures cornea thickness by determining the time it takes for a sound wave to reflect off the inner surface of the cornea. Measurements are taken by placing an anesthetic drop on the eye and gently touching the probe to the surface of the cornea.

Figure 6-5. Pachymetry. Corneal thickness is measured by administering an anesthetic drop to the eye and then gently placing the pachymeter probe against the outer surface of the cornea.

Intraocular pressure reading is performed using a topical anesthesia; it typically takes less than 15 seconds to measure the IOP in each eye in a cooperative patient. It is painless procedure when performed properly. A complete IOP assessment requires both the applanation tonometry as well as the corneal thickness measurement (or pachymetry).

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