Diagnostics

Basics of Tonometer Operation

A review of IOP measuring device usage can help technicians of all skill levels.

Sergina Flaherty, COMT, San Antonio, Texas

More than 2.2 million people have glaucoma and only half of them know it, says the Glaucoma Research Foundation. In the United States, 120,000 are blind from glaucoma, accounting for 9%-12% of all blindness. As technicians and nurses, we are primarily responsible for measuring IOP using a tonometer.

This article will explain the use of tonometers with a focus on performing applanation tonometry with a Goldmann tonometer.

The Aqueous Pathway

The pressure that forces fluid, electrolytes, and a small production of aqueous humor is the result of a complex sequence of events that occurs in the ciliary processes. This secretion from the ciliary body travels into the posterior chamber. Aqueous then passes between the iris and the lens and into the anterior chamber and is responsible for supplying nutrients and removing metabolic waste from the nonvascularized structures of the anterior segment of the eye. The angle refers to the junction of the cornea and the iris, and contains the drainage pathway for 90% of the aqueous (Figure 1).

Figure 1: Aqueous Humor Flow

Figure 2: A. Indentation B. Applanation Tonometry

The resistance of flow through the trabecular meshwork creates internal pressure in the eye reflecting a balance between the production of aqueous humor and its outflow through the trabecular meshwork.

Corneal Thickness

When tonometry was introduced, it was thought that all corneas had the same thickness. However, after thinning the corneas of our LASIK patients, it was found that thin corneas (thinner than 545µ) had tonometry readings artificially low and conversely thick corneas (thicker than 545µ) had readings artificially high.

Today most ophthalmologists accept the following: Add 1 mm hg to measured IOP for every 20µ of “thinness” below 545µ and subtract 1 mm hg from measured IOP for every 20µ of “thickness” over 545µ. There are charts that can be used to calculate this adjustment. We obtain corneal thickness measurement by ultrasound using a pachymeter on all new patients with glaucoma or those patients considered to be glaucoma suspects.

Types of Tonometers

The two most common types of tonometers are the applanation and non-contact. (Figure 2) They are based on the application of pressure to the center of the cornea and the measurement of the eye’s resistance to the external pressure.

Indentation tonometry measures the amount that the cornea is indented with a fixed weight as in Schiotz Tonometry. However, there are several disadvantages to indentation tonometry: improper instrument manipulation can affect the reading, you can obtain an artificially low IOP in young or highly myopic eyes, or an artificially high IOP due to sclera rigidity. Because of these disadvantages the Schiotz is not the tonometer of choice today.

Figure 3: Proper patient and tonometer position

Goldmann Tonometer

The Goldmann tonometer is based on 520µ thick cornea and flattens central cornea 3.06 circle diameter. This tonometer is still considered the industry standard, as the pressure readings are the most accurate, reliable and reproducible we can obtain. Its fluid displacement and corneal displacement are minimal as is its change of ocular volume. It also negates scleral rigidity as a factor since it displaces only 0.56 mm volume.

However, Goldmann readings are rendered inaccurate for patients with corneal irregularities, scars, pterygia, grafts, keratotomies or any disease that alters the smooth surface of the cornea.

Non-Goldmann Options

▪ The Tonopen action is between indentation and applanation principles and is an excellent substitute when faced with the corneal problems that make Goldmann applanation tonometry difficult. The Tonopen can also be very handy when patients are wheelchair bound or in the hospital. It is portable and handheld. Calibration must be checked at the beginning of everyday. First, hold the tip down and push the button twice. When CAL is displayed, turn the tip up and then turn instrument upside down. The display must read clearly twice. If a bad reading occurs, check manual for further directions. Documentation when using the Tonopen is TP and pressure reading to the right of the TP.

▪ The Icare tonometer is also handheld and portable. Icare is a rebound tonometer. Turn on the unit by pressing the measurement button. It will beep and then display LOAD. Place the single use probe into the collar and push the measurement button again. When the Icare tonometer is ready to use, it is brought to the patient’s eye with the central grove in the horizontal position. The distance of the eye to the tip of the probe is from 4-8 millimeters (Figure 4). Press the measurement button and take no less than six measurements. After the six measurements the IOP will be displayed.

Figure 4. The Icare tonometer in use

▪ The Pulsair non-contact tonometer requires no anesthetic. There’s no discomfort, just a gentle puff of air. You can perform a test on the patients hand to show them how gentle the puff is. This will put the patient at ease and there will be a reduction in the squeezing effect. It gives you consistent readings. The target is reflected off the cornea and a pulse of air is activated automatically when the instrument is perfectly aligned with the cornea (Figure 5). OP

Figure 5. The Pulsair non-contact tonometer in use

Ms. Flaherty is a Certified Ophthalmic Medical Technologist at Stone Oak Ophthalmology in San Antonio, Texas. She is owner of Ophthalmic Seminars of San Antonio and conducts instructional seminars to ophthalmic assistants and technicians in Texas and nationally. She is currently on the Board of Directors of the Association of Technical Personnel in Ophthalmology (ATPO) as a Director at Large. You may contact her by visiting her website www.ophthalmicseminars.com