Glaucoma is a group of eye diseases that result in damage to the optic nerve and vision loss. Factors that impact a glaucoma diagnosis include IOP, optic nerve appearance, central corneal thickness (CCT), and loss of peripheral vision on visual field (VF) testing.

When managing glaucoma, IOP is the most important “modifiable” risk factor. To properly treat a patient, the physician must have accurate readings. We can measure IOP with several methods, including contact tonometers like the Goldmann, non-contact tonometers, hand-held tonometers, and more. Each have their pros and cons depending on various patient factors, such as corneal scarring.

Here, I discuss the various tonometer options as well as factors that affect readings.

Tonometers

The Goldmann tonometer uses a prism to measure the force it takes to flatten a 3.06-mm diameter circle of the central cornea. When using a Goldmann tonometer, the following can cause erroneous readings:

• CCT
• Excessive or insufficient tear film
• Patients holding their breath
• Irregular or scarred corneas
• Pressure from holding eyelids.

In a patient with high cylinder correction, the minus cylinder axis should align with the red marking on the tonometer tip base for the best result. If working in plus cylinder, the prescription must be transposed to determine the minus axis.

The Pneumatonometer (Reichert) uses a fenestrated silicone probe tip that floats on a stream of air. The user gently touches it to an anesthetized cornea and applies increasing air pressure by the probe until the force is equal to that of the pressure in the anterior chamber. It is the preferred measurement when a patient has a keratoprosthesis or artificial cornea — in these cases, the IOP is read at the limbus.

The Tono-Pen (Reichert) is a hand-held device with a plunger protruding from the center of a small footplate. As it makes contact, the plunger gets resistance from the cornea when the force of resistance is shared by the footplate and plunger, and the measurement is read. Multiple readings are taken and an average measurement with a percentage of error is given. Errors can be made by applying the disposable tip cover too tightly and not cleaning the tip regularly, causing it to read higher.

The Icare ic100 tonometer works on the principle of rebound tonometry. It uses a dynamic electromechanical action to measure the IOP. A moving magnetized probe hits the ocular surface and rebounds immediately. The instrument processes and interprets the movement and converts an average of multiple readings to mmHg. The higher the IOP, the faster the rebound. Because of its rapid action, no anesthetic drops are needed.

Non-contact tonometers (commonly referred to as the air puff) use a column of air that gradually increases in intensity until the cornea is flattened. These are a good tool to use as a screening device and will be found in many optometric offices. They are easy to train assistants to use and have none of the “fear factor” of touching a patient’s cornea, but they tend to overestimate low IOP and underestimate high IOP.¹

The Ocular Response Analyzer (ORA, Reichert) is a non-contact tonometer that measures corneal hysteresis (CH). Using the stream of air common to non-contact tonometers, the increasing intensity of the force deforms the cornea to a concave shape. CH is calculated as the difference between the innermost and outermost cornea and is found to be significantly associated with glaucoma progression. One study found that eyes with lower CH had faster rates of VF loss than those with higher CH.²

Factors that affect readings

Several factors can cause fluctuations in IOP readings. For instance, CCT measurements should be recorded by performing pachymetry for all glaucoma patients as it can affect adjusted IOP by as much as 7 mmHg. The thicker the cornea, the higher the IOP reading, and in cases in which the CCT is low (post-LASIK patients, for example), the IOP readings may need to be adjusted upward. Many EMR systems auto-adjust after pachymetry info is added, or a common table is used (see the online version of this article).

Here are some of the factors that impact IOP readings:

• Diurnal variations. These patients usually have their highest recorded IOP in the early morning, and it decreases throughout the day. Ranges of fluctuations have been reported as much as 3 mmHg to 9 mmHg.³ Many physicians perform serial tonometry and have a patient return three to four times during the day to check for fluctuations. To get the best results, the same technician and equipment should be used on each visit.

• Diet, alcohol, and drugs. Increased intake of caffeine, cigarette smoking, and water can all temporarily increase IOP. In one study showed an increased IOP in 80% of glaucoma patients and only 20% of non-glaucoma patients after the consumption of a quart of water.⁴ Also, alcohol and marijuana can lead to a transient decrease in IOP. While some states have legalized marijuana, it is not a recommended treatment as the IOP-lowering effects are transient.

• Body positioning and external factors. In larger patients who lean forward, a Valsalva maneuver happens where air is forced against the epiglottis, which increases intracranial pressure. To avoid this, remind these patients to breathe, and be aware of their position.

• Exercise. Regular aerobic exercise has been shown to decrease IOP⁵ and is associated with less IOP increase in times of psychological stress. Weight lifting and yoga have been found to cause increases of IOP.

• Medications. I have found that systemic steroid medications (including some inhalers) can cause IOP increases. Also, beta blockers can reduce IOP. Although low blood pressure can be associated with glaucomatous damage, it is important to educate patients that these are two separate readings.

Conclusion

By educating ourselves on methods for taking IOP and what factors may influence measurement, we will be better educators and caregivers to our glaucoma patients and perform more informed and accurate readings for our physicians. OP

REFERENCES:

1. Turner MJ, Graham SL, Avolio AP, Mitchell P. Potential effects of systematic errors in intraocular pressure measurements on screening for ocular hypertension. Eye (Lond). 2013;27:502-506.
2. Medeiros FA, Meira-Freitas D, Lisboa R, Kuang TM, Zangwill LM, RN. Corneal hysteresis as a risk factor for glaucoma progression: a prospective longitudinal study. Ophthalmology. 2013;120:1533-1540.
3. Syam PP, Mavrikakis I, Liu C. Importance of early morning intraocular pressure recording for measurement of diurnal variation of intraocular pressure. Br J Ophthalmol. 2005;89:926-927.
4. Vasconcelos-Moraes CG, Susanna R Jr. Correlation between the water drinking test and modified diurnal tension curve in untreated glaucomatous eyes. Clinics (Sao Paulo). 2008;63:433-436.
5. Yan X, Li M, Song Y, et al. Influence of exercise on intraocular pressure, Schlemm’s canal, and the trabecular meshwork. Invest Ophthalmol Vis Sci. 2016;57:4733-4739.

Amanda Waggoner is a certified ophthalmic medical technologist with 25 years of experience. She is the Northern Colorado Program director for Rocky Mountain Ophthalmic Personnel, a CORE speaker for Allergan, and serves patients in Longmont, CO, for a multi-specialty practice.