INTRAOCULAR PRESSURE MEASUREMENT

Ophthalmologist Carolina Valdivia, MD answers a question about intraocular pressure measurement and why there might be variations between readings.

A 53 year-old woman from Florida, USA writes:

Hello Dr. Valdivia,

I have had myopia since age nine and presbyopia since age 40. My mother has glaucoma. I recently was diagnosed as a glaucoma suspect by a retinal ophthalmologist that I consulted for vitreous syneresis. This specialist found no vitreous problems or retinal detachment.

My eye pressure has been 16-18 for the past four years and my cup-to-disc ratios 0.4-0.45.

The optometrist that I initially consulted for the stationary blurry spot (syneresis) measured my eye pressure at 18. Four days later, the retinal specialist measured it at 23. Six weeks later this same specialist measured my eye pressure at 24. He also measured my cup-to-disc ratio at 0.6.

Later, I saw an optometrist (a new one) to have my eyeglass prescription checked, as per the retinal specialist's suggestion, since I was due for that and he didn't see anything in my vitreous. I told this optometrist the whole story, including my past pressures and cup-to-disc ratios. He measured my eye pressure and obtained a reading that was exactly the same as I told him that the retinal specialist had reported.

I am scheduled to see a glaucoma ophthalmologist in a few weeks. I'm a little suspicious of the fact that the first optometrist and the retinal ophthalmologist's eye pressure measurements were so different, but just four days apart. I also am suspicious that the second optometrist got the same exact intraocular pressure measurement as the ophthalmologist.

I have had both OCT and visual field studies and they were normal, so we're just trying to decide whether to treat me for ocular hypertension.

How much variation is there in different instruments for measuring eye pressure and different operators of the instruments? All of the measurements were made at appointments between at 9-10:30 am.

Thank you!

Thank you for your question and for your interest in glaucoma-eye-info.com.

Carolina Valdivia, MD In my clinical practice, I use the Goldmann Applanation Tonometer for intraocular pressure measurement. This instrument is considered the gold standard in the field.

There are other popular methods for intraocular pressure measurement, but, at this time, all instruments are compared up to the Goldmann Tonometer. This does not mean that the Goldmann Tonometer is flawless. In fact, all non-invasive methods are inherently imprecise. However, measurement error associated with the Goldmann Tonometer is considered to be more manageable compared to other methods.

Applanation means that the instrument touches the cornea to obtain the intraocular pressure measurement. However, eyes can have different corneal thickness and this points to the main weakness of the Goldmann Tonometer. It has a tendency to overestimate eye pressure when corneas are thicker than average and underestimate it when corneas are thinner than average. Ophthalmologists correct for this inherent imprecision by first performing a pachymetry test, which determines corneal thickness, and then plotting this value on a nomogram. A nomogram typically has three scales. Two of them represent known values, in this case the corneal thickness determination and the intraocular pressure measurement. The third scale allows for the calculation of a corrected intraocular pressure measurement that is highly accurate.

In your question, you mention that you are nearsighted (myopia). In some myopic eyes, the corneas can be thinner than average. This may result in an underestimation of the actual eye pressure. When deciding whether to initiate treatment for elevated intraocular pressure, this information can be decisive. For example, the degree of seriousness between eye pressure readings of 23 mmHg and 26 mmHg is different. Ophthalmologists generally are less in doubt about whether or not to start treatment when pressures are 25 mmHg or more.

The fact that an optical coherence tomography (OCT) test revealed no abnormalities in your optic nerve and a visual field evaluation also showed no significant problems is great news. These are two of the strongest procedures used to detect glaucoma. However, your family history of glaucoma combined with a personal history of myopia provide an additional rationale for why it might be advisable to have your corneal thickness determined through pachymetry.

The Ocular Hypertension Treatment Study (OHTS) sponsored by the National Eye Institute of the National Institutes of Health in the USA showed central corneal thickness to be a powerful predictor for the development of glaucoma. Eyes with corneal thickness of 555 microns or less (i.e. eyes with relatively thin corneas) had a threefold greater risk of developing glaucoma than those who had corneal thickness of more than 588 microns. The implication of this study is that a corneal thickness of less than 555 microns could be viewed as a risk factor for the development of glaucoma.

Taken together, eye pressure greater than 24 mmHg and a corneal thickness less than 555 microns may form the basis for starting treatment even if there is no evidence of optic nerve damage, the aim being to reduce the likelihood of developing glaucoma in the future. This rationale has sparked debate among ophthalmologists, with some in favor and others opposed.

Temporary increases in eye pressure can be induced by a number of things. Among these are caffeine intake, yoga positions where the head is lower than the hips, wearing tight neckties, and isometric exercises, such as weight lifting. In one study, eye pressure increased from a mean of 13 mmHg to a mean of 28 mmHg with maximum isometric contraction (weightlifting). Additionally, if you are nervous during an intraocular pressure measurement and you hold your breath, instead of breathing normally, then eye pressure readings may be measured higher than the true eye pressure.

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If the same Goldmann Tonometer is used by the same eye-care professional, readings can vary from 0-2 mmHg. The same holds true if the readings are taken using different examiners. However, remember that eye pressure both varies throughout the day and on different days. For this reason, I typically obtain several readings on different days and at different times during the day. This enables me to better identify the degree of fluctuation in a patient's intraocular pressure.

Intraocular pressure normally varies from 2 to 6 mmHg throughout the day. But in patients with glaucoma there are major variations. On average these are 10 mmHg. Because elevated eye pressure is so tightly linked to glaucoma, medications used to control it also help to reduce the progressive optic nerve damage observed in glaucoma.

When considering the differences in intraocular pressure measurement between the optometrist (18 mmHg) and the retinal ophthalmologist (23 mmHg) within a span of four days, there is no reason to suspect that either were in error. Remember that eye pressure vacillates. Different readings can be obtained if it is measured at the same time, but on different days.

The possibility also exists that the optometrist's reading was an underestimation of your actual intraocular pressure, not because of poor technique, but due to the method used. Many optometrists use non-contact tonometer methods (air puff tests), because of their practicality, ease of use, and the fact that they are less invasive. A non-contact tonometer can be useful for large-scale initial screenings at the population level. However, this method is not advisable for intraocular pressure measurement in glaucoma patients or glaucoma suspects. This is because non-contact methods tend to underestimate intraocular pressure that is high. For this reason, I would advise that you rely more on the readings obtained by the retinologist.

This does not mean that the optometrist measured your eye pressure incorrectly. Instead, I am suggesting that differences in methods used for intraocular pressure measurement could account for the difference and it is likely that your actual pressure is 23-24 mmHg.

It is important to determine average values for your intraocular pressure. Ocular hypertension management can be difficult, including the decision about when to start treatment. Intraocular pressure readings of 25 mmHg or greater are associated with progression to glaucoma in as much as 36% of cases over a period of 5 years. Other risk factors that may indicate progression to glaucoma in a person with ocular hypertension are age above 40 years, corneal thickness less than 555 microns, and large optic disc excavations.

Carolina Valdivia, MD

REFERENCE:

James B and Benjamin L. Ophthalmology: Examination and Investigative Techniques. Philadelphia, Pennsylvania: Butterworth Heinemann Elsevier, 2007:29-36.

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My goal in answering your questions is to provide you with information, clear up misconceptions, and to explain options so that you can have an informed conversation with your doctor. However, under no circumstances should my response to your question be considered a substitute for ongoing consultation and examination with your doctor. Since I have not examined you, I only can speak in terms of generalities, whereas your doctor has sufficient clinical details to evaluate your case specifically.

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