I have just examined a 50-year-old male patient. He is new to my practice. I measured his intraocular pressure as right eye 24, 26, 25 and left 25, 26, 26 using a non-contact (air puff) tonometer. His cup-to-disc ratio is 0.2 in each eye and automated visual field tests show no visual field defects. What should I do?
In my country of practice government health guidance advises that intraocular pressures above 24 mmHg should be repeated using a contact tonometer. In your patient’s case the right intraocular pressure average value is 25 and the left almost 26. They deserve to have a repeat intraocular pressure even though their cup-to-disc ratios are in the normal range and there is no visual field defect.
There is a range of normal intraocular pressure required to keep the eye healthy but for some people the pressure can increase slowly without the person realising. Intraocular pressure that remains above the normal range will damage nerve fibres in the retina leading to permanent visual field loss. Intraocular pressure is often measured during an eye examination on people 40 years and older but 35 years and older for people of African-Caribbean and Black African heritage. Base line values of intraocular pressure are used to make future comparisons in order to detect pressure above the normal range.
A difference between one and the other eye can be a sign of disease in the eye with the higher pressure. Intraocular pressure that rises slowly over many months or years does not cause symptoms and the person is unaware anything is wrong with their eyes even though nerve fibres are being damaged. This is why it is important for optometrists to carry out opportunistic screening of intraocular pressure.
High intraocular pressure can be reduced through the use of eye drops and/or surgery. Nerve fibres that are damaged during the period of high pressure are unlikely to recover full function and any visual field loss caused during the period of high pressure will remain. Intraocular pressure is usually measured by non-contact ‘puff-of-air’ instruments although some practitioners prefer to use anaesthetic eye drops and techniques which involve making contact with the eye.
Non-contact tonometry can result in values that are high due the patient blinking and/or not being in the correct position or narrowing their palpebral aperture. The eye specialists dilemma is whether the intraocular pressure is genuinely high or whether it is high because of the procedure itself.
In my opinion it is reasonable to repeat the intraocular measurements a few days later with a non-contact tonometer. Not all practitioners have access to a contact tonometer or are licensed to use anaesthetic eye drops. In my experience when a person with borderline pressures has a repeat test the repeat values are usually in the normal range. This is likely to be due to greater attention paid to the procedure and/or the patient is aware of what to expect and is less apprehensive. In these cases the patient can be put back onto the appropriate recall schedule.
There are some patients who on repeat non-contact tonometry have consistently high values. If there are other signs of potential glaucoma such as asymmetric (greater than 0.2 difference between the eyes) and/or high (over 0.3) cup-to-disc ratios and/or narrow peripheral anterior chamber angles and/or a visual field defect typical of glaucoma and/or a parent or sibling with glaucoma then it is an easy decision to make to refer the patient to an eye specialist that is licensed to treat glaucoma.
However, if the only sign of potential glaucoma is intraocular pressures around 25-28 mmHg then the management is more difficult. This person is likely to have ocular hypertension or higher than normal central corneal thickness. Higher than normal central corneal thickness causes all tonometers to give higher readings than if the central corneal thickness was average.
If the patient has higher than average central corneal thickness the glaucoma specialist can take this into account and calculate the actual intraocular pressure.
If the patient has ocular hypertension then referral to a glaucoma specialist would be appropriate as then can take into contact tonometry pressure, central corneal thickness, family history of glaucoma and life expectancy when deciding on the risk for visual impairment and whether to treat the high pressure or not.
In summary, for your patient a repeat intraocular pressure measurement would be clinically necessary and then decide on management depending on whether the repeat values are less than or greater than 24 mmHg.