Eye Pressure: What It Is and What Its Readings Mean

Intraocular pressure is the pressure of the fluid inside the eye, and it is checked at almost every ophthalmologist visit. In the report you will see numbers in mmHg and the word "tonometry." Let's break down what lies behind them and what these numbers do and do not tell us.

What eye pressure (IOP) is

IOP is the pressure of the fluid inside the eye. It is measured by tonometry, and the result is recorded in millimeters of mercury. For ophthalmology this is one of the basic indicators: pressure is linked to the risk of glaucoma—a group of diseases in which the optic nerve connecting the eye to the brain is damaged.

At the same time, the number itself does not equal a diagnosis. Some people have pressure above normal, but the optic nerve is not affected. The opposite also happens: glaucoma develops at pressure within the normal range. Each nerve has its own margin of safety, so the doctor looks not only at the number but also at the fundus and visual fields.

How pressure inside the eye is maintained

Simple hydraulics operate inside the eye: aqueous humor is produced and drains out, mainly through the trabecular meshwork in the angle of the anterior chamber. If outflow slows, fluid accumulates and pressure rises. Calculations take into account the rate of fluid formation, the rate of outflow, and the pressure in the episcleral veins.

The indicator is not constant: it changes over the course of a day, with a difference of 3–6 mmHg for a healthy eye, while fluctuations are greater in glaucoma. The numbers are affected by heart rate, breathing, fluid intake, and medications; caffeine can raise them. Another nuance is corneal thickness and rigidity: after refractive surgery, measurement may show normal values where the pressure is actually higher.

What is considered normal intraocular pressure

Ophthalmologists and optometrists use the range of 10–20 mmHg: the mean is about 15.5 with fluctuations of approximately 2.75. In the general population, values are close to 10–21 mmHg, with a mean of 15–16 plus or minus 3.5 over the course of a day.

• 10–20 mmHg — the generally accepted normal range.
• Pressure above normal without optic nerve damage or visual field loss — ocular hypertension.
• 5 mmHg and below — ocular hypotony; such a drop may indicate a fluid leak.
• A pressure difference between the two eyes may be associated with certain types of glaucoma, uveitis, or retinal detachment.

Why eye pressure can be elevated

The causes vary: anatomical features, eye inflammation, genetic factors, medication side effects; pressure usually rises with age. Often elevated pressure is not felt at all, and glaucoma is found during a routine check—sometimes already after some peripheral vision has been lost.

Glaucoma risk factors are considered separately—they do not explain elevated pressure on their own: age over 50, glaucoma in a brother, sister, or parent, African, Caribbean, or Asian ancestry, myopia, hyperopia, diabetes. Those with diabetes or high blood pressure are advised to have an examination with pupil dilation at least once a year.

How eye pressure is measured

Tonometry is part of a comprehensive examination. The doctor may instill drops that dilate the pupil, examine the fundus through a magnifying lens, and check visual fields—how the eye sees objects to the side. After the drops, vision is blurry and sensitive to light for several hours, so it is better not to drive home. If glaucoma is suspected, the patient is referred to an ophthalmologist for further evaluation; routine vision checks are done at least once every two years.

Eye pressure in children

Glaucoma is rare in children: it is a congenital form, and a child can be born with it. A child's IOP is assessed as part of an ophthalmologist examination.

Eye pressure by itself is not a reason to call an ambulance. 103 or 112 — if symptoms appear suddenly: severe eye pain, redness of the eye, tenderness around the eyes, rainbow-colored halos around bright light sources, blurred vision, nausea and vomiting, headache. This may be an acute attack of angle-closure glaucoma.

This material is for reference—the numbers on the form do not replace an ophthalmologist examination and do not substitute for a doctor's consultation.

This content is for reference and does not replace a doctor's consultation. Medical disclaimer