Normal Eye Pressure in Adults: What Values Are Normal

The result of tonometry — measurement of the pressure inside the eye — often arrives on a form without explanation, and the person is left alone with two numbers. Below is an explanation of what intraocular pressure is, which values in adults are considered normal, and when a deviation requires a doctor's examination.

What intraocular pressure is

Intraocular pressure is the pressure of the fluid inside the eye. Aqueous humor is constantly produced in the eye, and it drains through the drainage system in the angle of the anterior chamber. The pressure depends on how much fluid is produced and how quickly it leaves. When outflow slows, pressure rises and can damage the optic nerve — the bundle of fibers connecting the eye to the brain. This value is measured by tonometry, and the result is recorded in millimeters of mercury (mmHg).

Normal eye pressure in adults

Ophthalmologists and optometrists generally use the guideline that normal intraocular pressure lies between 10 and 20 mmHg. The average value is about 15.5 mmHg. Intraocular pressure usually rises with age, and heredity also affects it. An elevation above normal without damage to the optic nerve and without loss of visual field is called ocular hypertension. The opposite condition, ocular hypotony, is defined as a drop to 5 mmHg or below: pressure that low may indicate a fluid leak.

What values of 18 and 20 mmHg mean

Values of 18 and 20 mmHg fall in the upper part of the generally accepted range, that is, they formally remain within normal limits. Such a number does not indicate disease, but it does not eliminate the need for monitoring. Tolerance of pressure is individual: not everyone with elevated pressure develops glaucoma, and some people develop the disease at pressures within the normal range. The measurement is affected by corneal thickness and rigidity, so after some corneal surgeries the device may show a normal value while the true pressure is elevated. Therefore the number is assessed together with examination of the optic nerve and visual field testing.

How intraocular pressure is measured

Pressure is measured with a tonometer during a comprehensive vision examination. If glaucoma is suspected, an extended examination is performed: drops that dilate the pupil are instilled into the eye, and the tissues deep in the eye are examined through a magnifying lens. Visual field testing is also performed — it determines how well a person sees objects to the side without turning the eyes. If glaucoma is suspected, the person is referred to an ophthalmologist for additional examinations. It is advised to have vision checked at least once every two years, and more often if at increased risk.

Why values fluctuate during the day

Intraocular pressure changes over the course of the day and night. In a healthy eye, the difference between measurements is 3 to 6 mmHg, while in glaucoma the fluctuations may be greater. At night, pressure sometimes does not decrease, although fluid is produced more slowly. The result is affected by the amount of fluid drunk, medications a person takes by mouth or instills into the eye, alcohol, and caffeine. The difference between the two eyes also matters: it can be a sign of certain forms of glaucoma.

When deviations from normal require a doctor's examination

Glaucoma proceeds without symptoms for a long time, and most people are unaware of the disease: it develops over years and is often detected during a routine vision examination. Elevated pressure is the main risk factor for glaucoma, and very low pressure (5 mmHg or below) may indicate a fluid leak; the assessment is made by an ophthalmologist. People in risk groups require attention:

• age: glaucoma is more common in adults over 50, and age over 60 is considered an increased risk;
• heredity: if the disease is present in a brother, sister, or parent, it is advised to have vision checked more often;
• origin: increased risk is noted in people of African, Caribbean, and Asian origin;
• other diseases: myopia, hyperopia, diabetes, high blood pressure.

Vision loss begins with peripheral vision and remains unnoticed for a long time, so it is not a single number that decides, but the examination as a whole.

A single tonometer number by itself is not a reason to call an ambulance. Calling 103 or 112 is necessary if severe eye pain, redness of the eye, tenderness around the eye, rainbow-colored halos around bright light sources, blurred vision, nausea, vomiting, or headache suddenly appear: such signs require immediate help.

This article is for reference: it explains the numbers but does not replace a doctor's consultation.

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