Intraocular pressure (IOP) represents the internal fluid pressure of the eye, a measurement expressed in millimeters of mercury (mmHg). This pressure is maintained by a continuous balance between the production and drainage of a clear fluid called aqueous humor. The aqueous humor is produced by the ciliary body and flows through the pupil into the front part of the eye, where it eventually drains out through a meshwork of tissue. This constant flow is necessary to provide nutrients to the eye’s structures and ensure the globe retains its proper shape for clear visual function. An imbalance in this production and drainage system can lead to fluctuations in eye pressure.
Understanding the Standard Normal Range
The standard medically recognized range for healthy intraocular pressure in adults is between 10 and 21 mmHg. This range is static, meaning the numerical value of a normal reading does not change as a person gets older. The average IOP for most individuals falls in the middle of this range, around 15 or 16 mmHg.
The concept of a normal range “by age” is a common misconception. While the normal numbers remain the same throughout adult life, the risk profile associated with those numbers changes significantly with age. After age 40, the eye’s drainage system may become less efficient, leading to a tendency toward higher IOP readings and increased risk of conditions like glaucoma.
This heightened potential for disease is why eye care professionals recommend more frequent testing for older adults. For example, an IOP reading of 20 mmHg is technically within the normal range for any age. However, it prompts closer observation in an older adult because the focus shifts from the number itself to the health of the optic nerve in relation to that measurement.
How Intraocular Pressure is Measured
Measuring IOP is a procedure called tonometry, which is a fundamental part of a comprehensive eye examination. The goal of tonometry is to determine the force required to flatten a small, defined area of the cornea, and the pressure reading is calculated from this resistance.
The Goldmann Applanation Tonometer (GAT) is considered the gold standard for accuracy. This method requires numbing the eye with drops so a small, illuminated probe can gently touch the cornea to measure resistance.
The non-contact tonometer, often called the “air puff” test, is common for initial screenings. This device uses a quick burst of air to momentarily flatten the cornea, calculating the pressure without physical contact or anesthetic drops.
While convenient, non-contact tonometers are less precise than the Goldmann method. If a reading is elevated, a professional will typically use the GAT for a definitive measurement, as accuracy can be influenced by the thickness of the patient’s cornea.
Ocular Hypertension and Glaucoma Risk
When IOP consistently exceeds the normal range, typically above 21 mmHg, the patient may be diagnosed with ocular hypertension. This condition indicates high pressure inside the eye but does not yet involve damage to the optic nerve or vision loss. Ocular hypertension is considered a significant risk factor for developing a more serious disease.
Glaucoma, by contrast, is a group of conditions where elevated intraocular pressure has caused detectable damage to the optic nerve, leading to permanent vision loss. The distinction between ocular hypertension and glaucoma lies entirely in the presence of this nerve damage, not just the pressure reading alone. Patients with high pressure are closely monitored and often treated to prevent progression to glaucoma.
High pressure is only one factor in the development of the disease. In Normal Tension Glaucoma, optic nerve damage and vision loss occur even when IOP readings remain consistently within the normal range. This highlights that while pressure is a treatable risk, the overall health and sensitivity of the optic nerve are the ultimate determinants of the disease.
Daily Variables That Influence Readings
Intraocular pressure is not a fixed value but fluctuates throughout the day, following a natural circadian rhythm. For most people, IOP is highest in the early morning hours and tends to decrease as the day progresses. This fluctuation means a single measurement taken in the doctor’s office is merely a snapshot in time.
Acute changes in body position can temporarily affect a reading. Lying down flat, for instance, can cause a temporary increase in IOP compared to a measurement taken while sitting upright. Strenuous physical activities, such as heavy weightlifting, may also cause a momentary spike in eye pressure.
Dietary choices and certain systemic factors also influence readings. Consuming a large amount of fluid rapidly or ingesting high levels of caffeine can lead to small, temporary increases in pressure. Additionally, some prescription medications, particularly corticosteroids, are known to reduce the eye’s ability to drain fluid, resulting in a sustained rise in IOP.