What is the normal range for intraocular pressure?

The statistical normal range for IOP by Goldmann applanation tonometry is 10-21 mmHg. This represents the 95th percentile of a normal population distribution. The average IOP is approximately 15-16 mmHg. Values above 21 mmHg are termed ocular hypertension, but glaucoma can occur at any IOP, and many people with IOP above 21 mmHg never develop glaucoma.

What is normal-tension glaucoma?

Normal-tension glaucoma (NTG) is glaucoma that develops and progresses despite IOP consistently within the normal range (10-21 mmHg). It suggests that the optic nerve is unusually vulnerable to damage even at normal pressures. Vascular insufficiency to the optic nerve, systemic hypotension, and other factors may contribute. NTG demonstrates that IOP is just one risk factor for glaucoma, not its sole cause.

Why does the time of measurement affect IOP?

IOP follows a diurnal (24-hour) cycle, typically peaking in the early morning (upon waking, between 6-8 am) and declining through the day to a nadir in the afternoon or early evening. This variation averages 3-6 mmHg in normal eyes and can be larger in glaucoma patients. A patient measured in the morning may have an IOP that appears 5 mmHg higher than a measurement taken in the afternoon. Recording the time of measurement allows comparison of IOP values taken under similar conditions.

How does corneal thickness affect IOP measurement?

The Goldmann tonometer is calibrated for an average central corneal thickness (CCT) of approximately 520-540 micrometers. A patient with a thinner cornea (e.g., 480 micrometers) is easier to applanate, causing the tonometer to underestimate the true IOP. A patient with a thicker cornea (e.g., 600 micrometers) is harder to applanate, causing overestimation. Clinicians use published correction formulas or tables to adjust the measured IOP based on CCT.

What is ocular hypotony?

Ocular hypotony is IOP below 6 mmHg -- abnormally low pressure. This can occur after glaucoma filtration surgery (over-drainage), wound leaks following any intraocular surgery, cyclodialysis cleft (abnormal communication between the anterior chamber and the suprachoroidal space), or severe anterior segment inflammation. Hypotony causes the eyewall to soften, leading to corneal folds (hypotony keratopathy), choroidal effusions, and macular edema with vision loss.

IOP Interpretation for the CPOA Exam

Normal IOP and What It Means

Intraocular pressure (IOP) is the pressure of the aqueous humor inside the eye, measured in millimeters of mercury (mmHg). Aqueous humor is produced by the ciliary body, flows through the pupil, and drains primarily through the trabecular meshwork at the angle of the anterior chamber. The balance between production and drainage determines IOP.

Normal IOP: approximately 10-21 mmHg by Goldmann applanation tonometry, with a population mean of 15-16 mmHg. This range represents the 95th percentile of the general population -- not the range free from glaucoma risk.

Ocular Hypertension

Ocular hypertension (OHT) is defined as IOP consistently above 21 mmHg in the absence of any glaucomatous optic nerve damage or visual field loss. Ocular hypertension is a risk factor for developing glaucoma, but it is not glaucoma by itself.

Not all patients with elevated IOP develop glaucoma. The Ocular Hypertension Treatment Study (OHTS) found that only about 10% of untreated ocular hypertension patients develop glaucoma within 5 years. Risk factors that increase conversion include:

Higher baseline IOP
Thinner central corneal thickness (CCT)
Larger cup-to-disc ratio
Older age
Higher pattern standard deviation (PSD) on visual fields

Normal-Tension Glaucoma

Glaucoma can also develop and progress at IOP levels within the statistical normal range (10-21 mmHg). This is called normal-tension glaucoma (NTG). It is thought to involve increased optic nerve susceptibility to damage at normal pressure levels, vascular factors, and possibly immune mechanisms. NTG reinforces the point that IOP is just one of many glaucoma risk factors -- a normal IOP does not guarantee the absence of glaucoma.

💡 Clinical Tip: When recording IOP, always document the time of measurement. IOP has a diurnal (daily) variation of 3-6 mmHg in normal eyes and can be larger in glaucomatous eyes. IOP is typically highest in the early morning hours. A measurement taken at 7 am may be significantly higher than one taken at 3 pm in the same patient.

Factors That Affect IOP

Factor	Effect on IOP
Time of day (diurnal variation)	Typically peaks in morning, lower in afternoon
Central corneal thickness (CCT)	Thin cornea = underestimated IOP; thick cornea = overestimated IOP
Corneal rigidity	Rigid cornea = falsely high reading
Lid squeezing / Valsalva	Artificially elevates IOP during measurement
Exercise	Moderate aerobic exercise typically lowers IOP transiently
Caffeine	May slightly raise IOP
Alcohol	Acutely lowers IOP
Glaucoma medications	Reduce IOP (prostaglandins, beta-blockers, alpha-agonists, carbonic anhydrase inhibitors)
Systemic steroids	Can elevate IOP in susceptible individuals (steroid-response glaucoma)

Target IOP in Glaucoma Management

Once a patient is diagnosed with glaucoma or treated for ocular hypertension, the doctor sets a target IOP -- the pressure at which the optic nerve is unlikely to progress. Target IOP is individualized based on the baseline IOP at which damage occurred, the severity of existing damage, and patient age and life expectancy.

In general, the target is set 20-30% below the untreated IOP. The CPOA records the IOP at each visit and flags readings above target for the doctor's attention.

Low IOP (Hypotony)

IOP below 6 mmHg is considered hypotony. It can cause the eyewall to soften, leading to corneal folds, macular edema, and other complications. Causes include wound leaks after surgery, over-filtration from glaucoma surgery, or cyclodialysis cleft.

⚠️ Common Mistake: Recording only one IOP measurement and accepting it as accurate. A single measurement can be affected by patient movement, blink, or squeezing. Always take at least 3 readings and average them. If readings are highly variable, repeat the entire series and flag the variability for the doctor.

Key Takeaways

Normal IOP is 10-21 mmHg by Goldmann tonometry; population mean is approximately 15-16 mmHg.
Ocular hypertension = IOP above 21 mmHg without glaucomatous damage. It is a risk factor, not a disease by itself.
Normal-tension glaucoma occurs at IOP within normal range -- proving IOP alone does not define glaucoma.
IOP varies by time of day (highest in the morning), corneal thickness, patient cooperation, and many systemic factors.
Target IOP in glaucoma management is set individually, typically 20-30% below untreated baseline.
The CPOA records IOP at each visit and flags any reading above the patient's target for the doctor.