What Is Intraocular Pressure (IOP)?
Intraocular pressure (IOP) is the pressure created by the fluid inside the eye. A healthy level of pressure helps the eye maintain its shape and supports normal visual function. Measuring IOP is an important part of routine eye examinations. Raised IOP is a recognised risk factor for glaucoma and other conditions affecting long-term eye health.
How Is Intraocular Pressure Measured?
Intraocular pressure is measured using a test called tonometry. This assessment is routinely performed by an optometrist or ophthalmologist during an eye examination. Two methods are commonly used:
- Goldmann Applanation Tonometry (GAT): Goldmann applanation tonometry is widely regarded as the clinical gold standard for measuring IOP. The test determines eye pressure by measuring the force required to gently flatten a small area of the cornea. Before the measurement is taken, anaesthetic eye drops are used to numb the surface of the eye, making the procedure comfortable for the patient. NICE NG81 specifies Goldmann-type applanation tonometry as part of the required test battery for case-finding and diagnosis, and states that a referral decision should not be based solely on IOP measurement using non-contact tonometry.
- Non-Contact Tonometry (NCT): Non-contact tonometry measures eye pressure using a brief puff of air directed at the cornea. Because the instrument does not touch the eye, anaesthetic drops are not required. This method is widely used in routine optometry practice as a screening tool and can provide a quick assessment of IOP.
The thickness of the cornea can influence tonometry results. For this reason, clinicians may measure central corneal thickness and take it into account when interpreting IOP readings.
What Causes High Intraocular Pressure?
Raised intraocular pressure most commonly occurs when aqueous humour, the clear fluid produced inside the eye, does not leave the eye as efficiently as it is produced. This creates a build-up of pressure within the eye.
Factors associated with elevated IOP include:
- Reduced drainage through the eye’s drainage system, including the trabecular meshwork
- Increased production of aqueous humour by the ciliary body
- Long-term use of corticosteroid medications
- Eye injury or trauma
- Certain eye conditions, including uveitis and pigment dispersion syndrome
Raised IOP and glaucoma are not the same condition. Ocular hypertension refers to elevated eye pressure without evidence of glaucoma-related optic nerve damage or visual field loss.
However, elevated IOP is a recognised risk factor for developing glaucoma and is interpreted alongside optic nerve assessment, visual field testing, corneal measurements, and other clinical findings.
What Is the Link Between Intraocular Pressure and Glaucoma?
Elevated intraocular pressure is a significant risk factor for glaucoma. Glaucoma is a group of eye conditions that damage the optic nerve and can lead to permanent vision loss if left untreated.
Higher eye pressure increases the likelihood of optic nerve damage, but pressure alone does not determine whether someone develops glaucoma. Some people with ocular
hypertension never develop glaucoma, while others develop glaucoma despite having IOP readings within the normal range.
This latter form is known as normal-tension glaucoma. Because glaucoma can occur across a range of pressure levels, diagnosis relies on a combination of tests, including optic nerve examination, visual field assessment, and IOP measurement.
How Is Intraocular Pressure Managed?
Treatment is not required for every person with elevated IOP. When management is indicated, the approach is based on the individual’s overall risk profile and clinical findings.
Common treatment options include:
- Medicated eye drops that reduce fluid production or improve fluid drainage
- Laser procedures, including 360° selective laser trabeculoplasty (SLT), which can improve fluid outflow
- Surgical intervention for cases requiring additional pressure control
Regular monitoring by an optometrist or ophthalmologist remains a central part of long-term IOP management. Ongoing assessments help identify changes in eye pressure and detect any signs of optic nerve damage at an early stage.
Monitoring Intraocular Pressure and When to Seek Professional Advice
Intraocular pressure can only be measured accurately during an eye examination. Routine eye tests allow clinicians to monitor IOP over time and assess it alongside other indicators of eye health.
Anyone diagnosed with ocular hypertension, glaucoma, or another condition affecting eye pressure should attend all recommended follow-up appointments. Patients presenting with sudden changes in vision, severe eye pain or new ocular symptoms require prompt professional assessment.
Because elevated IOP often develops without noticeable symptoms, regular eye examinations remain the most effective way to identify pressure-related eye conditions early and support long-term eye health.