Intraocular pressure (IOP) is the fluid pressure maintained inside the eye. This internal pressure is necessary for the eye to keep its spherical shape, which is fundamental for proper visual function. If the pressure is too high, it can lead to damage, making IOP measurement a routine assessment of overall eye health.
Understanding Intraocular Pressure
The pressure inside the eye is determined by the fluid dynamics of aqueous humor. This clear, water-like fluid is continuously produced by the ciliary body, located behind the iris. Aqueous humor circulates to nourish the lens and cornea before draining out of the eye.
IOP is maintained by a balance between the fluid’s production rate and its drainage rate. The primary drainage route is through the trabecular meshwork, a specialized tissue located where the iris and cornea meet.
If outflow through this meshwork is obstructed or slowed, the fluid backs up, causing the intraocular pressure to rise. The vitreous humor, the gel that fills the large back part of the eye, does not significantly affect this pressure.
How Eye Pressure is Measured
The procedure used to measure intraocular pressure is called tonometry, a standard part of a comprehensive eye exam. Tonometers measure pressure in units of millimeters of mercury (mmHg). The normal range for IOP is generally between 10 mmHg and 21 mmHg.
The Goldmann applanation tonometer is the most accurate method. This technique involves numbing the eye and gently touching the corneal surface with a probe to measure the force required to slightly flatten, or applanate, the cornea.
Non-contact tonometry, sometimes called the “air puff test,” uses a rapid burst of air and is less invasive. If a screening test shows an elevated reading, eye care professionals usually use the Goldmann applanation method to confirm the measurement.
The Connection Between IOP and Glaucoma
Elevated intraocular pressure is the most significant modifiable risk factor for developing glaucoma, a group of conditions that cause damage to the optic nerve. The optic nerve is the bundle of millions of nerve fibers that transmits visual information from the eye to the brain. When IOP remains persistently high, the continuous pressure exerts mechanical strain on the optic nerve head, particularly at the lamina cribrosa.
This sustained pressure restricts blood flow and nutrient transport to the nerve fibers, leading to their gradual degradation and death. Because the damage is slow and affects peripheral vision first, a person may not notice symptoms until significant vision loss has occurred.
Not everyone with elevated IOP develops glaucoma; this condition is called ocular hypertension and requires monitoring. Conversely, some individuals develop optic nerve damage even with normal IOP measurements, a condition known as normal-tension glaucoma. Lowering the IOP is the established treatment approach to slow or prevent the progression of optic nerve damage.
Managing Elevated Intraocular Pressure
Treatment for high IOP focuses on reducing the pressure to a target level considered safe for the individual’s optic nerve. The most common first-line treatment involves prescription eye drops, which work through two primary mechanisms. Some drops, such as prostaglandin analogs, increase the outflow of aqueous humor through the drainage pathways. Other medications, including beta-blockers, decrease the production rate of aqueous humor by the ciliary body.
Often, a combination of different eye drop types is necessary to achieve the desired pressure reduction. For patients whose pressure remains too high, procedural interventions are considered. Laser trabeculoplasty is a minimally invasive procedure that uses a laser to open the drainage angle, improving the efficiency of the trabecular meshwork. If drops and laser treatment are insufficient, filtering surgery, such as a trabeculectomy, may be performed to create a new channel for fluid drainage.