The delicate fluid balance that keeps your vision from collapsing

Health · Published 17 October 2026 Swipe more like this →

In short: Your eyes are constantly balancing a delicate internal pressure. If the fluid inside your eye builds up too much, it can crush your optic nerve, leading to permanent vision loss. Understanding this pressure is the key to preventing glaucoma and maintaining your sight.

Intraocular pressure (IOP) is the force exerted by the fluid inside your eye. Specifically, it measures the pressure of the aqueous humor, a thin, water-like liquid that fills the space between your cornea and iris. Your body regulates this automatically: as the eye produces new aqueous humor, an equal amount must drain out through the drainage angle—the spot where the iris meets the sclera (the white of the eye).

Healthcare providers measure this pressure in millimeters of mercury (mmHg), the same unit used for blood pressure. A healthy range is generally considered to be between 10 mmHg and 21 mmHg, with an average of about 15.5 mmHg. When pressure consistently exceeds these limits without causing immediate damage, it is known as ocular hypertension. If left untreated, this chronic stress can damage the optic nerve, potentially causing glaucoma and permanent blindness.

Measuring this pressure can be complex because it is influenced by the thickness and rigidity of the cornea. Traditional tonometry works by puffing air at the eye to see how much the cornea flattens; however, certain refractive surgeries can make the cornea less rigid, potentially leading to inaccurate readings. To bypass this, newer methods measure pressure through the upper eyelid and sclera. Furthermore, IOP is not static; it fluctuates throughout the day and night, with normal variations of 3 to 6 mmHg. In some cases, medical interventions like Selective Laser Trabeculoplasty (SLT) are used to manually lower this pressure in patients with glaucoma.

The mechanics of IOP are governed by a specific quantitative relationship known as Goldmann's equation. It accounts for the rate of aqueous humor formation, the rate of resorption through the uveoscleral route, and the episcleral venous pressure. Notably, the vitreous humor—the thick gel at the back of the eye—has a relatively fixed volume and does not significantly affect the regulation of intraocular pressure.

General information, not professional advice. Spotted an error? Flag the card or see corrections and complaints.

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