Intraocular pressure (IOP) is the pressure of the fluid inside the eye. It is an important aspect in the evaluation of patients at risk of glaucoma. The eye is filled with fluid, and the chamber at the back of the eye is filled with vitreous humor, while a liquid called aqueous humor fills the area at the front of the eye, between the cornea and iris. Most of the aqueous humor flows out of the eye through the drainage angle, in front of the iris, and a small amount of new aqueous humor is always entering the eye while an equal amount drains out, maintaining a stable pressure.

IOP is measured in millimeters of mercury (mmHg) using a test called tonometry. Most tonometers are calibrated to measure IOP based on the resistance to flattening of a small area of the cornea. Normal IOP is usually considered to be between 10 and 20 mmHg, and pressures of between 11 and 21 mmHg are considered normal. However, measured values of IOP are influenced by corneal thickness and rigidity, and some forms of refractive surgery can cause traditional IOP measurements to appear normal when in fact the pressure may be abnormally high. A newer transpalpebral and transscleral tonometry method is not influenced by corneal biomechanics and does not need to be adjusted for corneal irregularities as measurement is done over upper eyelid and sclera.

Having high IOP is a health condition called ocular hypertension, and if it’s not treated, this added stress on the parts inside the eye can damage the optic nerve, leading to glaucoma and permanent vision loss. Elevated IOP is the crucial modifiable risk factor in the development of primary open-angle glaucoma. Therefore, measuring IOP is an important part of eye exams, and individuals diagnosed with high IOP should have regular comprehensive eye examinations by an eyecare professional to check for signs of the onset of glaucoma.