The fluid inside the eye is called the aqueous humor. The aqueous humor is produced and drains from the eye at approximately the same rate, resulting in a stable pressure inside the eye of 15 to 20 mmHg (millimeters of mercury). Glaucoma occurs as a consequence of a defect in the drainage angle resulting in inadequate outflow of aqueous humor and a subsequent buildup of pressure inside the eye. The resulting high pressure damages the optic nerve and results in blindness.

Gonioscopy is a technique used to evaluate the drainage angle. A goniolens (also known as a gonioscope) in conjunction with a slit lamp or operating microscope to gain a view of the drainage angle. The technique is essential to evaluate the non-glaucomatous eye for risk of a future attack of glaucoma.