Gonioscopy

Abstract

Gonioscopy is critical in the evaluation of all forms of glaucoma. It is unfortunately common that secondary forms of glaucoma are unrecognized because of the failure to perform gonioscopy. Not only is gonioscopy able to provide information about the potential causes of raised intraocular pressure, but it can also provide clues critical to the determination of therapy.

When light coming from the iridocorneal angle reaches the tear-air interface, it is reflected back into the eye. This is known as the principal of total internal reflection. The critical angle at which light from the angle structures can be seen is 46° (Shields MB, Textbook of glaucoma. Williams & Wilkins, Baltimore, 1992). In patients with keratoconus, the angle is sometimes open beyond this critical angle, and the iridocorneal angle can be seen with slit lamp biomicroscopy alone. In eyes with a normally vaulted cornea, a contact lens must be used to examine the iridocorneal angle. Two types of lenses are used: the direct lens and the indirect lens.