Gonioscopy, sometimes performed on dogs and cats, is the examination of the iridocorneal angle of the eye. The iridocorneal angle is where the base of the iris attaches to the cornea and sclera (the white, outer layer of the eyeball). It’s the site where aqueous humor (the fluid within the eye) drains from the front chamber of the eye.
A normal eye produces and drains watery fluid (called aqueous humor, which is the fluid produced by the ciliary process in the eye. This fluid nourishes the lens and cornea and maintains the proper ocular pressure. Poor drainage of this fluid can cause glaucoma. Pressure within the eye builds up if this fluid does not drain properly. This pressure can damage the optic nerve, leading to vision loss.
What Does Gonioscopy Reveal in Cats?
Gonioscopy is used in the diagnosis of primary glaucoma or ocular hypertension. It is often used when the results of tonometry is inconclusive. Gonioscopy provides information on how the iridocorneal angle appears. There are three categories: open, narrow (or closed) and dysplastic (malformed). Specifically, gonioscopy allows visualization of the base of the iris, the pectinate ligaments, and the base of the cornea.
Gonioscopy is also useful to diagnose the presence of tumors, foreign bodies, inflammatory exudates, or blockage caused by bleeding.
How is Gonioscopy Performed in Cats?
Gonioscopy is performed using a goniolens, which is a type of contact lens. In dogs, a goniolens is often necessary because the iridocorneal angle is not readily visible as it is in cats. Some lenses allow for direct visual examination while others use indirect viewing through mirrors. Some type of magnifier is often used to help view the angle.
With the goniolens resting gently on the eye, the room is darkened and a focal light source is shone through the lens towards the iridocorneal angle. The examiner may look at the angle through a slit lamp biomicroscope, or may wear a head loupe magnifier and use a transilluminator to examine the angle.
The appearance of the angle is then recorded in the medical record and photographs may be taken of the angle through the goniolens, using a special retinal camera.
Is Gonioscopy Painful to Cats?
Gonioscopy is not a painful procedure, and is very well tolerated in cats.
Is Sedation Required?
Most cooperative patients receive only a local anesthetic eye drop, such as proparacaine, to numb the eyeball. This allows the veterinarian to examine the eye without alarming the patient and for the cat not to feel the minor presence of the goniolens. Sedation is not required but may be necessary in some pets that will not stay stationary for the procedures.