Glaucoma in cats

©Barnett, KC & Crispin, SM Feline Ophthalmology (2002) Saunders

The cat has a deep anterior chamber despite the marked curvature of the anterior face of the lens. The cornea is large and the irido-corneal angle wide. Gonioscopy, examination of the angle of filtration, is possible in this species without the use of a goniolens due to the deep anterior chamber and large cornea. It reveals the slender, more widely separated and unbranching fibres of the pectinate ligament with numerous fibres behind in the trabecular mesh-work.

Congenital Anomalies

Congenital glaucoma, or buphthalmos, has been recorded on a number of occasions in the kitten as both unilateral and bilateral cases. It presents as a grossly enlarged globe(s) sometimes with intense corneal oedema. The cause is unknown but presumably is due to a developmental abnormality preventing adequate drainage. It is known that scleral stretching, with consequent globe enlargement, occurs more readily in young animals. A single case of congenital glaucoma in a Siamese kitten caused by iridoschisis (cleavage of the iris layers) has been reported.

Glaucoma

Glaucoma is defined as an increase in the intraocular pressure with resultant damage to  the retina and optic nerve causing total and irreversible blindness. It is not a simple condition but a complex. Glaucoma is one of the most challenging ophthalmic diseases we face as veterinarians. The condition is painful and often results in permanent vision loss. The condition is painful and often results in permanent vision loss. The elevation in intraocular pressure occurs due to impaired outflow of aqueous humour through the iridocorneal angle. Glaucoma may occur secondary to many conditions including lens luxation and uveitis but most cases are primary and inherited, usually affecting dogs between 3-6 years of age. The typical case presents with a history of redness and cloudiness of variable duration. When only one eye is affected, the symptoms may be overlooked for some time until globe enlargement is finally noted. This generally indicates that the pressure has been in excess of 40-50 mmHg (upper limit of normal is 25 mmHg) for weeks and that irreversible vision loss has resulted. When the second eye becomes affected (as is usually the case with inherited glaucoma), the patient then presents with a history of acute blindness. When only one eye is affected, examination of the "normal" eye by gonioscopy (use of a contact lens and a slit lamp to directly examine the iridocorneal angle) can help in predicting whether this eye is likely to become affected. In some cases topical medication may be used prophylactically.

Treatment for glaucoma is directed at restoring both vision and comfort. This requires an accurate assessment of the extent and duration of the pressure elevation. In eyes which have the potential for return of functional vision (no globe enlargement, duration less than one week), immediate medical therapy is used to initially reduce the pressure. This is followed by a choice of surgical options to attempt to permanently normalize the pressure. Medical management is rarely successful long-term due to recurrent pressure elevation and drug toxicity.