We define cataracts as an opacification of the crystalline lens. One must differentiate cataracts from nuclear sclerosis, a normal aging process of the lens. In general, nuclear sclerosis does not cause significant visual difficulties, except in the unusual case of very dense nuclear sclerosis.

Cataracts can be classified according to several criteria: etiology, age at presentation (congenital, infantile, juvenile, and senile), localization (capsular, subcapsular, zonular, cortical, nuclear, sutural, axial, equatorial), appearance (punctate, stellate, wedge-shaped), and stage (incipient, immature, mature, hypermature).

There exist several causes for cataracts in small animals. Hereditary or genetic cataracts are quite frequent, as also are diabetic cataracts (which are the result of an osmotic gradient due to glucose to sorbitol conversion within the substance of the lens). Endogenous causes of cataracts include: uveitis, retinal degeneration (PRA), and glaucoma. Chemical substances, radiation, electricity, and trauma are other causes of cataracts in small animal practice.

In order to classify the cataracts according to their stage and to consider if surgery is a treatment option, we shall adhere to a classification scheme adopted by veterinary ophthalmologists, where there exist four stages: incipient, immature, mature, and hypermature. As shown in the table below, these four stages are differentiated by the presence or absence of a tapetal reflex, as well as by the appearance of the anterior lens capsule (smooth versus wrinkled).

Cataract surgery is an elective procedure. Careful patient selection is imperative in order to achieve a successful result. Dogs that exhibit pre-existent keratitis, keratoconjunctivitis sicca, uveitis, glaucoma, and retinal degeneration are usually poor surgical candidates.

Pre-operative diagnostic examinations should include a complete hemogram, biochemical profile, sterile urinalysis, electroretinography (ERG), and ocular ultrasound. The ERG will rule-out the presence of retinal degeneration and progressive retinal atrophy. An ocular ultrasound can detect focal to diffuse retinal detachments and vitreal opacifications.

There exist three general techniques for cataract extraction: extracapsular lens extraction (ECLE), intracapsular lens extraction (ICLE), and phacofragmentation (also called phacoemulsification or "phaco" for short). Phacofragmentation is the preferred technique for cataract removal by veterinary ophthalmologists today. Intracapsular lens extractions are reserved for subluxated or luxated lenses, and the extracapsular technique is performed by some for extremely hard lenses, which are unable to be removed by phacofragmentation. The phaco technique utilizes ultrasonic waves to emulsify, to aspirate lenticular fragments, and to irrigate the eye. Implantation of intraocular lenses after cataract removal is routinely performed to return the patient to emmetropia (or "true vision"). Intraocular lenses are made for both dogs and cats. Foldable lenses, which are injected into the capsular bag, are also available.

Post-operative complications include uveitis, glaucoma, corneal endothelial damage, capsular opacification, posterior capsule rupture with vitreal prolapse, intraocular hemorrhage, endophthalmitis, retinal detachment, and blindness.

The percentage for a successful surgical outcome can approach 90% in carefully selected clinical cases.

References

1.  Davidson MG, Nelms SK. Diseases of the Lens and Cataract Formation. In: Veterinary Ophthalmology. Third edition ( ed. Gelatt KN ), Lippincott Williams & Wilkins, Philadelphia. 1999:797-856.

2.  Dziezyc J. Cataract Surgery. Current Approaches. Vet Clinics of North Amer 1990:(20): 737-754.

3.  Gelatt KN: Essential of Veterinary Ophthalmology. The Canine Lens. Lippincott Williams & Wilkins, Philadelphia. 2000: 227-252.

4.  Glover TD, Constantinescu GM. Surgery for Cataracts. Vet Clinics of North Amer 1997 (27): 1143-1173.