A cataract is technically any opacity in the lens; it may be a small focal spot or, at the other end of the spectrum, the whole lens may be involved. The most common causes of cataracts in dogs are hereditary and diabetes mellitus. In cats the most common cause is uveitis. All cataracts lead to intraocular inflammation or lens-induced uveitis (LIU). This occurs from leakage of altered lens proteins through the lens capsule. Normally, the lens proteins are sequestered from the intraocular environment, but with cataract formation they may leak through the capsule and the ocular immune system "sees" them as a foreign object leading to inflammation.

Cataract surgery has evolved and progressed since it was first developed in the 1960's. Today many techniques and improvements in equipment and intraocular lens implants allow more patients to be surgical candidates. Overall, cataract surgery is considered to have a 90-95% success rate.

A complete ophthalmic examination, including visualization of the cataract following dilation, is the initial step in evaluating a patient for cataract surgery. Good prognostic factors include a positive menace response, a brisk dazzle reflex and good PLR's. Additionally, notation in the client history of previous good vision may also help in evaluating the patient for surgery. If the cataract formation is minimal then surgery may not be recommended or re-evaluation of the cataract at a specific time point may be recommended. It is important to differentiate advanced lenticular sclerosis from cataract formation. Lenticular sclerosis may lead to depth perception issues and increased glare but does not generally necessitate surgery. Very advanced lenticular sclerosis may progress to a senile cataract and visual disturbance; in those cases surgery may improve vision, however there may be anesthetic risks associated with systemic disease as many of these patients are very advanced in age. Lenticular sclerosis may be diagnosed after inducing mydriasis. Central or lens nucleus cloudiness with a clear peripheral lens cortex is usually noted and importantly a fundic reflex is noted through the whole lens.

Lens-induced uveitis is controlled by using topical steroidal and/or non-steroidal anti-inflammatories. Prior to cataract surgery the frequency of the anti-inflammatory drops is increased. Glaucoma medications may also be instituted to address post-surgical glaucoma risk. Long term topical anti-inflammatory medication is necessary in all patients with LIU in the lecturer's opinion. Lack of uveitic control may lead to glaucoma or zonular degeneration and lens luxation. Uncontrolled LIU may decrease the surgical prognosis so treatment should be started in a timely manner.

Most veterinary ophthalmologists perform a scotopic electroretinogram (ERG) and ocular ultrasound, after controlling the lens-induced uveitis, for cataracts deemed surgical candidates. Low amplitude or "flat" ERG's are consistent with retinal dysfunction and surgery would not be recommended. Similarly, retinal detachments diagnosed on ocular ultrasound would preclude surgery for the cataract. Gonioscopy is also routinely used to evaluate the eye and the risk of glaucoma prior to surgery. A wide open or normal iridocorneal angle (ICA) with normal pectinate ligament architecture is ideal. Narrowing of the ICA or goniodysgenesis is a potential complication that needs to be addressed with the owners prior to surgery. Approximately 5%-10% of patients develop post-operative glaucoma so in predisposed candidates the eye may be treated with additional medication or alternatively prophylactic glaucoma surgery is an option at the same time as cataract surgery.

Surgery is recommended on immature and newly mature vision-compromising cataracts as it results in a shorter surgery time, fewer chronic changes associated with LIU and a faster healing time. The newer techniques and equipment have eliminated the need to wait until the cataract is mature or "ripe".