One of the most rewarding things for a veterinary ophthalmologist is to be able to perform cataract surgery on a blind animal and then witness the restored vision and delight that this delivers to both pet and owner. However, not all cataract surgeries will have that outcome, particularly in the long term. Complications can arise meaning vision is not restored, or more commonly is restored and then lost. In making the decision of when to perform cataract surgery for the best outcome there are several factors that need to be considered. These include the optimal stage of cataract formation for the best outcome, the likely progression or lack of progression of the cataract, other concurrent disease (ocular and non-ocular), the owner's availability and ability to apply postoperative medications.

The term cataract describes an opacity of the lens that may range from a tiny opacity (incipient cataract) that has no effect on vision to a total mature (or hypermature) cataract that effectively makes the animal blind. My personal philosophy is that prior to performing cataract surgery I need to be convinced that either the cataract is already extensive enough to significantly impair vision, or will very likely progress to do so. Not all cataracts will progress to maturity. To judge the progression of an early cataract repeated examinations are required. Careful recording of the extent of the cataract at each visit is important, typically using drawings and perhaps photographs. It is also important not only to predict the likely degree of vision loss from the extent of the cataract, but also to directly assess the effect the cataract has on vision by for example, use of an obstacles course. Cataract surgery does not have a 100% success rate, even in the hands of the most proficient cataract surgeon so if an eye that still has good vision is operated on and develops a serious complication it could mean that the animal's vision is made worse by the surgery. Typical success rates for phacoemulsification in dogs are quoted as between 90 and 95%. Success rates depend not only on the skill and experience of the surgeon but also on concurrent ocular disease.

Phacoemulsification of cataracts that are still immature is likely to have a better surgical outcome than those carried out on cataracts that have reached maturity or have become hypermature. Changes to the lens proteins that occur with cataract formation can result in leakage of lens proteins through the lens capsule. These proteins tend to induce a secondary inflammatory change (lensinduced uveitis). This may manifest as a mildly red eye (mild episcleral congestion often diagnosed as a mild conjunctivitis) and a darkening of the iris. More detailed examination may reveal a lowered intraocular pressure and possibly aqueous flare. Eyes with a previous lens-induced uveitis are likely to have a greater postoperative inflammatory reaction following cataract surgery than eyes without previous inflammation and therefore tend to have a lower success rate.

Many cataract patients are older animals so a full physical examination and appropriate laboratory work-up should be performed prior to anaesthesia. Cataract surgery is an elective procedure so any concurrent disease such as diabetes or systemic illnesses should be controlled/stabilised prior to surgery to reduce the risk of anaesthetic complications. If there are foci of infection (e.g., cystitis, severe dental disease etc.) these should be treated prior to surgery.

Examination for concurrent eye disease is also important. In addition to the lens-induced uveitis mentioned above the eyes should be examined/tested for other abnormalities. Corneal surface disease should be treated prior to surgery because firstly corneal opacity may make visualisation during surgery difficult and secondly because the surgery itself might exacerbate the condition. Low tear production may be a concern and may lead to postoperative ocular surface complications so should be treated appropriately prior to surgery. The presence of corneal oedema may indicate corneal endothelial dysfunction that could be exacerbated by cataract surgery possibly even leading to corneal decompensation. If the posterior segment of the eye cannot be examined ophthalmoscopically an electroretinogram and ocular ultrasonography should be performed.

Prior to cataract surgery I prefer to administer a course of topical anti-inflammatory drops. I feel that this helps to suppress any subclinical inflammation resulting from leakage of lens proteins. This regime appears to help reduce the severity of postoperative inflammation.

When selecting a surgeon to refer your patient to ensure that they have good ophthalmology training, are regularly performing cataract surgery, and will hospitalise the animal overnight on the day of surgery for monitoring postoperative intraocular pressures. Refer the patient early for evaluation for surgery: do not wait until the cataract is 'ripe' or 'mature'. An early specialist examination will detect any other abnormalities present and monitor for concurrent problems such as lens-induced uveitis. Do not have your local physician ophthalmologist do the surgery - dog's eyes have very significant differences from human eyes both in size of lens and other anatomical differences but also in the way they react to surgical intervention. One exception to this might be if your patient is a primate.