Cataracts - Causes and Classification

Cataract is an opacity of the lens. There are numerous potential causes for cataracts, including metabolism (diabetes), nutrition (use of milk replacers), developmental ocular defects, trauma, etc. However, the most common cause of cataracts in dogs appears to be genetic disposition. Cataracts have been demonstrated to be hereditary in a score of canine breeds, and are suspected of being hereditary in dozens of other breeds. Senility is probably another major cause of canine cataracts, though the exact prevalence (and causes) of age-related cataracts in dogs remains unknown.^1,2

There are additional methods of classifying cataracts, besides cause. The opacity may be described according to age of appearance (which ranges from congenital to elderly) or location in the lens. However, in many ways the most important classification is the degree of cataract maturation. Cataracts frequently begin as a small, focal opacity, which is incipient in nature. This usually progresses to involve most of the lens. At this stage, the immature cataract obscures funduscopic details (though a tapetal reflection is still present), and vision deteriorates. As the cataract becomes completely opaque (mature), tapetal reflection and vision are lost. In hypermature cataracts the lens proteins break down and leak through the capsule. Because lens protein is immunogenic to the eye, leakage into the anterior chamber (which also occurs in mature cataracts) will cause lens-induced uveitis (LIU) that must be treated.³

Selecting and Referring Surgery Candidates

There are several important considerations in choosing the surgical candidate. The first is the stage of cataract maturation. Patients with incipient cataracts do not require surgery, as the effect on vision is minimal. However, this may be an optimal time to refer the patient to a veterinary ophthalmologist who will assess the cataract (to determine the likelihood of progression), and more importantly, perform a thorough examination of the ocular fundus (see below). At the other end of the spectrum, patients with hypermature cataracts make poor surgical candidates, as they usually suffer from severe LIU that gravely impacts the surgical prognosis. Most ophthalmologists prefer to operate on immature cataracts, as at this stage there are significant visual deficits, yet the surgery is relatively easy (due to softness of the lens) and the LIU is minimal.^1,3,4

Obviously, cataract surgery will not restore vision to patients whose retina is not functioning, and therefore assessment of retinal function prior to surgery is of utmost importance. Ideally, such an evaluation is conducted by a thorough funduscopic examination by an experienced clinician. Unfortunately, by the time the patient reaches the ophthalmologist, the cataract is usually too advanced to permit such an examination. Therefore, retinal function (and visual prognosis) is usually determined through electroretinography, which records the electrophysiological response of the retina to light. A diminished or absent response implies retinal dysfunction, and thus save the patient from a surgery that will not restore its vision.

In this context it should be noted that the presence of pupillary light responses does not rule out retinal degeneration; these responses may be sluggish, but are usually present in the patient. An ultrasound examination does not test retinal function, either. It is used to diagnose retinal detachment and/or vitreal disease.¹

Other considerations include the overall health of the eye and the patient. As cataract is an elective surgery, it can be postponed so that LIU, or any other ocular and systemic diseases, may be treated prior to the operation. And a final consideration in selecting the patient is the personality of the dog and/or owner. Many cataract patients require vigorous or long-term post-operative treatment. Aggression in the dog, or a physical handicap of the owner, that would prevent administering such treatment to the patient will negatively impact the surgical outcome.

Peri-Operative Treatment & Surgical Procedures

An important aspect of cataract prevention is breeding, and mating of animals with cataracts suspected of being inherited should be strongly discouraged. Currently, there is no efficacious medical treatment for slowing, halting or reversing the progression of cataracts. Treatment with mydriatics may temporarily restore vision in cases of nuclear cataracts, and aldose reductase inhibitors are being used experimentally to slow the progression of diabetic cataracts. However, cataracts remain a surgical problem.

Referring veterinarians should be aware of the peri-operative medical treatment administered to cataract patients, though this treatment may vary between veterinary ophthalmologists. Typically it consists of various regimens of anti-inflammatory and mydriatic drugs. Antibiotics and prophylactic glaucoma therapy (to prevent post-operative intraocular pressure spikes), may also be administered.

For many years, extracapsular extraction was the routine operation practiced by veterinary ophthalmologists. This procedure has largely been replaced by phacoemulsification, which relies on ultrasonic waves to pulverize the cataract. A probe is inserted through a small corneal incision, and is used to alternately break up the lens, irrigate the capsular bag and aspirate the cataractous material. The thorough irrigation & aspiration performed by the machine decrease the severity of post-operative uveitis and capsular opacities, and the smaller corneal incision speeds up both the surgery and the healing.^1,4

There is a long list of potential intra-operative complications. However, technological advances enable most surgeons to claim a success rate > 90%. Unfortunately, this rate may decline with time due to long-term complications, notably LIU and glaucoma.^5,6

Optical Correction

Following lens extraction, the eye becomes far-sighted (hyperopic). This optical deficiency may be corrected by implanting artificial intraocular lenses (IOLs). The implantation greatly improves the post-surgical visual performance of the dog. However, IOLs are not "automatically" implanted in every cataract patient. Because of the great number of possible intraoperative complications, and because of the inevitable post-operative LIU, many surgeons will implant IOLs only if the surgery was "perfect". Still, it is important to remember that dogs will be able to see post-operatively even if they were not implanted with an IOL. Most of the refraction in the eye takes place at the cornea, with the lens providing only a third of the total refractive power of the eye. Therefore, even without an IOL, successful removal of the cataract will enable the patient to regain vision.^1,4

References

1.  Ofri R. Lens. In: Maggs DJ, Miller PE, Ofri R, eds. Slatter's Fundamentals of Veterinary Ophthalmology, 4th ed. St Louis: Saunders Elsevier 2007:258–276.

2.  van der Woerdt A, et al. Lens induced uveitis in dogs: 151 cases (1985–1992). J Am Vet Med Assoc 1992;120:921–926.

3.  Williams DL, et al. Current concepts in the management of canine cataract: a survey of techniques used by surgeons in Britain, Europe and the USA and a review of recent literature. Vet Rec 1996;138:347–353.

4.  Appel SL, et al. Evaluation of client perceptions concerning outcome of cataract surgery in dogs. J Am Vet Med Assoc 2006;228:870–875.

5.  Gelatt KN, Mackay EO. Prevalence of primary breed-related cataracts in the dog in North America. Vet Ophthalmol 2005;8:101–111.

6.  Sigle KJ, Nasisse MP. Long-term complications after phacoemulsification for cataract removal in dogs: 172 cases (1995–2002). J Am Vet Med Assoc 2006;228:74–79.