by Dr. Noelle La Croix, DVM, Dip. ACVO
A cataract is any opacification of the lens. Crystallins within lens fibers, or the lens fibers themselves, are disorganized within a cataract. Ultimately, crystallin disorder decreases transparency or light transmittance. Cortical cataracts are histologically associated with disordered, swollen, and ruptured lens fibers. These ruptured fibers are not normally repaired. In contrast, senile nuclear cataracts are characterized by disordered crystallins within dense but ordered fibers. Cataracts have many etiologies including genetic, metabolic, environmental, and senility factors.
Stages and Referral
Cataracts can be classified by etiology, age of onset, location, appearance, and stage of progression. The stage of progression is the most widely used and useful classification. The most common progression stages are incipient, immature, mature, and hypermature. Incipient cataracts are the earliest to appreciate clinically and usually take up no more then 10 to 15% of lens volume (Figure 1).
Patients with incipient cataracts are ideal for referral to an ophthalmologist. Incipient cataract progression is dependent upon both position and appearance, as evaluated by slit lamp biomicroscopy. Incipient cataracts allow for a clear view of the fundus for diagnosis of retinal and vitreal disease without ultrasonography.
Not all incipient cataracts progress. Generally, congenital incipient cataracts within the lens nucleus will not progress. In most cases, non-progressive incipient canine cataracts do not require surgical removal as they do not limit functional vision. Incipient cataracts near the active proliferative lens equator will often progress. In these cases, anti-inflammatories can help prevent phacolytic uveitis. Cataract surgery is most successful on eyes without a clinical history of phacolytic uveitis.
The immature cataract is at the ideal stage for cataract surgery. Immature cataracts are characterized by low density areas, with clear lens fibers, that allow for tapetal reflection through the lens. Immature cataracts are often swollen with fluid, forming large separation clefts within the lens. These clefts can be used as areas for lens fracture during cataract surgery. However, extreme swelling can result in intumescence. Intumescent lenses are associated with glaucoma, and the loss of vision via pupillary blockage and ciliary cleft closure (Figure 2). Severe uveitis is also associated with intumescent lenses which leak large amounts of proteins into the aqueous humor.
Mature cataracts typically involve the entire lens, completely obscuring tapetal reflection. These advanced cataracts can be associated with uveitis, capsular plaques, and lens instability. Increased intra-operative and post-operative complications, and decreased visual outcome, are associated with mature cataract surgery. The lens fibers of mature cataracts continue to break down and release lens proteins. Over time, phacolytic uveitis progresses, and the lens capsule wrinkles from epithelial cell proliferation and shrinkage of the lens volume. A hypermature cataract has a wrinkled lens capsule with a swollen milky cortex. This is the result of autolysis of the lens fibers of a mature cataract. During cataract surgery distorted capsules predispose them to tearing and prevent the implantation of artificial lenses. In addition, some mature and hypermature cataracts are not candidates for surgery because advanced phacolytic uveitis has already caused glaucoma, retinal detachment, and/or retinal degeneration.
Humans typically present earlier stages of cataracts then dogs. Human eyes contain an extremely dense area of cones (the macula and fovea) for sharp central vision. Any small opacity in the lens overlying this area is associated with a dramatic change in functional vision, quickly vocalized to an attending physician. In contrast, dogs contain a less dense area known as the “visual streak” and have an appreciably lower visual acuity then humans. If the typical human eye scores 20/20 on the Snellen eye chart, the typical canine eye would score 20/75. Anything below the 3rd line of the Snellen chart would be a blur to a dog. Therefore, typical domesticated dogs do not depend on fine visual acuity to survive, and will not usually show visual dysfunction with early stage cataracts.
In the veterinary world cataracts typically present in later stages, when first appreciated by owners. Ideally, incipient cataracts are managed medically to control phacolytic uveitis. Immature and mature cataracts can often be removed with phacoemulsification. Other ocular issues must be addressed including the control of phacolytic uveitis, intraocular pressure, and tear production. As with all anesthetic surgical procedures, pre-disposing conditions (diabetes; liver or kidney disease; elevated blood pressure, etc.) must be managed prior to cataract surgery. Infections (skin, oral, and bladder) should be resolved prior to cataract surgery to decrease the risk of post-operative endophthalmitis. Ocular and systemic issues can often be managed in tandem.
Post-operative care following cataract surgery is demanding. Owner and patient compliance with the large and precise regimen of medications should be considered prior to surgery. Cataract surgery is considered an elective procedure as most animals will adapt to visual losses. However, conditions associated with cataracts (uveitis, lens luxation, glaucoma, etc.) that affect an animal’s quality of life should always be addressed.
Figure 1: A posterior subcapsular cortical cataract in the left eye of a 1.5-year old Golden Retriever. This incipient cataract has a genetic basis and is associated with phacolytic uveitis.