Cataracts in dogs: what are the options?
The term cataract refers to any opacity of the optical lens that causes abnormal light scattering or prevents light from reaching the retina. Cataracts affect multiple species and if left untreated often lead to progressive blindness. In fact, according to the World Health Organization, cataract disease is the most frequent cause of visual impairment in humans, effecting 18 million people worldwide and causing 48 percent of all blindness.
Man’s best friend is not exempt; cataract disease is very common in dogs. However, unlike their human being counterparts, the cause and clinical manifestations of cataracts within the canine species vary with age and breed. Dogs are less likely to encounter cataract disease related to aging, which may be explained by the difference in life span between the two species. Oxidative damage and environmental triggers (such as exposure to UV light) become more important when the life span extends to multiple decades, as in humans. In dogs, heritable and metabolic cataracts are more common.
In fact, specific gene mutations have been identified in a few canine breeds, such as Staffordshire Bull Terriers, Boston Terriers and Australian Shepherds. In addition, dogs of any breed can develop cataracts that are considered genetic, although the precise gene mutations have not yet been elucidated. Age may also be a factor; large breed dogs develop lens opacities earlier than middle-sized and small dogs. When traumatic cataracts occur (frequently associated with penetrating injuries), they are usually related to a single eye and associated with other symptoms related to the eye. The most common metabolic cause for canine cataracts is diabetes mellitus.
Most canine cataract disease is considered progressive and likely to be associated with ocular complications (most commonly uveitis and secondary glaucoma). For this reason, the most useful criterion for classifying cataracts is the stage of progression.
Incipient cataracts are opacities that involve less than 10% of the lens volume. (Fig 1-A)
These small opacities usually go unrecognized by pet owners and are typically discovered incidentally. They are not clinically relevant although they should periodically be assessed for progression. Some incipient cataracts do not progress.
Immature cataracts are the next stage of progression. (Fig 1-B), involving 10 to 100 percent of the lens volume. Immature cataracts may allow partial vision and even if advanced, menace response, pupillary light reflex and dazzle reflexes will likely be present. Fundic reflection still allows determination of the color of the ocular fundus. Some owners may observe signs of visual impairment in their dog’s behavior at advanced stages. The immature stage is usually ideal for surgical treatment since associated inflammation and lens hardness do not increase the surgical challenge or the risk of complications.
Mature cataracts (Fig 1-C) are defined by a complete opacity of the lens, which completely obscures observation of the ocular fundus. If the cataracts are bilaterally symmetrical, a patient becomes blind. It is important to note that if the cataracts have developed slowly and the dog is living in a familiar environment, pet owners might not recognize the loss of slight. Often times, affected dogs may gradually adjust to their impairment and learn to navigate well in a known space. Most commonly, owners seek medical treatment because they become concerned about development of a white pupil.
Mature cataracts are usually associated with some ocular inflammation whose origin is still unknown, although it has historically been attributed to the release of lenticular protein into the anterior chamber. The inflammation is termed lens-induced uveitis (LIU) and if moderate or severe it may be a risk factor for post-surgical complications. In older patients, the hardness of the lens may present a surgical challenge.
Hypermature cataracts abundantly leak proteins into the aqueous and can be recognized by a characteristic lens capsule wrinkling. (Fig 1-D) This stage of cataract disease is commonly associated with moderate-to-severe uveitis and may occasionally lead to retinal detachment. Surgery is still possible at this stage, although it is associated with a higher incidence of complications.
Morgagnian cataracts are a further evolution of the hypermature stage and are characterized by complete reabsorption of the cortical material. At this advanced stage of cataract disease the nucleus becomes tightly enwrapped by the fibrotic lens capsule, surgery is not always a viable treatment option and an intraocular lens (IOL) cannot be used.
Diabetic cataracts often behave differently as they often appear rapidly. These cataracts develop because of a shifting water gradient within the lens due to accumulation of a specific sugar byproduct (sorbitol). Sorbitol is not soluble and it draws fluid into the lens. The lens becomes intumescent leading to two important considerations:
- Diabetic cataracts are usually easier to treat surgically because they are softer than other mature cataracts, reducing the length of surgery and the potential for complications.
- The relatively rapid swelling of the lens may lead to rupture of the lens capsule (which is non-distensible), with leakage of lens contents through the capsule, usually at the equator (periphery) of the lens. Massive exposure of lens material to the intraocular immune system may cause a severe inflammatory reaction (phacoclastic uveitis).
In general, cataract disease requires accurate clinical staging to determine the prognosis expected with surgery. Immature, early mature and uncomplicated diabetic cataracts have the best treatment outcomes. Complications increase with more advanced stages, although surgery is still possible. Despite the marketing of medical therapies, surgery remains the only truly effective treatment. Medical treatment, beyond being ineffective, often allows the stage of cataract to progress, increasing the risk of surgical complications.
Ninety to 95 percent of dogs with early-stage cataracts receive a good to excellent outcome from timely treatment. If uveitis is present, it should be addressed before performing surgery. Gonioscopy, ocular ultrasound and electroretinography are mandatory tests to predict the likelihood of complications and rule out diseases that might compromise vision despite surgery for the cataracts. The gold standard surgical treatment for cataracts is phacoemulsification and insertion of an artificial intraocular lens (IOL).