Definition

Opacification of the lens (focal or diffuse).

Pathophysiology

• The normal lens is composed of perfectly aligned lens fibers that create a transparent structure. A clear capsule surrounds the cortex and nucleus. New lens fibers are continually produced at the equatorial region of the lens cortex throughout life. The aqueous humor provides nutrition to the lens.
• A cataract occurs when there is derangement of lens fibers due to alterations in lens nutrition, energy metabolism, protein synthesis or metabolism, or osmotic balance.
• Anterior uveitis is a common cause of alteration of lens nutrition. Genetics can result in altered protein and energy metabolism, or protein synthesis, in the lens.
• Diabetes mellitus will affect the osmotic balance within the lens of the dog. Hyperglycemia increases glucose in the aqueous and lens overwhelming the normal glycolysis pathway; glucose is then shunted to the sorbitol pathway; when sorbitol is produced it creates an osmotic gradient that draws water into the lens, resulting in the rapid formation of a cataract from lens fiber swelling and derangement. Because the sorbitol pathway requires aldose reductase enzyme, the more aldose reductase in the lens, the more readily diabetic cataracts will form. Dogs have more aldose reductase than cats, making cats more resistant to developing diabetic cataracts. There is some variability between individual dogs, which may explain why some diabetic dogs are more resistant to cataract development.

Systems Affected

Ophthalmic

Genetics

• Inheritance has been established for many dog breeds (see “Suggested Reading”); the most common mode of inheritance is autosomal recessive.
• Inheritance has been established in the Himalayan cat (autosomal recessive).

Incidence/Prevalence

• Cataract is one of the leading causes of blindness in dogs.
• The prevalence of genetic cataracts varies significantly between breeds; it has been reported as high as 10% in some breeds.
• Most diabetic dogs will develop cataracts regardless of their diabetic control.
• Cataracts are rare in cats.

Signalment

Signs

Causes

• Hereditary-most common cause in dogs.
• Diabetes mellitus.
• Anterior uveitis-either by altered nutrition of the lens from the abnormal aqueous, or by posterior synechia and inflammatory debris causing opacification of the anterior lens capsule.
• Trauma-perforating injury that disrupts the anterior lens capsule, most commonly a cat claw injury, especially in puppies and kittens.
• Senile-slowly progressive cataract in geriatric animals usually beginning as dense nuclear sclerosis followed by gradual spoke-like opacities extending into the cortex.
• Congenital-due to heredity, in utero insult, or associated with other congenital ocular anomalies such as persistent pupillary membranes, persistent hyperplastic primary vitreous/persistent tunica vasculosa lentis, or a hyaloid artery attachment.
• Surgery-transpupillary laser energy, intraocular instrument trauma.
• Toxic-from long-term ketoconazole therapy; suspected secondary to toxic by-products of degenerating photoreceptors in dogs with progressive retinal atrophy.
• Radiation-when the eye is in the radiation treatment field for head neoplasia.
• Hypocalcemia-can cause bilateral, diffuse punctate or incipient cataracts.
• Nutritional-use of unbalanced milk replacers in bottle-fed puppies and kittens.
• Electrical shock-chewing electrical cords or lightning strike.

Risk Factors

• Diabetes mellitus is a great risk factor in dogs.
• Chronic anterior uveitis.
• Progressive retinal atrophy.

Differential Diagnosis

Lenticular nuclear sclerosis-normal aging change in the lens of dogs and cats starting at 6 years of age due to compression of older lens fibers in the center of the lens; gradually becomes more visible with age and can be mistaken for a cataract in geriatric patients; definitive diagnosis can be made using mydriasis (1% tropicamide) and the observation of a perfectly round, bilaterally symmetrical, homogeneous nucleus in the center of each lens, and the ability to view the fundus through the lens; vision is rarely affected and treatment is not indicated.

CBC/Biochemistry/Urinalysis

Dogs with diabetic cataracts may have hyperglycemia and glucosuria.

Imaging

Ocular ultrasound can be used to evaluate the posterior lens capsule for any sign of rupture and can evaluate for retinal detachment prior to cataract surgery.

Diagnostic Procedures

Electroretinogram is performed prior to cataract surgery to evaluate for retinal degeneration when the fundus is not visible due to the cataract.

Activity

For safety, blind animals should not be allowed access to an in-ground swimming pool or elevated decks with open railings; use caution near stairs; restrict outside activity to fenced yards or leash walks.

Client Education

• Cataract surgery is routinely performed with an overall 80–90% success rate.
• Once the cataracts are removed they cannot return.
• Artificial lens implants will restore essentially normal vision.
• Evaluation for surgery should be done early in the course of cataract development to avoid complications that may result in the cataract becoming inoperable, to allow time to plan for the surgery, and in some cases to eliminate the need and extra cost for an ocular ultrasound and electroretinogram.

Surgical Considerations

• Phacoemulsification (removal of the cataract through a 3 mm corneal incision using ultrasonic waves to emulsify and then aspirate the lens cortex and nucleus) is the most common technique for cataract removal.
• The ideal time for cataract surgery is the immature/early mature stage.
• Inherited, diabetic, and senile cataracts are potentially good candidates for surgery; cataracts secondary to anterior uveitis are normally poor surgical candidates.
• Artificial intraocular lenses are routinely placed inside the patient's lens capsule; lens implants restore normal focus and help minimize posterior capsular fibrosis; if a lens cannot be implanted (e.g., due to an unstable lens capsule or luxated lens), the dog or cat will still have very functional vision.
• Traumatic lens perforation with release of lens cortex into the anterior chamber requires immediate removal of the lens to avoid a severe granulomatous anterior uveitis and vision loss.

Drug(s) Of Choice

• Topical anti-inflammatory medication is recommended q6–24h to help prevent or treat lens-induced uveitis with immature, mature, and hypermature cataracts; this can be a topical NSAID such as flurbiprofen, diclofenac, or ketorolac, or a topical steroid such as prednisolone acetate 1% or dexamethasone 0.1%; topical NSAIDs may be preferable in diabetic patients unless the uveitis is severe.
• Topical atropine q8–24h is indicated for lens-induced uveitis; atropine is contraindicated with glaucoma.
• Oral NSAIDs (carprofen, meloxicam, deracoxib) are also used to treat lens-induced uveitis.
• Topical antioxidants are advertised as able to reverse cataract changes; to date there has been no published data conclusively showing a significant reversal, or delay in progression, of a cataract with antioxidant therapy; unfortunately time spent trying medical therapy will delay evaluation for surgery, resulting in surgery being performed at a suboptimal stage, or complications from the cataract making it inoperable.
• Topical aldose reductase inhibitors are currently under investigation and may prove helpful in delaying the onset of diabetic cataracts in dogs in the future.

Patient Monitoring

• Incipient or early immature cataracts should be monitored regularly for progression in order to select the ideal time for surgery and to avoid complications associated with cataracts.
• Postoperative monitoring by the surgeon is critical for the success of surgery and should be clearly discussed with the owner prior to surgery.

Prevention/Avoidance

Do not breed animals with cataracts.

Possible Complications

• Lens-induced uveitis-associated with hypermature cataracts and cataracts that progress very rapidly; caused by antigenic lens proteins leaking through the lens capsule. Clinical signs can be very subtle (e.g., low intraocular pressure) to extreme (granulomatous uveitis with dense aqueous flare, miosis, synechia, keratic precipitates); preoperative lens-induced uveitis increases the risk of postoperative complications.
• Secondary glaucoma-impaired aqueous outflow from intraocular changes associated with lens-induced uveitis, or from an intumescent cataract causing a forward displacement of the iris narrowing the iridocorneal angle.
• Retinal detachment-associated with hypermature cataracts and cataracts in young dogs with a rapid onset and cortical liquefaction.
• Lens luxation-associated with hypermature cataracts in which the lens and capsule shrink, causing the zonules to stretch and break, resulting in a lens subluxation or luxation.

Expected Course and Prognosis

• Most cataracts are progressive, although the rate of progression can vary widely depending on age, breed, and location of the cataract.
• Long-term prognosis following cataract surgery is very good; however, some patients have increased risk for postoperative complications; preexisting anterior uveitis (even when medically controlled), genetic predisposition for glaucoma, and unstable peripheral retina or undetected peripheral retinal tears increase the risk for postoperative chronic uveitis, glaucoma, and retinal detachment, respectively.

Associated Conditions

• Retinal detachment
• Lens-induced uveitis
• Congenital ocular anomalies

Age-Related Factors

• Immediate referral for cataracts in young dogs (<2 years of age) is recommended because the cataract can progress very rapidly with partial cortical liquefaction followed by retinal detachment.
• Nuclear sclerosis is prominent in geriatric animals; a dilated exam may be necessary to definitively distinguish nuclear sclerosis from cataract.

See Also

• Anterior Uveitis-Cats
• Anterior Uveitis-Dogs
• Diabetes chapters

Abbreviation

• NSAID = nonsteroidal anti-inflammatory drug

Client Education Handout Available Online