Cataracts are abnormal opacities in the lens of the eye and must be differentiated from normal, age-related, nuclear sclerosis. Whereas a cataract may involve many parts of the lens and often eventually results in impaired vision to total blindness, nuclear sclerosis is a homogeneous "graying" of the central (nuclear) part of the lens and does not affect overall vision until the pet is very old (15 to 20 years). Examination of the lens with a dilated pupil using a short-acting mydriatic agent, such as tropicamide (4- to 6-H dilation vs. several days with atropine), will help differentiate a true cataract from nuclear sclerosis. One final introductory note is that this article almost exclusively discusses dogs, as most cats develop cataracts secondary to severe, chronic uveitis and glaucoma that usually preclude any reasonable attempt at surgery.
Why & When to Refer
Cataracts come in a "variety of flavors," with some inherited types-such as the posterior subcapsular, triangular opacity in golden retrievers, which usually does not progress to cause vision problems, versus the rapidly progressive, mature cataracts in young cocker spaniels or diabetic patients that result in blindness. Some owners may note some presbyopic changes, such as difficulty in climbing stairs or seeing objects up close, but nuclear sclerosis rarely requires referral for surgery. These patients may benefit from simple dilation therapy to improve peripheral vision since the cortex is clear.
Early referral to the veterinary ophthalmologist provides several advantages, including the following:
• Staging of the cataract (incipient, immature, mature, hypermature) with possible determination of the cause and prognosis for progression or development in the opposite eye
• Opportunity to remove a unilateral cataract vs. waiting for both eyes to develop cataracts, which risks complications in the first eye and may preclude surgery
• Opportunity to directly view the fundus to evaluate for possible vitreal or retinal disease, such as progressive retinal atrophy
• Evaluation and early treatment for mild lens-induced uveitis, such as microscopic flare/cells or hypotony, thereby improving the chances for a successful surgical outcome
• Determination of cause: genetic, metabolic (diabetes), trauma, lens-induced, thereby helping to predict prognosis for vision
• Evaluation for concurrent diseases (dry eye, corneal dystrophy, PRA, glaucoma, synechiae), again helping to aid in determining overall prognosis and treatment for the patient, including surgical outcome.
The ophthalmologist will perform a comprehensive examination, including Schirmer's tear test, fluorescein corneal staining, tonometry, slit lamp examination for cataract staging, and ophthalmoscopy to evaluate the fundus. In addition, most ophthalmologists perform ERG and ocular ultrasonography to further evaluate the vitreous and fundus, especially in patients in which cataract formation precludes direct evaluation.
In some cases, the patient, usually a purebred (such as a Labrador retriever), has incipient or immature cataracts, but vision is poorer than what would be expected to be caused by the cataracts. These situations are often caused by comorbid conditions, such as PRA. Such patients do not benefit in the long run from cataract surgery, as there is no current treatment for PRA, and they will eventually go blind. The referring veterinarian should keep this in mind as he or she sees a patient with mild cataract formation but very poor functional vision.
Information to Supply
The referring veterinarian is welcome to submit ophthalmic findings to the ophthalmologist, but any general medical information on the patient is far more important (see the Box). In addition, specific information on diabetic patients is very important, such as information about insulin regulation. Most veterinary ophthalmologists use topical corticosteroids after surgery, so it is important to be sure insulin is well regulated beforehand. In addition, poorly controlled diabetes is more likely to cause postoperative complications, such as lipemic uveitis with secondary glaucoma. Any blood analysis results, such as a complete blood count, chemistry profile, cortisol tests for Cushing's disease, and fructosamine values for diabetics, are helpful to the ophthalmologist to fully evaluate the patient before surgery and anesthesia induction, as well as to avoid unnecessary duplication of recent blood analysis.
What to Expect & Request
The ophthalmologist should provide regular updates to the referring veterinarian, including the results of the initial ophthalmic examination, the outcome of the ERG and ultrasonography, surgery report, and postoperative visit examinations. If the patient is not a candidate for surgery or the client elects to postpone surgery, the ophthalmologist should document those findings and discussions with the client.
Postoperative reports should give intraocular pressure values as well as examination findings, such as the extent of postoperative inflammation, condition of the vitreous and retina, and appearance of either the aphakic (no lens replacement) or pseudophakic (intraocular lens replacement) eye.
Any intraocular surgical procedure will cause some postoperative uveitis, so the ophthalmologist should provide information on the type of postoperative medications. Ophthalmologists use many types of postoperative medical protocols, depending on training, the patient's medical status, surgical trauma, and the patient's overall healing process. Most ophthalmologists prefer a combination of topical antibiotics with corticosteroids, starting at Q 4 H dosing, and some surgeons also treat with a mydriatic agent, such as atropine or tropicamide, systemic nonsteroidal antiinflammatory drugs or steroids, and a pressure-reducing drug if postoperative intraocular pressure spikes.
Private ophthalmology practices tend to perform outpatient surgery, whereas university practices often hospitalize the patient for a matter of days. In any event, the patient is usually evaluated 3 to 4 times over a 6- to 8-week period, and medications are usually reduced in number and frequency of treatment during that period until the medications are either eliminated or used infrequently. A trend of continuing to use steroidal or nonsteroidal topical drugs once or twice daily indefinitely is developing among ophthalmologists.
One of the most common questions a client will ask is whether the cataracts will "grow back" over time and result in another surgery. Once the lens is removed, the cataract cannot grow back, but the practitioner should be aware of the possibility of "posterior capsular opacity," in which some lens epithelial cells could migrate onto the posterior capsule; over time, the patient may develop an opacity on the lens capsule, which can produce a noticeable haze. However, such formation usually does not have a clinically significant effect on the patient's vision and newer style intraocular lenses are now available to prevent this opacification.
The postoperative cataract patient should be evaluated at least yearly to monitor the surgical eye for any long-term concerns, such as glaucoma, uveitis, posterior capsular opacity, retinal detachment, or corneal dystrophy. If the patient had unilateral surgery, then the other eye should be evaluated for possible cataract formation or progression. Each time the patient is examined by the ophthalmologist, the referring veterinarian should expect a letter with the surgeon's findings, including prognosis for vision.
What the Veterinary Ophthalmologist Needs to Know
• Cardiac status
• Diseases related to anesthesia
• Metabolic illness, such as diabetes or Cushing's disease
• Internal medicine concerns
• Any drug sensitivities