The lens of the eye is a fascinating and remarkable tissue, derived embryologically from surface ectoderm early in gestation, avascular through all but its earliest existence, and maintaining its primordial cells life-long, sequestered by a basement membrane. The lens fiber cells lack both nuclei and organelles and consist of exquisitely ordered protein molecules within a relatively dehydrated cytosolic matrix, with virtually no extracellular space. For its metabolic needs the lens precariously depends upon the adjacent uvea, aqueous humor, and vitreous. Energy metabolism is low and predominantly through anaerobic glycolysis. Lens function depends on its refractive capabilities and as such its transparency.

Examination of the lens should be a routine component of the ophthalmic and thus any physical examination (especially puppy examinations). Dilation is required (1.0% tropicamide in dogs, 0.5% in cats), along with a bright light source (a direct ophthalmoscope, transilluminator, or penlight will do) and some form of magnification (the biomicroscope is optimal), and both direct and retroillumination should be utilized.

Diseases discussed will include aging changes, congenital anomalies, cataracts, and lens dislocation.

Aging Changes

The lens exists as an inverted epithelial structure confined by its own basement membrane that undergoes life-long fiber cell proliferation: as a result, the lens enlarges somewhat and becomes more compact with aging. Biochemically associations include an increase in insoluble lens proteins. This results in alterations in the optical density of the nucleus observed clinically as nuclear sclerosis, which is evident in dogs in middle age, later in cats. Clinically the condition is distinguished from cataracts by its location, well-defined margins, and minimal effect on functional vision. As the process evolves, true nuclear cataracts may develop, accompanied by cuneiform cortical opacities. These changes are very slowly progressive and it is a rare animal that requires cataract surgery for age-related cataracts; in animals with dense nuclear and minimal cortical opacification, dilation of the pupil with 1.0% atropine sulfate solution every other day may enhance visual function as an option to surgical intervention.

Congenital Anomalies

Congenital anomalies of the lens can be classified mechanistically as: 1) those that involve abnormalities in the formation and differentiation of the lens placode and subsequently the lens epithelial cells; 2) those that are associated with anterior segment dysgenesis; or 3) those that are associated with abnormalities of the fetal vasculature.

The first would incorporate aphakia, a rare event indeed but which has been described in a cat; microspherophakia, in essence a failure of the secondary lens fibers to form; congenital cataract; and posterior lenticonus and posterior capsular rupture (a failure of basement membrane production by the primary posterior lens epithelial cells). The second includes a spectrum of abnormalities related to incomplete or late separation of the lens vesicle from the surface ectoderm (Peters' anomaly), pigment flecks on the anterior capsule, and capsular/subcapsular cataracts associated with dysplastic pupillary membranes. The third includes posterior capsular opacities, posterior capsular defects, and retrolental choristomatous plaques, once referred to as PTVL/PHPV, preferentially labeled as PEV (persistent embryonic vasculature). The latter condition has been elegantly defined by Stades and Bovee.

Lens "colobomas" occur secondarily to colobomatous defects of the adjacent ciliary body with resultant focal zonular agenesis and flattening of the lens equator due to absence of zonular tension.

Congenital lesions that involve the capsule and nucleus tend to be stable and non-progressive; cortical involvement suggests progression. A congenital cataract confined to the nucleus is indicative of an early transient insult and progression will not be observed as long as the secondary lens fibers are uninvolved, while cortical involvement implies an ongoing insult and cataract progression.

Cataracts

A cataract is an opacification of the normally transparent lens due to alteration of lens proteins, cell membranes, or both. They can be classified by location, appearance (vacuoles, spokes, etc), age of the patient at time of onset, stage of development, biologic behavior, or etiology.

Description by location is straight forward and as mentioned above may have prognostic implications. The appearance of a cataract is extremely variable and the descriptive terminology limited only by the imagination of the examiner. Congenital (present at birth), developmental (appearing between birth and 8 years of age, and age-related (occurring after 8 years of age) are terms that can be applied relative to the age of the patient at time of onset that may have etiologic implications. Incipient, immature, mature and hypermature are somewhat subjective terms that describe the continuum of cataract development that is not invariably progressive from one stage to the next. "Incipient" means beginning to exist, that is small. An immature cataract is a bit larger but through which a fundus reflex can be visualized. A mature cataract is total, with obscuration of the fundus reflex, and is accompanied by visual impairment in the affected eye. A variant of a mature cataract is the intumescent cataract, in which retention of water causes the lens to swell with resultant shallowing of the anterior chamber and chamber angle and characteristic separation of the lens sutures. Hypermaturity occurs when lens cortex liquefies and is usually accompanied by lens-induced uveitis; in a Morgagnian hypermature cataract, the nucleus is displaced inferiorly within the lens capsule by gravity.

Cataracts are by nature somewhat unpredictable but based upon location and etiology can usually be ascribed a predictive behavior; for instance, congenital nuclear cataracts do not progress, developmental cataracts progress very slowly if at all in the Golden Retriever and Siberian Husky, slowly in American Cocker Spaniels and Miniature Poodle, and diabetic cataracts progress rather rapidly, over days to weeks.

Hypermaturity with trans-capsular diffusion of soluble lens proteins and associated lens-induced uveitis is important and common clinical entity; usually mild and non-granulomatous, it can be diagnosed by clinical features (episcleral injection, a homogeneous bluish appearance to the cataract, wrinkling of the equatorial lens capsule, resistance to dilation, low IOP, iris hyperpigmentation and/or marginal cysts) and should be treated with topical corticosteroids and mydriatics to prevent significant complications, the most dreaded of which is secondary glaucoma. These eyes have a somewhat higher complication rate during and following cataract surgery. Occasionally a severe granulomatous uveitis can result (diabetic miniature schnauzers seem predisposed) that is difficult to control medically and requires prompt lens removal to attempt salvage of vision. Lens-induced uveitis associated with spontaneous or traumatic capsular rupture generally demands equally aggressive intervention.

The majority of cataracts in dogs are inherited and as such bilateral (see Table 1 for clinical features). Cataracts are much less frequently encountered in cats and most commonly occur secondary to chronic idiopathic uveitis. Other etiologies include blunt or penetrating trauma, with or without rupture of the lens capsule; electric shock; radiation; other intraocular diseases including lens luxation, glaucoma, and retinal degeneration; nutritional deficiencies (arginine deficiency in young wolves); metabolic disease (diabetes mellitus, Cushing's disease); and toxicities.

Table 1. Clinical Features of Inherited Canine Cataracts modified after Bedford and Jones in Small Animal Ophthalmology, Peiffer and Petersen-Jones, eds, 3rd ed

Refinement of technologies and methodologies have made cataract extraction and visual rehabilitation with intraocular lenses a predictable (90-95% success rates) and rewarding procedure. Glaucoma and retinal detachment are the most common reasons for failure.

Lens dislocation

Zonular disinsertion with loss of lens stability can occur as a primary disorder of the zonules or secondary to blunt or penetrating trauma, chronic ocular inflammation, or stretching of the zonules with enlargement of the globe in chronic glaucoma or microspherophakia. If the zonular disinsertion is partial and the lens still rests in the patella fossa of the anterior vitreous, it is described as a posterior subluxation. If the zonular disinsertion is total, with migration of the lens posteriorly onto the floor of the vitreous, or anteriorly through the pupil, into the anterior chamber, the lens is described as luxated. Pathogenesis of sequelae involve the distortion of the visual optics, physical irritation by the displaced lens to retina, anterior uvea, and/or cornea, and pupillary block by the lens itself of the adherent vitreous with resultant secondary glaucoma. Luxated lenses become cataractous if they are not already, likely due to altered nutrition.

Primary luxation occurs in the terrier breeds; dogs are usually middle-aged and the condition is always bilateral although not necessarily concurrently so. The affected eye may present with iridodonesis, an aphakic crescent, and irritation manifested by redness, blepharospasm, and tearing if the lens is subluxated; IOP may be variable. Anterior luxation is characterized by visualization of the lens equator and obscuration of the pupil; IOP will usually be elevated and if it is not soon will be. Posterior luxations are diagnosed by observing a deep anterior chamber with a concavity to the iris surface, and the lens on the floor of the posterior segment; IOP is usually not elevated. Examine fellow eyes closely; iridodonesis may be present and dilation may reveal an aphakic crescent. Dilation may turn an innocuous posterior subluxation into an emergent anterior luxation. In cats the majority of luxations occur secondary to chronic uveitis; secondary glaucoma is uncommon compared to the dog, related to the deep feline anterior chamber and the liquefaction of the vitreous that accompanies chronic inflammation.

Medical therapy of anteriorly luxated lenses involves aggressive mydriatic therapy to break the pupillary block and osmotic diuretics and topical prostanoids to reduce IOP, but prompt lens extraction with radical anterior vitrectomy is essential to preserve a visual globe. Likewise, posterior subluxations warrant aggressive surgical intervention. Removal of stable posterior luxations with normal IOP may be considered as an elective procedure; while many of these lenses are well-tolerated and vision preserved, the possibility of lens-induced uveitis and /or local retinal fibrosis swing the pendulum towards radical vitrectomy and intracapsular lens removal in the majority of cases. Surgery is somewhat less predictable than extracapsular extraction, with success rates approximating 80% with glaucoma and retinal detachment the most common complications.