The lens is derived entirely from surface ectoderm that thickens to form the lens placode. The lens placode invaginates to form the lens pit and eventually the lens vesicle. As the result of early embryologic sequestration, lens protein is in an immunologically privileged site, and is capable of inciting a significant inflammatory reaction. The lens is nourished during embryologic life by the tunica vasculosa lentis, which posteriorly is comprised of the hyaloid artery (a part of the primary vitreous), and anteriorly by the embryologic pupillary membrane.
Anatomy and Physiology
The lens capsule is the basal lamina of the lens epithelium and is thickest anteriorly. The epithelium lines the lens capsule anteriorly and at the equator and produces the basal lamina (capsule). The lens fibers are produced at the equator and compressed toward the nucleus as new cells are formed thus contributing to leticular sclerosis. Lens metabolism is most active at the equator and mediated predominantly by anaerobic glycolysis, and to a lesser extent, by the hexose monophosphate shunt, sorbitol pathway, and the Krebs cycle. Lens proteins are immunologically sequestered (large amounts of lens material which gain access to the anterior chamber is capable of inciting a destructive immunologic reaction. Anything that alters the metabolism or structure of the lens is capable of producing a cataract.
Lens congenital defects include aphakia, microphakia, lenticonus, lentiglobus, coloboma, vascular anomalies (persistent pupillary membranes, persistent hyaloid artery, Mittendorf's dot, Bergmeister’s papillae, and persistent hyperplastic primary vitreous (PHPV). Multiple ocular abnormalities associated with lens changes are seen in the Australian Shepherd (congenital cataracts associated with microphthalmia, microcornea, equatorial staphylomas and retinal detachments); St. Bernard (microphakia and aphakia associated with microphthalmia, retinal detachment, and retinal dysplasia); Bedlington Terrier; Sealyham Terrier; and Labrador retriever (congenital cataracts associated with retinal detachment and dysplasia).
Cataract is defined as any opacification of the lens, regardless of cause, size or location. Most cataracts in dogs are inherited, although they may be caused by congenital defects, nutritional deficiencies, toxic substances, uveal adhesions, and diabetes mellitus. The basic abnormality in cataract formation is degeneration of the normal protein structure of the lens fibers. As such, cataract formation affects predominantly the lens cortex. Cortex changes include fiber swelling (bladder cells, Morgagnian globules). These are the earliest indicators of cataract formation, usually seen in the peripheral (subcapsular) cortex. Bladder cells are swollen, nucleated lens cells, and Morgagnian globules are spherical clumps of degenerating lens protein. As lens proteins degenerate, liquefaction of lens fibers is seen. Liquefied lens material may or may not leak out of the lens capsule. Mineralization is seen in extremely advanced cataracts. Epithelial changes include posterior migration of epithelium, fibrous pseudometaplasia (lens epithelial cells can undergo fibrous metaplasia to function as fibroblasts) and subcapsular fibroplasia. Size and shape changes are intumescence (lens swelling) and lens resorption (hypermature cataract). Resorption is associated with flattening of the normal lens curvature as the lens becomes smaller. Only in very young dogs (< 2 years) can the lens resorb completely. One of the most common sequelae to resorbing lens material is phacolytic uveitis. If significant resorption occurs, the capsule becomes wrinkled. The term “after cataract” refers to a diverse group of lens changes that occur after surgical lens extraction. In the most common method used to remove lenses from animals (phacoemulsification), most of the lens capsule remains in the eye. In the event the lens epithelium is still viable at the time of surgery (young animals and immature cataracts), surgical stimulation invariably stimulates the epithelium to replicate and secrete lens material. Pathologic features include epithelial proliferation, regeneration of new cortex, Elschnig’s pearls, (large, globular, malformed lens cells), and Soemmering’s ring (a ring shaped donut of abnormal secondary cortex in the periphery of the capsular bag. Fibrous metaplasia of lens epithelium also occurs with chronicity to cause capsule fibrosis and wrinkling. The collagen secreted by the metaplastic lens epithelium has a characteristic tendency to line the lens capsule. As it matures and contracts, the lens capsule becomes wrinkled.
Lens Capsule Rupture
Phacoclastic uveitis may occur if the lens capsule of an animal is ruptured as a result of a penetrating injury (e.g., cat scratch) because massive amounts of highly antigenic lens material can gain access to the immune system. The subsequent immune response is characterized histologically by the combination of lens capsule rupture, intra-lenticular neutrophils, and perilenticular mononuclear cells. With chronicity, a dense zone of fibroplasia will form around the ruptured lens. This form of uveitis, unless treated early by lens extraction, almost always results in loss of the eye due to secondary glaucoma. Spontaneous lens capsule rupture is occasionally seen secondary to chronic uveitis.
Classification of Cataracts
Cataracts may be classified by age of onset: congenital (present at birth), juvenile (developmental, less than eight years of age), and senile (generally over eight years of age). Cataracts are also described in terms of location as determined by biomicroscopy: capsular, subcapsular, cortical, nuclear, axial and polar. However, their degree of maturation is the most important feature relative to lens extraction: incipient (earliest changes, 10% tapetal reflex obstructed); immature (incipient until mature); mature (completely solid, no tapetal or fundus reflex visible, lens capsule smooth and regular); and hypermature (lens material, particularly cortical, may undergo liquefaction, may be “complete” cataract or may see a partial fundus reflex). Hypermature is recognized by: 1) rough or irregular anterior lens capsule; 2) or a deep anterior chamber; or (3) signs of lens-induced uveitis. If enough cortex liquefies, the nucleus will settle to the bottom of the lens and is termed a Morgagnian cataract.
Causes of Cataracts
Genetic defects are the most common cause of cataracts in dogs. Many breeds are affected. For example, recessive cataracts are suspected in the Miniature Schnauzer—congenital, American Cocker Spaniel—congenital to juvenile, Afghan—juvenile, Standard Poodle—juvenile, Old English Sheepdog—congenital, Miniature Poodle—adult, Terrier breeds and many brachycephalic breeds. Dominant cataracts are suspected in the Labrador Retriever—adult, Beagle—congenital, and Golden Retriever—congenital to juvenile (some disagree and list this as unknown inheritance). Cataracts whose inheritance pattern is unknown are seen in the Chesapeake Bay Retriever, Labrador Retriever, and red Cocker Spaniel. Associated ocular diseases include progressive retinal atrophy (PRA) (Miniature Poodle, Cocker Spaniel, and Miniature Schnauzer), central PRA (Labrador Retriever), retinal dysplasia (Labrador Retriever), and multiple ocular defects (red Cocker Spaniel, Beagle, old English Sheepdog). Senility accounts for spontaneous cataract formation in aged dogs of all breeds. The most common metabolic cataract is caused by diabetes mellitus. As glucose levels increase in the eye, hexokinase, the regulatory enzyme, becomes saturated; glucose accumulates in the lens and begins to be metabolized through the sorbitol pathway. The sugar alcohols, sorbitol and fructose, accumulate within the cells of the lens since they penetrate cell membranes (including the lens capsule) poorly. The result is an intracellular accumulation of solutes and hypertonicity, which results in an accumulation of water within the lens fibers. Swelling of the lens fibers progresses and the fibers rupture, forming vacuoles in the lens cortices. This continues until the entire lens becomes cataractous. A large majority of diabetic dogs eventually develops cataracts. Puppies and kittens may develop cataracts from nutrient deficiency (i.e., amino acid). This usually does not occur unless puppies are orphaned within the first two weeks of life and fed exclusively milk replacer. Cataracts may or may not regress. Toxic cataracts are seen after exposure to some drugs (uncommon), or secondary to PRA (cataracts occur secondary to by-products of retinal degeneration (dialdehydes) which diffuse through the vitreous to the lens). Cataracts may also occur secondary to inflammation (anterior uveitis), persistent vascular remnants (PPMs, hyaloid remnants), and trauma (usually requires lens capsule penetration).
Spontaneous Cataract Resorption
In hypermature cataracts, the lens proteins liquefy but may or may not diffuse out through the lens capsule. In very young dogs enough resorption may occur such that limited vision is restored. The incidence is higher in young dogs: if not resorbed in one to two years probably will never resorb. Complications include lens-induced uveitis (common) and lens-induced glaucoma (uncommon). Hypermature cataracts also predispose to retinal detachment.
Patient Selection for Cataract Surgery
Cataract surgery is generally indicated in any patient with significant vision impairment, or when significant vision impairment is impending. Retinal disease is ruled out by electroretinogram (ERG). The presence of a PLR does not necessarily indicate absence of PRA as PLRs are preserved until late in the normal disease. Ultrasonography is indicated in animals with hypermature cataracts. Gonioscopy is advisable in breeds predisposed to primary glaucoma. Lens induced uveitis, if present, should be suppressed with preoperative corticosteroid therapy.
Contemporary Surgical Technique
The current standard for canine lens extraction is phacoemulsification. A conventional lens extraction follows the following sequence: 1) patient positioning; 2) ventilator controlled respiration; 3) neuromuscular blockage to achieve neutral globe position (atracurium or pancuronium); 4) 2/3 depth, 7–8 mm corneal groove; 5) 3 mm corneal stab wound; 6) instillation of viscoelastic material; 7) anterior capsulectomy; 8) neclear sculpting with phacoemulsification (ultrasonic lens fragmentation at 30,000-50,000 cycles per second); 9) cortical material cleanup by irrigation/aspiration; 10) intraocular lens insertion; and 11) wound closure with continuous sutures of 8-0 or 9-0 Vicryl. Many different approaches are taken to control postoperative complications. Our protocol consists of one week of q6h Pred Forte, q8h tropicamide, and q8h 0.3% tobramycin. Thereafter topical q8h Pred Forte is used for up to three months.
Refinements in phacoemulsification technique have improved the short-term success to 95%. The most common immediate complications are uveitis, glaucoma and endophthalmitis. Other complications such as retinal detachment, hyphema, hypopyon, IOL luxation, and posterior capsule tears are largely the consequence of poor surgical technique. Long-term success decreases to approximately 70%. Chronic postoperative uveitis is a major risk factor for development of secondary glaucoma.
Subluxation and Luxation of the Lens
Lens displacement occurs secondary to damage to or spontaneous degeneration of the ciliary zonules. The specific causes are: 1) trauma (uncommon); 2) secondary to glaucoma; 3) secondary to chronic uveitis (especially in cats); 4) secondary to hypermature cataracts; and 5) senile zonular degeneration. Most treatable lens luxations are primary (familial) and are seen in wirehaired Fox Terriers, Sealyham Terriers, Manchester Terriers, Welsh Terriers, Poodles, and Jack Russell Terriers. The clinical signs are aphakic crescent, iridodonesis (quivering of iris with eye movement), deep anterior chamber, shallow anterior chamber and vitreous in the anterior chamber. Complications of lens luxation are corneal endothelial damage, corneal edema, secondary glaucoma, anterior uveitis, and retinal detachment. Lens subluxations are treated by intracapsular lens extraction with suture fixation of an intraocular lens. The prognosis is best if the lens is removed prior to the onset of glaucoma.
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