NAVLE Special Senses

Canine Cataracts Study Guide

📅 March 28, 2026 ⏱ 25 min read 👁 1 views

Cataracts are opacities of the lens or lens capsule that interfere with light transmission to the retina, leading to visual impairment and potentially blindness.

Overview and Clinical Importance

Cataracts are opacities of the lens or lens capsule that interfere with light transmission to the retina, leading to visual impairment and potentially blindness. They are one of the most common causes of vision loss in dogs and a high-yield NAVLE topic. The lens is a transparent, biconvex, avascular structure composed of lens fibers (crystallin proteins) surrounded by the lens capsule.

Etiology	Key Features
Hereditary/Genetic	Most common cause. Bilateral, symmetrical. HSF4 gene mutations. Juvenile onset (1-5 years). Affected dogs should not be bred.
Diabetic	Second most common. Rapid onset (75-80% within 1 year of diagnosis). Bilateral. Sorbitol accumulation via aldose reductase. Risk of capsule rupture.
Senile/Age-Related	Third most common. Dogs greater than 7-8 years. Slow progression. Must differentiate from nuclear sclerosis.
Secondary to Uveitis	Chronic inflammation alters aqueous. Inflammatory mediators damage lens epithelium. Usually begins in cortex.
Traumatic	Penetrating injuries (cat claw most common). Capsule rupture causes phacoclastic uveitis. Usually unilateral. Emergency.
Secondary to PRA	Toxic dialdehydes from degenerating retina. Posterior cortical location. Surgery NOT indicated (non-functional retina).
Congenital	Present at birth. May be hereditary or in utero insults. Nuclear location. May associate with other anomalies.
Nutritional	Rare. Arginine deficiency in orphan puppies. Usually reversible if corrected early.

Lens Anatomy and Physiology

The lens is embryologically derived from surface ectoderm, while other ocular structures form from mesoderm. This is clinically significant because lens proteins are immunologically sequestered, leading to potential immune-mediated inflammation when exposed (lens-induced uveitis).

Key Anatomical Components

Lens Capsule: Elastic basement membrane encasing the lens; thicker anteriorly
Lens Epithelium: Single layer of cuboidal cells beneath anterior capsule; divides into lens fibers
Cortex: Outer, younger lens fibers; softer consistency
Nucleus: Central, older lens fibers; firmer, denser
Zonules: Suspensory ligaments attaching lens to ciliary body

High-YieldThe lens is avascular and receives nutrients from aqueous humor via diffusion. Changes in aqueous composition (uveitis, glaucoma) can affect lens metabolism and contribute to cataract formation.

Breed	Age of Onset	Notes
Boston Terrier	Juvenile to adult	High prevalence (11.1%); increased glaucoma risk post-op
Cocker Spaniel	1-5 years	American and English; increased glaucoma risk
Miniature/Toy Poodle	1-5 years	High prevalence; also predisposed to PRA
Labrador Retriever	Variable	Also predisposed to PRA; increased retinal detachment risk
Siberian Husky	Juvenile	Posterior subcapsular location common
Bichon Frise	Adult	High prevalence; increased retinal detachment risk

Etiology of Canine Cataracts

Breed Predispositions

NAVLE TipBoston Terriers and Cocker Spaniels have increased risk of post-operative glaucoma. Labrador Retrievers, Bichon Frises, and Poodles have increased risk of post-operative retinal detachment.

Stage	Lens Involvement	Tapetal Reflection	Vision/Significance
Incipient	Less than 15%	Fully visible	No visual deficit; monitor
Immature	15-99%	Partially visible	Variable impairment; best surgical prognosis
Mature	100%	Not visible (blocked)	Blind; PLR intact if retina functional
Hypermature	100% with resorption	Partially visible (resorption)	Wrinkled capsule; high LIU risk; poorer prognosis

Pathophysiology of Diabetic Cataracts

Approximately 75-80% of diabetic dogs develop cataracts within the first year of diagnosis, regardless of glycemic control.

The Aldose Reductase (Polyol) Pathway

Hexokinase pathway becomes saturated in hyperglycemia
Excess glucose shunted to aldose reductase pathway
Aldose reductase converts glucose to sorbitol
Sorbitol cannot cross lens capsule (large, polar molecule)
Osmotic gradient draws water into lens
Lens fibers swell and rupture, causing opacity

Clinical Features of Diabetic Cataracts

Rapid onset: Complete cataract can develop within days to weeks
Bilateral and symmetrical
Intumescent (swollen): Lens absorbs water
Characteristic 'waterclefts': Visible along Y-suture lines
High risk of capsule rupture: Causes phacoclastic uveitis

High-YieldDogs have high aldose reductase levels, making them susceptible to diabetic cataracts. Cats have LOW aldose reductase, which is why diabetic cataracts are RARE in felines. This species difference is commonly tested!

Feature	Nuclear Sclerosis	Cataract
Appearance	Bluish-gray haze; translucent	White opacity; opaque
Tapetal Reflection	FULLY VISIBLE	Obstructed (partial or complete)
Vision	No significant impairment	Variable to complete loss
Age	Greater than 7-8 years	Any age
Complications	None	LIU, glaucoma, luxation
Treatment	None needed	Surgical if indicated

Classification by Maturity

Exam Focus: The key differentiator between immature and mature cataracts is the TAPETAL REFLECTION. If visible = immature. If completely blocked = mature.

Drug Class	Examples	Notes
Topical Corticosteroids	Prednisolone acetate 1%, Dexamethasone 0.1%	Most potent; avoid if corneal ulcer
Topical NSAIDs	Diclofenac, Flurbiprofen, Ketorolac	Can use with ulceration; often combined with steroids
Mydriatics	Atropine 1%, Tropicamide 1%	Break/prevent synechiae; cycloplegia for comfort

Differentiating Cataracts from Nuclear Sclerosis

Nuclear sclerosis is a NORMAL age-related change causing bluish-gray lens haziness. It is NOT a disease and does NOT require treatment.

High-YieldThe diagnostic key is RETROILLUMINATION. Dilate pupil, hold transilluminator at arm's length. If fundic reflection is fully visible = nuclear sclerosis (normal). If blocked = cataract (pathologic).

Complication	Mechanism
Lens-Induced Uveitis	Lens protein leakage triggers immune-mediated inflammation
Secondary Glaucoma	LIU debris/synechiae obstruct aqueous outflow. Difficult to manage; may require enucleation
Lens Luxation	Chronic LIU degrades zonules. Anterior luxation = emergency (pupillary block)

Lens-Induced Uveitis (LIU)

LIU is the most common complication of untreated cataracts, with prevalence up to 71% in dogs screened for cataract surgery.

Types of LIU

Phacolytic Uveitis: Lens proteins leak through INTACT capsule. Most common. Typically mild. Associated with hypermature cataracts.

Phacoclastic Uveitis: Lens proteins released through RUPTURED capsule. More severe, vision-threatening. Requires aggressive treatment. Causes: trauma, intumescent diabetic cataract rupture.

Clinical Signs of LIU

Episcleral/conjunctival injection (redness)
Corneal edema
Aqueous flare (Tyndall effect) and cells
Miosis
Posterior synechiae
Hypotony (low IOP) in acute cases

Treatment of LIU

NAVLE TipMedical therapy does NOT cure LIU - as long as the cataractous lens remains, protein leakage continues. Surgical removal (phacoemulsification) is the definitive treatment. Pre-existing LIU significantly reduces surgical success rates.

Complication	Details
POH	Common within 24-72 hours. Usually transient. NOT chronic glaucoma.
Glaucoma	Risk less than 10% in 3 years. Boston Terriers/Cockers at higher risk.
Retinal Detachment	1-2%. Bichon Frises, Labs, Poodles at higher risk.
PCO	Common long-term. Residual lens epithelial cells proliferate. Young dogs at higher risk.

Diagnostic Evaluation

Standard Ophthalmic Examination

Menace response: Tests vision; variable with cataracts
Dazzle reflex: Present even with mature cataracts if retina functional
PLR: Tests subcortical pathway, not vision
Tonometry: IOP normal 15-25 mmHg. Low = uveitis; High = glaucoma
Slit-lamp biomicroscopy: Detailed lens and anterior segment exam

Preoperative Screening for Cataract Surgery

Electroretinography (ERG): MANDATORY before cataract surgery. Tests retinal function. Detects PRA, SARDS. Normal ERG does not guarantee vision (post-retinal blindness can have normal ERG).

Ocular Ultrasound: B-scan evaluates posterior segment. Rules out retinal detachment, vitreous degeneration, posterior capsule rupture.

Gonioscopy: Evaluates iridocorneal angle for glaucoma risk.

High-YieldRemember 'GEU' for preoperative cataract evaluation: Gonioscopy (angle), ERG (retinal function), Ultrasound (posterior segment).

Surgical Treatment: Phacoemulsification

Surgical removal is the only treatment to restore vision. Phacoemulsification is the gold standard with 85-95% success rates.

Surgical Procedure

General anesthesia with neuromuscular blockade
Small corneal incision (2.8-3.2 mm)
Anterior capsulotomy (continuous curvilinear capsulorhexis)
Phacoemulsification: Ultrasonic handpiece fragments lens (40,000 vibrations/second)
Aspiration of lens material
Intraocular lens (IOL) implantation into capsular bag
Corneal incision sutured

Without IOL, the patient is aphakic and severely hyperopic (farsighted). Aphakic dogs can navigate but cannot focus clearly.

Postoperative Care

E-collar at all times
Topical corticosteroids/NSAIDs: 4-6 times daily initially, tapered over 4-8 weeks
Topical antibiotics
Activity restriction for 2-4 weeks
Rechecks: Day 1, week 1, month 1, then every 3-6 months long-term

NAVLE TipMany dogs require LIFELONG topical anti-inflammatory therapy after surgery (especially diabetic dogs). Posterior capsular opacification (PCO) is a common long-term complication from residual lens epithelial cell proliferation.

Complications

Untreated Cataract Complications

Postoperative Complications

Memory Aids

Cataract Stages: 'I'IM Mature-Hypermature'

I = Incipient (less than 15%); I = Immature (15-99%); M = Mature (100%); Hypermature (resorption)

Diabetic Cataracts: 'SUGAR SWELLS'

Sorbitol accumulates
Uncontrolled glucose overwhelms hexokinase
Glucose shunted to aldose reductase
Aldose Reductase is the key enzyme
R SWELLS = Rapid onset; osmotic SWELLING of lens fibers

Preoperative Tests: 'GEU'

G = Gonioscopy (drainage angle)
E = ERG (retinal function)
U = Ultrasound (posterior segment)

Species Difference: 'Dogs DO, Cats DON'T'

Dogs DO get diabetic cataracts (high aldose reductase). Cats DON'T (low aldose reductase).

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