Cataracts are opacities of the crystalline lens that interfere with the transmission of light to the retina, resulting in varying degrees of visual impairment.
Overview and Clinical Importance
Cataracts are opacities of the crystalline lens that interfere with the transmission of light to the retina, resulting in varying degrees of visual impairment. In horses, cataracts represent a significant cause of blindness and are important to recognize during prepurchase examinations and routine ophthalmic evaluations.
Cataracts are estimated to be present in 5% to 7% of all horses with otherwise normal eyes. If we consider that approximately 8% of all horses are affected by equine recurrent uveitis (ERU), and nearly all of these horses develop some degree of cataract formation, the clinical significance of this condition becomes apparent.
High-YieldCataracts are the most common congenital ocular abnormality in foals, with a 33-35% incidence among all congenital ocular defects. They are also the most common cause of blindness secondary to ERU in adult horses.
| Structure |
Clinical Significance |
| Lens Capsule |
Thin, transparent, elastic membrane surrounding the lens; secreted by lens epithelial cells. The posterior capsule is thinner than the anterior capsule. |
| Lens Epithelium |
Single layer of cuboidal cells lining the anterior capsule. Responsible for metabolic functions, enzyme activity, and new fiber production. Damage leads to cataractogenesis. |
| Lens Cortex |
Outer, softer portion of lens fibers. Cortical cataracts are more likely to progress than nuclear cataracts. |
| Lens Nucleus |
Central, denser portion containing the oldest lens fibers. Nuclear cataracts involving embryonic or fetal nuclei are typically non-progressive. |
| Zonular Fibers |
Suspensory ligaments connecting lens to ciliary body. Chronic uveitis can cause zonular degeneration leading to lens luxation or subluxation. |
Anatomy and Physiology of the Equine Lens
The crystalline lens is a biconvex, transparent structure located posterior to the iris and pupil, suspended by zonular fibers (lens zonules) attached to the ciliary body. In horses, the lens is notably large compared to other domestic species.
Key Anatomical Features of the Equine Lens
High-YieldThe lens is avascular and derives all nutrition from the aqueous humor. Changes in aqueous composition (as occurs in uveitis) compromise lens metabolism and cause loss of transparency, leading to cataract formation.
| Etiology |
Characteristics |
Clinical Notes |
| Congenital/Inherited |
Present at birth or shortly after; usually bilateral; documented in Morgans, Belgians, Thoroughbreds, Quarter Horses, Standardbreds |
Nuclear cataracts are typically non-progressive; autosomal dominant inheritance pattern in many breeds |
| Uveitis-Induced (ERU) |
Most common cause in adult horses; cortical cataracts predominate; often associated with posterior synechiae, corpora nigra atrophy |
Usually progressive; poor surgical candidates due to ongoing inflammation; look for other signs of ERU |
| Traumatic |
Secondary to blunt or penetrating ocular trauma; typically unilateral; may see lens capsule rupture |
Best surgical prognosis if no other ocular damage; may develop rapidly following trauma |
| Senile/Geriatric |
Age-related lens changes in horses greater than 20 years; nuclear sclerosis is a normal aging change (not true cataract) |
Slowly progressive; may not significantly impair vision; distinguish from nuclear sclerosis |
Classification of Equine Cataracts
Cataracts in horses can be classified by multiple criteria. Understanding these classification systems is essential for determining prognosis and treatment options.
Classification by Etiology
NAVLE TipOn the NAVLE, if you see a question about an adult horse with cataracts, think ERU first! ERU is the most common cause of acquired cataracts and the leading cause of blindness in horses.
Classification by Age of Onset
Classification by Stage of Maturity
| Age Category |
Definition |
Key Points |
| Congenital |
Diagnosed before 6 months of age |
May be inherited or due to in-utero insult; often bilateral; most common congenital ocular defect in foals |
| Juvenile |
Develops after 6 months but before skeletal maturity |
May be inherited; Standardbreds overrepresented; consider removing from breeding if cause unknown |
| Senile/Geriatric |
Develops in horses greater than 20 years of age |
Typically slowly progressive; may be incidental finding |
Breed Predispositions
Several breeds have documented predispositions to congenital and juvenile cataracts. Understanding these associations is important for prepurchase examinations and genetic counseling.
Exam Focus: Remember the key breed associations: Morgan = inherited nuclear cataracts (autosomal dominant); Rocky Mountain Horse = MCOA syndrome (Silver gene); Appaloosa = high risk for ERU and secondary cataracts; Standardbred = recently identified predisposition to congenital/juvenile cataracts.
| Stage |
Lens Involvement |
Fundic Exam |
Visual Impact |
| Incipient |
Less than 10% |
Easily visible |
Minimal to none |
| Immature |
10-99% |
Visible through or around opacity |
Variable |
| Mature |
100% |
Not visible |
Blind in affected eye |
| Hypermature |
Resorbing/liquefying |
May become visible again |
Blind; capsule wrinkled |
Equine Recurrent Uveitis and Cataract Formation
Equine recurrent uveitis (ERU) is the most common cause of acquired cataracts in adult horses and the leading cause of blindness in horses worldwide. Understanding this relationship is critical for the NAVLE.
Pathophysiology of ERU-Associated Cataracts
- ERU is an immune-mediated, T-lymphocyte driven disease characterized by recurring episodes of intraocular inflammation
- Chronic inflammation alters aqueous humor composition, compromising lens epithelial cell metabolism
- Inflammatory mediators (prostaglandins, leukotrienes) directly damage lens fibers
- Posterior synechiae formation prevents normal aqueous flow around the lens
- Chronic zonular degeneration can lead to lens luxation or subluxation
Clinical Signs of ERU (Associated Findings)
High-YieldHorses with ERU-associated cataracts are POOR surgical candidates! The ongoing inflammation leads to high complication rates and poor visual outcomes following cataract surgery. Always look for signs of ERU before recommending surgery.
| Breed |
Type of Cataract |
Inheritance/Notes |
| Morgan |
Congenital nuclear cataracts; perinuclear cataracts |
Inherited; autosomal dominant pattern documented |
| Belgian |
Congenital cataracts |
Inherited; documented breed predisposition |
| Standardbred |
Congenital and juvenile cataracts |
Significantly overrepresented in studies; both trotting and pacing lines affected |
| Rocky Mountain Horse |
Congenital nuclear cataracts with MCOA syndrome |
Part of Multiple Congenital Ocular Anomalies syndrome; associated with Silver (PMEL17) gene mutation |
| Quarter Horse |
Aniridia-associated cataracts |
Associated with iridial hypoplasia (aniridia) and limbic dermoids |
| Appaloosa |
ERU-associated cataracts |
8.3x more likely to develop ERU than other breeds |
Clinical Signs and Diagnostic Approach
Clinical Signs
Many horses with cataracts show no outward clinical signs until vision impairment becomes significant. Owners may report:
- Cloudiness or white appearance in the eye
- Spooking or hesitation in unfamiliar environments
- Bumping into objects, especially on the affected side
- Difficulty with light changes or poor vision in dim conditions
- Changes in behavior during work or competition
Diagnostic Evaluation
A complete ophthalmic examination is essential for diagnosis. Pharmacologic pupil dilation is REQUIRED to adequately evaluate the lens.
Standard Ophthalmic Examination
- Menace response: Assess gross vision; may be positive with focal cataracts, absent with mature cataracts
- Pupillary light reflexes (PLR): Direct and consensual; should be positive if retina is functional
- Dazzle reflex: Tests subcortical visual pathway; positive indicates functional retina even with mature cataract
- Pupil dilation: Tropicamide 1% (0.1-0.2 mL topically, may repeat); essential for lens evaluation
- Direct illumination: Focal light reveals cataract as white opacification of lens
- Retroillumination: Obtain tapetal reflection from distance; cataract blocks areas of tapetal reflection
- Fundoscopy: Evaluate retina and optic nerve; determines if cataract surgery would be beneficial
- Tonometry: Assess intraocular pressure; rule out secondary glaucoma
Advanced Diagnostics
NAVLE TipOn the NAVLE, remember that the dazzle reflex tests the subcortical visual pathway. A positive dazzle reflex in a horse with a mature cataract indicates the retina is still functional, suggesting the horse may be a surgical candidate.
| Acute Phase Signs |
Chronic/Quiescent Phase Signs |
| Blepharospasm, epiphora
Corneal edema
Episcleral injection
Aqueous flare
Fibrin in anterior chamber
Miosis |
Posterior synechiae
Corpora nigra atrophy
Lens capsule pigmentation
Cataract formation
Vitreous degeneration/opacities
Retinal degeneration/detachment
Secondary glaucoma
Phthisis bulbi (end-stage) |
Treatment Options
No medical therapy can reverse lens opacification. Surgical removal of the lens is the only definitive treatment for visually significant cataracts.
Conservative Management
For small, non-progressive cataracts that do not significantly impair vision:
- Periodic monitoring (every 6-12 months) to assess progression
- Environmental modifications for visually impaired horses
- Management of underlying conditions (e.g., control of ERU if present)
Surgical Treatment: Phacoemulsification
Phacoemulsification is the surgical technique of choice for cataract removal in horses. It uses ultrasonic energy to liquefy and fragment the lens, which is then aspirated from the eye.
Surgical Candidate Selection
Surgical Outcomes
- Immediate postoperative success: 95-98% of eyes have evidence of vision immediately after surgery
- Short-term (less than or equal to 1 month): 87% of horses remain visual
- Long-term (greater than 2 years): 26-50% of horses maintain vision
- Best outcomes: Traumatic cataracts (up to 90% success); Worst outcomes: ERU-associated cataracts
Intraocular Lens (IOL) Implantation
Foldable intraocular lens implants (+14 diopter) have been developed specifically for equine use. IOL implantation improves postoperative visual outcomes by correcting the hyperopia that results from lens removal. However, IOL implantation is not performed in all cases, particularly in foals where globe size continues to change.
Postoperative Care
Intensive postoperative management is critical for surgical success:
- Topical anti-inflammatory medications: Corticosteroids and/or NSAIDs for several weeks to months
- Topical antibiotics: Prevent postoperative infection
- Mydriatics: Atropine to maintain pupil dilation and prevent synechiae
- Subpalpebral lavage system: Often placed to facilitate medication administration
- Stall rest: Darkened stall with limited exercise during healing
- Recheck schedule: 1 day, 7-10 days, 4-6 weeks, 3 months, 6 months, 12 months, then yearly
Complications
High-YieldRetinal detachment is the primary cause of long-term blindness after cataract surgery in horses. This is why ERG evaluation of retinal function and careful patient selection are so critical.
| Diagnostic Test |
Purpose and Indications |
| Ocular Ultrasound |
Evaluate posterior segment when fundoscopy is not possible (mature cataract); identify retinal detachment, vitreal degeneration, lens luxation, or intraocular masses |
| Electroretinography (ERG) |
Objectively assess retinal function prior to cataract surgery; identifies retinal degeneration that may not be visible; critical for surgical candidate selection |
| Leptospiral Serology |
Test serum, aqueous, or vitreous for Leptospira antibodies if ERU suspected; helps confirm etiology but negative titers do not rule out ERU |
Memory Aids and Board Tips
"CATARACT" Mnemonic for Causes
C - Congenital (most common in foals)
A - Age-related (senile, greater than 20 years)
T - Trauma
A - Autoimmune (ERU - most common cause in adults)
R - Radiation (rare)
A - Antibiotics (rare, toxicity)
C - Congenital syndromes (MCOA in Rocky Mountain Horses)
T - Toxins/metabolic
"MATURE" Mnemonic for Stages
M - Minimal opacity = Incipient (less than 10%)
A - Almost complete = Immature (10-99%)
T - Total opacity = Mature (100%)
URE - Ultra-Resorbing = Hypermature (lens liquefying)
NAVLE TipWhen you see a question about cataract surgery prognosis in horses: Traumatic cataracts = BEST prognosis (90% success). Congenital cataracts in foals = GOOD prognosis. ERU-associated cataracts = POOR prognosis (DO NOT recommend surgery).
| Good Surgical Candidates |
Poor Surgical Candidates |
| Healthy foals with congenital cataracts
Traumatic cataracts (no other ocular damage)
Normal retinal function on ERG
No evidence of uveitis
Normal intraocular pressure |
ERU-associated cataracts (active or quiescent)
Retinal detachment or severe degeneration
Secondary glaucoma
Lens luxation
Phthisis bulbi
Severe posterior synechiae |
| Complication |
Clinical Significance |
| Postoperative Uveitis |
Most common complication; can lead to synechiae, secondary cataract, and phthisis bulbi |
| Retinal Detachment |
Major cause of long-term vision loss; higher risk with ERU-associated cataracts |
| Glaucoma |
Secondary to inflammation or synechiae; may require enucleation if uncontrolled |
| Corneal Complications |
Edema, ulceration, or abscess; may occur with prolonged inflammation or medication toxicity |
| Endophthalmitis |
Rare but serious; intraocular infection; typically requires aggressive treatment or enucleation |