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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • The transparency of the lens is made possible by layers of perfectly aligned linear cells or lens fibers
  • Disruption of the precise anatomic arrangement of these lens fibers results in opacification or cataract formation of the lens
  • The basic mechanism of cataract formation is a decrease in soluble lens proteins, failure of the lens epithelial cell sodium pump, a decrease in lens glutathione, lens fiber swelling, and fiber membrane rupture
  • These lens opacities or cataracts can vary in size depending on the number of lens fibers damaged. Very small incipient lens opacities are common and not associated with blindness
  • As cataracts develop or mature, they become more opaque, and blindness develops
  • Cataracts can be classified by the degree of lens involvement (incipient, immature, mature, hypermature, Morgagnian), lens location (capsular, nuclear, cortical, anterior/posterior) age of onset (congenital, juvenile, senile), and cause of cataract (e.g. inherited, traumatic)

Incidence/Prevalence!!navigator!!

  • 5–7% of horses are reported to have cataracts as a primary ocular disease
  • 33.6–35.3% of foals with congenital ocular anomalies are reported to have cataracts

Signalment!!navigator!!

All ages and breeds of horses are at risk for cataract development. Cataracts are a frequent congenital ocular defect in foals.

Signs!!navigator!!

  • Horses manifest varying degrees of blindness as cataracts mature. The tapetal reflection is seen with incipient, immature, and hypermature cataracts but not with mature cataracts. The rate of cataract progression and development of blindness cannot be predicted in most instances
  • Blepharospasm and lacrimation may accompany cataracts in horses due to associated ERU or lens-induced uveitis; the latter is seen less commonly in horses than in other species such as dogs
  • Assessment of visual function can be made by observation of the horse walking, feeding, and interacting with other horses. Visually impaired horses may demonstrate a reluctance to run or even walk, although some horses with bilateral cataracts appear to do quite well in a familiar environment. Differences in head posture may be associated with cataracts, as a unilaterally blind horse may attempt to keep its sighted eye toward activity in its environment.

Causes!!navigator!!

  • Heritable, traumatic, toxic, nutritional, and postinflammatory etiologies have been proposed
  • Congenital cataracts may be associated with other congenital defects such as microphthalmos, aniridia, persistent pupillary membranes, persistent fetal vasculature, colobomas, and anterior segment dysgenesis
  • Cataracts secondary to ERU or trauma are frequently seen, while juvenile-onset cataracts are uncommon in horses. True senile cataracts that interfere with vision are found in horses older than 20 years

Risk Factors!!navigator!!

  • Congenital cataracts are reported in Thoroughbreds, Belgian draft horses, and Quarter Horses. Morgan horses can have congenital, nonprogressive, nuclear, bilaterally symmetrical cataracts that do not generally interfere with vision
  • Rocky Mountain Horses can develop cataracts in association with multiple congenital ocular anomalies syndrome

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Increased cloudiness of the lens occurs with age and is called nuclear or lenticular sclerosis. It is common in older horses, but vision is clinically normal as nuclear sclerosis does not cause vision loss
  • Older horses may develop brunescence in association with lenticular sclerosis, which gives the nucleus of the lens a yellow discoloration
  • Lens luxation, fibrin associated with uveitis
  • Blindness in horses can also occur secondary to ERU, glaucoma, and retinal disease

CBC/Biochemistry/Urinalysis!!navigator!!

CBC, chemistry, and fibrinogen levels are recommended as part of the preoperative evaluation for cataract surgery.

Other Laboratory Tests!!navigator!!

There are no tests currently available to screen for inherited cataracts in horses.

Imaging!!navigator!!

B-scan US is beneficial in assessing the anatomic status of the retina if a cataract is present.

Other Diagnostic Procedures!!navigator!!

Afferent pupillary defects in a cataractous eye cannot be attributed to the cataract alone, and normal pupillary light reflexes do not necessarily exclude some degree of retinal or optic nerve disease. ERG is beneficial in assessing the functional status of the retina if a cataract is present.

Pathologic Findings!!navigator!!

Epithelial cell dysplasia/metaplasia, migration of lens epithelial cells along the posterior lens capsule, liquefaction of lens material, Morgagnian globules, and lens fiber swelling is noted.

Treatment ⬆ ⬇

TREATMENT

  • Most veterinary ophthalmologists recommend surgical removal of cataracts in foals <6 months of age if the foal is healthy, no uveitis or other ocular problems are present, and the foal's personality will tolerate aggressive topical therapy. Adult horses with visual impairment due to cataracts are also candidates for surgery
  • Therapy for cataracts is necessarily surgical, although some degree of spontaneous cataract resorption may occur with hypermature cataracts. Horses considered for lens extraction should be in good physical condition. Complete ophthalmic and general physical examinations should be performed, and intraocular pressure should be measured. Preoperative CBCs and serum chemistries are important for evaluating systemic organ function
  • Any signs of anterior uveitis should delay cataract surgery until the cause of the inflammation is diagnosed and has been successfully treated. Cataract surgery should also be delayed in the presence of active eyelid, conjunctival, or corneal disease
  • Phacoemulsification is the recommended surgical technique for equine cataract surgery. Immature, mature, and hypermature cataracts have been successfully removed in horses with this technique
  • Intraocular lenses of 18 diopters (D) are recommended in horses

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Subpalpebral lavage placement assists in the delivery of topical medications, particularly in a postoperative patient
  • Topically applied corticosteroids, such as prednisolone acetate (1%), are beneficial in treating lens-induced uveitis as well as pre- and postoperative uveitis
  • Topically administered NSAIDs, such as flurbiprofen and diclofenac, can be used TID to QID in treating lens-induced uveitis, as well as pre- and postoperative uveitis
  • Topically applied 1% atropine is effective in stabilizing the blood–aqueous barrier, minimizing pain from ciliary muscle spasm, and causes pupillary dilation. Atropine may be used as often as every 6 h, with the frequency of administration reduced as soon as the pupil dilates. This can be used pre- and postoperatively as indicated based on the degree of uveitis present
  • Systemically administered NSAIDs, such as flunixin meglumine (1.1 mg/kg IV or PO BID), are effective in reducing anterior uveitis in horses with cataracts, as well as treating postoperative intraocular inflammation
  • Broad-spectrum topically applied antibiotics such as neomycin–polymyxin B–gramicidin ophthalmic solutions are used starting the day before surgery, and are continued postoperatively. These are typically administered every 4–6 h
  • Systemically administered antibiotics are indicated peri- and postoperatively. Penicillin and gentamicin are typically administered IV perioperatively, followed by trimethoprim–sulfa PO postoperatively

Precautions!!navigator!!

Horses receiving topical atropine should be monitored for colic.

Contraindications, Possible Interactions!!navigator!!

  • Cataract surgery is not recommended in any patient that has uncontrolled systemic disease
  • Animals with poor retinal function, as determined by preoperative ERG and US, are not candidates for cataract surgery
  • Animals with cataracts secondary to controlled ERU may be candidates for cataract surgery; however, horses with ERU may also be at higher risk for postoperative complications, particularly glaucoma

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Horses with cataracts should be monitored for blepharospasm and lacrimation as cataracts can be associated with uveitis
  • Blind horses should be monitored in their environment
  • Horses that have had cataract surgery should be monitored for signs of ocular discomfort, self-trauma, recurrence of blindness, and colic

Prevention/Avoidance!!navigator!!

Breeding of horses with cataracts should be avoided.

Possible Complications!!navigator!!

Postoperative complications can include persistent iridocyclitis and plasmoid aqueous, fibropupillary membranes, synechiae, iris bombé, corneal ulceration, corneal edema, posterior capsular opacification, wound leakage, glaucoma, vitreous presentation into anterior chamber, retinal degeneration, retinal detachment, and infectious endophthalmitis. General anesthesia with its attendant risks is required for cataract surgery.

Expected Course and Prognosis!!navigator!!

  • Surgery is easiest to perform on foals because the globe size is small enough that the standard cataract surgical equipment is of a satisfactory size, general anesthesia is less of a risk in foals, and foals tend to heal quickly following cataract surgery. Early return of vision is paramount in foals for development of the higher visual centers
  • Slight corneal edema is usually present from 24 to 72 h postoperatively; 1 week following surgery the pupil should be functional, any fibrin in the anterior chamber resorbing, and the fundus visible; 3 weeks following surgery the eye should be nonpainful, the patient visual, pupillary movement normal, and the ocular medium clear
  • Most reliable reports of vision in successful cataract surgery in horses indicate that vision is functionally normal postoperatively. From an optical standpoint, the aphakic eye should be quite far-sighted or hyperopic postoperatively, and was +9.94 D in 1 study. Images close to the eye would be blurry and appear magnified. Placement of an artificial intraocular lens should improve acuity

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Retinal detachment
  • ERU
  • Microphthalmos
  • Iridal hypoplasia

Age-Related Factors!!navigator!!

  • Foals may present with congenital cataracts
  • Older horses may present with senile cataracts

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

N/A

Abbreviations!!navigator!!

  • ERG = electroretinography
  • ERU = equine recurrent uveitis
  • NSAID = nonsteroidal anti-inflammatory drug
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Brooks DE.Ophthalmology for the Equine Practitioner, 2e. Jackson, WY: Teton NewMedia, 2008.

Gilger BC. Equine ophthalmology. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary Ophthalmology, 5e. Ames, IA: Wiley Blackwell, 2013:1560–1609.

Gilger BC, ed. Equine Ophthalmology, 3e. Ames, IA: Wiley Blackwell, 2017.

Author(s) ⬆

Author: Shari M. Greenberg

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author and editor acknowledge the prior contribution of Dennis E. Brooks.