2 Equine Ophthalmology Topics

Iris Abnormalities: Persistent Pupillary Membranes

• These are remnants of embryonic vasculature commonly found on the iris surface.
• Usually an incidental finding but may attach to the cornea or lens causing focal opacity.

Iris Abnormalities: Iris Hypoplasia

• A dark area of the iris bulges forward, typically dorsally and more commonly in blue eyes.
• It has a thinned iris stroma that can retroilluminate and needs to rule out neoplasia.

Iris Abnormalities: Iris Neoplasia

• Melanoma is the most common form, especially in gray horses.
• Manifests as a dark mass filling the anterior chamber.
• Use ultrasound to differentiate from cysts.
• Treatment is typically benign but may require enucleation.

Iris Abnormalities: Iris or Corpora Nigra Cysts

• Locations include the corpora nigra, pupil margin, and are free-floating.
• May enlarge over time and should transilluminate.
• Rule out neoplasia as part of diagnosis.
• Treatment includes benign neglect unless vision is compromised, where diode laser or aspiration can be used.

Uveitis

• Most common cause of blindness in horses.
• Primary acute uveitis can affect any horse and is diagnosed at the first episode.
• ERU occurs mostly in Appaloosa and Draft breeds.
• ERU involves two or more episodes of uveitis typically spaced months apart and is immune-mediated.

Uveitis: Clinical Signs

• Acute uveitis clinical signs encompass pain (blepharospasm, epiphora, photophobia), conjunctival hyperemia, corneal changes, aqueous flare, miosis, vitreous discoloration, and hypotony.
• Chronic uveitis clinical signs include hyper- or hypo-pigmentation of the iris, rubeosis irides, atrophy of corpora nigra, synechiae, dyscoria, lens issues, vitreous syneresis, secondary glaucoma, and fundus lesions.

Uveitis: Diagnosis

• Rule out primary corneal disease with fluorescein stain!.
• Rule out by performing a general PE.
• Perform a CBC/chemistry panel during first episode.
• Elevated serum fibrinogen is not a reliable biomarker for uveitis.
• Serologic tests involve testing for L. bratislava, L. canicola, L. grippotyphosa, L. hardjo, L. icterohaemorrhagiae, and L. pomona with >/=1:400 considered positive and recommend paired sera samples 2-3 weeks apart with 4-fold rise.

Equine Recurrent Uveitis (ERU)

• Also known as “moon blindness” or “periodic ophthalmia”.
• Reported prevalence is 2-25% of horses in the US, with 8-10% in Europe and <1% in the UK.
• Client education should warn owners of recurrence or opposite eye risk.
• Owners should never attempt to treat without veterinary examination.
• Diagnosis of ERU is 2 or more observed episodes.
• Appaloosa and draft breeds are predisposed.
• Equine Recurrent Uveitis disease can be unilateral in ~50% of horses, but bilateral in 80-87% of Appaloosas.

ERU Stages

• Classic: Active inflammatory episodes separated by quiescent periods in any horse.
• Insidious: Persistent low-grade inflammation in Appaloosas and Draft breeds.
• Posterior: Inflammation in the vitreous, retina, choroid.
• Active: Signs of active inflammation.
• Quiescent: No signs of active inflammation ('quiet eye').
• End-stage: Blind eye: Phthisis bulbi, dyscoria, posterior synechiae, cataract, lens luxation, retinal detachment.

ERU Pathogenesis

• Immune-mediated, specifically T-helper type 1.
• Disruption of the blood-aqueous barrier (BAB) allows CD4+ T cells to enter the eye.
• Recognition of self-antigens (intraocular tissues).
• Primary uveitis can be a cause in the development of ERU.
• Leptospirosis, other infectious etiologies and trauma play a role.
• Environmental and genetic influences, Appaloosas and German warmbloods, are markers for ERU risk with coat color, leopard complex spotting locus for insidious ERU, in brown and black warmbloods.

General Uveitis Therapy

• Dexamethasone (NeoPolyDex) is administered topically every 4-6 hours for active anterior uveitis only if stain is negative and cornea is normal.
• Flurbiprofen 0.03% and Diclofenac 0.1% are administered topically every 12-24 hours for quiescent periods, but must be compounded.
• Atropine 1% is administered topically every 12-24 hours, not exceeding TID, for active anterior uveitis but must monitor for colic.
• Flunixin meglumine is administered orally at 0.5-1.1 mg/kg BID short term and used for active anterior uveitis.
• Phenylbutazone is administered orally at 1 gram every 12-24 hours (1000 lb horse) for quiescent periods.
• Aspirin is administered orally at 10-25 mg/kg every 12-24 hours for quiescent periods.
• Firocoxib? administered orally at 0.1 mg/kg every 24 h in horses at risk for adverse effects of systemic NSAIDs.

ERU Treatment

• Goals are to preserve vision, decrease pain, and prevent or minimize recurrence.
• Treat active lepto infection with systemic antibiotics like doxycycline, minocycline or enrofloxacin.
• Intravitreal injections of low dose gentamicin (4 mg).
• Surgical treatment: Suprachoroidal cyclosporine implant.
• Poor long term prognosis.
• Lepto EQ Innovator vaccine by Zoetis is newly developed vaccine.
• L. interrogans serovar pomona should not be used an adjunctive treatment and is not supported at this time.

Glaucoma

• Uncommon in horses, typically secondary to uveitis, but primary, congenital, and other secondary causes exist.
• Clinical findings include elevated IOP (normal is 15-30 mm Hg), PLR deficits or fixed and dilated pupil, buphthalmos, and corneal changes.

Glaucoma Treatment

• Medical therapy uses topical carbonic anhydrase inhibitors like dorzolamide 2% BID-TID.
• Topical beta-blockers such as timolol 0.5% BID.
• Uveitis treatment.
• Laser treatment of ciliary body (cyclophotocoagulation) for eyes that maintain vision.
• For end stage options for blind, painful eyes use enucleation or chemical ablation.

Cataracts

• The majority of equine cataracts are non-progressive.
• Can be developmental (including congenital), in which case congenital may be associated with other abnormalities or acquired.
• Inherited cataracts are found in the Belgian, Thoroughbred, and QH breeds.
• Acquired cataracts are secondary to uveitis (ERU) and are commonly anterior capsular/subcapsular +/- synechiae, secondary to trauma and also senile cataracts.
• Senile cataracts (horses >20 years) need to DDx nuclear sclerosis.
• Cataract surgery is only recommended for young foals and not generally recommended for cataracts secondary to ERU.

Equine Motor Neuron Disease

• Neurodegenerative condition caused by Vitamin E deficiency.
• Clinical signs include severe weakness and muscle atrophy, and fasciculations.
• 50% of horses have fundic changes.
• Fundic changes include a "Honeycomb mosaic" pattern of dark brown to yellow pigmentation (ceroid-lipofuscin in retinal pigment epithelium) with no visual deficits.

Ischemic & Traumatic Optic Neuropathies

• Ischemic: sudden hypoxemia of optic nerve due to thromboembolic disease, acute hypovolemia, or surgical occlusion of the carotid artery.
• Traumatic: blunt trauma to head.
• Clinical signs include blind eye with normal optic nerve initially, papilledema, focal hemorrhages, hyperemia, accumulations of axoplasmic materials, eventual optic nerve degeneration.
• Limited treatment options.

Proliferative Optic Neuropathy

• White or gray "mass" protruding from the optic disc into the vitreous.
• Histologically resembles schwannoma.
• The eye is visual and an incidental finding.
• Typically occurs in older horses (>15 years).
• Diagnosis is by benign neglect and needs to DDx neoplasia.

Pre-purchase Examination

• Evaluate menace response for each eye, and Nasal and temporal visual fields.
• Includes an anterior segment exam.
• Must also include a dilated lens and posterior segment exam.
• Record and discuss findings such as corneal opacities, "footprints of synechiae,” cataracts, chorioretinal lesions, and evidence of historic ERU.
• It is the buyer’s decision to purchase or not given the information provided.

Equine Vision

• Total horizontal visual field is up to 350°, with a binocular visual field of ~60°.
• Minimal accommodation (<2 diopters).
• Dichromatic color vision as compared to the trichromatic color vision of humans.

Managing a Blind Horse

• Inform owners of the horse having visual deficits.
• Considerations are horse’s function, temperament, individual preferences, and management issues.
• Recommendations include finding a dedicated equine owner, a buddy system, a safe, stable environment, a regular schedule, and do not clip facial whiskers.
• Riding is not recommended, with some exceptions.