Strabismus

Strabismus is a deviation of the globe from its normal position. In foals, this condition occurs more commonly in the Appaloosa breed and can accompany stationary night blindness (STNB) (Figure 15.2). The globe is usually rotated upward (hypertropia), or dorsomedial or medial (estropia). If both eyes are affected, the foal may position its head as if to compensate for the strabismus. Stumbling is a common complaint. STNB should be excluded when a young animal presents with strabismus. In this condition the ocular fundus appears normal by ophthalmoscopy and the animal has clinically normal vision in daylight but night vision is profoundly impaired. This is easily noted by behavior changes when the ambient illumination is reduced. STNB has a very distinct pattern on electroretinography. Primary strabismus in foals has been treated by surgery of the involved rectus muscles (either recession or resection).

Congenital Entropion

Congenital entropion or the inversion of the eyelid margin is an infrequent abnormality in foals (Figure 15.3). The lower eyelid is most often affected. Microphthalmia, dehydration, prematurity/dysmaturity, and eyelid trauma are contributing disorders. Presenting signs are increased lacrimation, blepharospasm, inverted lid margin, conjunctivitis, and occasionally corneal ulceration. Temporary eversion of the lid until the underlying disease state has resolved or the foal’s growth alleviates the entropion is indicated. Temporary surgical repair should be delayed until the foal has approximated its adult size. Permanent entropion repair is rarely necessary in the horse unless the condition has resulted from cicatrix formation.

Lid and Conjunctivae Dermoids

Dermoids or choristomas can affect the foal’s eyelids, conjunctiva, nictitating membrane, or cornea (Figure 15.4). Most often the cornea and limbal bulbar conjunctiva are affected. The dermoid presents as a raised, pigmented, hair‐covered mass (which can be confused with a limbal‐based melanoma). Treatment is surgical removal.

Nasolacrimal Duct Atresia

The development of the nasolacrimal system starts as two buds that eventually become the lacrimal puncta and canaliculi, and extend through the developing facial bones and nasal cavity to exit at the muctocutaneous junction on the floor of the external nares (easily observed). In nasolacrimal duct atresia, the distal end of the nasolacrimal duct stops prematurely and ends in a blind pouch (Figure 15.5). Clinical signs include a persistent conjunctivitis (which can respond temporarily to topical antibiotics) and epiphora, but these may not be readily apparent until the foal is several months of age. Dacryocystitis with mucoid or mucopurulent discharge develops with time. Fluorescein applied on the eye does not exit at the external nares and close examination of the external nares reveals absence of the nasal punctum. Nasolacrimal flush through the upper or lower lacrimal punctum results in the accumulation of the irrigation solution in the enlarged blind end. Dacryorhinocystography facilitates the diagnosis and localizes the endpoint of the atretic duct. Restoration of nasolacrimal duct patency is by surgery.

Irial Anomalies

Congenital disorders of the iris in the foal are rare, but include aniridia (in Thoroughbreds, Belgians, and Quarter Horses) and persistent pupillary membranes. In heterochromia iridis, the color of the iris is blue, blue and white, or some combination of brown, blue, and white, and affects one or both eyes (Figure 15.6). The term “wall eye” refers to a blue–white iris with a black corpora nigrum (or granula iridica), while “china eye” refers to a white iris with a black corpora nigrum. Heterochromia iridis is related to hair coat color and occurs in Appaloosa, gray, white, palomino, spotted, and chestnut horses. It can be accompanied by hypoplasia or colobomatous malformations of the iris.

Congenital Glaucoma

Congenital glaucoma is rare in foals and usually signals anomalous development of the anterior chamber angle and aqueous humor outflow pathways (Figure 15.7). Presenting signs include buphthalmia (an enlarged globe), mydriasis, variable corneal edema, lens luxation, and variable degeneration of the optic nerve head and retina. Tonometry indicates elevated intraocular pressure (IOP). Treatment is not usually attempted, but includes topical drugs to lower IOP and laser cyclophotocoagulation.

Iridocyclitis

Iridocyclitis in foals can be secondary to other ocular diseases (usually corneal ulcers) or associated with systemic diseases such as Salmonella sp., Rhodococcus equi, Escherichia coli, Streptococcus equi, Actinobacillus equuli, adenovirus, and equine viral arteritis as well as any other cause of septicemia, pneumonia, or any systemic inflammatory condition (Figure 15.8). The iridocyclitis is often bilateral and the foal will exhibit lacrimation, blepharospasm, and photophobia. Ophthalmic findings include eyelid swelling, corneal edema, conjunctival hyperemia, ciliary injection, aqueous flare, hypopyon and/or hyphema, and miosis. Fibrin and contact of the iris and lens can result in posterior synechiae formation and secondary cataracts. A complete general physical examination as well as a complete blood count, clinical chemistry, and thoracic radiographs should be performed. Treatment should be aimed at the underlying disease but should include symptomatic therapy for the iridocyclitis (mydriatics, and topical and/or systemic anti‐inflammatory medications).

Congenital Cataracts

Congenital cataracts and microphthalmia are the two most frequent congenital ophthalmic anomalies in foals (Figure 15.9). Inherited cataracts occur in the Belgian, Thoroughbred, Morgan, and Rocky Mountain Horse breeds. The cataracts in the Belgian may be related to aniridia (actually hypoiridia) and are inherited as a dominant trait. Cataracts are also thought inherited in the Thoroughbred breed as a dominant trait. The Morgan Horse has inherited and bilateral nonprogressive nuclear cataracts that do not progress and interfere with vision. Congenital cataracts in the Rocky Mountain Horse are associated with multiple ocular anomalies such as megalocornea, goniosynechiae, cysts of the iris, ciliary body and peripheral retina, and retinal dysplasia.

Often, a foal from a normal‐eyed mare and stallion is presented to the veterinarian because of either white opacities within the eye or vision impairment to a variable degree. Ophthalmic findings include normal or slightly smaller globes, normal pupillary light response, and mature and complete cataract formation. Ultrasonography and electroretinography are usually normal. Cataract surgery is performed most commonly in the foal for congenital cataracts (adult horses usually have uveitis cataracts) and phacoemulsification is the technique of choice. Young foals used to handling have surgery at young ages (at 1–2 months before lens‐induced uveitis occurs and the foal is more difficult to manage); success rates are over 80%.

Optic Nerve Hypoplasia

Optic nerve hypoplasia is a rare cause of congenital blindness in foals (Figure 15.10). The condition is unilateral or bilateral. Diagnosis is made with ophthalmoscopy. The optic nerve head is smaller than normal, white, and contains few retinal blood vessels. There is no treatment.