Equine Ophthalmology

Equine Ophthalmology

This chapter is divided into ophthalmic diseases in foals and adult horses. Manual restraint in foals is usually possible while in the adult horse some sedation and eyelid nerve blocks are necessary for a complete ophthalmic examination. Topical medical therapy for foals is quite easy (especially with two adults), but intensive topical therapy in the adult horse usually requires some type of subpalpebral delivery system.

Ophthalmic Diseases in Foals


Microphthalmia is one of the most frequent congenital defects in foals, and is often associated with congenital cataracts (Figure 15.1). The condition is unilateral or bilateral, and the Thoroughbred appears to be predisposed. The presenting signs are reduced palpebral fissure size, prominent nictitating membrane, and a smaller than normal eye. Vision is normal or impaired, depending on the size of the eye, and other abnormalities. There is no treatment for microphthalmia.

Photo displaying a foal’s eye with severe microphthalmia.

Figure 15.1 Severe microphthalmia in a foal. Histologically, all ocular tissues were present in this globe, but were reduced in size and disorganized.


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).

Photo displaying strabismus in an Appaloosa yearling’s eye.

Figure 15.2 Strabismus in an Appaloosa yearling. Note the strabismus is dorsomedial. It affects clinical vision, producing frequent stumbling.

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.

Photo displaying a Thoroughbred foal’s eye with entropion and secondary blepharospasm worsened the lid defect.

Figure 15.3 Entropion in a Thoroughbred foal. Secondary blepharospasm has worsened the lid defect.

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.

Photo of a foal’s eye displaying a pigmented dermoid, affecting the ventral conjunctiva and cornea with the long coarse hair emerging from its surface.
Photo of a horse’s eye displaying an example of corneal dermoid with the pigmented lesion along the dorsal cornea.

Figure 15.4 (A) A pigmented dermoid affecting the ventral conjunctiva and cornea in a foal. Note the long coarse hair emerging from its surface. (B) Another example of a corneal dermoid in a horse. The pigmented lesion along the dorsal cornea is skin with small, fine hairs.

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.

Photo displaying a foal with nasolacrimal duct atresia swelling on the floor of the nasal cavity.
Photo displaying an 8‐month‐old Thoroughbred colt with nasolacrimal duct atresia and a 4‐month history of mucopurulent ocular discharge.
Image described by caption.

Figure 15.5 (A) Nasolacrimal duct atresia in a foal. The lower, blind end of the nasolacrimal duct appears as a swelling (arrow) on the floor of the nasal cavity just caudal of the nares following orthograde instillation of fluid. (B) An 8‐month‐old Thoroughbred colt with a 4‐month history of mucopurulent ocular discharge had nasolacrimal duct atresia. (C) Dacryorhinocystography study of the same horse as in part B. The contrast material does not exit the distal nasal punctum, confirming the location of the distal end of the duct and the reason for the lack of patency.

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.

Photo of an eye of Appaloosa foal displaying heterochromia iridis.
Photo of a blue-eyed horse displaying iris hypoplasia with the ventral equator of the lens visible behind the thin iris.
Photo displaying a blue-eyed horse with iris hypoplasia.

Figure 15.6 (A) Heterochromia iridis in an Appaloosa foal. Note the iridal periphery is white and its central portion blue. (B) Iris hypoplasia (nearly a coloboma) in the typical position in a blue‐eyed horse. The ventral equator of the lens is visible behind the thin iris. (C) Iris hypoplasia in a blue‐eyed horse. Note the dorsal bulge of the iris. This is commonly mistaken for a mass lesion. Instead, the iris is bulging as a result of thinning or weakness of the hypoplastic tissue.

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.

Photo displaying a foal’s eye with congenital glaucoma and lens subluxation, with two arrows depicting the periphery of the lens within the pupil.

Figure 15.7 Congenital glaucoma and lens subluxation in a foal. The periphery of the lens (arrows) can be seen within the pupil.


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).

Photo displaying iridocyclitis in a foal's eye secondary to bacteremia associated with Salmonella infection, with fibrin filling the anterior chamber.
Photo of a foal’s eye displaying fibrinous uveitis with Rhodococcus pneumonia and a large fibrin clot in the anterior chamber obscuring the view of the iris.

Figure 15.8 (A) Iridocyclitis in a foal secondary to bacteremia associated with Salmonella infection. Fibrin filled the anterior chamber. The pupil has been pharmacologically dilated. Salmonella organisms were cultured from the anterior chamber. (B) Fibrinous uveitis in a foal with Rhodococcus pneumonia. The large fibrin clot in the anterior chamber is obscuring view of the iris.

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.

Photo displaying a foal’s eye with congenital cataract affecting the nucleus (center) of the lens.
Photo displaying a foal’s eye with arrow depicting the red hyaloid vessels on the posterior lens surface.
Photo displaying a late immature cataract in a Quarter Horse weanling filly’s eye.
Photo displaying a hypermature resorbing cataract in a yearling Thoroughbred’s eye.

Figure 15.9 (A) Congenital cataract in a foal affecting the nucleus (center) of the lens. Progression of the cataract is unlikely. (B) Congenital cataract secondary to persistent hyaloid remnants in a foal. Note the red hyaloid vessels (arrow) on the posterior lens surface. (C) Late immature cataract in a Quarter Horse weanling filly. (D) Hypermature, resorbing cataract in a yearling Thoroughbred.

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.


Figure 15.10 Optic nerve hypoplasia in a Quarter Horse foal. The optic disc is tilted, smaller than normal, and pale white.

Equine Ophthalmology in Adult Horses

Orbital Diseases

Orbital Inflammations

Orbital cellulitis occurs occasionally in horses, and can be associated with trauma, foreign bodies, and the extension of infections from the adjacent nasal and sinus cavities as well as tooth root infections (Figure 15.11). The presenting clinical signs are acute swelling of the eyelids and supraorbital fossa, blepharospasm, exophthalmia, orbital pain, epiphora, conjunctival hyperemia, and protrusion of the nictitating membrane. Ultrasonography can assist with characterization and localization of disease as well as sample acquisition.

Photo displaying an orbit cellulitis in a Standardbred horse’s eye with eyelids swelling and supraorbital fossa, blepharospasm, and copious mucopurulent discharge.

Figure 15.11 Orbit cellulitis in a Standardbred horse. Note the marked swelling of the eyelids and supraorbital fossa, blepharospasm (from pain), and copious mucopurulent discharge.

Orbital Trauma and Fractures

Orbital trauma is more frequent in young horses maintained in groups. Blunt as well as penetrating trauma occurs. Orbital swelling produces a prominent or partially luxated globe, nictitans protrusion, conjunctival hyperemia and chemosis, variable corneal edema, miosis, and hyphema. An impaired blink reflex can occur, and corneal exposure with ulceration rapidly develops. Treatment consists of placement of the subpalpebral system, a complete temporary tarsorrhaphy, and topical and systemic antibiotics, mydriatics, and systemic nonsteroidal anti‐inflammatory drugs (NSAIDs).

Orbital trauma can also produce fractures of the orbital bones (Figure 15.12). Clinical signs include asymmetry of the orbits, epistasis, exophthalmos, conjunctival and lid swelling, crepitus, and pain on orbital palpation. Ultrasonography and radiology are essential diagnostic modalities. Surgical treatment includes bone plates or wires for the orbital fractures with displacement, subpalpebral lavage system, and complete temporary tarsorrhaphy to protect the cornea.

Photo displaying an orbital trauma and fracture of the supraorbital process in a horse’s eye with the upper lid swelling, exophthalmia, ventral strabismus, and dorsal subconjunctival hemorrhage.
Photo of a horse’s eye displaying an orbital trauma with exophthalmos and severe conjunctival swelling as well as an impaired blink and exposure keratitis.
Image described by caption.

Figure 15.12 (A) Orbital trauma and fracture of the supraorbital process in a horse. Note the upper lid swelling, exophthalmia, ventral strabismus, and dorsal subconjunctival hemorrhage. (B) Orbital trauma resulted in a displaced fracture of the dorsal orbital bones which resulted in exophthalmos and severe conjunctival swelling as well as an impaired blink and exposure keratitis. (C) Computed tomography image of the same horse as in part B revealing displaced fracture and globe compromise.

Orbital Neoplasia

Orbital neoplasms are rare in horses, but include most histopathological types (Figure 15.13). The most frequent are squamous cell carcinoma (usually extending from the nictitating membrane and conjunctiva) and lymphoma (often bilateral). Clinical signs are progressive exophthalmia, strabismus, orbital and supraorbital fossa swelling, and blindness. Prognosis is usually poor.

Photo displaying orbital lymphosarcoma in a horse’s eye with protrusion of the nictitating membrane and hyperemia of the ventral conjunctiva.
Photo displaying an orbital lymphoma in an aged Quarter Horse gelding’s eyes with a unilateral focal conjunctival swelling.
Photo of a horse’s eye displaying an orbital squamous cell carcinoma with tumor spreading throughout the orbit.

Figure 15.13 (A) Orbital lymphosarcoma in a horse with protrusion of the nictitating membrane and hyperemia of the ventral conjunctiva. The mass affected the ventral orbit and lower eyelid. (B) Orbital lymphoma in an aged Quarter Horse gelding. This horse presented several years previously with a unilateral focal conjunctival swelling that was excised and diagnosed as lymphoma. (C) Orbital squamous cell carcinoma. The tumor originated on the third eyelid. Excision of the nictitans was incomplete and the tumor spread throughout the orbit.

Atrophy of the Globe or Phthisis Bulbus

Phthisis bulbus or atrophy of the globe is a not uncommon condition in older horses (Figure 15.14). It results from severe trauma, chronic anterior uveitis, and glaucoma that eventually impair aqueous humor formation. Reduced IOP results (usually less than 5 mmHg) which can cause corneal decompensation (corneal edema), cataract formation, and retinal detachment. The condition does not respond to therapy.

Photo displaying phthisis bulbus in a 10-year-old horse’s eye.

Figure 15.14 Phthisis bulbus of a 10‐year‐old horse. Note the reduction in the globe size, diffuse corneal edema and superficial vascularization, and mature cataract. The globe has very low intraocular pressure and the horse is blind.

Diseases of the Eyelids

Eyelid Lacerations

Eyelid lacerations are common in horses. Either the upper or lower eyelid can be affected; however, because of its size, mobility, and prominence, the upper lid is most often affected (Figure 15.15). The laceration often involves the lid margin, and must be repaired carefully to avoid a postoperative notch or defect. The cornea should be carefully examined as corneal lacerations may be also present (especially if hyphema is present). Treatment consists of two‐layer closure starting with the eyelid margin. A temporary complete tarsorrhaphy is recommended to protect the cornea during lid healing and when the blink reflex is impaired.

Photo displaying an eyelid laceration in a young horse’s eye with the lateral eyelid being partially torn and remains attached medially.

Figure 15.15 Eyelid laceration in a young horse. The lateral upper eyelid has been partially torn and remains attached medially. Surgical apposition with no debridement is recommended.

Eyelid Neoplasia

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Jul 24, 2020 | Posted by in INTERNAL MEDICINE | Comments Off on Equine Ophthalmology

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