Disorders of the eye

Disorders of the eye

Part 1 Orbit and globe

Congenital/developmental conditions

Microphthalmos/anophthalmos (Figs. 10.1–10.4)

Microphthalmos is a developmental anomaly that presents as a congenital defect in foals. Thoroughbreds are most commonly affected, although the heritability of the condition is unknown. One or both eyes are affected, and the globe is smaller in size than a normal globe. The small size of the globe may be the only defect, or there may be a wide range of accompanying abnormalities within the eye, such as colobomas within the eye or heavy pigmentation of the globe itself. The palpebral fissure is usually small also. Vision may be normal if small size is the only ocular abnormality, however accompanying defects or very small globes may result in blindness.

Anophthalmos is diagnosed if no eye tissue can be located. It is an extreme form of microphthalmos and is actually very rare. The main differential diagnosis for microphthalmos is non-congenital phthisis bulbi (see below). The microphthalmic eye should be examined carefully to assess whether there is vision present, using the menace response. The dazzle reflex and pupillary light reflex should also be tested. The globe is carefully evaluated for additional abnormalities such as lens coloboma or cataract.

Anterior segment dysgenesis (Figs. 10.5 & 10.6)

Anterior segment dysgenesis is failure in the normal development of the tissues of the anterior segment of the eye. It leads to abnormalities in the structure of the adult anterior segment, which can result in corneal opacity, visible intraocular abnormalities and an increased risk of glaucoma. Multiple congenital ocular abnormalities can occur as a congenital condition in Rocky Mountain horses. These usually feature megalocornea (an abnormally large cornea) along with a syndrome of ocular abnormalities including cysts of the iris, ciliary body and retina, iris hypoplasia, cataracts and retinal dysplasia. The condition is likely to be inherited, and therefore affected animals should not be used for breeding. However, any breed of horse can be affected with congenital anterior segment dysgenesis and multiple ocular abnormalities.

Non-infectious conditions

Phthisis bulbi (Figs. 10.7–10.9)

Phthisis bulbi is a shrunken, atrophic globe as a result of injury or inflammation. The affected eye is always permanently blind. The third eyelid usually protrudes due to the reduction in orbital volume. Entropion may occur. Ocular discharge often builds up due to incomplete blinking.

Orbital fat prolapse (Fig. 10.10)

The extraorbital fat pad is positioned around the base of the third eyelid. Occasionally, this fat may herniate through weak fibrous connective tissue. This appears as a smooth conjunctival swelling arising from behind the third eyelid. The cause is not known, and one or both eyes might be affected.

Orbital neoplasia (Figs. 10.11–10.15)

The presence of neoplasia within the orbit can affect the eye, and the owner may be first alerted to the problem because of an abnormal eye. Types of orbital neoplasms include neuroendocrine tumors, extra-adrenal paraganglioma, anaplastic sarcoma, lymphosarcoma and squamous cell carcinoma. Orbital neoplasia can arise in adjacent structures such as the sinus or nasal cavity.

Orbital trauma (Fig. 10.16)

Orbital and periorbital trauma may be caused by kicks or collisions. The periorbital bones which might be fractured include the zygomatic arch, orbital rim and supraorbital process. Globe rupture might also occur. The eyelids may not be able to blink completely due to nerve damage or because of the malaligned bones. Diagnosis is made by visualizing and then palpating the deformity of the periorbital bones. This can be confirmed with radiology. Bony fragments may rub against the globe, causing irritation and threatening penetration. Treatment depends on the extent of the injury and on the level of cosmesis required. It may be possible to manually reposition displaced fragments, or surgically remove them. Surgical repair is possible, and preferred soon after the injury.

Part 2 Eyelids, third eyelids, conjunctiva and nasolacrimal diseases

Congenital/developmental disorders

Entropion (Figs. 10.17 & 10.18)

Entropion occurs when the eyelid margin is inverted, causing the eyelid to roll inwards. The lower eyelid is affected most commonly. This allows for the facial hairs to contact the globe (trichiasis), leading to ocular discharge, conjunctivitis, keratitis and potentially corneal ulceration. It is the most common ocular abnormality in foals. The cause can be a primary anatomical problem, but also can be secondary to dehydration or cachexia in the neonatal foal. It may arise as a result of loss of orbital volume such as with microphthalmos or phthisis bulbi. At any age, eyelid trauma, eyelid scarring (cicatricial entropion) or blepharitis might result in entropion.

Cilia disorders (Fig. 10.19)

Cilia disorders are uncommon in horses. Distichia are hairs that arise from meibomian gland orifices. These may contact the globe and result in frictional irritation, manifested as blepharospasm, ocular discharge and keratitis, occasionally becoming ulcerative. Ectopic cilia are cilia that arise from the palpebral conjunctiva and contact the globe. A cases series of seven cases has been published (Hurn et al. 2005), and ophthalmic examination revealed a single translucent cilium in the upper eyelid palpebral conjunctiva, emerging approximately 5 mm from the eyelid margin. Post-traumatic trichiasis might occur as a result of inappropriately healed eyelid trauma.

Nasolacrimal duct atresia (Figs. 10.20 & 10.21)

Lacrimal punctum agenesis and lacrimal duct atresia are congenital conditions in the foal. They may occur unilaterally or bilaterally. Typically the condition is not noticed in the very young foal, but when the animal is several months old it develops a chronic copious mucopurulent ocular discharge.

Non-infectious disorders

Eyelid injury (Figs. 10.22–10.25)

Traumatic injury of the eyelids is a relatively common occurrence in horses, often resulting in eyelid laceration. The severity of injury varies, but all full-thickness lacerations require surgical repair. The eyelid has an excellent blood supply, and therefore heals very well and repair is normally successful.

Diagnosis, treatment and prognosis

• Diagnosis is usually obvious but the orbit should be carefully assessed for fractures by gently palpating the orbital rim. The conjunctiva and cornea should also be assessed for injury. If the wound is near the medial canthus, the nasolacrimal puncta and canaliculi should be carefully assessed for injury.

• Differential diagnosis (of a chronic laceration): ulcerated mass such as squamous cell carcinoma or severe solar blepharitis.

• Treatment involves prompt surgical repair. This can be achieved with topical and regional anesthesia, with sedation. The eyelid should be cleansed with a dilute disinfectant which is not harmful to the eye, such as 10% povidone iodine solution, at a 1 : 50 dilution. Minimal debridement is required, and hanging eyelid pedicles should be replaced rather than amputated.

• Full-thickness laceration may require two-layer closure, and a deep conjunctival layer should first be gently apposed using a continuous pattern with an absorbable suture (for example 6-0 polyglactin 910).

• The eyelid margin should be carefully re-aligned using a figure-of-eight suture. The external skin is then repaired with simple interrupted sutures. Poor realignment or second intention healing can result in incomplete eyelid closure which can lead to drying to the exposed cornea and subsequent ulceration.

• The prognosis is normally very good with meticulous and timely surgical repair.

Acquired nasolacrimal duct obstruction (Figs. 10.26–10.29)

Nasolacrimal duct occlusion or stenosis may be acquired due to dacryocystitis (inflammation of the duct), trauma, foreign body, parasite obstruction, tooth root disease or sinusitis. Clinical signs are chronic epiphora, which occurs if there is no infection, or mucopurulent to purulent ocular discharge if infection occurs.


• Careful flushing of the nasolacrimal duct, which may be cannulated at either the proximal end and flushed towards the nose, or at the nasal ostium and flushed towards the eye. If the blockage is successfully alleviated, the duct should be carefully but copiously flushed.

• An indwelling catheter or tubing might be sutured in place for 2–3 weeks to retain patency.

• An ophthalmic eye drop containing an antibiotic and a steroid is prescribed for application up to four times daily until the inflammation has fully settled and then at reduced frequency for several days.

• Systemic antibiotics may also be required.

• If an underlying cause such as a sinus or tooth root problem is identified, this is treated appropriately.

• Surgical correction is occasionally required if repeated flushing cannot alleviate the obstruction, establishing drainage with a more invasive procedure such as a conjunctivorhinostomy or a conjunctivosinusotomy.

Non-infectious/infectious disorders

Blepharitis (Figs. 10.30–10.34)

Blepharitis involves inflammation of the eyelids. The main types of blepharitis that occur in the horse are infectious, traumatic, allergic, immune-mediated, actinic and parasitic blepharitis.

• Infectious blepharitis may be due to bacterial infection with Moraxella equi, Listeria monocytogenes or opportunistic bacteria. Fungal blepharitis might be caused by many organisms, depending on geographical exposure, and include Trichophyton, Microsporum and Histoplasmafarciminosus.

• Traumatic blepharitis is also a common cause of inflammation.

• Allergic blepharitis might arise as a result of a local or a systemic allergy, and blepharoedema may be the most striking sign. The allergen might be environmental, e.g. mold, dust, pollen, or it could be due to topical or systemic medication or shampoos, or a food allergy.

• Immune-mediated blepharitis is uncommon, but might arise as part of the pemphigus foliaceus or bullous pemphigoid complexes, which will also affect other mucocutaneous junctions.

• Solar blepharitis affects eyelids with little or no protective pigment which are exposed to UV light. Actinic blepharitis could be a precursor for later development of squamous cell carcinoma.

• Parasitic infestation can result in blepharitis, and those that might affect the eye include Onchocerca cervicalis, Habronema muscae, Habronema microstoma and Draschia megastoma and Thelazia lacrymalis. Onchocerca principally affects the conjunctiva, with focal conjunctivitis present as nodules lateral to the limbus or as regions of de-pigmentation. Habronema causes mainly blepharitis, with thickened ulcerative caseous nodules on the eyelid margin or palpebral conjunctiva. Thelazia infestation might cause dacryocystitis but is often asymptomatic.

Blepharitis symptoms vary depending on the underlying cause but include some of the following: blepharoedema, hyperemia of the palpebral conjunctiva, eyelid hyperemia, pruritis and focal swellings on the eyelid margins or palpebral conjunctiva.


• Infectious blepharitis should be treated as appropriate, based on culture and sensitivity. Systemic anti-inflammatory treatment might be required for allergic blepharitis, along with exclusion from the underlying trigger factor.

• It is recommended that horses with solar blepharitis are shielded from the sun with UV-protecting eye wear, the provision of shade and with the application of sun block.

• Habronema is effectively controlled with ivermectin (0.2 mg/kg) but the lesions may also need to be treated with topical compresses, de-bulking, topical antibiotics or lubricants and systemic anti-inflammatories if severe.

• Systemic ivermectin at the same dose is effective at controlling Onchocerca microfilaria, and this treatment will need to be repeated, as it is not effective against adult worms. Thelazia might be treated with topical irrigation with 0.5% iodine solution and 0.75% potassium iodide. Topical application of 0.03% echothiophate iodide or 0.025% isoflurophate has also been reported to be successful.


Eyelid sarcoids (Figs. 10.35–10.37)

Equine sarcoids are benign fibroblastic cutaneous tumors which are very common in horses. Metastasis of sarcoids is rare but recurrence is common. They can affect the skin of the eyelid and periocular region, where they may affect the eye by direct contact with the globe. Location next to the eye can make surgical removal challenging. Retrovirus or palpillomavirus may be involved in the etiology.


• Periocular sarcoids present a therapeutic challenge as some of the treatment modalities, which are amenable to other parts of the body, are contraindicated around the eye because of the possibility of damaging the globe.

• Historically, sarcoids were treated with topical irritants, such as engine grease, tea tree oil, and oil of rosemary, which served to stimulate the local immune system to mount a response against them. However they would cause a severe keratitis if they were allowed to come into contact with the cornea.

• Surgical excision is usually difficult because of the size of the mass and the lack of available tissue for blepharoplasty procedures. It has also been suggested that sarcoids that recur after surgery are less amenable to further adjunctive treatments.

• Immunotherapy using BCG injections at fortnightly intervals has been very successful at treating periocular sarcoids, although local and systemic anaphylaxis is a risk and should be anticipated with pre-treatment with systemic NSAIDs.

• Chemotherapy with intralesional cisplatin or topical 5-fluorouracil has been reported to be successful, although 5-fluorouracil is irritant to the globe.

• Cryotherapy and hyperthermia have both been reported with some success. Brachytherapy is reported to be very successful although facilities for radiation treatment must be available.

• The prognosis is poorer when recurrence develops, therefore the initial treatment should be well thought out and thorough.

Squamous cell carcinoma of the eyelid and third eyelid (Figs. 10.38–10.42)

Squamous cell carcinoma (SCC) is the most common neoplasm to affect the eye and adnexa of the horse. An increased incidence has been observed in draft breeds and the Appaloosa. The average age at diagnosis is about 11 years. Horses with light-colored coats are more susceptible than those with darker pigmentation. Actinic solar blepharitis may transform into a squamous cell carcinoma. UV light may mutate the tumor suppressor gene p53, allowing the neoplasm to develop.


• Treatment of periocular SCC is challenging. While complete surgical resection is the treatment of choice, extensive eyelid re-construction is often not possible.

• Removal of the entire third eyelid, over-sewing the remaining conjunctiva to prevent fat prolapse, may be curative for third-eyelid SCC.

• Eyelid SCC may require surgical debulking with adjunctive treatment, or adjunctive treatment alone. Methods successfully employed include immunotherapy (BCG injections), chemotherapy (e.g. intralesional cisplatin, 5-fluorouracil), cryotherapy (double or triple freeze–thaw cycle to –25°C), brachytherapy (radioactive isotopes gold, iridium, radon, etc.) or hyperthermia (41–50°C).

• The most recently reported treatment with a favorable outcome is photodynamic therapy (Giuliano et al 2008) although it is still undergoing further investigation as a suitable treatment modality.

• There is a poor prognosis for eyelid SCC compared with third-eyelid SCC, and reported metastasis rates vary from 6% to 15%.

Other adnexal tumors (Figs. 10.43–10.51)

Other adnexal neoplasms arise occasionally. In all cases, the diagnosis might be suspected from the clinical appearance but can only be confirmed by fine-needle aspirate, biopsy or excisional biopsy. Other non-neoplastic swellings can occur in the area, such as Habronema granulomas or orbital fat prolapse.

Eyelid and subconjunctival melanomas are most common in gray horses. They may cause frictional irritation on the globe. They present as firm pigmented nodules on the eyelid margin or beneath the conjunctiva. The adjacent area should be carefully examined to determine the full extent of the mass. Surgical resection is the preferred treatment, and cryotherapy might also be considered. Oral cimetidine has been used to treat cutaneous melanomas but there are no reports of using it for periocular melanomas.

Lymphosarcoma is an uncommon tumor of the adnexal region. It has a variety of presentations. There may be infiltration of the conjunctiva and eyelids. Orbital involvement may result in exophthalmos. Nodular swellings of the conjunctiva may occur occasionally, and have been referred to as pseudotumors. A full systemic examination is indicated. Most cases represent multicentric lymphosarcoma which makes the prognosis very poor. Treatment with systemic dexamethasone 30 mg every second day has been reported.

Mast cell tumors are not common. They are associated with eosinophilic infiltration and may resemble a Habronema granuloma. They may result in a diffuse thickening of the affected eyelid. Surgical resection is indicated if this is possible.

Other less common adnexal tumors include hemangioma, hemangiosarcoma, basal cell carcinoma, lacrimal gland carcinoma, fibroma and fibrosarcoma.

Part 3 Cornea

Congenital/developmental conditions

Dermoids (Fig. 10.52)

A dermoid is normal skin that is present in an abnormal location (a choristoma). They are congenital but may become more obvious as the animal gets older, and the hair grows a little longer. They may be located on the conjunctiva, cornea or eyelid, and may be unilateral or bilateral. Associated clinical signs include the physical presence of haired skin on the eye or adnexa. They most commonly occur on the dorsal limbus but may be located elsewhere. The skin may be pigmented, making their presence more obvious. The consequences of its presence are also visible as irritation results in epiphora or mucopurulent ocular discharge, and occasionally blepharospasm.

Linear keratopathy (Fig. 10.53)

Paired parallel well-defined corneal striae in a linear pattern occasionally present in an apparently normal eye. These linear opacities occur at the level of Descemet’s membrane, and there is no associated corneal edema. It may travel in any direction, but most often extends from limbus to limbus across the center of the cornea, and may branch one or more times. The cause is unknown, but they are thought to arise due to a transient increase in intraocular pressure during parturition. Identical opacities are found in some horses with glaucoma, and in others that have suffered blunt ocular trauma.

Non-infectious disorders

Corneal trauma (Figs. 10.54–10.60)

Horses are very vulnerable to both blunt and sharp penetrating ocular trauma, and this can have a variety of effects on the globe. Sharp penetrating corneal trauma will cause laceration. If the wound is full thickness, there may also be damage to the iris or lens. Blunt force energy is transmitted throughout the globe. Corneal perforation may also occur but it is more likely that there will be more serious structural damage, worsening the prognosis. The traumatized eye presents with ocular pain, manifested as blepharospasm, alteration in the angle of the eyelashes and epiphora. The eye is usually cloudy, due to corneal edema and/or aqueous flare, and red due to inflammation or hyphema. Corneal trauma invariably results in secondary iridocyclitis, with miosis, iritis, aqueous flare and possibly hypopyon. Full-thickness corneal laceration may be associated with a shallow anterior chamber, hyphema and iris prolapse. If the corneal laceration is peripheral, it should be carefully checked if it traverses the limbus as the sclera may also be ruptured. Scleral rupture is more difficult to see clinically because of the posterior location and because it is covered by conjunctiva which will be inflamed and swollen.


• Diagnosis is made with the history of trauma and by careful examination of the eye. As the eye will be painful, sedation, regional nerve blocks and topical anesthesia will be required for a thorough examination.

• The cornea should be examined with magnification to determine if any foreign body is still present.

• Fluorescein stain is applied to assess the presence of corneal ulceration. The Seidel’s test is carried out simultaneously as follows. The corneal wound is examined carefully after liberal application of fluorescein dye. If the wound is leaking, a small stream of clear aqueous will flow through the concentrated fluorescein, appearing brighter green as it is diluted. This is a positive Seidel’s test.

• If the cornea is stable and the eye cannot be examined thoroughly due to opacity in the ocular media, B-mode ultrasound is a useful method to look for lens luxation, lens capsule rupture, hemorrhage or inflammatory membranes in the vitreous and retinal detachment.

Treatment and prognosis

• Treatment depends on the extent of the injury. Small corneal lacerations, which are not full thickness, can be treated medically as for corneal ulcers. This involves topical antibiotics and mydriatics, along with systemic NSAIDs.

• If the edges of the corneal wound are edematous and therefore everted or irregular, or if the corneal wound is full-thickness, the wound should be surgically repaired. Direct simple interrupted corneal sutures are normally sufficient, but occasionally a conjunctival graft might be necessary to support healing. Systemic antibiotics and NSAIDs will be required as well as topical antibiotics and mydriatics.

• Iris prolapse requires surgical repair. Ocular survival has been reported to be 80% after surgery with vision in 33% of horses with iris prolapse due to corneal lacerations.

• The prognosis is best for corneal lacerations that do not involve any other ocular structures, and that are presented promptly for treatment. Corneal lacerations could potentially become complicated by bacterial or fungal infection, corneal melting, anterior synechiae, glaucoma, or uncontrolled uveitis due to lens capsule disruption causing lens-induced uveitis. Intraocular bacterial inoculation might result in endophthalmitis.

Corneal foreign bodies (Figs. 10.61–10.67)

Corneal foreign bodies are not unusual in the horse. Regions of the north-eastern USA have a plant burdock pappus with sharp bristles that can lodge in the conjunctiva and damage the cornea. They result in ocular pain and chronic non-healing ulcers. The eye needs to be examined with magnification to determine whether any bristles are retained. If there are, they are gently removed with a forceps after application of topical anesthesia, and both motor and sensory nerve blocks. Treatment as for a corneal ulcer is then given, with topical antibiotics and atropine, with systemic NSAIDs until the eye is comfortable and the cornea is negative for fluorescein uptake.

Feb 27, 2017 | Posted by in EQUINE MEDICINE | Comments Off on Disorders of the eye
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