Ophthalmic emergencies are common in horses. Emergent ocular conditions are acute and vision-threatening; they include acute periocular swelling, eyelid lacerations, corneal ulcers and stromal abscesses, blunt or sharp trauma to the globe, uveitis, and glaucoma.
Because clinical signs of ocular disease are nonspecific and vision is critical to horse safety and performance, as well as to human safety, concerns about a horse’s ocular health warrant prompt examination by a veterinarian. Accurate diagnosis and fast, effective treatment are critical to preserve the globe and minimize longterm impacts on vision.
Many ophthalmic emergencies, including complicated eyelid lacerations and some corneal ulcers, stromal abscesses, and traumatic injuries to the globe, are best managed by immediate referral to a board-certified veterinary ophthalmologist with advanced microsurgical training.
Ophthalmic Examination of Horses
Before the horse is approached or touched, the ophthalmic examination should begin with observation of its head carriage, signs of ocular discomfort, facial symmetry, and how the horse interacts with and navigates its environment.
The neuro-ophthalmic examination—including assessment of globe position and testing of the menace response and palpebral, dazzle, pupillary light, and vestibulo-ocular reflexes—should be performed before sedatives are administered. The ophthalmic examination is performed in a systematic manner, most often moving from anterior to posterior; it includes assessment of the eyelids, third eyelid, conjunctiva, sclera, cornea, anterior chamber (including looking for aqueous flare), iridocorneal angle, iris and corpora nigra, lens, vitreous, and fundus.
Measurement of intraocular pressure and application of fluorescein stain are considered part of a complete ophthalmic examination. A Schirmer tear test should be considered in cases with mucoid or mucopurulent discharge, conjunctivitis, or corneal disease. If the globe is potentially fragile (in cases of deep corneal ulcers, descemetoceles, or ruptures), intraocular pressure measurement and Schirmer tear testing are contraindicated.
Most horses with nonpainful ocular disease tolerate a complete ophthalmic examination without sedatives or periocular nerve blocks. Horses with painful eye disease or those intolerant of examination should be administered sedatives and/or periocular nerve blocks to minimize stress and pain. The auriculopalpebral nerve block (the most commonly performed periocular block) inhibits motor input to the orbicularis oculi muscle, making the horse unable to blink (see auriculopalpebral nerve block image).
Courtesy of Dr. Kelly Knickelbein.
The auriculopalpebral block does not provide desensitization. A short-acting local anesthetic should be used for this block to minimize the amount of time the horse cannot cover its cornea. This block is used to aid in examination of the eye and for procedures of the eyelids and globe.
The supraorbital (frontal) nerve block, which desensitizes the medial canthus, is indicated for various procedures (see supraorbital nerve block image).
Courtesy of Dr. Kelly Knickelbein.
Acute Periocular Swelling in Horses
Etiology
Acute periocular swelling is a common reason for horses to be presented on an emergency basis (see acute periocular swelling image). In such cases, severe blepharoedema (swelling of the eyelids) and chemosis (edema of the conjunctiva) can preclude evaluation of the globe.
Courtesy of Dr. Kelly Knickelbein.
Potential causes of periocular swelling include the following:
traumatic blepharitis
conjunctivitis (allergic, eosinophilic, neoplastic)
foreign material embedded in the conjunctiva or periocular tissues
orbital cellulitis
inflamed orbital neoplasia
orbital fracture(s)
corneal disease
uveitis
Clinical Findings
Horses with severely swollen eyelids might be quieter than normal when first approached, likely because of ocular discomfort. Sedation is indicated to minimize discomfort during the ophthalmic examination; however, the following clinical signs can be readily observed beforehand:
blepharoedema
chemosis
conjunctival hyperemia
ocular discharge (ranging from epiphora to mucopurulent discharge)
Diagnosis
Digital palpation of the bony orbit and conjunctival surfaces
Ultrasonography and radiography
A thorough ophthalmic examination can often result in a definitive diagnosis of acute periocular swelling. Sometimes the diagnosis remains speculative or cannot be made until treatment has been initiated and clinical signs improve.
For the ophthalmic examination, sedation, auriculopalpebral perineural anesthesia, and topical anesthesia are indicated after the neuro-ophthalmic examination. Even if the swelling is severe enough to obscure the globe, an intense light should be shone toward the affected globe to test the indirect pupillary light reflex (PLR). Confirming the presence or absence of neural input conveyed from the affected to the unaffected eye provides important prognostic information for the potential for vision in the affected eye.
The ophthalmic examination in cases of periocular swelling should always be performed before palpation of the bony orbit and globe retropulsion, in case the globe is fragile. When globe integrity is compromised, as in the case of a descemetocele, retropulsion could result in rupture of the eye. When retropulsed, the globe should readily move posteriorly within the orbit. A lack of posterior movement of the globe when pressure is applied indicates retrobulbar disease such as hemorrhage, orbital fracture, cellulitis, abscess, or neoplasia.
Transcutaneous palpation of the bony orbit is indicated in all cases.
Transconjunctival digital palpation of the bony orbit and all conjunctival surfaces (including the conjunctival fornices and anterior and posterior aspects of the third eyelid) should be performed when it has been confirmed that the globe is not fragile. This palpation should be performed only after sedation, an auriculopalpebral nerve block, and application of topical anesthetic, using a gloved and well-lubricated (with an ophthalmic lubricant) finger.
For many cases of acute periocular swelling, imaging (including ocular and orbital ultrasonography and skull radiography) is indicated. Ocular ultrasonography should be performed when the globe cannot be directly assessed to determine the structural integrity of the ocular structures and prognosis for globe maintenance and vision. Findings of extensive intraocular hemorrhage, lens luxation, or retinal detachment carry a poor prognosis for vision.
Orbital ultrasonography and/or skull radiography should be performed in cases that have decreased retropulsion, orbital fracture, or fragility of the globe.
Treatment
Systemic anti-inflammatories
Ophthalmic ointments
The mainstay of treatment for acute periocular swelling is systemic anti-inflammatories. NSAIDs are indicated in all cases. For severe cases, systemic steroids can be administered as a sole agent or in combination with an NSAID. Selection of an NSAID versus a steroid should be based on the severity of disease, horse signalment, and perceived or known risk of unwanted adverse effects of these drugs.
In most horses, periocular swelling will decrease rapidly after NSAID administration, and a thorough ophthalmic examination should be repeated within 24–48 hours to more fully assess the globe. Additional treatment recommendations are based on the specific diagnosis.
If a deep corneal ulcer, descemetocele, or globe rupture is ruled out, treatment with a topical antibiotic ointment (without steroid) is appropriate. The goal of treatment is to prevent infection of corneal ulcers (some of which might not be evident, because usually the entire cornea cannot be assessed), as well as to lubricate the ocular surface.
Topical steroids should be avoided until the cornea can be fully assessed and confirmed not to have fluorescein retention or evidence of a stromal abscess. In cases with severe chemosis, application of 5% sodium chloride ophthalmic ointment should be considered to help decrease chemosis to enable a more thorough ophthalmic examination at the time of reevaluation.
Eyelid Laceration in Horses
Etiology
Eyelid lacerations in horses frequently occur when the eyelid skin is caught on the edge of a bucket handle, hook, nail, or other object and the horse pulls away, shearing the eyelid. Given their etiology, all eyelid lacerations are assumed to be contaminated.
Clinical Findings
Traumatic eyelid lacerations in horses are often in the form of a pedicle that is torn parallel to the eyelid margin, sometimes affecting the entire length of the eyelid. The extent of desiccation and devitalization of the affected tissue depends on the time elapsed since the injury. In every case, however, immediate primary closure with minimal debridement is the only option to maintain a functional eye. Loss of even small portions of the eyelid margin or adjacent skin can result in exposure keratitis.
Diagnosis
Examination of a horse with an eyelid laceration should involve assessment of the vitality of the eyelid tissue, the integrity of the globe, and the health of the cornea and intraocular structures.
Treatment
Immediate primary closure
Postoperative supportive care
The goal in repairing an eyelid laceration is to restore the normal anatomy of the eyelid to preserve its function of protecting the globe and distributing tear film. There is no redundant tissue around the eye; the skin and conjunctiva must be preserved and not excised. Therefore, the skin should be minimally debrided, and generally the conjunctiva should not be debrided at all.
The subcutaneous tissue should be debrided until it appears healthy and bleeding. The wound should be closed promptly in two layers (subcutaneous and skin; see large and small eyelid laceration images), and the following adjunct treatments should be provided:
tetanus prophylaxis
systemic NSAIDs
systemic antibiotics
ophthalmic antibiotics
ophthalmic atropine in cases of concurrent corneal ulcer
Courtesy of Dr. Kelly Knickelbein.
Courtesy of Dr. Kelly Knickelbein.
Laceration repair consists of the following steps:
Sedation
Auriculopalpebral perineural anesthesia
Topical anesthesia (tetracaine or proparacaine 0.5% ophthalmic solution)
Eyelash trimming (if needed)
Lavage of the wound until clean with diluted povidone-iodine solution (1:50 povidone-iodine solution:saline)
Local anesthetic infiltration of wound edges
Minimal debridement (leaving skin and conjunctiva) of excessively swollen or devitalized subcutaneous tissue
Eyelid lacerations should be closed in two layers:
For the deep layer, a row of absorbable sutures in a simple interrupted or simple continuous pattern should be placed to appose subcutaneous tissue (taking care that no suture material passes through the conjunctiva). The first suture of the deep layer should be placed close to the eyelid margin to ensure appropriate wound alignment.
The skin should be closed with nonabsorbable sutures, beginning with a figure-of-eight suture at the eyelid margin, followed by simple interrupted or cruciate sutures along the remainder of the wound. Suture material ranging in size from 3-0 to 5-0 is appropriate for eyelid laceration closure.
When a portion of the eyelid margin is completely devitalized or avulsed, referral to a veterinarian with advanced ophthalmic microsurgical training for blepharoplasty is indicated. If referral is not an option and eyelid function is lost, resulting in corneal exposure, frequent lubrication, permanent partial tarsorrhaphy, or enucleation should be considered.
Postoperative treatment of eyelid lacerations includes continuation of systemic NSAIDs and ophthalmic antibiotics, as well as a rigid protective eye mask (see visor and eye protector images).
Courtesy of Dr. Kelly Knickelbein.
Courtesy of Dr. Kelly Knickelbein.
Corneal Ulceration or Laceration in Horses
Etiology
Acutely painful eyes in horses are commonly due to corneal disease. Corneal ulcers are often traumatic and vary widely in severity.
Corneal ulcers are typically classified as simple or complicated:
Simple corneal ulcers do not have stromal loss, cellular infiltrate, or malacia (corneal softening or liquefaction), and with appropriate treatment, they typically heal within approximately 7 days.
Complicated ulcers have any of the following: stromal loss, WBC infiltrate, malacia, or failure to heal within 7 days.
Complicated ulcers should be presumed to be infected (bacterial, fungal, and mixed infections are all common) and treated aggressively. An underlying cause for corneal ulceration—including conjunctival foreign bodies, facial nerve paresis/paralysis, low tear production, poor corneal sensitivity, and pituitary pars intermedia dysfunction (PPID)—should be considered when a corneal ulcer is diagnosed.
Corneal lacerations are traumatic and can be partial thickness or full thickness into the anterior chamber, possibly involving the iris and/or lens. Corneal lacerations frequently lead to implantation of fungi and/or bacteria into the cornea or intraocular structures (see ulcerative fungal and bacterial keratitis images).
Courtesy of Dr. Kelly Knickelbein.
Courtesy of Dr. Kelly Knickelbein.
Clinical Findings
Corneal injuries are painful; the affected horse will usually hold its eye tightly closed (blepharospasm) and have excessive tearing (epiphora). Once the eye can be examined, other clinical signs will be evident and can include the following:
focal/geographic/linear corneal edema
stromal loss
WBC infiltrate
keratomalacia (corneal melting; see keratomalacia image)
moderate to severe reflex uveitis
Courtesy of Dr. Kelly Knickelbein.
Diagnosis
Fluorescein staining
Diagnosis of a corneal ulcer or laceration is based on clinical signs that are confirmed via fluorescein staining. Before stain is applied, the cornea should be thoroughly examined to determine whether there is corneal thinning (stromal loss), white/yellow corneal opacification (WBC infiltration), or malacia. The extent of reflex uveitis should be assessed; it can range from mild (miosis and subtle aqueous flare) to severe (fibrin and/or hypopyon in the anterior chamber). An accurate interpretation of these findings is essential to determining an appropriate treatment plan.
Fluorescein stain is best applied to the horse's eye as a liquid via a small syringe. The plunger of a 3-mL syringe is removed and the white paper torn from the fluorescein strip and discarded. The orange portion of the fluorescein strip is placed into the syringe, and the plunger is reseated. A small-gauge needle is attached to the syringe, and approximately 1 mL of eye wash solution is drawn into the syringe; then the needle is broken off at its hub and discarded. The result is a concentrated fluorescein solution. Painful eyes generally have markedly increased tear production, which immediately dilutes any stain applied to the eye. Therefore, fluorescein should be liberally applied in a concentrated form so that a corneal ulcer will not be overlooked.
Treatment
Prevention or treatment of corneal infection
Pain management
Treatment of reflex uveitis
Management of simple corneal ulcers focuses on preventing infection and addressing pain. The mainstay of treatment is application of an ophthalmic broad-spectrum antibiotic ointment (neomycin–polymyxin B–bacitracin ophthalmic ointment or similar) every 6–8 hours. The antibiotic does not need to penetrate the cornea, because these ulcers are not infected, and quick reepithelialization of the cornea will restore its barrier to infection.
Ophthalmic atropine solution or ointment is applied only one time or as prescribed for once-daily administration, depending on the extent of pain and reflex uveitis. Atropine carries a risk of GI stasis and colic; however, uncontrolled ocular pain carries the same risk.
Systemic NSAIDs are administered as needed to control pain.
Because corneal infections are common in horses, frequent rechecks are important. The eye should be reassessed every 5–7 days, and the client should be advised to call if the horse's ocular comfort does not quickly improve.
Medical management of complicated corneal ulcers and partial-thickness corneal lacerations is more intensive than that of a simple corneal ulcer. If any stromal loss, infiltrate, or malacia is noted, cytological examination and bacterial and fungal cultures should be performed to determine the infection's etiology.
Placement of a subpalpebral lavage (SPL) system might be necessary for the treatment of such ulcers.
Administration of a broad-spectrum ophthalmic antibiotic with corneal penetration every 2–6 hours is indicated. Because no available single agent provides a complete antibacterial spectrum (gram-positive, gram-negative, aerobic, anaerobic) plus good corneal penetration, more than one antibiotic is generally necessary. Examples of appropriate empirical antibiotic selection include ofloxacin in combination with cefazolin, or chloramphenicol in combination with tobramycin.
Antibiotic selection should be guided by results of culture and susceptibility testing; however, empirical treatment should be started immediately at the time of diagnosis to attempt to prevent corneal perforation. Horses are prone to fungal infection of the cornea, and an ophthalmic antifungal with corneal penetration applied every 4–6 hours is indicated in the treatment of complicated corneal ulcers.
The most commonly used antifungals are azoles (voriconazole, miconazole, itraconazole/dimethyl sulfoxide).
Administration of an anticollagenase every 2–6 hours is indicated to halt or prevent corneal malacia in an effort to prevent corneal perforation. Commonly used anticollagenases include autologous or heterologous equine serum, 1% EDTA, and N-acetylcysteine.
Ophthalmic atropine is administered every 12 hours until pupillary dilation is achieved and ocular comfort is good.
Systemic NSAIDs are administered for pain and reflex uveitis. Flunixin meglumine (1.1 mg/kg, PO or IV, every 12 hours as needed) is anecdotally the most effective NSAID for controlling ocular pain and uveitis in horses.
Systemic antibiotics are appropriate for treating corneal disease when the corneal lesion is vascularized or when there is corneal perforation. Unnecessary use of systemic antibiotics should be avoided to minimize risk to the horse and to prevent the development of resistance. Systemic antifungal medications are rarely indicated for treatment of corneal disease in horses, because the only affordable options are ineffective against the most common corneal fungal isolates.
Complicated corneal ulcers in horses require frequent monitoring because they can progress to perforation rapidly. If possible, the patient should be referred to a veterinarian with advanced training in equine ophthalmology to determine whether surgical stabilization of the cornea is the best course of treatment. Surgical stabilization is generally indicated when the depth of the corneal ulcer or laceration is 50% or more of the corneal thickness and when there is progressive stromal loss.
Treatment of complicated corneal ulcers can be summarized by the five A's:
antibiotics
antifungal
anticollagenase
atropine
systemic anti-inflammatory
The most appropriate treatment for full-thickness corneal lacerations is determined by the extent of intraocular damage. If the eye maintains a dazzle reflex or indirect PLR (from the affected to the unaffected eye), surgical stabilization is indicated. If referral to a veterinarian with advanced ophthalmic microsurgical training is not an option, treatment with a systemic anti-inflammatory and a systemic antibiotic should be initiated.
The indications for treatment of any ruptured globe are based on the potential for maintenance of vision. Ophthalmic treatments for a ruptured globe are the same as those for a complicated corneal ulcer or partial-thickness corneal laceration and should be administered via SPL to avoid rerupture of the eye. Great care should be taken during SPL system placement to prevent globe rerupture. If the ruptured eye has no indication of light perception, enucleation is likely the most humane treatment option.
Corneal Stromal Abscesses in Horses
Etiology
Corneal stromal abscesses in horses arise after inoculation of the corneal stroma with fungi, bacteria, or both, either via microtrauma or upon reepithelialization of a contaminated or infected corneal ulcer. The infection then proliferates within the avascular corneal stromal and often moves deeper, in some cases penetrating through Descemet's membrane into the anterior chamber.
Clinical Findings
Stromal abscesses in horses generally cause moderate to severe ocular pain. Owners commonly report a painful eye (blepharospasm) with excessive tearing (epiphora). The key clinical sign is a focal or multifocal corneal stromal WBC infiltrate that usually appears white or yellow with "fluffy" (indistinct) margins.
Corneal abscesses are usually accompanied by reflex anterior uveitis, which can be severe. Fluorescein stain is not retained by the cornea, because the epithelium has regrown over the area of inoculation.
Stromal abscesses accompanied by reflex uveitis must be differentiated from primary uveitis, because the treatment of these two diseases is drastically different, and misdiagnosis can lead to vision loss and necessitate enucleation.
Pearls & Pitfalls
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Depending on the location of the abscess, corneal vascularization can occur in a focal area or be circumferential (see corneal stromal abscess image).
Courtesy of Dr. Kelly Knickelbein.
The key clinical signs of corneal stromal abscesses are as follows:
lack of fluorescein stain uptake
moderate to severe blepharospasm
focal corneal opacity (white/yellow cellular infiltrate, edema)
often severe reflex uveitis
corneal vascularization in some cases
Diagnosis
The diagnosis of corneal stromal abscesses is based on clinical signs that generally include acute onset of ocular pain, a focal or multifocal region of stromal cellular infiltrate, ocular pain, reflex uveitis, and lack of fluorescein stain uptake by the cornea.
The primary differential diagnosis for corneal stromal abscesses is immune-mediated keratitis, which can appear similar but lacks reflex uveitis and generally results in mild to no ocular pain. Immune-mediated keratitis generally has a more insidious onset and extensive corneal vascularization, indicating chronic disease.
Treatment
Ophthalmic antibiotic and antifungal
Pain management
Treatment of reflex uveitis
The medical management of corneal stromal abscesses is similar to that of complicated corneal ulcers. Drug penetration into the cornea is critical for effective treatment of corneal stromal abscesses; fluoroquinolones and chloramphenicol are the only antibiotics that adequately penetrate the cornea. An appropriate treatment regimen is as follows:
ophthalmic antibiotic every 2–4 hours (fluoroquinolone or chloramphenicol)
ophthalmic antifungal (azole such as voriconazole, miconazole, itraconazole/dimethyl sulfoxide) every 2–4 hours
ophthalmic atropine every 12 hours
systemic NSAID every 12 hours
systemic antibiotic (after the lesion is vascularized)
Uveitis in Horses
Etiology
Recurrent uveitis is the most common cause of blindness in horses (see uveitis image) and a devastating problem with substantial financial impact on the horse industry.
Courtesy of Dr. Kelly Knickelbein.
Common etiologies of uveitis in adult horses include trauma, inflammation secondary to corneal disease (ie, reflex uveitis), Leptospira sp–associated infection, other infectious etiologies, and immune-mediated disease. Uveitis can be a manifestation of systemic disease, so a thorough physical examination and diagnostic workup, including infectious disease testing appropriate for the geographical region and for the patient's travel history, is generally warranted.
Regardless of the initial inciting cause, recurrence of uveitis is common, and a plan for longterm monitoring and management should be discussed with the owner from the outset.
Uveitis in foals should always raise concern about underlying systemic disease, particularly sepsis in young foals. In older foals and weanlings, infection by Rhodococcus sp is a common cause of uveitis.
Clinical Findings
Clinical signs of uveitis in horses vary according to the severity and location of inflammation within the uveal tract. In general, these signs can include blepharospasm, epiphora, conjunctival and episcleral hyperemia, focal or diffuse corneal edema, keratic precipitates, miosis, aqueous flare (increased protein content in the aqueous humor), fibrin in the anterior chamber, hyphema, hypopyon, vitritis, and low intraocular pressure.
In horses with light-colored irides, a color change might be precipitated by serum leakage into the iris. When uveitis is chronic, additional findings can include iridal hyperpigmentation or pigment loss, corpora nigra atrophy, posterior synechiae (see synechiae image), pigment on the anterior lens capsule, cataract, lens subluxation or luxation, chorioretinal scars, retinal degeneration, retinal detachment, phthisis bulbi (shrunken, disorganized, blind eye), and glaucoma.
Courtesy of Dr. Kelly Knickelbein.
Diagnosis
Biochemical and serological testing
Complete physical examination
The key diagnostic findings confirming uveitis include aqueous flare and miosis, which are observed during a thorough ophthalmic examination.
It is critical to distinguish primary uveitis from reflex uveitis induced by a corneal ulcer or stromal abscess because treatment of the two varies drastically. Primary uveitis cases are fluorescein stain negative and have no corneal opacities consistent with cellular infiltrate. However, chronic uveitis can also predispose the horse to poor corneal health and corneal ulceration.
Pearls & Pitfalls
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For adult horses with uveitis, the diagnostic workup consists of the following:
thorough physical examination
CBC
biochemical profile
fibrinogen, serum amyloid A concentration
Leptospira microagglutination test on serum to assess exposure, as well as other infectious disease testing appropriate for the geographical region
For foals, the diagnostic workup consists of these components:
physical examination, including thorough assessment of joints, umbilicus, and lungs
CBC
biochemical profile
IgG concentration
fibrinogen, serum amyloid A concentration
thoracic ultrasonography
blood cultures
Treatment
Stabilization of blood-ocular barrier
Pain management
Accurate diagnosis and prompt treatment of uveitis in horses via administration of ophthalmic and systemic anti-inflammatories is critical to minimize permanent damage to the delicate intraocular structures. Without prompt treatment, uveitis can result in substantial intraocular changes (synechiae, cataracts, retinal detachment, glaucoma) that result in visual impairment or blindness.
Topical corticosteroids (prednisolone acetate 1% ophthalmic suspension or neomycin–polymyxin B–dexamethasone ophthalmic solution or ointment) every 6–8 hours are the mainstay of treatment. For severe cases, they can be combined with a topical NSAID (diclofenac, flurbiprofen, etc) at the same frequency.
Ophthalmic atropine solution or ointment is critical to the management of uveitis because it provides important pain relief from ciliary muscle spasms, dilates the pupil to aid in the breakdown and prevention of posterior synechiae, and helps to stabilize the blood-aqueous barrier. Atropine is generally administered every 12 hours until pupil dilation is achieved and ocular comfort restored; then it is tapered to the minimum frequency required to maintain pupil dilation and comfort, and it is continued until uveitis is completely controlled.
During treatment for uveitis, the horse should be monitored carefully for inadequate fecal output, anorexia, and signs of abdominal discomfort. Systemic NSAIDs are a staple of uveitis treatment because of their analgesic and anti-inflammatory properties; however, systemic steroids are indicated in severe cases of uveitis and can be administered in combination with systemic NSAIDs in select cases.
Glaucoma in Horses
Etiology
Glaucoma is a disease of the optic nerve that results in blindness. The most common cause of glaucoma in horses is elevated intraocular pressure resulting from an aqueous humor outflow obstruction secondary to recurrent uveitis. Other causes of glaucoma in horses exist but are not as well recognized.
Clinical Findings
The most common clinical sign of glaucoma in horses is corneal edema, which can be focal, geographic, or diffuse. Additional clinical signs include a partially or fully dilated pupil that is minimally responsive or nonresponsive to light stimulation.
With chronically elevated intraocular pressure, the globe enlarges (hydrophthalmos), and breaks in Descemet's membrane can be apparent as linear gray opacities (Haab striae; see glaucoma image). Eyes with glaucoma often show evidence of chronic or active uveitis. If the posterior segment is visible, optic nerve atrophy or degeneration may be evident.
Courtesy of Dr. Kelly Knickelbein.
The key clinical finding of glaucoma is elevated intraocular pressure with tonometry. Horses generally tolerate moderate elevations in intraocular pressure for longer periods than do other species, and they may maintain some vision, despite evidence of chronicity.
Diagnosis
Tonometry
Diagnosis of glaucoma is generally based on documentation of elevated intraocular pressure (> 25 mm Hg) via tonometry. Both applanation and rebound tonometry are commonly used to estimate intraocular pressure in horses. Rebound tonometry is generally thought to be more straightforward and more accurate, particularly in nonsedated horses.
Intraocular pressure in horses is greatly affected by head position. Measurements should be obtained with the horse’s head above the heart in a normal head carriage position. Sedation, pressure placed on the globe or the eyelids, and jugular compression also affect intraocular pressure readings.
Treatment
Antiglaucoma medications
Ophthalmic or systemic anti-inflammatories
The mainstay of glaucoma treatment is ophthalmic antiglaucoma medications, which decrease intraocular pressure via various mechanisms. The combination of dorzolamide hydrochloride (a carbonic anhydrase inhibitor) and timolol maleate (a beta blocker) is generally considered to be the most effective antiglaucoma medication in horses. This drug is commercially available as an ophthalmic solution and can be compounded into an ointment for ease of administration. Notably, antiglaucoma drugs that are quite effective in some other species are not as effective and cause irritation in horses (the prostaglandin analogs, such as latanoprost).
Ophthalmic anti-inflammatories are also a critical part of glaucoma management, because most cases are driven by underlying uveitis. Topical steroids are generally the most effective; however, topical NSAIDs are also appropriate.
The use of ophthalmic atropine to treat uveitic glaucoma in horses is controversial. For glaucoma in most species, atropine is contraindicated because of its potential to exacerbate pressure elevations by narrowing the aqueous humor outflow pathway. However, atropine was found to lower intraocular pressure in normal horses in one study, and there are anecdotal reports of its effectiveness in lowering intraocular pressure in equine glaucoma cases (1, 2). Therefore, atropine should be used on a case-by-case basis.
Glaucoma can often be managed, but it cannot be cured. A blind eye with uncontrolled glaucoma should be enucleated to restore the horse's comfort.
Subpalpebral Lavage System Placement in Horses
Subpalpebral lavage (SPL) systems enable the administration of ophthalmic solutions to the eye without the need to open the eyelids and treat the eye directly (see images showing an SPL system and an SPL system protected with a spandex hood).
Courtesy of Dr. Kelly Knickelbein.
Courtesy of Dr. Kelly Knickelbein.
SPL systems are well tolerated by most horses, make treatment easy for the caretaker, and ensure that medications reach the ocular surface. Importantly, treatment via SPL systems can improve patient comfort by obviating the need for frequent eyelid manipulation in a painful eye.
Drugs administered through SPL systems should be aqueous solutions. Suspensions lead to occlusion of the SPL catheter.
SPL systems can be placed in the inferior or superior eyelid; however, numerous studies show fewer complications with inferior-eyelid SPL (3, 4). After medications are instilled to the inferior fornix, blinking distributes them across the entire ocular surface.
Indications
Any horse with ocular disease in need of ophthalmic treatment is a candidate for SPL. Many horses with nonpainful eye disease and even some with painful eyes tolerate direct application of medications to the ocular surface well. However, SPL should be considered when the horse is intolerant of ophthalmic medication application.
Horses with fragile globes, those that require frequent doses of ophthalmic medications, and those that undergo corneal or intraocular surgeries are generally best treated with SPL.
Supplies
The following supplies are needed for insertion of an SPL system:
an SPL kit, which includes the SPL attached to a trocar, a 20-gauge, 3.2-cm (1.25-inch) catheter; a wooden tongue depressor (which should be covered with white tape); and an injection port
sterile gloves
proparacaine or tetracaine 0.5% ophthalmic solution
lidocaine or mepivacaine 2% injectable anesthetic
diluted povidone-iodine solution
4×4 gauze
sedative(s)
small rubber bands to braid the mane
white tape
nonabsorbable suture
suture scissors
Preparation for Placement
The preparatory steps for SPL placement are as follows:
Braid the forelock and mane.
Sedate the patient.
Prepare the periocular skin with diluted povidone-iodine solution.
Administer local anesthesia:
Prepare the ocular surface with diluted povidone-iodine solution.
Placement
After all supplies have been assembled and the preparatory steps (see above) have been completed, the SPL system should be placed as follows, using sterile technique:
Test the horse’s tolerance of pressure on the intended site of placement with a gloved finger. If pressure is not well tolerated, place a nose twitch and/or administer additional sedative doses.
Make sure the SPL line is well attached to the trocar and not tangled.
Guard the trocar with the dominant index finger to protect the globe, and position the point of the trocar in the conjunctival fornix (nasally, if placing in inferior eyelid; centrally, if superior). Make sure the trocar does not penetrate the conjunctiva close to the lid margin.
Push the trocar through the eyelid skin at the site of local anesthetic infiltration with the dominant hand, and grasp the trocar as it exits the skin with the nondominant hand to pass it through the thick eyelid skin while continuing to protect the globe with the index finger of the dominant hand.
Keep the SPL line clean while pulling it through the eyelid skin until the footplate is settled in the fornix. Check placement with a gloved finger to make sure the footplate will not contact the cornea.
With the SPL line attached, pass the trocar through the braids of the forelock and mane, and then remove the trochar.
Insert the catheter into the end of the SPL line by retracting the stylet just behind the tip of the catheter and feeding the line onto it.
Place an injection cap onto the catheter.
Secure the catheter end to the provided tongue depressor (wrap the depressor in white tape so that it does not splinter if broken), and secure it to the braid near the withers with white tape.
Make sure the line is not under tension when the horse lowers its head fully to the ground.
Key Points
Because clinical signs of ocular disease are nonspecific, emergent evaluation of painful eyes is always indicated in horses.
Acute periocular swelling, eyelid lacerations, corneal ulcers, corneal stromal abscesses, blunt or sharp trauma to the globe, uveitis, and glaucoma are each considered ophthalmic emergencies.
Subpalpebral lavage systems are often indicated for the treatment of ocular disease. They are well tolerated by most horses and simplify treatment.
For More Information
Dwyer A. Ophthalmic emergencies in the field. Vet Clin North Am Equine Pract. 2021;37(2):441-460. doi:10.1016/j.cveq.2021.04.011
Gilger BC, ed. Equine Ophthalmology. 4th ed. John Wiley & Sons; 2022.
Also see pet owner content regarding eye emergencies.
References
Mughannam AJ, Buyukmihci NC, Kass PH. Effect of topical atropine on intraocular pressure and pupil diameter in the normal horse eye. Vet Ophthalmol. 1999;2(4):213-215. doi:10.1046/j.1463-5224.1999.00081.x
Herring IP, Pickett JP, Champagne ES, Troy GC, Marini M. Effect of topical 1% atropine sulfate on intraocular pressure in normal horses. Vet Ophthalmol. 2000;3(2‐3):139-143. doi:10.1046/j.1463-5224.2000.3230139.x
Giuliano EA, Maggs DJ, Moore CP, Boland LA, Champagne ES, Galle LE. Inferomedial placement of a single‐entry subpalpebral lavage tube for treatment of equine eye disease. Vet Ophthalmol. 2000;3(2‐3):153-156. doi:10.1046/j.1463-5224.2000.3230153.x
Quéré E, Chahory S. Outcomes and complications associated with superior and inferior subpalpebral lavage treatment systems placed in 61 equine eyes (2004–2021). J Equine Vet Sci. 2022;117:104076. doi:10.1016/j.jevs.2022.104076
