Glaucoma is an eye condition in horses that is characterized by increased intraocular pressure. [1] Glaucoma develops when there is an imbalance in fluid drainage at the front of the eye, leading to increased pressure.

In horses, glaucoma may be caused by inflammation, traumatic injuries, and improper development of the eye. Symptoms can be subtle; the main symptom is haziness or cloudiness in the eye, but many horses also experience squinting, tearing, and redness of the eye.

Without treatment, glaucoma can lead to blindness in horses by damaging the retina and optic nerve, two structures that are involved in vision.

Glaucoma is diagnosed using a specialized device that measures intraocular pressure called a tonometer. Treatment may include topical medications, surgical correction, or enucleation (complete removal of the eyeball).

The long-term prognosis for vision is poor in eyes with glaucoma, as treatment typically only slows the progression of disease. However, many horses manage well with loss of vision in one or both eyes with appropriate lifestyle and management changes.

Glaucoma in Horses

Glaucoma in horses is characterized by increased pressure within the eye, leading to potential vision loss and other ocular complications.

This elevated pressure can compromise blood flow to the retina and optic nerve, resulting in progressive damage to these ocular structures.

Equine Eye Anatomy

Given the high rates of occurrence of eye injury and disease in horses, it can be helpful for horse owners to familiarize themselves with eye anatomy.

horse-eye-anatomyIllustration:

Intraocular Pressure

The eye contains fluid called aqueous humour, which is a water-like substance found at the front of the eyeball. [1] This fluid maintains a specific fluid pressure, which is how the eyeball holds its spherical shape within the orbital socket. [1]

The ciliary body, a structure found behind the iris (colored part of the eye), produces aqueous humour continuously. [1]

The aqueous humour flows through the pupil towards the front of the eye, into the anterior chamber between the cornea (the clear front surface of the eye) and the lens (the structure that directs light to the retina). [1]

The fluid drains out of this chamber through the iridocorneal angle, the junction between the iris and the cornea. [1]

Constant production of aqueous humour means that drainage through the iridocorneal angle must be maintained to prevent fluid buildup. [1] If drainage is impaired, the fluid pressure within the anterior chamber can increase, resulting in symptoms of glaucoma.

Effect of Glaucoma

As the intraocular pressure increases, the structure of the eyeball distorts significantly. [1] The increased pressure also compresses small blood vessels and nerves within the eye, resulting in functional disturbances. [1]

The most significant outcomes of glaucoma in horses are: [1]

  • Retina: Damage or death of the retina, the layer of the eye where visual signals are received
  • Optic Nerve: Damage to the optic nerve, the main nerve transmitting visual signals from the retina to the brain

The combination of these effects can ultimately result in vision impairment or blindness. [1]

Causes

The underlying cause of glaucoma is an abnormality in aqueous humour drainage, resulting in increased fluid and pressure within the anterior chamber. [1]

In horses, there are several possible causes of poor aqueous humour drainage, including: [1][2]

  • Scarring after inflammation within the eye
  • Active inflammation causing white blood cells to clog the iridocorneal angle
  • Disruption of the iris structure, causing obstruction of the iridocorneal angle
  • Lens luxation (dislocation), where the lens moves into the anterior chamber
  • Improper development of the iridocorneal angle preventing drainage
  • Traumatic injuries damaging structures within the eye

The most common cause of glaucoma in horses is likely equine recurrent uveitis (ERU), an inflammatory condition where the immune system attacks structures within the eye. [1] Studies show that between 85-91% of horses with glaucoma have equine recurrent uveitis. [1]

Risk Factors

There are no specific risk factors for developing glaucoma, other than increasing age. [3] Horses over the age of 15 have an increased risk of glaucoma compared to younger horses. [3]

However, there are specific risk factors identified for equine recurrent uveitis. Common risk factors for developing ERU include: [4]

Symptoms

The initial symptoms of glaucoma can be subtle and may be missed by owners until more severe signs develop. [1]

Common initial symptoms in horses include: [1][5]

  • Hazy appearance to the eye
  • Squinting
  • Tearing
  • Swelling of the eyelids
  • Redness of the eye
  • Avoiding bright light
  • Prominent vessels on the whites of the eyes

Severe Cases

As the disease worsens, symptoms may include: [1][3][5]

  • Enlarged eye that may protrude significantly
  • Linear streaks across the cornea
  • Cloudy appearance to the eye
  • Large pupil that does not contract in response to bright light
  • Blood vessels on the eye surface
  • Corneal ulcers due to inability to completely close eyelids around the enlarged eye

Unlike other species, most horses diagnosed with glaucoma still have some vision. [3][5] One study showed that 41 out of 42 eyes with glaucoma still had vision present at the time of diagnosis. [3] However, blindness can develop in severely affected eyes if left untreated. [3][5]

Obvious signs of vision loss require emergency veterinary attention. Urgent medical intervention provides the best chance of preserving vision.

Diagnosis

Glaucoma is diagnosed using a tonometer, a device that measures intraocular pressure. [1] These devices push a soft probe against the eye surface and measure the amount of resistance. [6]

Some horses require sedation and nerve blocks to prevent eyelid closure for a successful tonometry reading. [6]

After identifying the presence of glaucoma, a thorough investigation is necessary to determine the cause. [1]
Further diagnostics may include: [1]

  • Ophthalmic examination, including specific evaluation of the iridocorneal angle
  • Ultrasound of the eye

Referral to a specialist may be required in some cases.

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Treatment

The goal of glaucoma treatment is to reduce intraocular pressure. [1] By reducing pressure, the damage to the retina and optic nerve is minimized and the maximum amount of vision is preserved. [1]

There are several treatment options for glaucoma, including eye medications, surgical intervention, or enucleation (removal of the eye). [1] Factors that may influence the chosen treatment plan include: [1]

  • Level of remaining vision in the affected eye
  • Presence of other eye conditions or complications
  • Age of the horse
  • Work demands for the horse
  • Cost and budget
  • Long-term feasibility of administering medication multiple times per day

Medical Treatment

The medications used for treating glaucoma are typically topical (applied to the eye surface), which some horses do not tolerate well. [6] The frequency of administration varies between medications but is usually between one to three times a day. [6]

Veterinarians may place a subpalpebral lavage system to facilitate medication administration. These systems allow the owner to administer medications through from a greater distance than directly applying the medication to the eye.

In a subpalpebral lavage system, a tube is placed through the eyelid then run along the head and neck, where the owner can dose medication through the open end. [1]

There are several medications available for treating glaucoma, each with a different function. Some veterinarians may combine these medications to maximize their effects. [6] Functions of glaucoma medications include: [6]

  • Reducing aqueous humour production
  • Increasing aqueous humour drainage through alternative routes
  • Improving blood flow within the eye
  • Anti-inflammatories to reduce the effect of scarring

The long-term prognosis for medications alone is poor, with most horses eventually requiring surgery to control their glaucoma. [2][3] Glaucoma in Appaloosa horses appears particularly difficult to control with medications only. [2]

Surgical Treatment

There are several surgical options available for glaucoma. The choice of surgery primarily depends on whether the affected eye has retained any vision at the time of treatment. [1]

Surgery on eyes with vision aims to restore normal intraocular pressure and maintain vision through targeted, minimally invasive treatment. [1]

Blind eyes are considered for more aggressive surgical treatment since vision does not need to be preserved. [1] In these cases, surgery focuses on reducing pain associated with glaucoma by removing the obstruction causing pressure buildup. [1]

Laser cyclophotocoagulation

Laser cyclophotocoagulation is a method where the surgeon uses a specialized laser to destroy the ciliary body. [1] With a damaged ciliary body, the eye produces less aqueous humour, restoring normal pressure within the anterior chamber. [1]

Laser cyclophotocoagulation has a high success rate, with 70% of eyes showing a reduction in intraocular pressure at 6 months after treatment. [1][7] 58-60% of eyes retain vision after laser treatment. [6]

Currently, laser cyclophotocoagulation is the gold standard for treating glaucoma surgically in cases of glaucoma where the affected eye has preserved some vision. [6]

Gonioimplants

Gonioimplants are implants placed within the iridocorneal angle to restore drainage. [1][8] These implants are typically a short-term solution, as they typically become blocked by scar tissue, rendering them ineffective. [1][9]

However, they can be a useful addition to other surgical procedures to immediately reduce intraocular pressure. [1]

Gonioimplant use is uncommon in horses, however case reports show that they can effectively reduce intraocular pressure for up to 400 days after surgery. [8][9]

Ciliary Body Ablation

Ciliary body ablation uses an irritating medication to destroy the ciliary body tissues. [6] Veterinarians only perform this procedure in blind eyes, as the medication can also cause damage to other structures within the eye. [1]

In this procedure, the medication is injected directly into the eyeball. Some horses may require multiple injections to successfully reduce intraocular pressure and pain. [6]

Ciliary body ablation can be painful when the irritating substance takes effect. [1] Most veterinarians use anti-inflammatory medications to provide some pain control during and after the procedure. [1]

The appearance of the eye after ciliary body ablation varies. [6] The end result is typically some degree of phthisis bulbi, shrinking of the eye as a whole. [1]

Enucleation

Horses that do not respond to medication or surgical treatment often require enucleation (complete removal of the eyeball) to be comfortable. [1] Enucleation is also the preferred treatment for horses with tumors within the eye, infections, or other eye conditions in addition to glaucoma. [1]

Owners with cost concerns may also elect to perform enucleation as a first treatment, as it is a single procedure that usually does not require ongoing expense. [1]

Surgeons can perform enucleation under standing sedation or general anesthesia. [1] During the procedure, the surgeon carefully dissects around the eye, leaving the structure intact. After removing the eye, the surgical site is sutured closed. [1]

Eye Implants

Surgeons or owners may choose to place a prosthetic implant within the surgery site that resembles the shape of the eye. [1] While implants may be preferable for cosmetic reasons, they carry a higher risk of complications including surgical site infections and separation of the incision. [6]

There are two main types of implants: intrascleral (within the existing eye) and intraorbital (implant replaces the eye completely). [6]

Intrascleral implants are more difficult to place, as the surgeon must open the existing eye structure, remove all of the contents, insert the implant, and close the incision. [6] The final result is a cloudy or opaque eye that otherwise retains its normal appearance. [1]

Intraorbital implants are balls of silicone that are a similar shape and size to the original eye. [6] The surgeon places these implants within the orbit (bony cavity where the eye sits) after the eye is removed. [6]

These implants can provide a better cosmetic appearance by preventing the skin and tissues surrounding the eye from sinking into the skull. [1][5][6]

Prognosis

The prognosis for vision in horses with glaucoma is poor. [6] Although medical treatment or surgical intervention can reduce intraocular pressure to some extent, damage to the retina and optic nerve from glaucoma often continues. [6][10]

Eventually, horses develop blindness and may require enucleation or more aggressive surgical intervention to prevent pain and discomfort. [6][10]

Managing Blind Horses

Horses typically manage very well with blindness in either one or both eyes. [11] After a few days to weeks, many horses learn to cope with their blindness and adapt to their environment. [11]

Management strategies for blind horses include: [11]

  • Setting a predictable daily routine
  • Keeping the horse in a familiar environment
  • Pairing the horse with a familiar horse or animal that is not aggressive
  • Advising individuals handling the horse on safe handling practices
  • Ensuring the horse’s environment is free from nails, sharp objects, etc.
  • Placing gravel around immovable objects like trees or fences to alert the horse of an obstacle

Summary

Glaucoma in horses is characterized by increased aqueous humour within the eyeball, resulting in increased intraocular pressure.

  • Causes of glaucoma in horses include inflammatory diseases, traumatic injuries, and improper development of the eye
  • Symptoms of glaucoma include a hazy or cloudy appearance to the eye, squinting, tearing, and blindness
  • Treatment of glaucoma in horses can involve medications, surgery, or removal of the eye itself
  • Glaucoma is a progressive disease that typically results in blindness even with appropriate treatment

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References

  1. Wilkie, D. A. et al. Glaucoma, in Equine Ophthalmology. 1st ed. Ed. B. C. Gilger. Wiley, 2016, pp. 453–468.
  2. Ollivier, F. J. et al. Equine Glaucomas: A Review. Equine Veterinary Education. 2009.
  3. Wilkie, D. A. Equine Glaucoma: State of the Art. Equine Veterinary Journal. 2010.View Summary
  4. Kingsley, N. B. et al. A Review of Investigated Risk Factors for Developing Equine Recurrent Uveitis. Veterinary Ophthalmology. 2023.View Summary
  5. Ollivier, F. and Monclin, S. Equine Glaucomas. Equine Veterinary Education. 2010.
  6. Michau, T. M. Equine Glaucoma. Veterinary Clinics of North America: Equine Practice. 2017.View Summary
  7. Whigham, H. M. et al. Treatment of Equine Glaucoma by Transscleral Neodymium:Yttrium Aluminum Garnet Laser Cyclophotocoagulation: A Retrospective Study of 23 Eyes of 16 Horses. Veterinary Ophthalmology. 1999.View Summary
  8. Wilson, R. et al. Use of a Baerveldt Gonioimplant for Secondary Glaucoma in a Horse. Equine Veterinary Education. 2015.
  9. Townsend, W. M. et al. Feasibility of Aqueous Shunts for Reduction of Intraocular Pressure in Horses. Equine Veterinary Journal. 2014.View Summary
  10. Cullen, C. L. and Grahn, B. H. Equine Glaucoma: A Retrospective Study of 13 Cases Presented at the Western College of Veterinary Medicine from 1992 to 1999. Can Vet J. 2000.View Summary
  11. Dwyer, A. E. Management of Blind Horses, in Equine Ophthalmology. 1st ed. Ed. B. C. Gilger. Wiley, 2016, pp. 629–648.