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Glaucoma is an eye condition in horses that is characterized by increased intraocular pressure. [1] Glaucoma develops when there is impaired 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 identify