Dilated Pupils (Mydriasis) in Horses

Key takeaways

Dilated pupils (also referred to as mydriasis) describes when the hole in the middle of the iris of the eye is very large. Pupils normally change size in reaction to light changes or a particular emotional state such as anxiety or excitement. 

  • If the pupil is constantly dilated, it could be a sign of eye, nerve, or brain disease
  • In horses, common causes of abnormal pupil dilation include eye tumors, uveitis, glaucoma, and cataracts
  • Other possible causes include brain tumors or injuries, rabies, and cyanide poisoning
  • Diagnostics consist of a physical and ophthalmic examination
  • Treatment varies depending on the underlying condition
  • Some conditions can be managed with medication, while others may require removal of the affected eye
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A closer look: Dilated Pupils (Mydriasis) in Horses

The pupil is the hole in the middle of the cornea which allows light into the eye. The size of the pupil is controlled by muscles in the iris (the colored part of the eye). The pupils dilate and constrict normally in response to changing light levels and some physical states such as during fear or stress. Dilated pupils are an uncommon symptom in horses.

In most cases, dilated pupils result from stress or anxiety as a normal change. In cases of normal pupil dilation, the pupils respond to light and return to normal size once the stressor is removed or light levels shift again.

If dilation of the pupils continues for long periods of time, more serious conditions such as eye disease, optical nerve damage, or tumors are more likely.

Continuously dilated pupils warrant prompt veterinary attention as there is a risk of permanent blindness associated with some possible underlying causes.

Possible causes

Risk factors

The severity of mydriasis varies depending on whether the pupils respond to light. If there is minimal constriction of the pupils in bright light, it usually is an indicator of complete blindness in the eye.

In horses, complete blindness is typically related to retina or optic nerve diseases where the eye can no longer transmit signals to the brain appropriately.

Horses that show some pupil constriction in response to bright light likely have some vision remaining, however conditions associated with dilated pupils may rapidly lead to blindness if left untreated.

Testing and diagnosis

The diagnostic process usually involves a complete physical and ophthalmic examination. Tests include:

  • Pupillary light reflex
  • Vision testing
  • Intraocular (within the eye) pressure testing

Other tests might be needed if other conditions involving the brain or the optical nerve are suspected.

Treatment largely depends on the underlying condition. Topical medications such as eye drops might be prescribed. In some cases, removal of the eye is necessary.

Similar symptoms

Dilated pupils as a symptom can be mistaken for the normal process of dilation of the pupil in situations of low light or when the horse is excited or anxious.

Associated symptoms


No Author - Writing for Horse Side Vet Guide
The Brooke - Writing for The Working Equid Veterinary Manual

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