Squinting and photophobia can affect one or both eyes simultaneously. Sometimes both eyes are squinty even if only one eye is painful. However, the squinting is usually more dramatic in the diseased eye. It is important to monitor pets closely for subtle behavioral changes since it is instinctual for many animals to withdraw and become more reclusive when they are experiencing eye pain or discomfort.

Numerous ophthalmic diseases result in eye pain. Diseases involving the eyelids, cornea, conjunctiva, iris and front chamber of the eye, and the orbit result in the most overt signs of eye pain. A thorough ophthalmic and physical examination is necessary to determine which diagnostic tests are needed to determine the cause of the pain.

In younger animals, ocular trauma, inherited or congenital diseases, and infections are common causes of eye pain. In older pets, chronic inflammations, immune-mediated and neoplastic (cancerous) diseases may be more common.         

Causes

The following are the most important causes of eye pain:

Proptosis, or forward displacement of the eye out of the orbit following some sort of trauma

Corneal scratches, lacerations and foreign bodies

Corneal injuries that result in surface abrasions and ulcers

Recurrent or chronic corneal ulcers that arise from healing defects within the cornea

Inward rolling of the eyelid margins (entropion), resulting in hair rubbing against the cornea

An extra row of eyelashes growing from the edge of the eyelid (distichiasis) and rubbing on the cornea

Eyelashes growing out from underneath the eyelid (ectopic cilia) and rubbing on the cornea

Eyelids injuries and infections

Sudden or gradual decline in tear production (keratoconjunctivitis sicca) resulting in drying of the eye

Certain forms of inflammation of the cornea (keratitis), especially those that interrupt the top layer of the cornea

Conjunctival scratches, lacerations and foreign bodies

Exposure of the eye to chemicals, smoke, fire, and other noxious materials

Anterior uveitis from a variety of causes (anterior uveitis)

Penetrating trauma to the eye

Bleeding in to the eye (hyphema)

Forward dislocation of the lens (luxation), with the development of acute glaucoma

Glaucoma (elevated pressure within the eye), especially when the pressure rise is sudden in onset

Orbital diseases, such as orbital inflammation or infection, orbital abscess, nasal infection or disease with extension into the orbit

Trauma and fractures to the bones around the eye