Can vision problems cause a horse to spook more?

Q:My 17-year-old gelding is becoming increasingly spooky as he ages and I’m wondering if he could have a vision problem. He seems particularly wary of places where footing changes color—like onto a shadowy spot on the ground. He wasn’t like this when he was younger. My veterinarian has looked at my horse’s eyes and says he doesn’t see anything obviously amiss. Are there standard “eye tests” for horses that can determine how well a horse sees or a way to detect a loss in visual acuity?

A:Spooking is a relatively common reason for calls to primary veterinarians and veterinary ophthalmologists. Many eye abnormalities may contribute to this behavior, including:

• opacities of the cornea, the normally clear ‘window’ at the front of the eye,

• inflammation inside of the eye (uveitis),

• cysts arising from the corpora nigra, the part of the iris that functions to limit light entering the back of the eye,

• opacities of the lens (cataracts),

• changes to the normally clear gel (vitreous humor) that fills the back of the eye, or to the sensory structures that detect light and transmit signals to the brain to allow for vision (the retina and optic nerve).

If a standard ophthalmic examination fails to reveal any eye abnormalities but a visual deficit is suspected, a veterinary ophthalmologist may conduct additional tests. For example, an electroretinogram (ERG) can be used to assess retinal function. Typically done with the horse sedated, an ERG involves placement of a contact-lens electrode on