The posterior segment of the eye comprises the vitreous, retina, optic nerve and choroid, all contained within the fibrous continuation of the cornea - the sclera. The clear gel-like vitreous humor behind the lens makes up the greatest volume of the eye, maintains the ocular shape and helps to keep the retina attached in the eye. The neurosensory retina lines the back of the eye and is responsible for light absorption and conversion to nerve potentials which are transmitted out of the eye and back to the brain via the optic nerve. The choroid is a dense layer of blood vessels behind the retina which provides a large part of the nutrition to the metabolically active retina.
We see (or recognize and understand) fewer diseases of the retina and adjacent structures in the horse than in people or small animals. The most important chorioretinal (choroid and retina combined) disease is probably chorioretinitis (inflammation of the choroid and retina) seen as a component of equine recurrent uveitis
. Chorioretinitis can also be caused by various viral, bacterial, and protozoal infections. Chorioretinitis may be detected during an ophthalmic examination and can be either active or may appear as scarring once the inflammation has subsided (or been treated). Active inflammation or scarring (with areas of retinal degeneration) often result in varying degrees of loss of vision depending on severity.
Developmental abnormalities affecting the retina and ciliary body are seen as an inherited problem in the Rocky Mountain Horse (and most likely some other equine breeds). Affected animals have retinal dysplasia (folds in the retina), retinal detachment, iris cysts and cataracts. No treatment is available but screening of potential breeding animals makes sense to try and reduce the incidence of affected animals in the population.
Congenital stationary night blindness (CSNB) is an inherited disease seen in Appaloosas. It is present at birth and manifests as visual impairment in dim light. It rarely progresses to affect day vision.
Retinal detachment - separation of the retina from the back of the eye occurs most often associated with the inflammation seen with ERU but can also occur after ocular trauma (including intra-ocular surgery). Retinal detachment causes blindness in the affected eye and can be very difficult to treat in the horse.
The optic nerve can become inflamed ( optic neuritis) as part of ERU and viral infections resulting in visual loss. Chronic inflammation of the nerve results in degeneration (optic nerve atrophy). Stretching of the optic nerve causing acute and usually irreversible blindness can occur in horses which rear up and fall backwards hitting the back of the head on the ground.
Ischemic optic neuropathy occurs associated with occlusion of major arteries in the head as part of a surgical procedure to treat guttural pouch infections - blindness occurs in the eye on the surgically treated side of the head. Exudative optic neuritis is seen in older horses which develop optic neve swelling and hemorrhage - the cause is unknown but the condition causes blindness. Older horses can also develop proliferative optic neuropathy - white masses gradually develop over the edge of the optic nerve head - these do not affect vision