Fundus examination of the horse.

This article outlines various conditions affecting the fundus of horses, their symptoms, and associations. The conditions discussed include retinal hemorrhages in newborns, retinal detachments, inflammation, optic neuritis, and congenital stationary night blindness.

Understanding the Normal Equine Fundus

• The fundus (back part of the eye) in horses has a specific type of structure called paurangiotic, marked by about 30 to 60 small retinal blood vessels radiating from the optic disc.
• The optic disc itself is elliptical and typically exhibits a salmon-pink color.
• This study emphasizes the need to visualize the optic disc for accurate examination.

Conditions Affecting the Fundus

• Newborn foals may experience retinal hemorrhages due to trauma during their birth. These hemorrhages usually originate from the choroidal circulation and often resolve with no lasting visual damage within a week or so.
• Retinal detachments can occur at the weakly attached points which are between the retinal photoreceptors and the retinal pigment epithelium. Detachments are predominantly associated with choroiditis, leading to the buildup of exudative fluid and inflammatory cells.
• Inflammation of the retina, known as chorioretinitis, seldom happens without the accompanying inflammation of the choroidal vasculature. This inflammation can cause necrosis of photoreceptor cells, pigment cell layer hyperplasia, and resultant atrophy, all leading to a loss of retinal functionalities.
• Optic neuritis is generally associated with chorioretinitis. In this condition, the optic disc appears swollen and there’s usually significant visual loss. This is often due to the vitreous humor filling up with proteins and inflammatory cells which obstruct clear fundus visualization.

Specific Disorders

• Congenital stationary night blindness (CSNB) is a condition seen in Appaloosa horses, which resembles the Schubert-Bornschein type of the disorder in humans.
• This disorder means the horse struggles with night vision, however, the fundus looks normal when examined.
• An electroretinogram (ERG) shows certain irregularities like the absence of a normal scotopic β wave and no increase in β-wave amplitude with further dark adaptation.