Do We Need to Worry about Retinal Abnormalities?

Examination of the equine fundus can be challenging, and proficient detection of a lesion requires suitable equipment and an understanding of normal variation. Indirect ophthalmoscopy necessitates the use of a light source and condensing (‘Volk’) lens to generate a wide-field, virtual, inverted image. Direct ophthalmoscopy produces a magnified, upright, real image for the examiner and can be used to provide detailed assessment of localised lesions. The administration of topical tropicamide will induce mydriasis and increase the field of view during fundoscopy.

The fundus includes the optic nerve head, neurosensory retina and choroid. The equine retina is paurangiotic with 50–80 small peripapillary retinal vessels radiating from the elliptical optic disc. The majority of the equine fundus is devoid of retinal vessels and retinal tissue relies on the underlying choroid for nutritional support. The optic disc should be pale pink, and a small pyramidal ‘notch’ at 6 o’clock on the optic disc is a normal finding.

The choroid is composed of the choriocapillaris, choroidal vessels, tapetum, and pigmented tissue. Tapetal colour can be related to the coat and iris colour of the horse and will range within yellow, blue and green hues. The nontapetal fundus is dark brown due to retinal pigment epithelium, but can be absent in part or in its entirety with variations of iris and coat colour patterns. The choroidal vascular pattern can often be visualised in subalbinotic and albinotic eyes, and the tapetum may be absent.

Classification of fundic abnormalities can be congenital, acquired, primary ophthalmic, secondary to systemic disease, active and inactive. Accurate identification of a lesion and its location within the fundus can aid the examiner to determine the possible pathogenesis and prognosis for the patient. Chorioretinopathy lesions can range from incidental isolated focal ‘bullet-hole’ lesions through to extensive peripapillary chorioretinitis. Chorioretinitis can be a manifestation of equine recurrent uveitis or systemic disease and lesions within or close to the optic disc will have a greater impact on vision, compared with those in the peripheral fundus. Retinal detachment, congenital stationary night blindness, equine motor neuron disease, retinal degeneration and optic neuritis are all conditions that can limit the vision of a patient.

If a lesion is detected within the equine fundus we must consider the current and future impact on vision, and that the loss of retinal function can affect behaviour and safety for the horse and the client.

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