Discussion

The equine vitreous and fundus remain a relatively unexplored region in the horse. There are an increasing number of surveys published but little in the form of histopathology to back up the mainly presumptive diagnoses made in the surveys, case reports and other literature.
In addition to a lack of definitive description of vitreal and fundic lesions there is little evidence to correlate these findings with their effect on vision. As a consequence it may be assumed that attributing significance to abnormalities is fraught with difficulty. Despite this a good knowledge of the anatomy and relevant neurological tests can allow the examiner to make a more effective assessment of any lesions and their role in causing visual deficits.

Vitreal anatomy.
The vitreous is an amorphous clear gel composed of water, collagen fibrils, glycoproteins and the occasional hyalocyte and fibroblast. The primary vitreous contains the hyaline vessels that supply the developing lens. The secondary vitreous forms the bulk of the vitreal structure and the tertiary vitreous differentiates to form the lens zonules.

Vitreal disease.

Congenital.
Remnants of the hyaloid vasculature may persist into adulthood and sometimes the vessels may be patent.

Acquired.
The vitreous acts as a conduit for cellular migration from the choroid to the posterior lens capsule during inflammation and uveitis will promote this cellular activity. Colour change and opacity are indicators of this process. Other vitreal pathology may occur as a result of disease in adjacent tissues. Changes seen include haemorrhage or pigment shedding from the ciliary body or prolapse of lens material from a posterior lens capsule rupture. Because of the slow turnover of vitreal material, changes that would be considered acute in other tissues may be much more chronic or historic in nature. Ageing changes seen include cholesterol crystals or mineral-lipid complexes within the vitreous.

Fundus anatomy.
From the vitreous the layers of the retina are: inner limiting membrane, nerve fibre layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, outer limiting membrane, photoreceptor cell layer, retinal pigment epithelium (RPE). Deeper to this the choroid supplies nutrition to the neurosensory retina through the RPE.
The optic nerve head is the visual portion of the passage of nerves from the nerve fibre layer through the sclera and away from the eye. Due to the thickness of the retina at this point the choroid is unable to supply the nerve fibre layer with oxygen and an alternative blood supply is required. This is visible as 30- 60 blood vessels arising from the edge of the optic nerve head and extending 1- 2 optic nerve head widths away. These blood vessels are usually not present at the 6 o'clock position. It lies within the nontapetal fundus.

Many photoreceptors supply the outer nuclear layer which in turn converges to supply the ganglion cells layer. Due to this converging system a single ganglion cell and therefore nerve fibre to the brain may take its signal from many photoreceptors. By measuring ganglion cell density a rough estimate of the most important areas of the fundus can be made. This has led to the identification of the visual streak horizontally positioned dorsal to the optic nerve head. How this signal is processed by the brain and which area has the highest visual acuity has yet to be described.

Fundus disease.

Congenital.
Dysplasia associated with multiple ocular abnormalities, retinal detachment and haemorrhage associated with parturition, and colobomas are readily identified on examination, optic nerve hypoplasia is more challenging to identify, and congenital stationary night blindness cases will have a normal fundus.

Acquired.
Chorioretinitis is the description given to a range of inflammatory diseases which may involve the choroid or retina or a combination of the both. Active chorioretinitis in the horses is rarely described as it requires examination to be performed at the time of active inflammatory disease. Inactive chorioretinitis describes the appearance of the fundus post inflammation. Changes include hyperpigmentation in either focal or linear distributions, retinal vasculature degeneration, tapetal hyperreflectivity and choroidal depigmentation.

Retinal detachments.
Non-rhegmatogenous or bullous detachments can occur as a result of inflammation or trauma leading to sub-retinal exudate formation, sub-retinal haemorrhage, over-hydration, hypoproteinaemia or neoplasia. Rhegmatogenous detachments occur due to retinal tears occurring as a result of vitreal traction band formation, lens luxation or trauma

Senile retinopathy.
This has been described as depigmentation of the RPE within the nontapetal fundus.

Ischaemic optic neuropathy.
This is recognised as a result of a failure of blood supply to the optic nerve.

Traumatic optic neuropathy.
Head trauma may lead to optic nerve degeneration due to either a compressing fracture of, or haemorrhage at, the base of the skull, or neuropraxia of the optic nerve due to sudden movement of the brain.

Exudative neuropathy.
This has been described as a protrusion of white/grey material with or without haemorrhages. It appears to be clinically similar to the appearance of the optic nerve head in ischaemic disease a few days after the initial insult.

Optic nerve atrophy.
This is seen as a result of chronic conditions causing compression of the optic nerve, the optic nerve head (glaucoma) or inflammation.

Equine motor neurone disease.
This has an appearance similar to senile retinopathy

Proliferative optic neuropathy.
This has been described on a number of occasions in the literature as a pink fleshy lobulated mass arising from the edge of the optic nerve head. It is thought to be a schwannoma like mass with a high concentration of lipid laden glial cells.

Astrocytomas
These are similar in appearance to proliferative optic neuropathy but have been described as arising within the fundus as well as at the edge of the optic nerve head. They tend to be smaller in appearance and not lobulated.

Medulloepitheliomas and neuroepitheliomas
These have been described in young horses and are seen as pink vascular masses that involve the optic nerve and are associated with blindness.

Melanomas
These have been described in association with anterior melanomas.