The retina is a multilayered structure which is basically an extension of the central nervous system (CNS). It provides the opportunity to directly examine not only a portion of the CNS but also, in many species, blood vessels. This highly specialised neuronal structure consists of a complex of neuronal cells, the outer layer of which houses the photoreceptors. Photoreceptors are ciliated cells; they have an outer segment which is connected to inner segments by a cilium which transports proteins to and from the outer segment. Phototransduction occurs on membrane discs within the photoreceptor outer segment. Light focused on to the outer segment layer results in high levels of oxidative stress meaning that slight metabolic imbalances that might be tolerated by cells elsewhere can lead to death of photoreceptors. The photoreceptors are end-differentiated cells which unfortunately means that they do not regrow and loss of the cells is permanent.

Degeneration of the retina results in deterioration of vision and, if extensive, even blindness. Focal areas of degeneration with remaining normal retinal regions may be seen in some conditions and may not have any detectable effect on vision (depending on the extent and positioning of the degenerate regions). Panretinal degeneration will result in loss of vision. Some progressive degenerative conditions may result in loss of specific photoreceptor types, most commonly rod photoreceptors. Loss of rods results in loss of dim light vision. The remaining cones may allow for vision under brighter lighting conditions although, typically, if rod photoreceptors are lost, cones will eventually also degenerate as they rely on a surrounding population of rods for their survival. There are many potential causes of retinal degeneration including environmental and inherited factors and in instances a combination of both.

Hereditary Retinal Degeneration

Of the inherited causes of retinal degeneration, progressive retinal atrophy (PRA) is recognised as the commonest form in purebred dogs. It occurs to a lesser extent in cats and even crossbred dogs. It is a genetically heterogeneous group of conditions potentially caused by mutations in many different genes. In dogs, autosomal recessive, autosomal dominant and X-linked forms are recognised, whereas in cats only the autosomal forms of inheritance have been described. The PRAs that have been investigated histopathologically have been further subdivided, usually based on the photoreceptor changes. In some forms the photoreceptor development becomes arrested and then degeneration starts to occur as with the rod-cone dysplasia forms of PRA. Mutations in three different genes are recognised as causing rod-cone dysplasia (rcd types 1, 2 and 3). In a later-onset form of PRA, progressive rod-cone degeneration (prcd), photoreceptors were shown to develop to maturity and then progressively die. Many breeds have PRA-causing gene mutations that are specific for their breed (or very closely related breeds). The major exception to this is the prcd form of PRA. This is now known to cause PRA in many different dog breeds and when the gene mutation underlying it was finally identified it was in a previously unknown gene that was given the name prcd (for progressive rod-cone degeneration). Although great advances in gene identification have been made, the gene mutations underlying PRA in many dog breeds remain to be identified. It has become apparent that several different breeds of dog have more than one form of PRA.

Feline PRA is less prevalent than canine PRA. An autosomal dominant early-onset form in Abyssinians was described but is not now seen in the general cat population. An autosomal recessive form has been studied in the same breed and following the identification of the underlying gene mutation the same mutation has been identified as segregating in several other cat breeds.

In addition to PRA other hereditary retinal diseases are also associated with retinal degeneration. In Briards a condition, which has been named a congenital stationary night blindness or retinal dystrophy of Briards, results in a marked reduction in retinal function and yet a very slow retinal degeneration. This is caused by a mutation in a gene RPE65. Mutations in the same gene cause a severe childhood-onset loss of vision in human patients. Retinal dysplasia can result in focal areas of retinal degeneration. The term retinal dysplasia encompasses a group of conditions in which there is an abnormality in retinal formation. In some instances the dysplastic retinal region will degenerate. However, unless the lesions are extensive or retinal detachment occurs, as it does with some of the forms of retinal dysplasia, vision is rarely lost. Canine multifocal retinopathy (cmr) is a more recently recognised condition in which multifocal areas of retinal elevation develop with a tan-coloured subretinal fluid. Over time the retina in these regions will degenerate. Causal mutations for cmr have been found in a gene called BEST1. Mastiff breeds, Coton de Tulear and Lapponian Herder have been identified with cmr due to BEST1 mutations.

Acquired Retinal Degeneration

Sudden Acquired Retinal Degeneration Syndrome (SARDS)

This is a poorly understood condition that is a cause of a relatively rapid vision loss in dogs. Middle-aged dogs are affected and typically have a history of losing vision either as rapidly as overnight or over the course of about a week. Initially there are no ophthalmological signs of retinal degeneration, although there is an absence of retinal function (and vision). In some affected dogs there may be some retention of a sluggish pupillary response, while in others the pupils will be dilated and fixed. The differential diagnoses for this presentation include central causes of blindness. An electroretinogram can be used to differentiate between retinal and central causes of blindness. Over a period of several weeks dogs with SARDS develop retinal thinning that can be detected ophthalmoscopically. The aetiology of SARDS remains unknown and once the retina is degenerate there is no treatment to restore vision.

Hypertensive Retinopathy

This is a common disease in older cats. Vasoconstriction affecting retinal arterioles leads to alterations in vessel walls allowing leakage and resulting in retinal oedema and possibly leading to retinal haemorrhage, retinal degeneration and even retinal detachments. Ophthalmoscopically, changes in the diameter of retinal arterioles can often be detected - there may be focal constriction and dilation along the length of the arterioles, while the venules are unaffected. It is recommended that fundus examinations and blood pressure screening are performed routinely in elderly cats.

Other vascular diseases such as hyperviscosity syndrome also result in detectable dilation of retinal arterioles and venules. It may progress to retinal haemorrhages, retinal detachments and degeneration. Potential causes of hyperviscosity syndrome include polycythaemia and multiple myeloma (due to monoclonal gammopathy).

Retinal Detachment

This can result in retinal degeneration: unless a detached retina reattaches relatively quickly it will start to degenerate. Reattachment surgery can restore vision up to about 4 weeks after detachment, but the degeneration of photo-receptors starts as soon as the detachment develops so the sooner the condition is treated the more likely some vision may be restored. One specific and treatable form of retinal detachment is what has been called a 'steroid-responsive' retinal detachment. This is usually a bilateral condition in younger dogs that does not have an obvious cause (e.g., no evidence of systemic disease) and presents as a sudden-onset blindness. Presumably there is an inflammatory component to the retinal detachment because treatment with systemic steroid can result in resorption of the subretinal fluid and reattachment of the retina. Retinal detachments associated with retinal tears will not spontaneously reattach. Surgery for treatment of retinal detachments is becoming more commonplace in veterinary medicine, but remains a definite speciality surgery. Without reattachment the retina will degenerate because it requires the intimate connection with the retinal pigment epithelium for continued health and function.

Inflammation

Inflammation involving the posterior segment of the eye (retina and choroid, e.g., chorioretinitis/posterior uveitis) can result in retinal damage and possibly detachment and degeneration. Potential causes range from infectious disease to immune-mediated conditions. Some of the infectious diseases such as fungal and tick-borne disease can vary in prevalence with geographic location. Vision loss may be the major presenting sign of posterior uveitis unless there are accompanying anterior segment changes, and the inflammation would have to be generalised to result in loss of vision. Once the presence of uveitis is identified, a work-up to identify potential infectious causes is warranted and, so long as it is not contraindicated, treatment to suppress the inflammation (typically systemic steroids) started promptly.