Retinal Degeneration

Progressive retinal atrophy (PRA) is an inherited disease which results in degeneration of the rods and cones in the retina with irreversible vision loss. In all breeds in which the mode of inheritance has been identified, it is autosomal recessive. The inherited forms of PRA are broadly classified as to whether the rods or cones are primarily affected and as either a form of dysplasia (developmental abnormality of the photoreceptors - typically a very early onset of clinical signs prior to one year of age) or degeneration (a later onset condition occurring after normal maturation of the photoreceptors).

The age of onset and rate of progression varies from breed to breed. In most cases, clinical signs of vision impairment occur when the dog is 6-8 years of age. The first sign observed is usually a loss of night vision, which progresses, usually within a few months, to a loss of daylight vision as well. At this time, the pupillary dilation may also be noted.

Although most cases of retinal degeneration are inherited, there is a variant of this condition called Sudden Acquired Retinal Degeneration (SARD). Affected animals present with a history of acute blindness and bilateral pupillary dilation; there is usually no history of exposure to toxins or symptoms of systemic or neurologic disease. Ophthalmoscopic examination of the retina and optic nerve reveals no abnormalities in the early stages of the disease.

Diagnosis of inherited retinal degeneration can usually be made by clinical examination using an ophthalmoscope to examine the retina. Thinning of the retina and reduced size of the retinal vessels are observed. In some cases, a blood test may be diagnostic as well (Optigen). Occasionally, cataracts may occur secondary to retinal degeneration. In such cases, cataract surgery is obviously of no benefit. However, often a dog may not present for ophthalmic examination until the cataracts have progressed so that detailed fundus examination is prevented by the lens opacity. In such cases, electroretinography is essential to make this diagnosis and prevent an unnecessary surgery.

Electroretinography is the electrical response recorded when the retina is stimulated by light. Even in the presence of a corneal opacity or cataract, sufficient light reaches the retina to cause a response provided that the retina is functional. Indications for electroretinography include:

1) preoperative evaluation of retinal function before cataract extraction in order to detect those animals affected concurrently with PRA.

2) identification of retinal diseases which do not have ophthalmoscopically apparent lesions; e.g. Sudden Acquired Retinal Degeneration (SARD), hemeralopia; particularly to distinguish these from CNS causes of vision loss (amaurosis).

3) identification of animals genetically affected with PRA before they exhibit clinical signs or are detectable by ophthalmoscope examination. This is particularly important in selecting animals which would be used for breeding before the age of onset of most forms of PRA (6-9 years).

Electroretinography requires general anesthesia and specialized equipment. A contact lens electrode is placed on the surface of the eye and reference needle electrodes on the pinna of the ear and the bridge of the nose. Most tests are performed using a single flash of white or red light. To detect cone abnormalities, a technique called flicker fusion is used. This identifies the frequency of stimulation beyond which individual ERG responses are not detectable. Normally this value is 75-80/second. In dogs affected with hemeralopia, in which the cones are selectively affected, the flicker fusion frequency may be as low as 5-10/second.

Breeds Affected with Progressive retinal atrophy (PRA)

Normal canine retina

Progressive retinal atrophy (PRA). Note the tapetal hyperreflectivity, vascular attenuation, and optic nerve atrophy.

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