Introduction

• Retinal pigment epithelial dystrophy (RPED) is now the preferred name.
• Cause: hereditary; autosomal recessive inheritance.
  • Genetic defect leading to dysfunction of retinal pigment epithelium (RPE), the cell layer adjacent to the photoreceptors that is responsible for maintaining photoreceptor function → inability of dystrophic RPE cells to degrade photoreceptor outer segments (POS) → photoreceptor degeneration.
• Diagnosis: clinical signs, ophthalmoscopy, electroretinography. Nutritional influence → vitamin E deficiency gives similar lesions and maybe implicated here too.
• Treatment: Vitamin E supplementation may be recommended but therapeutic effects unknown.
• Prognosis: deterioration is probably influenced by many factors; affected animals lose their central field of vision but maintain peripheral sight.

Pathogenesis

Etiology

• CPRA is a genetically inherited disease.

Predisposing factors

General

• Breed.

Pathophysiology

• Inefficient degradation of photoreceptor outer segments (POS) by dystrophic PRE cells in conjunction with an inability to recycle substances used in the synthesis of new POS prevents normal photoreceptor function.
• Accumulation of breakdown products of the POS in the cytoplasm of the PRE cells has a detrimental effect on their other functions, particularly neuroretinal support.
• Melanin has a protective effect hence the neuroretina overlying the non-tapetal fundus will continue to function long into the disease process.

• Basic materials for regeneration of exhausted photoreceptor outer segments are provided by the degradation of POS by RPE cells.
• Dysplastic RPE cells are inefficient in both breakdown and production of POS and therefore accumulate phagocytosed POS material in their cytoplasm.
• Other functions of retinal pigment epithelial cells, such as neuroretinal support, are subsequently inhibited by POS accumulation.
• In combination, failure of POS regeneration and loss of neuroretinal support results in degeneration of photoreceptors and subsequent loss of sight.
• As neuroretinal cells are protected by melanin, non-tapetal photoreceptor function and hence peripheral vision persists long into the disease process.
• One study showed that both affected and unaffected Briards had elevated cholesterol levels, suggesting a primary hypercholesterolemia within this breed, although any link to RPED was unproven.
• In the cocker spaniel breed, an association has been made between RPED and low serum vitamin E levels, suggesting a systemic disorder.
• Neurological defects have been described in cocker spaniels in addition to ophthalmoscopic signs. Histopathological examination of the nervous system in post mortem investigations of affected dogs showed central neuronal fiber degeneration with prominent neuroaxonal dystrophy, particularly within the sensory relay nuclei of the brainstem.

Timecourse

• Most types of CPRA develop slowly, starting at 3-4 months of age.

Diagnosis

Treatment

Prevention

Outcomes