Delayed-onset cord1 progressive retinal atrophy in English Springer Spaniels genetically affected with the RPGRIP1 variant.

Abstract

Cone-rod dystrophy (cord1) is a form of progressive retinal atrophy. It is linked to an RPGRIP1 genetic variant which is the third most common canine disease variant thus far. While the variant affects various breeds, it is highly prevalent in English Springer Spaniels (ESSs). Yet its clinical and pathological implications remain equivocal. Herein, we study the retinal phenotype in ESSs genetically affected with the RPGRIP1 variant. Over 4 years, 494 ESSs (123 affected) were enrolled. Owner-perceived vision was collected via a questionnaire. Ophthalmic examination included fundus photography. In selected ESSs, retinal function and structure were assessed using electroretinography (ERG, 148 dogs) and optical coherence tomography (OCT, 4 dogs). Ophthalmoscopic changes included peripheral hypo-reflective lesions often with distinct borders progressing centripetally culminating in generalized retinal atrophy. Cross-sectional study revealed declining photopic ERG amplitudes with age in the affected group but not in controls. OCT indicated progressive photoreceptor loss. Despite ophthalmoscopic, ERG, or OCT abnormalities, most affected dogs were not visually impaired per their owners. In a fraction of afflicted ESSs, vision/globe-threatening complications were documented including cataracts, lens luxation, and glaucoma. In ESSs, the RPGRIP1 variant is associated with insidious pathology with delayed-onset visual defects. The subtle phenotype without apparent visual deficit until the final years of life, if at all, may have caused underdiagnosis of cord1. Still, DNA testing remains informative, and ERG and OCT indicate progressive pathology. Peripheral fundus examination and photopic ERG are particularly useful for early detection and monitoring of cord1.