History: a male Briard, 7-month-old, was referred due to night blindness since he was born. Complete ophthalmic examination was performed showing normal pupillary light reflexes; the menace test was present in bright light and absent in dim one. A similar result was observed with the maze test. The rest of the exam was unremarkable.

Electroretinography (ERG) was performed using a protocol which included 10 minutes of dark adaptation and flash ERG with blue light, 10 minutes of light adaptation and flash ERG with white light, and photopic 30 Hz flicker. No a-, b-, or c-waves could be detected at any stage of the ERG protocol. A DNA mutation-based gene test for Congenital Stationary Night Blindness was performed confirming the presence of the specific gene mutation.

Discussion: The Congenital Stationary Night Blindness (CSNB) is an autosomal recessive inherited retinal disease characterized by congenital rod dysfunction with a variable cone affection. The disease was primarily described in Sweden in Briards¹ as a different disease from the Retinal Pigment Epithelial Dystrophy (RPED) in the British Birard. Further clinical and morphologic studies in the Scandinavian Briard showed that there is a progressive component, and both the neural retina and the retinal pigment epithelium are affected by a dystrophy with an extremely slow progression. This disease is called Hereditary Retinal Dystrophy (HRD). Nowadays, both diseases, CSNB and HRD, are considered to be produced by the same gene mutation (RPE65).²

Clinically affected dogs are congenitally night blind, whereas daylight vision could vary from normal to more or less severely impaired. Responses to 30Hz flicker recordings usually are 50% reduced in affected dogs with recordable b-waves responses; dogs with nonrecordable scotopic ERGs did not have recordable flicker responses.³

Conclusion: in this case, the first described in Argentina, photopic, scotopic and flicker responses were nonrecordable. However, the ocular fundus was normal in appearance, and visual impairment of this dog was observable only at night. The absence of a-wave and the nonrecordable waves with photopic flicker stimulation indicated that there was not cone response.

References

1.  Narfstrom K, et al. The Briard dog: a new animal model of congenital stationary night blindness. Br J Ophthalmol 1989, 73: 750-6

2.  Aguirre GD, et al. Congenital stationary night blindness in the dog: common mutation in the RPE65 gene indicates founder effect. Mol Vis 1998, 4: 23-9

3.  Narfstrom K, et al. Hereditary Retinal Dystrophy in the Briard Dog: Clinical and Hereditary Characteristics. Vet & Comp Ophthalmol 1994, 4: 85-92