Abstract
Congenital stationary night blindness (CSNB), apparently inherited in an autosomal recessive manner, was observed in a litter of Briard dogs in Sweden. Of nine litter mates five had nyctalopia. The results of different clinical tests, including electroretinography (ERG), were compared with the results found in four human cases of CSNB, three of which were most probably associated with autosomal recessive inheritance and one with X-linked inheritance. The congenital and stationary nature of the disease, ophthalmoscopically normal appearing fundi, and recordable but reduced photopic flicker responses were some of the similarities found between canine and human cases. The single-flash ERG response was abnormal in the humans as well as in the affected Briards. However, the human cases showed a 'negative' ERG, whereas in the Briards both the a and b waves were extremely reduced and present only at a photopic level. Cases similar to these Briards have been described also in man, where rhodopsin concentration and regeneration were found to be normal, suggesting a disturbed transduction mechanism. It thus appears that the Briard dog may become a valuable model of human CSNB. A group of retinal disorders in man known collectively as congenital stationary night blindness (CSNB) is mainly characterised by a normal fundus appear- ance, loss of scotopic function, and an abnormal electroretinogram (ERG).' Though not very common, it was first described over 300 years ago as a dominant disease in the Nougaret family in France.2 Three modes of genetic transmission have since been described: a dominant form, an autosomal recessive form,3 and a sex linked recessive type.4 photopic responses could be recorded from the dark adapted retina. There seems to be no correlation between these two ERG types and the mode of inheritance of CSNB.'7-10 Recently, the Schubert Two types of CSNB have been differentiated by means of the ERG. In the first type, described by Schubert and Bornschein,' the dark-adapted response is characterised by a normal or slightly subnormal a wave, while there is no (or perhaps a minimal) scotopic b wave above baseline. This 'negative type' of ERG or type 1 ERG, is the most common type of ERG in CSNB. In the type described by Riggs' the a and b waves are proportionally more reduced, and the amplitudes may vary substantially (type 2 ERG). In some cases only reduced Correspondence to Dr K Narfstrom, Department of Ophthalmology, University of Linkoping, S-581 85 Linkoping, Sweden. photopic responses could be recorded from the dark- adapted retina. There seems to be no correlation between these two ERG types and the mode of inheritance of CSNB. '7-10 Recently, the Schubert- Bornschein type of CSNB has been further subdivided, mainly on the basis of ERG findings and psychophysical testing, into the complete type, which competely lacks rod function, and the incomplete type with some remaining rod function.' Furthermore, in all forms of CSNB the implicit times for scotopic and photopic b waves were found to be almost the same,' whereas in normal persons the implicit time of the scotopic b wave is at least twice as long as that of the photopic b wave. Although the cause of CSNB is still not clear, the results of ERG and fundus reflectometric studies have contributed to our understanding of the disease. Carr and associates showed that both the concentration and regeneration of rhodopsin were normal in two cases of CSNB affected human retinae.'2 One of the cases showed a 'negative' ERG, and in the other only a minimal photopic response could be elicited. It seems that the first case represents a defective transmission from the rods to the inner retina and 750 copyright. on August 22, 2023 by guest. Protected by http://bjo.bmj.com/ Br J Ophthalmol: first published as 10.1136/bjo.73.9.750 on 1 September 1989. Downloaded from that the second case involves a failure in the transduc- tion process at some point between the initial event and the change in plasma membrane potential.'2-'4 The few CSNB affected eyes that have been examined histologically have not shown any morphological alterations by either light or electron micro- scopy.'5 16 Female carriers of X-linked recessive CSNB have shown reduced oscillatory potentials. '7 The pathophysiology of specific retinal disease processes in man may be elucidated by study of animal models with similar defects. A form of night blindness with similarities to the Schubert- Bornschein type of CSNB was reported in the Appaloosa horse. 18 In this animal the trait was recessively inherited and there was behavioural evidence that the animals were night-blind. Electro- physiologically only the a wave of the ERG was recordable. Histological studies of one nyctalopic horse showed a normal retina. Transmission densito- metry of an isolated retina revealed an ample supply of rhodopsin. In the dog recent reports of sporadic cases of night- blind Briards have been obtained, mainly from France (Lescure F, personal communication, 1987) as well as from the USA (Riis RC, Aguirre GD: The Briard problem. Unpublished: for abstract see Transactions of the Fourteenth Annual Scientific Program of the College of Veterinary Ophthalmologists, Chicago, 1983). In Sweden such a defect has also been observed and further studied in a complete litter of Briard dogs and in their parents. This paper describes some of the clinical characteristics of the disease in these dogs and compares the results with the findings in cases of CSNB in man.
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