Evidence supportive of a functional discrimination between photopic oscillatory potentials as revealed with cone and rod mediated retinopathies

We report on a family where four of the eleven children presented with redu ced visual acuities, a red-green deficit at the Farnsworth-Munsel FM 100-hu e test, normal appearing fundi and unexpected electroretinographic findings . Light- (photopic) and dark- (scotopic) adapted electroretinograms (ERG) a nd oscillatory potentials (OPs) were obtained following an accepted standar d protocol. The b-wave of their photopic ERG was significantly more attenua ted than the a-wave due to the specific abolition of OP4, while the amplitu des of OP2 and OP3 were within the normal range, giving to the b-wave a tru ncated appearance reminiscent of that seen in congenital stationary night b lindness (CSNB) with myopia. Interestingly in the latter condition, which i s believed to result from an ON-retinal pathway anomaly, it is OP2 and OP3 which are specifically abolished while OP4 is of normal amplitude thus resu lting in an OP response pattern which complements that seen with our patien ts. Also of interest is the fact that, in our patients, the amplitude of th e dark-adapted OP2 was, on average, 240% larger than that measured in light -adaptation while, in normal, a non-significant 14% increase is noted; a fi nding which is in keeping with other studies reporting supernormal scotopic ERGs in some forms of cone dystrophies. Based on the photopic OP response pattern, our patients represent the electrophysiological complement of pati ents affected with CSNB. Interestingly their symptoms are also complementar y, a finding which could support a functional discrimination between the ph otopic OPs.