Objective: To test the hypothesis that carriers of Bardet-Biedl syndrome have abnormal rod-mediated responses.
Methods: Parents (n = 26) of children with Bardet-Biedl syndrome (BBS), who are obligate carriers of BBS, consented to scotopic, full-field electroretinography (ERG). Responses were recorded over a 4 to 5 log unit range, up to a maximum stimulus of approximately +3.6 log scotopic troland seconds. The parameters of activation of the rod photoresponse, S (sensitivity parameter) and Rmp3(amplitude of the saturated rod response), were derived by fitting a transduction model to the ERG a-waves. For the b-wave, the stimulus producing a half maximum response (log sigma) and the saturated amplitude (Vmax) were determined. The model of the rod photoresponse was subtracted from the intact ERG to demonstrate a corneal positive potential (P2), and log kP2 and P2max were determined. The carriers' ERG responses were compared with those of healthy control subjects (n = 26).
Main outcome measures: Sensitivity (S, log sigma, and log kP2) and saturated amplitude (Rmp3, Vmax, and P2max) of receptoral and postreceptoral response components.
Results: All parents had decreased P2 sensitivity, and most (15 [60%] of 26) had decreased b-wave sensitivity. The rod photoresponse sensitivity and the saturated amplitudes of the rod cell response, b-wave and P2, did not differ significantly between carriers and controls.
Conclusions: Diminished P2 sensitivity is characteristic of the carriers of BBS. The site of the primary defect in the BBS rod pathway appears to be proximal to the outer segments, most likely before the rod-bipolar cell synapse.