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Gerald F. Cox, MD, PhD; Ronald M. Hansen, PhD; Nicole Quinn, OD; Anne B. Fulton, MD

Arch Ophthalmol. 2003;121:804-810.

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 R_mp3(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 ) and the saturated amplitude (V_max) were determined. The model of the rod photoresponse was subtracted from the intact ERG to demonstrate a corneal positive potential (P₂)_, and log k_P2 and P_2max were determined. The carriers' ERG responses were compared with those of healthy control subjects (n = 26).

Main Outcome Measures  Sensitivity (S, log , and log k_P2) and saturated amplitude (R_mp3, V_max, and P_2max) 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.

From the Departments of Medicine (Division of Genetics) (Dr Cox) and Ophthalmology (Drs Hansen, Quinn, and Fulton), Children's Hospital, Boston; the Departments of Pediatrics (Dr Cox) and Ophthalmology (Drs Hansen, Quinn, and Fulton), Harvard Medical School, Boston; Genzyme Corporation, Cambridge (Dr Cox); and the New England College of Optometry, Boston (Dr Quinn), Mass. The authors have no relevant financial interest in this article.

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