In the light-adapted state (background of 30 cd/m
2), the single flash cone ERG (energy of 3.0 cd-s/m
2) responses (
Fig. 3, first column) of patients A-1 and B-1 (first and second rows, respectively) were characterized by a prolonged a-wave implicit time with normal amplitude, and a prolonged b-wave implicit time and subnormal amplitude. The ERG responses to the bright (30 cd-s/m
2) white light stimulus (
Fig. 3, second column) were of supernormal amplitudes and prolonged implicit times of the a-wave and the b-waves relative to the normal response (
Fig. 3, seventh row). These were qualitatively similar to the corresponding responses of the ESCS patient (
Fig. 3, third row), except the latter had larger amplitudes of the photopic b-waves. Flicker responses (
Fig. 3, third column) of A-1, B-1, and the ESCS patients were delayed and markedly subnormal, smaller than the a-wave amplitude of the light-adapted ERG response to 3.0 cd-s/m
2, which is a typical finding in ESCS patients.
9–12 The isolated rod response (
Fig. 3, fourth column), elicited by a dim blue stimulus in the dark-adapted state, was nonrecordable in A-1, B-1, and ESCS patients. The ISCEV standard mixed rod–cone responses
26 in A-1, B-1, and ESCS patients (
Fig. 3, fifth column) were of small amplitude and prolonged implicit times, very similar to their ERG responses for the same stimulus in the light-adapted state (
Fig. 3, first column). The dark-adapted ERG responses to bright (30 cd-s/m
2) white light stimuli (
Fig. 3, sixth column) of these patients were of large amplitudes and delayed implicit times of both a-waves and b-waves, but were characterized by different waveform. In patients A-1 and B-1, the a-wave dominated the waveform, and the b-wave was difficult to identify reliably. In fact, we selected the peak b-wave according to a small notch in the rising phase of the large a-wave. In the ESCS patient, the ERG to bright flash had a normal a-wave to b-wave waveform. Another ERG criterion that has been suggested as typical for ESCS patients is reduction in the function of M- and L-cones. This criterion was met in patients A-1 and B-1, as evident by the single flash and flicker responses in the light-adapted state (
Fig. 3, first and second columns). Furthermore, we typically record the dark-adapted ERG response to a red stimulus that elicits a characteristic X-wave, reflecting cone function,
28 almost exclusively that of L-cones.
29 The amplitudes of the X-wave in patients A-1, B-1, and ESCS were subnormal—26 μV, 15.4 μV, and 43.5 μV, respectively—while our lowest limit for the normal range was 50 μV. The ERG responses of the three heterozygotes (individuals A-2, A-3, and C-1) (
Fig. 3, rows 4–6) were very similar to the corresponding ERG responses of the normal subject (
Fig. 3, seventh row) for all recording conditions.