A 7-year-old presents for evaluation because of unexplained subnormal visual acuity. Although abnormal, the decreased vision is thought to have been stable, not progressive. There is no history of color or peripheral vision difficulties, but his family has noted that he has difficulty adapting to dimly lit environments, such as movie theatres, and that he has some degree of photophobia. There is no family history of eye diseases. The child is systemically well, has a well-balanced diet, and has three unaffected sisters.
On examination, best-corrected visual acuity was 20/40 right eye and 20/60 left eye with correction of mild hyperopic astigmatism. Color vision was normal by Ishihara testing and no nystagmus was observed. Anterior segments were normal as were the fundi, except for a possible reduction of the foveal light reflex as seen in the photos. FAF and OCT were essentially normal.
Full-field ERG scotopic responses revealed strong flash a-waves within normal limits, but b-wave amplitudes that were subnormal with the waveform being electronegative. Photopic 30Hz flicker was delayed and subnormal bilaterally. Single flash photopic a-waves were within normal limits, however b-wave amplitudes were subnormal with the waveform having a low b:a ratio. What could be happening?