Abnormal cortical excitability in Rett syndrome.

Visual and somatosensory evoked potentials (VEPs and SEPs) were studied in 9 patients with Rett syndrome and compared with those in 6 patients with photosensitive progressive myoclonus epilepsy (PPME). In Rett syndrome, a giant III-IV amplitude of VEPs was present in 8 patients, although none exhibited giant II-III amplitudes. Four of 6 patients with Rett syndrome who demonstrated giant SEPs did not have a C reflex. Conversely, the patients with PPME demonstrated giant II-III and III-IV amplitudes in VEPs, and giant SEPs with concurrent positive C reflexes. It is concluded that the mechanism of altered cortical excitability in Rett syndrome is different from that in PPME.