Fast-wave periodic complexes in a mentally retarded child who later developed subacute sclerosing panencephalitis: a modification of a classic EEG by preexisting brain damage?

The EEG of a 12-year-old girl with stage II subacute sclerosing panencephalitis (SSPE), who had also suffered from a non-progressive mental retardation of unknown aetiology since early childhood, revealed periodic generalised stereotyped fast wave bursts synchronous with myoclonic jerks. The background activity was nearly normal. The diagnosis of SSPE was established by raised serum and measles antibody titres, raised CSF IgG, and brain biopsy. This rare type of periodic complex has only once been described in the literature, again in a mentally retarded child who had developed SSPE. We suggest a mechanism of origin of this type of periodic complex drawn from observations in these two cases, and discuss its significance.