EEG — SSPE Periodic Complexes (Subacute Sclerosing Panencephalitis — Late Measles Complication)

An 8-year-old boy from rural Maharashtra presents with a 3-month history of progressive behavioral decline, declining school performance, and recent onset of stereotyped myoclonic jerks of the head and trunk occurring every 5–10 seconds. His mother recalls he had measles at age 18 months, which was not vaccinated against. EEG shows high-amplitude (500–1000 μV) polyphasic periodic complexes recurring every 4–15 seconds, bilaterally synchronous, time-locked to his clinical jerks. CSF analysis reveals elevated measles-specific IgG with a CSF:serum ratio of 50:1, and oligoclonal bands are present. The periodic complexes marked **A** in the EEG diagram are characteristic of which condition, and what is the underlying pathophysiology?