Subacute sclerosing panencephalitis (SSPE) is a chronic, progressive neurodegenerative disorder caused by persistent wild-type measles virus infection of the central nervous system. Early in its course, SSPE may mimic autoimmune or infectious encephalitis, making diagnosis challenging. This case highlights the diagnostic pitfalls and underscores the importance of considering SSPE in children with encephalopathy and myoclonus who are underimmunized or may have been exposed to measles before vaccination.

Electronic medical records were reviewed, including progress notes, laboratory studies, MRI reports and images, and EEG tracings.

A 9-year-old boy, recently immigrated from a measles-endemic region, presented with acute confusion, behavioral changes, ataxia, and myoclonic seizures. Neurologic examination showed lethargy, delayed responses, right Babinski sign, and right lower-extremity weakness. Routine labs, metabolic and infectious panels, and brain/spine MRI/MRA were unremarkable. CSF revealed mild pleocytosis and elevated IgG index. Broad infectious, demyelinating, and autoimmune testing—including evaluation for autoimmune encephalitis—was negative. Immunization records were unavailable, though the family reported routine vaccinations. With a working diagnosis of seronegative autoimmune encephalitis, corticosteroids, IVIG, and plasmapheresis were administered with minimal improvement. By hospital day 12, he demonstrated only minimal withdrawal to pain. Repeat EEG showed generalized background slowing with periodic discharges every 3–5 seconds, suggestive of SSPE. On hospital day 20, CSF measles IgG titers obtained early in admission returned markedly elevated (>300 AU/mL), confirming SSPE. Immunotherapy was discontinued, and multidisciplinary teams provided supportive and palliative care planning.

This case illustrates the importance of maintaining a high index of suspicion for SSPE in children with progressive encephalopathy and myoclonus, particularly those with uncertain immunization status or residence in endemic regions. Early CSF measles antibody testing and EEG findings can expedite diagnosis, prevent unnecessary immunotherapy, and guide appropriate family counseling and supportive management.