Background/Aims: Subacute sclerosing panencephalitis (SSPE) is a fatal neurodegenerative disease caused by persistent measles virus (MeV). Its rarity in vaccinated populations often leads to diagnostic delays. We aim to present the diagnostic algorithm for a fulminant SSPE case in a vaccinated child, highlighting the critical role of dynamic serological testing.

Methods: We present a retrospective case analysis of a 5-year-old girl with prior MMR (measles-mumps- rubella) vaccination. Neurological symptoms developed three weeks after a chickenpox infection. She underwent clinical evaluation, magnetic resonance imaging (MRI), serial video-electroencephalography (EEG), and two paired cerebrospinal fluid (CSF) and serum analyses for MeV-specific immunoglobulin G (IgG) over a three-week interval.

Results: Initial symptoms (motor aphasia, atonic seizures) prompted a broad differential diagnosis. The first CSF analysis showed MeV-IgG (6.66 IU/ml) 2.5 times higher than in serum, which was deemed inconclusive. Crucially, repeat testing three weeks later revealed a pathognomonic finding: despite a drop in total CSF IgG to 0 g/L, the specific MeV-IgG titer rose to 33.28 IU/ml, seven times higher than the serum level (4.75 IU/ml). This confirmed isolated, super-intensive intrathecal synthesis. Serial EEGs demonstrated progressive deterioration with the emergence of pathognomonic periodic Radermecker complexes (high-amplitude sharp- slow wave discharges). MRI revealed progressive diffuse cerebral atrophy. The patient's condition deteriorated to a vegetative state, unresponsive to intrathecal interferon-alpha and isoprinosine.

Conclusions: A history of chickenpox may act as a non-specific trigger for SSPE, but the diagnosis requires high vigilance even in vaccinated children. While Radermecker complexes on EEG and neuroimaging are supportive, the definitive in-vivo diagnosis hinges on demonstrating dynamic intrathecal MeV-specific antibody synthesis. A single negative or equivocal CSF result does not exclude SSPE; repeat CSF analysis can reveal a paradoxical immune response pathognomonic for the disease. This case underscores that maintaining high vaccination coverage to eliminate wild-type MeV circulation remains the only definitive strategy to eradicate SSPE.

Bzhemukhova Syuzanna Aidamirovna is a Pediatric Neurologist at the V.F. Voyno-Yasenetsky Scientific and Practical Center for Specialized Medical Care for Children and serves as Secretary of the Society of Young Scientists at the Center. Her research interests include autoimmune encephalitis in children, neuroimmunology, and rare diseases of the nervous system. Bzhemukhova specializes in the diagnosis and treatment of autoimmune encephalitis in children. She combines clinical practice with research activities and coordinates the work of the Society of Young Scientists at the V.F. Voyno-Yasenetsky Scientific and Practical Center. She is an active participant in specialized conferences and seminars.