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.

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