Early detection of subtle hemispheric atrophy in Rasmussen encephalitis using FreeSurfer-based sequential volumetric analysis.

Rasmussen encephalitis (RE) is a progressive disease characterised by unilateral brain atrophy, drug-resistant epilepsy, epilepsia partialis continua, hemiparesis and cognitive decline. Early initiation of immunomodulatory therapy is crucial to slow disease progression. However, early formal diagnosis is challenging as it typically requires hemispheric atrophy or brain biopsy. This case reports on a preschool-aged boy with RE who began immunotherapy before demonstrating clear hemispheric atrophy. Follow-up MRI did not indicate global hemispheric atrophy; however, FreeSurfer-based volumetric analysis revealed a decreased left:right hemispheric volume ratio, suggesting early left hemispheric atrophy. Subsequently, intensive immunotherapy was administered. Over 3 years of treatment, the patient exhibited gradual hemispheric atrophy on MRI, along with mild motor and cognitive impairments. Serial FreeSurfer-based volumetric analysis may contribute to detecting subtle hemispheric volume changes in RE, facilitating prompt diagnosis and early initiation of immunotherapy-potentially limiting disease progression.