The definite diagnosis of
central nervous system vasculitis requires pathological verification by biopsy or surgical resection of the lesion, which may not always be feasible. A 74-year-old woman with a history of
allergic rhinitis, but not
asthma, presented with slowly progressive left
hemiparesis. Magnetic resonance imaging of the head revealed a heterogeneously enhancing mass involving the right internal capsule and corona radiata. Histological examination of the resected specimen revealed eosinophil-rich non-granulomatous small vessel
vasculitis with no neutrophil infiltration or foci of microbial
infection. Epstein-Barr virus in situ hybridization was negative, and polymerase chain reaction tests for both
T-cell receptor gamma and
immunoglobulin heavy-chain variable region genes did not show rearrangements, excluding the possibility of
lymphoma and
lymphoproliferative disorders. Blood
hypereosinophilia and elevated erythrocyte sedimentation rate were observed; however,
anti-neutrophil cytoplasmic antibodies were not detected. A biopsy of the
erythema in the hips and thighs revealed perivasculitis with eosinophilic infiltration within the dermis. Chest computed tomography revealed multiple small nodules in the lungs. Her symptoms, aside from
hemiparesis, disappeared after
corticosteroid administration. The clinicopathological features were similar to
eosinophilic granulomatosis with polyangiitis but did not meet its current classification criteria and definition. This patient is the first reported case of idiopathic eosinophilic
vasculitis or
idiopathic hypereosinophilic syndrome-associated
vasculitis affecting the small vessels in the brain. Further clinicopathological studies enrolling similar cases are necessary to establish the disease concept and unravel the underlying pathogenesis.