Teaching NeuroImage: Shrimp Sign in Ataxic Cerebellar Progressive Multifocal Leukoencephalopathy

Abstract

An 80-year-old woman presented to the emergency department with vertigo, right hand incoordination, and postural instability 5 months after rituximab treatment for chronic lymphocytic leukemia. Physical examination revealed dysarthria, horizontal nystagmus, gait ataxia, and right upper and lower limb dysmetria. Brain magnetic resonance imaging showed a T2 hyperintense (Figure, A) and T1 hypointense (Figure, B) lesion in the right cerebellar white matter that demarcates the dentate nucleus (Figure, C), with pontine and middle cerebellar peduncle extension. This imaging finding corresponds to the “shrimp sign”: white matter lesion that preserves the curvilinear-shaped dentate nucleus, giving a shrimp-like appearance.1 HIV serologic testing was negative, and JC virus DNA was detected in the CSF, establishing the diagnosis of HIV-negative progressive multifocal leukoencephalopathy (PML). Rituximab was discontinued, and she showed no radiologic and clinical progression after 1-year follow-up. The “shrimp sign” is a reliable indicator of infratentorial PML in patients with cerebellar ataxia.