From radiological finding to diagnosis: the shrimp sign in cerebellar progressive multifocal leukoencephalopathy

Abstract

Case Presentation: A 42-year-old man with human immunodeficiency virus (HIV), without recent viral load or CD4 T-lymphocyte count assessment, presented with sudden dysarthria, vertigo, and gait instability. Neurological examination revealed multidirectional nystagmus, dysarthria, bilateral dysmetria, ataxic gait, and impaired balance without motor or sensory deficits. Brain magnetic resonance imaging (MRI) showed T2/FLAIR hyperintense lesions with diffusion restriction in the cerebellar white matter and middle cerebellar peduncles. Given the clinical stability and exclusion of central nervous system (CNS) infection, the patient was discharged with viral load and CD4 testing already requested. However, one month later, he returned with progression of symptoms. Laboratory tests revealed a viral load of 97,400 copies/mL and a CD4 count of 17 cells/mm³. Given the advanced HIV infection and the progressive clinical course, the MRI was reviewed, raising suspicion for cerebellar progressive multifocal leukoencephalopathy (PML) based on the “shrimp sign”. JC virus was confirmed in the cerebrospinal fluid (CSF). Discussion: PML is a demyelinating CNS infection caused by the JC virus, representing one of the most feared complications of HIV infection, particularly in patients with a low CD4 T-lymphocyte count, such as this patient (17 cells/mm³). Diagnosis relies on clinicoradiological correlation combined with JC virus detection in the CSF. The progressive clinical course, incompatible with ischemic stroke, prompted diagnostic reassessment. From a radiological standpoint, certain imaging patterns, such as the “shrimp sign,” may aid in recognition. The sign is characterized by lesions in the deep cerebellar white matter that are hyperintense on T2-weighted images and hypointense on T1-weighted images, surrounding but sparing the dentate nucleus, thereby accentuating its serrated and curved configuration. The broader portion within the middle cerebellar peduncle resembles the shrimp’s “head,” the dentate nucleus outlines the “body,” and the tapering within the white matter corresponds to the “tail". This finding may occur unilaterally or bilaterally and is typically asymmetric. In this case, the identification of this pattern on MRI strengthened suspicion of cerebellar PML, which was later confirmed in the CSF. Final Comments: In patients living with HIV, PML should always be considered in the differential diagnosis, even in the presence of seemingly typical presentations of common conditions. Recognition of imaging findings such as the shrimp sign may raise diagnostic suspicion, especially when associated with a compatible clinical presentation. Diagnosis can be confirmed by polymerase chain reaction (PCR) for JC virus in the CSF.