The shrimp sign is an MRI marker of cerebellar progressive multifocal leukoencephalopathy, characterised by T2-hyperintensity in the cerebellar white matter abutting but sparing the dentate nucleus.

The white matter lesion resembles a shrimp, with the dentate nucleus outlining the belly of the shrimp, the broader area of involvement at the middle cerebellar peduncle corresponding to the head end of the shrimp, and the narrower area of involvement in the medial cerebellar white matter corresponding to the tail. There are detailed diagnostic criteria to maintain reliability and accuracy of the sign ¹.

Inclusion criteria

The presence of the following features qualifies a white matter lesion as the shrimp sign ¹:

• well-defined

• located in the cerebellar white matter

• hyperintense on T2-weighted and T2-FLAIR sequences

• hypointense on T1-weighted sequences

• abutting and sharply outlining the dentate nucleus (in axial, sagittal, and/or coronal planes)

• encompassing at least 50% of the dentate nucleus (not necessarily contiguous)

Exclusion criteria

The white matter lesion does not meet the shrimp sign if any of these features are present ¹:

• not hypointense on T1-weighted imaging

• hazy and ill-defined on T2-weighted and T2-FLAIR sequences

• cavitation within the lesion

• prominent focal, diffuse, or ring enhancement

• involvement of the dentate nucleus

• displacement of the dentate nucleus

• enhancement of the dentate nucleus

• severe atrophy of the dentate nucleus early in the disease course

Optional features

Some features are compatible or permissible with the diagnosis ¹:

• mottled appearance of the lesion on T2-weighted imaging

• involvement of the white matter hilum of the dentate nucleus

• co-occurrence with or independence of cerebral and brainstem lesions

• minimal enlargement (<2–3 mm) of the middle cerebellar peduncle

• minimal mass effect on the fourth ventricle

• faint enhancement of the white matter lesion or the hilum of the dentate nucleus

• olivopontocerebellar atrophy in late-stage disease

• bilateral, but usually asymmetric, white matter lesions

Differential diagnosis

Fragile X-associated tremor/ataxia syndrome, the cause of the middle cerebellar peduncle sign, can look like the shrimp sign but is typically bilateral symmetric, rather than unilateral or asymmetric like the shrimp sign. Moreover, the clinical context should help distinguish these entities whereby fragile X-associated tremor/ataxia syndrome occurs in older men with a familial history, while most cases of progressive multifocal leukoencephalopathy occurs in patients with HIV infection.