MRI Shrimp Sign in Cerebellar Progressive Multifocal Leukoencephalopathy: Description and Validation of a Novel Observation

N. Adra; A.E. Goodheart; O. Rapalino; P. Caruso; S.S. Mukerji; R.G. González; N. Venna; J.D. Schmahmann

doi:10.3174/ajnr.A7145

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FIG 1.
Flow diagram outlining the methodology for defining and validating the shrimp sign. In the preliminary stage, axial planes of T1-weighted imaging, T2-weighted imaging, FLAIR, and, when available, postgadolinium T1-weighted images were studied. In subsequent stages, these images were also examined in the sagittal and coronal planes.
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FIG 2.
Representative MRIs from 5 patients with PML demonstrating the essential features of the MR imaging shrimp sign. Each row represents images taken from 1 patient, numbered cases 1–5: T2 axial (A), T1 axial (B), and T1 sagittal images (C). White arrows identify the white matter lesion; black arrows, the dentate nucleus. FLAIR axial (D), T1 axial (E), T1 sagittal (F), FLAIR axial (G), T2 axial (H), T1 coronal (I), FLAIR axial (J), T2 axial (K), T1 sagittal (L), T2 axial (M), FLAIR axial (N), and T1 sagittal (O) views. In case 5, the bilaterally symmetric shrimp sign is somewhat atypical in this patient with PML in the setting of confirmed HIV positive for CSF-JCV.

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Table 1:

Diagnostic criteria for the MR imaging shrimp sign of cerebellar PML

Core Inclusion Criteria of the Shrimp Sign	Core Exclusion Criteria of the Shrimp Sign
White matter lesion Well-defined lesion in the cerebellar white matter The lesion is hyperintense on T2-weighted and FLAIR imaging The lesion is hypointense on T1-weighted imaging The lesion abuts and sharply demarcates the dentate nucleus and outlines the dentate nucleus in horizontal, parasagittal, and/or coronal views The lesion must encompass at least 50% of the dentate nucleus (partial shrimp); if there are multiple lesions adjacent to the dentate, they do not need to be contiguous	White matter lesion Is not hypointense on T1-weighted imaging Is hazy and ill-defined on T2-weighted or FLAIR imaging Has cavitation within the substance of the lesion Demonstrates prominent focal, diffuse, or ring enhancement Dentate nucleus Is invaded by the white matter lesion Is displaced by the white matter lesion Shows enhancement of the nucleus itself (specifically excluding the hilum) Is severely atrophic early in the course of the disease
Other features compatible with PML	Permissible atypical features^a
There may be a mottled appearance of the white matter lesion on T2-weighted imaging The white matter hilum of the dentate nucleus may be involved The lesion may occur together with, or independent of, cerebral hemisphere and brainstem PML lesions	Minimal enlargement of the MC P (up to 2–3 mm) 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 PML Bilateral, usually asymmetric white matter lesions

Note:—MCP indicates middle cerebellar peduncle.
↵a In the right clinical and imaging context and when all core inclusion and exclusion criteria are met.

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Table 2:

Patient demographics^a

	Definite PML (n = 16)	Possible PML (n = 4)	Control Cohort(n = 40)
Sex (F/M)	5:11	1:3	16:24
Age (mean) (SD) (yr)	46.4 (9.21)	45.8 (8.46)	43.6 (21.04)
Cause of PML
HIV	13 (81.25%)	3 (75%)	–
Cancer	2 (12.5%)	0 (0%)	–
Immunosuppressant therapy	1^b (6.25%)	1^c (25%)	–
Ethnicity
Hispanic	1 (6.25%)	0 (0%)	2 (5%)
Non-Hispanic	8 (50%)	1 (25%)	25 (62.5%)
Unknown	7 (43.75%)	3 (75%)	13 (32.5%)
Race
White	10 (62.5%)	0 (0%)	30 (75%)
African American	2 (12.5%)	2 (50%)	3 (7.5%)
Asian	0 (0%)	0 (0%)	1 (2.5%)
Unknown	4 (25%)	2 (50%)	6 (15%)

↵a Patients with PML with positive JCV-CSF findings were defined as having definite PML. Patients with PML without CSF samples but with clinical and MR findings indicative of PML were defined as having possible PML.
↵b Rituxan and fludarabine.
↵c Mycophenolate mofetil (Cellcept), tacrolimus (FK-506), and prednisone.

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Table 3:

Diagnoses in the 40 patients in the control cohort

Diagnosis	No.
Acute disseminated encephalomyelitis	5 (12.5%)
Multiple sclerosis	5 (12.5%)
Neurofibromatosis	4 (10%)
PRES	4 (10%)
Fragile X–associated tremor ataxia syndrome	3 (7.5%)
Lymphoma	3 (7.5%)
Neuro-Behçet disease	3 (7.5%)
Cerebrotendinous xanthomatosis	2 (5%)
HIV encephalopathy	2 (5%)
Susac disease	2 (5%)
Alexander disease	1 (2.5%)
Krabbe disease	1 (2.5%)
Langerhans cell histiocytosis	1 (2.5%)
Metachromatic leukodystrophy	1 (2.5%)
Neurosarcoidosis	1 (2.5%)
Toxoplasmosis	1 (2.5%)
Wilson disease	1 (2.5%)