An 86-year-old man presented with 1 month of abnormal involuntary movements. He had a 17 lb unintentional weight loss, no exposure to neuroleptics or anti-emetics, and no family history of chorea. Exam: facial plethora, diffuse ecchymoses, generalized chorea. Labs 3 months before developing chorea: high WBC 11.8 (normal differential), low Hgb 11.4 (chronic), normal platelets 202. Further workup: low erythropoietin level, normal flow cytometry, positive JAK2V617F mutation. Negative workup for other causes of chorea. JAK2V617F mutation-positive myeloproliferative disorder accounted for elevated WBC with associated chorea. Both improved with hydroxyurea, tetrabenazine, and aspirin.

A PubMed search with terms “chorea” and “polycythemia vera” showed 31 case reports with PV, 1 with essential thrombocythemia, 1 with pancytopenia, 1 with normal CBC and JAK2V617F mutation with contralateral stroke, 1 with high altitude erythrocytosis, and 1 using oral contraceptive pills. A search with “chorea” and “JAK2V617F mutation” showed 5 case reports with PV (all included in prior search results) and 1 with JAKV617F mutation without PV. A search for “JAK2” and “striatum” showed studies demonstrating JAK2 expression in striatal progenitor cells and neuroprotection by JAK2 inhibition.

Chorea associated with the JAK2V617F mutation can manifest with any form of myeloproliferative disorder, not just PV, and can precede the development of abnormal blood cell counts. Our case and literature review demonstrate the importance of checking a CBC in the workup of late-onset chorea. The classic proposed mechanism to generate chorea is hyperviscosity in the basal ganglia. However, expression of JAK2 in striatal progenitor cells suggests a direct mechanism.