Neuronal intermediate filament paraneoplastic autoimmunity complicating avelumab therapy of Merkel cell carcinoma

A 67-years-old woman developed subacute oculomotor nerve palsy and cerebellar gait instability while receiving avelumab as immunotherapy for Merkel cell carcinoma. Brain MRI revealed oculomotor nerve T2/FLAIR hyperintensity and contrast enhancement, CSF cell number and protein concentration were slightly increased. Antibodies against intracellular and surface antigens were excluded through commercial assays, but home-made immunohistochemistry on rat brain sections showed a "neurofilament-like" pattern. Antibodies against neuronal intermediate filament (NIF-IgG) were thus tested and resulted positive in both serum and CSF, confirming the diagnosis of NIF-IgG autoimmunity. Avelumab was discontinued and treatment with steroids and intravenous immunoglobulins led to partial improvement.