Neuronal intermediate filament neurologic autoimmunity in a patient with Merkel cell carcinoma receiving pembrolizumab

Background Neuronal intermediate filament (NIF) autoimmunity is an uncommon paraneoplastic neurologic syndrome that has increasingly been recognized in association with immune checkpoint inhibitor (ICI) therapy. Merkel cell carcinoma is among the malignancies most frequently associated with NIF antibodies, although reported cases remain limited. Case Presentation A 55-year-old female with stage IIIA Merkel cell carcinoma developed vertigo, gait instability, dysarthria, and persistent right hemifacial and cervical spasms after receiving second dose of pembrolizumab for recurrent disease. Brain MRI was unremarkable, while continuous electroencephalography demonstrated encephalopathy without a definitive electrocerebral correlate, raising suspicion for subcortical myoclonus. Cerebrospinal fluid (CSF) analysis revealed lymphocytic pleocytosis, elevated protein. CSF and positive NIF antibodies using cell-based assay. Pembrolizumab was discontinued, and the patient was treated with high-dose corticosteroids followed by monthly intravenous immunoglobulin, resulting in near complete resolution of her neurologic symptoms after 6 months. Conclusion This case expands the growing spectrum of ICI-associated NIF paraneoplastic neurologic autoimmunity complicating Merkel cell carcinoma. To our knowledge, persistent hemifacial and cervical spasm has rarely been reported in this clinical setting. Early recognition of this syndrome, prompt antibody testing, discontinuation of ICI therapy, and initiation of therapy may improve neurological outcomes while informing further oncologic management.