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Paraneoplastic cerebellar degeneration is a rare disabling syndrome typically associated with an underlying malignancy. It is an immune-mediated process whereby onconeural antibodies cause inflammation and subsequent destruction of cerebellar Purkinje cells. Symptoms include dizziness, double vision, and impairment of balance.

Merkel cell carcinoma is a rare neuroendocrine tumor which has been associated with paraneoplastic syndromes. Nivolumab, an immune-checkpoint inhibitor used as adjunctive treatment of metastatic disease, has been linked to a case of acute cerebellar ataxia.

A 59-year-old male was diagnosed with Merkel cell carcinoma by shaved biopsy of a lesion on his left hand. He was treated with wide local excision with flap resection. He subsequently developed dizziness, diplopia, and problems with balance. Patient had down-beating nystagmus on primary gaze, gaze-evoked nystagmus, and ataxia. The patient did not have fever or signs of meningeal irritation. Patient could not get an MRI due to a spinal stimulator. CT head with contrast was unremarkable. Initially, patient opted for conservative treatment with Baclofen.  Patient received Nivolumab as adjunctive treatment for cancer. Patient reported severe worsening of balance and was further evaluated. Autoimmune encephalopathy panel was negative. Pt was treated with IVIG with improvement of nystagmus and diplopia and mild improvement of balance. Patient was followed up outpatient three months after IVIG course with no worsening or new symptoms.

Diagnosis of paraneoplastic cerebellar degeneration relies on history and exam. While detection of antibodies makes the diagnosis definite, recognizing the signs and symptoms and the proximity of their onset to the diagnosis of cancer is essential in its identification. Merkel cell carcinoma and Nivolumab have been associated with deleterious neurologic complications and it is vital to watch for subacute onset of dizziness, diplopia, nystagmus, and ataxia as they often point closely to paraneoplastic cerebellar degeneration.