Pembrolizumab-associated Neuronopathy with Improved Lower Limb Somatosensory Evoked Potential After Intravenous Methylprednisolone Treatment.

Cases of neuronopathy associated with immune checkpoint inhibitors (ICIs) have rarely been reported. We herein report a case of ICI-associated neuronopathy. A 54-year-old man underwent chemotherapy for right maxillary sinus cancer. Two months after pembrolizumab treatment, diarrhea, worsening of abnormal sensations, and severe ataxia of the lower limbs were observed. Somatosensory evoked potentials (SEPs) with tibial nerve stimulation showed disappearance of the N21 waveform. A colonic biopsy suggested ICI-associated colitis. Based on these findings, the patient was diagnosed with ICI-associated neuronopathy. Clinical symptoms and SEP findings improved markedly after two courses of intravenous methylprednisolone.