Pembrolizumab-associated neuronopathy with improved lower limb somatosensory evoked potential after intravenous methylprednisolone treatment: A case report

Abstract Background: Peripheral neuropathy associated with immune checkpoint inhibitor (ICI) has been reported in several cases; however, cases of ICI-associated neuronopathy are rare. Herein, we report a case of ICI-associated neuronopathy with acute sensory ataxia. Case presentation: A 54-year-old male was undergoing chemotherapy for right maxillary sinus cancer. Pembrolizumab had been completed 2 months prior, and the drug was switched to cetuximab and paclitaxel due to tumor enlargement. Anorexia has been observed since the current chemotherapy, and he was admitted to the department of otorhinolaryngology. Immediately after admission, diarrhea and worsening of abnormal sensations in the lower limbs were observed. He was unable to walk with severe ataxia. Neurological examination revealed normal muscle strength but severe deep sensory impairment. Cerebrospinal fluid examination revealed elevated cell counts with a predominance of mononuclear cells, elevated proteins and interleukin 6. Nerve conduction studies have shown sensory neuropathy, predominantly in the lower limbs. Somatosensory evoked potentials (SEPs) with tibial nerve stimulation showed a markedly prolonged P15-N21 latency difference. A colonic biopsy suggested ICI-associated colitis. Based on these findings, the patient was diagnosed with ICI-associated neuronopathy. After two courses of intravenous methylprednisolone, sensory ataxia and diarrhea improved, and lower-limb SEP showed marked improvement in the P15-N21 latency. Conclusions: Lower-limb SEP is useful not only for the diagnosis of ICI-associated neuropathy but also for post-treatment assessment.