Inflammatory Myopathy and Axonal Neuropathy in a Patient With Melanoma Following Pembrolizumab Treatment

Immune-mediated adverse effects of immune checkpoint inhibitors are rather common, but neuromyopathic immune-related adverse events are very rare. In this report, we present a unique case of a patient with a complex neuromyopathic syndrome with axonal neuropathy and inflammatory myopathy after a single dose of pembrolizumab. An 82-year-old patient with a previously untreated stage IIIc melanoma developed ptosis in the left eye, generalized weakness, and neck and shoulder pain 15 days after pembrolizumab administration. He had left-sided ptosis and miosis, with a normal pupillary light reflex, horizontal diplopia, and voice hoarseness, along with weakness of the neck muscles and a hypokinetic right vocal cord at laryngoscopy. The laboratory evaluation was remarkable for the marked increase in the serum lactate dehydrogenase and creatine phosphokinase levels. Further evaluation revealed findings compatible with axonal neuropathy and inflammatory myopathy. The patient was treated with corticosteroids, immunoglobulin, and plasmapheresis, with a minor response; the patient eventually died. This case represents a newly described syndrome probably associated with pembrolizumab administration.