To report a case of immune checkpoint inhibitor (ICI)-induced Chronic Inflammatory Demyelinating Polyneuropathy (CIDP).

Case: 22-year-old man with Stage 3A Melanoma developed subacute-onset lower extremity weakness a month after a dose of Nivolumab. His initial exam revealed decreased strength in hip flexion, knee flexion, and ankle extension with intact reflexes. His oncologist prescribed steroids for ICI-induced myositis and Nivolumab was discontinued. Three weeks later, his exam showed diffuse weakness in his extremities with absent reflexes, however, intact vibration and pinprick sensation in all distal extremities. Same-day nerve conduction study (NCS) and EMG revealed findings consistent with a subacute motor-predominant neuropathy. He underwent a 5-day course of IVIg for a presumed ICI-induced immune-mediated GBS.  His strength initially improved, however, three months later he had slowly progressive bilateral distal extremity numbness and recurrent weakness. Repeat EMG/NCS showed demyelinating polyneuropathy with absent sensory responses in his upper extremities but preserved sural responses. IVIg therapy was initiated. Three months later, his exam revealed normal upper extremity strength with mild lower extremity weakness and persistent decreased pinprick sensation in his feet. The relapse and progression of his symptoms was most consistent with CIDP.

Neurologic ICI-related toxicities may be difficult to accurately diagnose at presentation and may transcend usual diagnostic categories. This case initially resembled AMAN (acute motor axonal neuropathy) but his relapses and longer period of progression, as well as demyelinating physiology, show a transition to CIDP. Immune-mediated toxicities can persist despite discontinuation of ICI therapy and immunosuppression. Therefore, patients with ICI-neurologic toxicities require consideration of atypical presentations of neuroinflammatory disorders and close follow-up after treatment initiation.