A 67-year-old man presented with one-week history of progressive left facial droop, unsteady gait, slurred speech, double vision, and patchy numbness; he reported Ad26.COV2.S vaccination three weeks before presentation. On initial examination, he had left facial palsy, partial left abducens nerve palsy, generalized areflexia, severe lower extremities ataxia, and diminished vibratory sensation in toes. Cerebrospinal fluid analysis showed albuminocytologic dissociation. He received intravenous immunoglobulin (IVIG) for five days for presumptive diagnosis of Guillain-Barré syndrome (GBS) . Electrodiagnostic studies demonstrated impersistent F-waves, decreased recruitment in lower extremities, and bilaterally absent blink reflexes.

He developed right facial weakness on day three and asymmetric appendicular muscle weakness on day five of treatment. Seven weeks after symptom onset, his lower extremities weakness worsened, and received a second cycle of IVIG. The lower extremity weakness did not improve but remained stable until seventeen weeks after symptom onset when he returned with progressive upper extremities weakness and dyspnea. Electrodiagnostic studies demonstrated demyelinating features consistent with CIDP. Neuromuscular ultrasound demonstrated multifocal median nerve enlargements at non-entrapment sites. At this point, given the clinical course, electrodiagnostic and ultrasonographical findings, CIDP diagnosis was established.