To describe a patient with colorectal carcinoma who developed an acquired polyneuropathy which manifested as progressive proximal muscle weakness over a period of approximately 5 months following treatment with cetuximab. 

Cetuximab is an epidermal growth factor receptor inhibitor monoclonal antibody used in the treatment of colorectal and head and neck carcinomas. It has not been associated with significant neurotoxicity in the past, but two prior case reports have described development of CIDP with use of cetuximab. 

A 24-year-old patient with colorectal cancer due to familial adenomatous polyposis was frequently treated with cetuximab for varying durations from mid-2020 to mid-2024. Beginning in early 2024, she began experiencing progressive proximal muscle weakness. Neurologic examination demonstrated hyporeflexia as well as proximal weakness that was worse in the arms than in the legs. Extensive workup for an alternative etiology of the patient’s symptoms, including rheumatologic testing, antibody testing for paraneoplastic etiologies, testing for disorders of the neuromuscular junction, and MRI of the cervical spine and thighs was negative. Electromyographic testing was consistent with a diagnosis of CIDP. She was treated with corticosteroids and had complete resolution of her symptoms.

Two prior case reports have suggested a relationship between the use of cetuximab and the development of CIDP. While more research is needed to firmly establish a causative relationship between the two, clinicians should consider CIDP as a possible etiology of motor weakness or sensory loss in patients treated with cetuximab.