Disclosures: Sharon R. Ong, MD: No financial relationships or conflicts of interest

Case Diagnosis: Isolated medial antebrachial cutaneous nerve lesion

Case Description or Program Description: A 28-year-old right-handed male mechanic presented with 2 weeks of burning pain in his right medial forearm. This started without specific inciting factor. He was unable to work due to severe pain. He has a history of labral surgery for multiple daily right shoulder dislocations, and wears a shoulder brace at work. There was no associated neck pain, weakness, and negative family history of peripheral nerve disease. Examination was normal except for decreased sensation focally in the right medial antebrachial cutaneous nerve (MACN) distribution. Pain worsened with shoulder extension and abduction.

Setting: Outpatient musculoskeletal clinic

Assessment/Results: X-ray of the humerus, shoulder and cervical spine were normal. Ultrasound and MRI of the MACN did not reveal any focal entrapment. Electrodiagnostic study demonstrated an isolated axonal MACN lesion. He was treated with oral corticosteroid, Gabapentin, and physical therapy with minimal symptom improvement. He had transient resolution of pain after perineural corticosteroid injection and hydrodissection but persistent numbness at 6 months.

Discussion (relevance): Isolated MAC neuropathy is rare. The anterior and posterior branches branch at the elbow to innervate the anterior medial forearm and proximo-medial region of the posterior forearm, respectively. MACN runs closely with the ulnar nerve and can commonly be injured during elbow surgery. Repetitive arm use has been described as a rare cause of isolated MAC lesion. No specific cause of the lesion could be identified in our patient. Etiology may include traction or scar entrapment due to shoulder instability and repetitive overhead work versus a compression injury from the shoulder brace he wore at work.

Conclusions: MAC neuropathy is rare but can be severe and disabling. Diagnosis includes ultrasound or MRI imaging to rule out compressive lesions and electrodiagnostics to rule out lower plexus disorders or C8 or T1 radiculopathy. Nerve blocks can provide successful but transient pain relief.

Level of Evidence: Level IV

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/mechanic-with-non-iatrogenic-macn-lesion/