Ultrasound-Confirmed Median Nerve Pseudoconduction Block due to Anatomical Variation

Conduction block (CB) at non-entrapment sites is an important electrodiagnostic (EDX) feature supporting acquired demyelinating neuropathies. However, anatomical variation may produce pseudoconduction block, mimicking true CB. After encountering a patient with median nerve pseudoconduction block due to anatomical variation, we aimed to assess the frequency of this variation, its impact on EDX interpretation, and the role of neuromuscular ultrasound (NMUS) in improving diagnostic accuracy.

We describe an index patient with median nerve pseudoconduction block in the forearm caused by anatomical variation, confirmed by ultrasound. We then retrospectively reviewed upper limb NMUS performed by a single EDX medicine specialist at University Hospitals Cleveland Medical Center. At the antecubital fossa, the distance between the midpoint of the brachial artery and the midpoint of the median nerve was measured. Available EDX values were reviewed for ultrasounds with marked nerve-artery separation.

Sixty-eight patients were included. In all patients, the median nerve lay medial to the brachial artery (range: 2.5-26 mm). Seven patients (10.3%) had nerve-artery separation greater than 12.5 mm. Of these, five had available EDX data for retrospective review and three (60%) reported either complete or partial CB.

True CB suggests significant pathology. NMUS can help distinguish pseudoconduction block from true CB and prevent misdiagnosis. When pseudoconduction block of the median nerve in the forearm is suspected, stimulation 15-25 mm medial to the brachial artery pulse, or slightly more medial and proximal (between the bicep tendon and brachialis muscle) may improve accuracy.