Ulnar-Median Nerve Transposition: Embryology and Clinical Implications

Abstract

Branches of the brachial plexus situated within the flexor compartment of the arm, the musculocutaneous, median and ulnar nerves maintain a predictive topography. The musculocutaneous nerve courses lateralward, the ulnar medial-ward, and the median between these. The present case features bilateral occurrence of ulnar and median transposition in a 74-year-old female cadaver. This transposition is related to dissimilar lengths of the lateral and medial roots of the median nerve. Both lateral roots of the median nerve were found to be unusually longer than usual. In addition, both medial roots coursed infero-medially instead of its usual superolateral coursing from their respective medial cords. These anomalies resulted in the median nerve being placed postero-inferior and medial to the ulnar nerve both lateral roots were observed crossing anterior to and impinging on their respective ulnar nerve. Clinical implications of the observed transposition include possible neurographic or MR ultrasonography misinterpretation of images. In addition such transposition may pose challenges to trauma surgeons engaged in nerve reconstruction following crush or mangled arm injuries. Anesthesiologists engaged in selective median or ulnar nerve blocks may witness increased procedural time and failures, as well as unexplained failure of analgesia in the skin areas normally supplied by their intended blocked nerve, a clinical situation that can be resolved by performing a neurostimulation.