Fenestrated Basilic Vein Traversed by a Rare Variant of the Medial Brachial Cutaneous Nerve.

Variations of the upper limb's neurovascular structures hold significant clinical relevance for surgical, anesthetic, and vascular procedures. Although variations in the formation and branching of the brachial plexus and venous architecture of the arm are common, neurovascular crossings involving venous fenestrations are exceptionally rare. During a routine cadaveric dissection of a 68-year-old male, a fenestrated basilic vein (BV) was identified on the right arm, approximately 12.7 mm distal to the inferior border of the pectoralis minor muscle. The fenestration measured 17.6 mm in length and consisted of distinct medial and lateral limbs. One brachial vein drained into the lateral limb, while the other joined the BV distal to the fenestration. At the same level, the medial brachial cutaneous nerve (MBCN) was formed by its typical medial root and a lateral variant branch communicating with the ulnar nerve (UN). Notably, this communicating branch traversed the fenestrated segment of the BV. The contralateral arm did not have any neurovascular variation. This rare configuration likely results from incomplete condensation of the primitive venous plexus combined with aberrant axonal guidance during brachial plexus development. Such variants are of clinical importance due to their potential impact on venous catheterization, axillary block anesthesia, and vascular access surgery.