Introduction

The medial antebrachial cutaneous nerve, along with the posterior and lateral antebrachial cutaneous nerves, is responsible for providing sensation to the skin of the forearm.[1] Specifically, the medial antebrachial cutaneous nerve provides sensory innervation of the medial forearm as well as the skin overlying the olecranon.[2] It emerges from the medial cord of the brachial plexus and has sensory cell bodies located in C8 and T1. It travels distally along the upper arm running through the brachial fascia along with the basilic vein approximately 10 cm proximal to the medial epicondyle. As the medial antebrachial cutaneous nerve exits the brachial fascia, it divides into two major branches, anterior and posterior, which then continue distally as far as the wrist.

Structure and Function

Originating in the cervical and thoracic spine from spinal nerves C5 to T1, the roots of the brachial plexus divide sequentially into roots, trunks, divisions, cords, and terminate as branches.[3] The medial antebrachial cutaneous nerve is one of three non-terminal branches of the medial cord, which itself is the continuation of the anterior division of the inferior trunk of the brachial plexus. The other two non-terminal branches of the medial cord are the medial brachial cutaneous nerve, which provides sensory innervation for the medial arm and the medial pectoral nerve, which provides motor innervation to pectoralis major and minor. The medial cord also contributes fibers to the median nerve and ultimately continues distally as the ulnar nerve.[4] The medial antebrachial cutaneous nerve can be found superficial to the axillary artery and vein within the axillary fossa, near the median and ulnar nerves.[5] As it courses distally, it accompanies the basilic vein as it crosses between the triceps brachii and brachialis muscles. It then enters the brachial fascia overlying the biceps brachii and runs at the ulnar side of the brachial artery. From there, the nerve runs with the basilic vein at the level of the elbow, where it ultimately divides distally to the elbow into the volar and ulnar branches, which provide sensory innervation to the medial forearm and skin of the olecranon.[2][3]

Embryology

In the fifth week of development, mesoderm-derived limb-buds grow outward from the developing embryo while the ectoderm-derived brachial plexus, and thus the medial antebrachial cutaneous nerve, forms as segmental nerves from the spinal cord penetrate these limb-buds.[3] The brachial plexus originates from the fifth through eighth cervical segments, as well as the first thoracic segment. Two branches of the brachial plexus emerge: one dorsal branch which innervates extensor muscles and one ventral branch which innervates flexor muscles. Growth factors like homeobox (Hox) genes, sonic hedgehog (SHH), and fibroblast growth factor (FGF) aid in the positioning of the limbs at birth. Homeobox genes help in proximodistal limb patterning, Sonic hedgehog signaling molecules regulate limb growth, and fibroblast growth factors assist in anterior-posterior limb patterning.[3]

Nerves

The anterior and posterior branches of the medial antebrachial cutaneous nerve usually appear at the level of the medial and lateral epicondyles.[1] The anterior branch gives off 7 to 10 secondary branches and is primarily distributed in the middle one-third of the anterior medial forearm. The posterior branch gives off 10 to 12 secondary branches, primarily distributed in the proximo-medial region of the posterior forearm.[1] In addition to the two major branches, several cutaneous branches of the medial antebrachial cutaneous nerve have been identified, and all emerge medially coursing away in an anterolateral direction.[6]

Physiologic Variants

There have been several reported variations in the anatomy of the medial antebrachial cutaneous nerve as well as its branching pattern. There are cases where the nerve itself arises from the inferior trunk of the brachial plexus instead of the medial cord.[7] The posterior branch is highly variable as reports have described anastomosis between it and the ulnar nerve as well as its palmar cutaneous branch, the medial brachial cutaneous nerve, and the posterior antebrachial cutaneous nerve.[2] There is another variation in which the described four brachial branches of the medial antebrachial cutaneous nerve emerging before the separation into anterior and posterior branches. These branches travel to the medial, distal region of the upper arm, which is an area normally innervated by the medial brachial cutaneous nerve.[2]

Surgical Considerations

Understanding the anatomy of the medial antebrachial cutaneous nerve is important in transplant surgery using forearm flaps as donor tissue. Recipient tissues for such flaps have included the mouth, penis, and hands. Thus, successful sensory recovery for these areas relies on knowledge of the cutaneous innervation of the donor tissue.[1] One such example is the forearm free flap phalloplasty. This surgical technique is used in transgender surgeries to create a neophallus. The flap involved in this procedure contains the skin of the anterior forearm and much of the surrounding tissue, including the medial and lateral cutaneous nerves of the forearm.[8] The medial antebrachial cutaneous nerve itself can also be used in surgical grafts and gets frequently used in brachial plexus reconstructions.[9] 

Posterior branches of the medial antebrachial cutaneous nerve cross the ulnar nerve, normally 2 cm distal to the medial epicondyle, and are therefore at risk of damage during ulnar nerve releases at the elbow to treat cubital tunnel syndrome. Complications of damage to the medial antebrachial cutaneous nerve in such procedures include hyperesthesia and hyperalgesia, as well as painful neuromas.[10][11] The anterior branch of the medial antebrachial cutaneous nerve is also important surgically as it is used as the “cable graft” in the repair of peripheral nerves such as the digital nerve.[12] Knowledge of the anatomy of the medial antebrachial cutaneous nerve plays a significant role in several vascular surgeries involving the upper arm. Examples of these surgeries include arteriovenous fistula formation between the brachial artery and basilic vein as well as the brachial artery and vein, arteriovenous fistula steal syndrome surgery, thrombectomies, and embolectomies of the brachial artery.[13]

Clinical Significance

Knowledge of the anatomy of the medial antebrachial cutaneous nerve is vital clinically as the nerve is prone to damage during various procedures such as venipunctures, therapeutic injections for elbow pain, and cubital tunnel releases.[2][14] Accidental damage to the nerve following such procedures has been found to result in painful paresthesias, dysesthesias, and numbness.[7] Snapping elbow refers to a condition where flexion and extension of the elbow causes a painful, popping sensation which at times can even be heard, seen, and palpated. This condition is most often the result of ulnar nerve dislocation over the medial epicondyle. There are reports, however, of patients who described a similar painful popping sensation of the elbow and were found to have a snapping of the medial antebrachial cutaneous nerve.[15][16] The medial antebrachial cutaneous nerve, along with many other nerves of the upper extremity, is prone to injury in athletes who perform repetitive throwing motions. Throwing motions damage these nerves by creating compression and traction, leading to various neuropathic syndromes.[17] Rare causes of damage to the medial antebrachial cutaneous nerve include subcutaneous lipomas, laceration, and neuritis caused by tuberculoid leprosy.[7]