Lateral Antebrachial Cutaneous Nerve Anatomy

Origin. The lateral antebrachial cutaneous nerve (LACN, also LABCN) is the terminal sensory continuation of the musculocutaneous nerve, which arises from the lateral cord of the brachial plexus (C5-C6). The musculocutaneous nerve pierces coracobrachialis and runs between biceps and brachialis, supplying coracobrachialis, biceps and brachialis. Those motor branches are given off proximally, so what continues past the elbow is purely sensory.

Emergence at the elbow. The nerve pierces the deep fascia at about the level of the elbow crease, coming out from beneath the lateral border of the biceps in the interval between biceps and brachialis/brachioradialis. There it lies lateral to the biceps tendon and immediately deep or adjacent to the cephalic vein. That relationship is the key to its clinical vulnerability.

View larger

Branches. In the forearm the LACN typically divides into two. The larger anterior (volar) branch descends with the cephalic vein along the radial border of the volar forearm to the wrist and the base of the thenar eminence, and its course alongside the vein is why venepuncture and cannulation injure it. The smaller posterior (dorsal) branch runs over the dorsoradial forearm towards the wrist.

Territory. Together the branches supply the skin of the lateral (radial) forearm from just below the elbow to the wrist: the volar branch the radial-volar surface and thenar base, the dorsal branch the dorsoradial surface. The nerve does not supply the hand beyond the radial wrist and thenar base, because the dorsoradial hand and thumb web belong to the superficial radial nerve. With no motor function, it gives nothing to test on power examination.

Communications. Near the wrist the LACN communicates with the superficial radial nerve and with the palmar cutaneous branch of the median nerve. That is the anatomical basis for the clinical overlap with superficial radial nerve symptoms.

Where it is injured. Its proximity to the distal biceps tendon and the cephalic vein makes the LACN the nerve most commonly injured in two settings, and it is also at risk in other lateral elbow and anterior forearm approaches.

• Distal biceps tendon repair, particularly the anterior single-incision approach and the anterior limb of the two-incision approach
• Antecubital venepuncture or cephalic vein cannulation ('traumatic phlebotomy')

Protecting it. In the anterior approach to the distal biceps the nerve lies lateral to the tendon and just deep to the fascia, so it is at its most vulnerable during the initial fascial incision and lateral retraction rather than at depth. Awareness of its position and careful retraction reduce injury. At venepuncture, avoid the area lateral to the biceps tendon where the nerve lies with the cephalic vein.

Presentation. Injury causes numbness, paraesthesia or dysaesthesia over the lateral forearm, sometimes with a painful neuroma or a Tinel's sign over the nerve at the elbow. The deficit is sensory only, without weakness. Because the symptoms are sensory and overlie the lateral forearm, they can mimic superficial radial nerve symptoms or lateral elbow pathology.

Diagnosis. It is usually a clinical diagnosis, made from the history (recent distal biceps surgery or venepuncture) and lateral forearm sensory signs. Nerve conduction studies can record LACN sensory responses, but they alone are insufficient, so the diagnosis rests on anatomy and clinical correlation. Bilateral absence of LACN sensory responses can suggest an inflammatory cause within a wider musculocutaneous neuropathy.

Imaging. Ultrasound or MRI can show LACN enlargement when needed. Ultrasound makes the diagnosis practical because the nerve is superficial and the other arm is a free control: an injured nerve is fusiform, shows increased echogenicity and is roughly four times the cross-sectional area of the contralateral normal.

Management of injury. Most neurapraxic and traction injuries recover. A symptomatic neuroma may need desensitisation, injection, or excision/relocation.

Because the LACN territory clinically overlaps neighbouring nerves, the examiner expects a structured differential of the patient with lateral-forearm or radial-sided sensory symptoms.