Musculocutaneous Nerve Injury - Diagnosis and Management

Introduction:

The musculocutaneous nerve is the large terminal branch of the lateral cord of the brachial plexus. It receives fibers from C5, C6, and C7 nerve roots. It emerges from the inferior pectoralis minor muscle's inferior border and later passes to the axillary and upper part of the brachial artery. It leaves the axilla and pierces the coracobrachialis. It then enters the elbow by piercing the deep fascia and terminates in the forearm as a lateral cutaneous nerve or lateral antebrachial cutaneous nerve.

What Are the Branches of the Musculocutaneous Nerve?

The branches of the musculocutaneous nerve that run through the upper arm provide a motor function to the muscle it innervates. In contrast, it only plays a sensory part when it enters the lateral cutaneous nerve.

The motor branches to:

• Coracobrachialis.

• Brachialis.

• Biceps brachii.

All three muscles innervated by the nerve work together to provide motion to the arm.

Sensory branches to:

• Lateral forearm.

Articular branches to:

• Humerus.

• Elbow joint.

How Is the Musculocutaneous Nerve Injured?

The musculocutaneous nerve is not injured often as it is well protected in the axilla and elsewhere along its course. Thus isolated injuries are rare.

However, the nerve may become injured by the following:

• Injury to brachial plexus.

• Compression injuries such as weight lifting or other sports involving forearm flexion and supination.

• Shoulder dislocation.

• Stabbing or gunshot wound.

• Shoulder surgery.

• Nerve entrapment at the elbow.

• Repetitive stress injury in athletes.

Damage to the nerve can lead to:

• Loss of sensation in the skin in the region of nerve supply.

• Weakened movement, such as flexion at the shoulder joint and elbow joint.

• Weakened rotation of the arm.

How to Examine the Musculocutaneous Nerve Injury?

The following examinations can be done:

• Checking the strength of the muscles innervated by the nerve.

• Checking sensation on the lateral forearm.

• Checking nerve entrapment.

• Cross-checking and ruling out differential diagnoses.

What Is the Mechanism Behind Musculocutaneous Nerve Injury?

The mechanism behind the injury is not entirely known. Some describe it as secondary to iatrogenic causes, such as prolonged positioning of the arm and direct injury to nerves during surgery. Other contributing factors include repetitive, vigorous upper extremity exercises such as weight lifting, carrying, or throwing heavy objects in sports. Most reported injuries occur in the coracobrachialis muscle due to strong contraction of muscle or hypertrophy, resulting in ischemic nerve injury.

Peripheral nerve injuries range in severity. Neuropraxia injuries are more commonly seen and involve a conduction block without damaging the neural element. This injury usually occurs following a mild crush, compression, traction, or ischemia. Axonotmesis occurs from axonal injury, usually following a severe trauma that causes a crush injury. Nerve regeneration or repair can be done depending on the severity of the injury.

What Is the Differential Diagnosis of Musculocutaneous Nerve Injury?

It includes:

• Biceps tendon injury or rupture.

• Strain or tearing of biceps or brachialis muscle.

• Cervical 5 or 6 (C5 or C6) radiculopathy.

• Brachial plexus injury.

What Is Diagnostic Testing Used to Detect Musculocutaneous Nerve Injury?

• Upper extremity entrapment neuropathy is diagnosed through clinical assessment. However, a variety of diagnostic tests are available to confirm clinical findings.

• Electromyography and nerve conduction studies are used to determine the location and severity of the nerve injury. These techniques are invasive, dependent on the operator, and do not determine the structural causes of denervation.

• Ultrasonography (USG) and magnetic resonance imaging (MRI) may sometimes be used to confirm the etiology and location of nerve compression. Ultrasound is a quick, noninvasive, and cost-effective method. It allows the visualization of long nerve segments. It is generally used to rule out biceps tendon injury. Magnetic resonance imaging helps demonstrate specific patterns of muscle denervation, identifying edema at the earliest within 24 to 48 hours of denervation.

• Motor and sensory nerve conduction studies are also used.

• Needle electromyography identifies the presence of denervated muscles and shows the absence of motor unit recruitment during movement.

• Electrodiagnostic studies should be performed after 10 to 21 days of a peripheral nerve injury.

• Magnetic resonance neurography (MRN) is a new imaging modality generally done to confirm clinical suspicion of peripheral neuropathy. It helps demonstrate nerve abnormalities, entrapment, impingement lesions, or diffuse peripheral nerve lesions. It helps explain morphological changes of nerves, such as contour, calibre, and continuity in relation to other nerves, muscles, or bones.

Abnormally high signals in nerve fascicles demonstrate nerve injuries such as compression neuropathies, peripheral nerve tumors, or traumatic injuries. These findings help manage severe injuries.

How to Manage Musculocutaneous Nerve Injuries?

Both surgical and non-surgical treatment strategies exist. Conservative management is first implemented, and then the surgical approach is used if no improvements are seen.

Non-operative techniques are as follows:

• Rest.

• Reduced activity.

• Pharmacological agents such as non-steroidal anti-inflammatory drugs or anticonvulsants.

• Local inhibition methods such as injections of corticosteroids or botulinum toxin.

• Physical therapy.

Many people recover completely from the non-surgical technique. However, some cases require more intense conservative management, which includes:

• Education.

• Soft tissue interventions and therapies.

• Neurodynamic mobilization.

• Mobilization and manipulation of local joints.

• Myofascial therapies such as active release technique (ART).

Numerous surgical options have been applied if the nerve injury fails to respond to conservative management.

Operative management includes:

• Epineurotomy (a lengthwise incision in the epineurium of a nerve without harming the fascicles, commonly performed under magnification).

• Decompression.

• Nerve graft.

• Nerve transfer.

Conclusion:

Musculocutaneous nerve injuries are less often seen. Although this condition is rare, timely diagnosis and appropriate treatment are essential to prevent further nerve damage and muscle atrophy. The healthcare worker identifies the signs and symptoms and chooses the approach that has to be implemented. The conservative approach is the first line of treatment used. In case of nerve damage, some people experience spontaneous recovery through a conservative approach. But even this takes several months. Conservative treatment includes rest, reduced movement, ice, anti-inflammatory medicines, physical therapy, etc. However, surgical procedures are considered when the conservative approach fails or does not respond quickly. In some cases, nerve grafting or nerve transfer is done to restore the function.