Index
He is responsible for a very important function that we use every day; bend and flex your elbows.
The musculocutaneous nerve is a major peripheral nerve in the upper limb. The musculocutaneous nerve arises from the lateral cord of the brachial plexus, opposite the lower border of the pectoralis major, and its fibers are derived from C5, C6, and C7.
Tasks like raising a cup and brushing our teeth can be very difficult if the nerve is not working well.
The word musculocutaneous gives us a clue as to what the nerve does. “Muscle” implies its role in the innervation of the muscles (motor part) and “cutaneous”, that is, the skin, suggests that it also has a sensory role.
Motor functions : innervates the muscles in the anterior compartment of the arm; the coracobrachialis / coracobrachialis, the biceps brachii and the brachialis.
These muscles flex the upper arm at the shoulder and elbow. In addition, the biceps brachii also supinates the forearm.
Sensory functions : gives rise to the lateral cutaneous nerve of the forearm, which innervates the skin on the lateral surface of the forearm.
This nerve initially enters the deep forearm, but then pierces the deep fascia to become subcutaneous. In this region, it can be found very close to the cephalic vein.
The musculocutaneous nerve is well known to have a varied anatomical course. It can interact with the median nerve, adhere to the nerve, and exchange fibers.
Structure
The musculocutaneous nerve arises from the lateral cord of the brachial plexus, runs along the front of the arm, and ends 2 cm above the elbow as the lateral cutaneous nerve of the forearm.
The musculocutaneous nerve arises from the lateral cord of the brachial plexus with a root value of C5 to C7 from the spinal cord.
It follows the course of the third part of the axillary artery (part of the axillary artery distal to the pectoralis minor) laterally and enters the front face of the arm where the coracobrachialis muscle penetrates.
It then passes downward and laterally between the biceps brachii (above) and the brachialis muscles (below), to the lateral side of the arm; 2 cm above the elbow, it pierces the deep fascia lateral to the biceps brachii tendon and continues into the forearm as the lateral cutaneous nerve of the forearm.
On its course through the arm, it innervates the coracobrachialis, the biceps brachii, and most of the brachialis. Its terminal branch, the lateral cutaneous nerve of the forearm, provides the sensation of the lateral side of the forearm from the elbow to the wrist.
In addition, the musculocutaneous nerve also gives articular branches to the elbow joint and the humerus.
Throughout its course, the musculocutaneous nerve gives rise to two kinds of branches: collateral and terminal.
The collateral branches of the coracobrachialis, biceps, and brachii muscles, a branch to the shaft of the humerus, the vascular branches, and an articular branch to the anterior aspect of the elbow originate in the upper arm.
Terminal branches appear when the musculocutaneous nerve becomes subcutaneous: one branch is posterior and the other is anterior.
The posterior branch goes more laterally and passes behind the cephalic vein to successively reach the outer and posterior layers of the forearm and descend to the carpus, sending branches to the skin.
The anterior branch continues downwards in front of the cephalic vein, advancing towards the anterior aspect of the forearm and ending as small branches that go to the skin in the anterolateral region of the forearm.
Variations
The musculocutaneous nerve presents frequent variations and communication with the median nerve. It can adhere some distance to the median and then pass outward under the biceps brachii rather than through the coracobrachialis.
Some of the fibers of the median may run for a certain distance in the musculocutaneous and then leave it to join its proper trunk; less frequently the reverse occurs, with the median sending a branch to join the musculocutaneous.
The nerve can pass under the coracobrachialis or through the biceps brachii. Occasionally it gives a filament to the pronator teres, supplying the dorsal surface of the thumb when the superficial branch of the radial nerve is absent.
Clinical relevance
Various surgical procedures on the shoulder involve mobilization or separation of the muscles that attach to the coracoid process. These operations include osteotomy and transfers of the coracoid process, subcoracoid loops, and muscle transfers.
How it commonly occurs : An injury to the musculocutaneous nerve is relatively rare, as it is well protected within the armpit. The most common cause is a stab wound in the armpit region.
Motor functions : the coracobrachialis, biceps brachii and brachii muscles are paralyzed. Flexion at the shoulder is weakened, but can still occur due to the pectoralis major. Flexion at the elbow is also affected, but can still be done due to the brachioradialis muscle.
In addition, the supination of the affected limb is greatly weakened, but it is produced by the supinator muscle.
Sensory functions : loss of sensation on the lateral side of the forearm.
The fibrous sheath arrangements of the corocobrachialis muscle allow for a “telescopic” effect relative to the musculocutaneous nerve.
Any factor that reduces this sliding effect can expose the nerve to the mechanical effects of muscle contraction, with the possibility of compression syndrome.
Nerve injuries produce weakness of flexion at the elbow and weakness of supination. The biceps is an important supinator. There is sensory loss on the lateral side of the forearm.
The brachialis muscle receives innervation from both the musculocutaneous and radial nerves. One study found that the musculocutaneous nerve contributes 42% of the muscle power that flexes the elbow.
Nerve injuries
Musculocutaneous nerve injuries are associated with flexion weakness of the arm and sensory loss along the lateral forearm.
In children, musculocutaneous neuropathies are rare and usually caused by compressive injuries or overuse, or are associated with hereditary neuropathy with pressure paralysis (NHPP).
The musculocutaneous nerve can be damaged by compression, leading to loss of sensation on the outside of the forearm. This type of damage can occur during activities like weight lifting.
Some people who experience this type of injury may recover spontaneously after a few months, but others may require treatment. Damage to the shoulder or brachial plexus can also affect the nerve.
Musculocutaneous nerve injury is often associated with severe brachial plexus injuries and rarely occurs in isolation.
Isolated musculocutaneous nerve injury has less associated disability than any other major upper extremity nerve injury.
The injury is also not common since the nerve has a protected location and course, deep in the arm. It is vulnerable in the armpit, which is the most common site of isolated injury.
Mechanisms include penetrating trauma such as a stab wound, anterior dislocation of the shoulder, and fracture of the neck of the humerus.
Complete division of the nerve may be missed because sensory loss may be ill-defined, and flexion of the elbow by the brachioradialis may be strong enough to mask biceps palsy.
In these cases, it is essential to palpate the biceps while testing its function to identify specific muscle contractions.
Musculocutaneous nerve injury can be caused by three mechanisms: repeated microtrauma, indirect trauma, or direct trauma to the nerve. Overuse of the coracobrachii, biceps, and brachii muscles can cause stretching or compression of the musculocutaneous nerve.
Those who suffer from it may have pain, tingling, or a feeling of reduction on the lateral side of the forearm.
This symptom can be reproduced by pressing on the region below the coracoid process (positive Tinel sign). Pain can also be reproduced by flexing the arm against resistance.
Other differential diagnoses that can mimic the symptoms of musculocutaneous paralysis are:
C6 radiculopathy (pain can be caused by neck movement), biceps longus tendinopathy (without motor or sensory deficit), bicipital groove pain (relieved by shoulder joint) injection).
Electromyography test shows mild neuronal damage in the biceps and brachialis muscles with slower motor and sensory conduction over Erb’s point.
In indirect trauma, violent abduction and retroposition of the shoulder can stretch and damage the musculocutaneous nerve.
Those with this type of injury have pain, reduced sensation, and tingling in the anterolateral part of the arm and the lateral part of the forearm with a reduction in the force of elbow flexion.
Tinel’s sign can be positive. The differential diagnosis includes injuries to the C5 and C6 nerve root of the brachial plexus in which abduction, external rotation, and flexion of the elbow are lost.
On the other hand, the rupture of the biceps can cause the loss of flexion of the elbow without sensory deficits. The electromyography test is negative.
In direct trauma, humerus fracture, pistol shot, glass piece injuries and more, it can cause musculocutaneous nerve injury.
Iatrogenic nerve injuries (for example, during orthopedic surgery involving internal fixation of the humerus) are relatively common and in a certain percentage of cases probably unavoidable, although an adequate knowledge of the surgical anatomy can help reduce their frequency.
Neurolysis and nerve grafting are the treatment options for the above injuries.
Exams
Electromyogram (EMG) evaluation of the musculocutaneous nerve may include the lateral antebrachial cutaneous sensory nerve action potential (SNAP), a compound muscle action potential (CMAP) of the biceps brachii and the examination of the needle of the biceps brachii and the coracobrachialis.
The following symptoms are characteristic:
- Elbow flexion weakness and forearm supination.
- Sensory loss on the lateral and volar aspect of the forearm.
- Weak or absent biceps tendon reflex.
Being a lower motor neuron injury, other expected signs include poor muscle tone, marked loss, and possibly fasciculation.
Nerve conduction studies, electromyography, and MRI should confirm a lower motor neuron and sensory nerve injury and are helpful in differentiating musculocutaneous nerve injuries from cervical spine nerve root impingement.
Symptoms of musculocutaneous neuropathy can be similar to cervical spinal nerve root impingement or brachial plexus injuries. Therefore, MRI and electrodiagnostic studies can be helpful in differentiating these conditions.
Note : The musculocutaneous nerve is often missed with an axillary block because the musculocutaneous nerve leaves the brachial plexus high in the axilla, which may be proximal to the insertion of the locking needle.
Therefore, the local anesthetic may not reach the nerve, especially if a low-volume technique is used.
If musculocutaneous nerve block is necessary, a separate injection is made by reinserting the superior needle into the axillary artery and injecting 5 to 8 ml of local anesthetic into the substance of the coracobrachialis muscle.
Treatment
Once a diagnosis of musculocutaneous neuropathy has been made, treatments include relative rest, non-steroidal anti-inflammatory drugs, splinting, physical therapy, and surgical decompression in cases that do not respond to conservative treatment.
Spontaneous recovery is possible, but it can take several months. In injuries not directly due to trauma, rehabilitation measures may be appropriate. If conservative therapy fails, surgical decompression should be considered.
Surgical decompression is indicated as first-line treatment if there is paresthesia, as this suggests that the affected nerve still has some function.
Traumatic injuries to the musculocutaneous nerve are amenable to surgical repair. They are classified as open, neat, open, messy, and with closed traction injuries.
Neat, open injuries get the best results and traction injuries are the worst. There is a worse prognosis if there is an associated vascular injury. Repair within 14 days and nerve grafts less than 10 cm in length give best results.
Since the early 1990s, Oberlin’s technique of transferring bundles from the ulnar nerve to the motor branch of the musculocutaneous nerve has been the preferred surgical technique for reinnervation and restoration of function of the biceps muscle.
However, median nerve bundle transfer has been developed as an option and in some cases leads to a better functional outcome.
Risk factor’s
Isolated injury to the musculocutaneous nerve or lateral antebrachial cutaneous nerve is rare. Damage to the shoulder and brachial plexus can affect the musculocutaneous nerve.
Compression of the nerve by the aponeurosis and the biceps tendon against the brachialis muscle fascia causes a sensory loss below the elbow on the lateral side of the forearm. Nerve entrapment is a cause of elbow pain.
Nerve dysfunction (entrapment) of the musculocutaneous nerve can occur proximally where it pierces the coracobrachialis muscle or distally in the deep fascia lateral to the biceps tendon at the elbow.
Athletes who participate in airdrop sports and intense training activities (especially intense bicep training) may be especially at risk for problems with this nerve.
During shoulder joint replacement, before placing a retractor on the medial side of the incision to retract the attached muscles and the pectoralis major, it is necessary to palpate and identify the nerve to avoid damage.
Using an anterolateral approach to the humerus during orthopedic surgery also carries risk.
There may be a complaint of weak elbow flexion, poor supination power, or numbness in the lower lateral forearm. This can follow trauma to the shoulder or upper limb or overuse or training.
It presents with loss of power to the biceps and brachii muscles without disturbing pain. The injury usually occurs after strenuous exercise and can be demonstrated by electrophysiology.
Vigorous exhaustion of the elbow and pronation of the forearm are risky exercises for injury. The presentation may be pain in the elbow or “burning” in the forearm.
Discussion of clinical cases and studies
Compression of the nerve proximally at the level of the coracobrachii muscle manifests with pain and weakness of the biceps brachii and dysesthesia on the radial side of the forearm. Typically, the pain can be reproduced with full extension in the elbow.
Another case of musculocutaneous nerve injury after exercise is reported in the literature in a 20-year-old man who developed marked biceps brachii weakness the morning after vigorous weight training.
His examination revealed an absent biceps tendon reflex, reduced biceps brachii muscle tone, and mild hypoaesthesia on the radial aspect of the volar forearm.
Musculocutaneous neuropathy is also described in the literature in a 37-year-old man who presented 5 weeks after moving large coils of thread.
The rolls were between 65 and 80 pounds in weight, and the patient curled his arm around the rolls on his shoulder to stabilize himself. The next morning, he noticed an inability to flex his right elbow.
Compression may also be a cause of isolated musculocutaneous neuropathy as in a 22-year-old man who developed painless biceps brachii weakness the morning after recreational basketball.
Over the next 9 days, symptoms progressed to inability to contract the biceps brachii and sensation significantly reduced along with intensive paresthesia in the right volar forearm.
Finally, a 2 × 2 cm exostosis was identified that affected the musculocutaneous nerve after its origin in the lateral cord.
Compression of the lateral antebrachial cutaneous nerve after placement of a long cast was also reported in the literature in a 25-year-old man.
The patient experienced painful dysesthesia on the radial aspect of the forearm within 3 days after casting that was not relieved by removal of the cast.
A fibrous constriction band was identified and, finally, a surgical decompression was performed.
Trauma can also cause musculocutaneous nerve injury. For example, Jay Liveson reported cases of such neuropathies resulting from a shoulder injury.
Five of the 11 patients studied had musculocutaneous nerve injury. Three of these cases occurred after anterior shoulder subluxation and one after a dislocated humeral fracture.
No details were given about the last patient’s injuries. However, only one of these five patients had an isolated musculocutaneous nerve injury.
Two of the patients also had axillary nerve damage (the axillary nerve is the most commonly injured in anterior shoulder dislocation).
Two other patients had extensive brachial plexus damage. The author concluded that “the musculocutaneous nerve appears particularly vulnerable to dislocation of the shoulder.”
Inflammation secondary to osteoid osteomas can also present with these symptoms. There is a report of a 5-year-old boy who developed moderate biceps brachii muscle weakness with no evidence of progression after 7 months of shoulder pain and functional limitations.
A CT scan showed an osteoid osteoma at the base of the coracoid process with inflammation of the surrounding soft tissue.
An isolated musculocutaneous nerve injury has also been reported in the literature in a 21-year-old man after a 10-hour surgical procedure in which both arms were externally rotated and abducted at approximately 90 °.
The surgical table was placed in the Trendelenburg position with the patient’s head approximately 2 inches below their feet.
Fourteen days after surgery, the patient presented tenderness in the left arm, and the manual muscle strength test revealed 3/5 in the flexors of the left elbow.
In addition to the above causes of musculocutaneous and lateral antebrachial cutaneous nerve neuropathy, brachial neuritis, also known as Parsonage Turner syndrome, has been shown to produce similar nerve injuries.
Parsonage Turner syndrome is a rare disorder of unknown etiology, usually presenting with pain and weakness in the shoulder and upper limb. It is one of the most common atraumatic causes of brachial plexopathy.
Although brachial neuritis was first described as such by Parsonage and Turner in 1948, reports of similar clinical presentations date back to 1897.
Parsonage Turner syndrome usually affects the upper trunk of the brachial plexus or the peripheral nerves in the shoulder girdle with rare involvement of the middle and lower trunks.
The suprascapular, long thoracic, and axillary nerves are the most commonly involved peripheral nerves with the musculocutaneous, anterior interosseous, ulnar, and median nerves that are also occasionally involved.
The exact etiology of the disorder is not fully understood, but 25% of cases occur after a viral infection, and 15% occur after immunization. Parsonage Turner syndrome has also been seen after exercise and surgery.
The diagnosis of lateral antebrachial cutaneous nerve entrapment depends on a good history and a physical examination with knowledge of the nerve pathway within the arm.
A diagnostic injection of local anesthetic can help differentiate between elbow pain secondary to entrapment of the lateral antebrachial cutaneous nerve and other causes, such as lateral epicondylitis or radial tunnel syndrome.
When the lesion is below the coracobrachialis muscle, the predominant symptom is weakness of the brachii and biceps muscles in addition to paresthesia along the distribution of the lateral antebrachial cutaneous nerve.
The nerve is compressed between the distal biceps tendon and the brachialis muscle. This compression mechanism has been attributed to windsurfing when the arm is flexed for long periods of time or during a forced extension.
Vigorous exercise consisting of elbow extension and forearm pronation has also been associated with this condition.
Symptoms may include pain, paresthesia, and numbness in the radial aspect of the forearm. Although the lateral antebrachial cutaneous nerve is a purely sensory nerve, most patients complain of pain rather than paresthesia.
Symptoms of lateral antebrachial cutaneous nerve entrapment can mimic other causes of lateral elbow pain, such as lateral epicondylitis and radial tunnel syndrome.
This condition must be differentiated from a ruptured biceps tendon or a brachial plexopathy, which can sometimes follow minor trauma and is often painful.
These conditions should be further differentiated from possible cervical radiculopathy in which other muscles are affected in addition to the biceps brachii and brachii.
Neuropathic pain can be complicated and involves several molecular pathways, making it difficult to treat. Currently available drugs do not act on all the mechanisms underlying the generation and spread of this type of pain.
They often act only on the properties of temporary pain rather than targeting the various mechanisms responsible for its generation and spread.
Attempts to control pain at the molecular level have led to the introduction of antisense strategies, as well as genetic, stem cell, and viral therapies.
Given the complex nature of pain, a multifaceted approach appears to be necessary for successful pain management. Cytokine activation or dysregulation has been shown to be involved in multiple disease states.
Experimental studies have shown that pro-inflammatory cytokines induce or facilitate neuropathic pain.
Cytokine levels are increased in peripheral nerves, dorsal root ganglia, spinal cord, and in certain brain regions after peripheral nerve injuries.
Anticytokine therapies currently on the market are effective primarily for inflammatory pain conditions and need to be tested to determine their efficacy for neuropathic pain.
Agents that more specifically target downstream signals have been identified that may provide new tools for more specific therapies.
Other studies have observed that the peripheral nerves synthesize and metabolize neuroactive steroids, which also express classical and non-classical steroid receptors.
Neuroactive steroids modulate the expression of transcription factors for Schwann cell function, regulate Schwann cell proliferation, and promote the expression of myelin proteins involved in the maintenance of myelin multilamellar structures.
Neuroactive steroids, such as testosterone, progesterone, and their metabolites, may also represent a promising therapeutic option.
Although the experimental treatments mentioned above may one day lead to better treatment of neuropathic pain.
Current treatments for musculocutaneous neuropathy and its branches are limited to relative rest, non-steroidal anti-inflammatory drugs (NSAIDs), a posterior elbow splint to prevent full extension, and physical therapy.
If symptoms persist beyond 6 weeks, steroids and local anesthesia may be injected into the musculocutaneous tunnel to possibly relieve the inflammatory component of the pain.
After 12 weeks of successful nonoperative treatment for these neuropathies, surgical decompression is often recommended.
In a series of 15 patients who presented symptoms of compression of the lateral antebrachial cutaneous nerve, 11 were successfully treated with surgical decompression that consisted of resecting a triangular aponeurosis wedge covering the nerve.
Musculocutaneous nerve injury may present with an atypical clinical picture and therefore an electromyography / nerve conduction study can help confirm the diagnosis to initiate appropriate treatment in a timely manner.