Peripheral nerve injury in the upper extremity is common, and certain peripheral nerves are at an increased risk of injury because of their anatomic location. Risk factors include a superficial position, a long course through an area at high risk of trauma, and a narrow path through a bony canal. The anatomy and function of upper extremity nerve roots, as well as specific risk factors of injury, are described in Online Table A. The most common nerve entrapment injury is carpal tunnel syndrome, which has an estimated prevalence of 3 percent in the general population and 5 to 15 percent in the industrial setting.¹ Given the potential for longstanding impairment associated with nerve injuries, it is important for the primary care physician to be familiar with their presentation, diagnosis, and management.

Pathophysiology

The three categories of nerve injuries are neurapraxia, axonotmesis, and neurotmesis. Neurapraxia is least severe and involves focal damage of the myelin fibers around the axon, with the axon and the connective tissue sheath remaining intact. Neurapraxia typically has a limited course (i.e., days to weeks). Axonotmesis is more severe, and involves injury to the axon itself. Regeneration of the nerve is possible, but typically prolonged (i.e., months), and patients often do not have complete recovery. Neurotmesis involves complete disruption of the axon, with little likelihood of normal regrowth or clinical recovery.^2,3

Most nerve injuries result in neurapraxia or axonotmesis. Mechanisms of nerve injury include direct pressure, repetitive microtrauma, and stretch- or compression-induced ischemia. The degree of injury is related to the severity and extent (time) of compression.⁴

General Diagnostic Approach

Nerve injury should be considered when a patient reports pain, weakness, or paresthesias that are not related to a known bone, soft tissue, or vascular injury. Symptom onset may be insidious or acute. The location of symptoms, the type of symptom (i.e., paresthesias, pain, weakness), and any relation between a symptom and specific activity should be determined. Table 1 outlines the differential diagnosis of upper extremity nerve injury by symptom and area of the body.^5,6

Initial physical examination of a patient with an upper extremity injury includes looking for the presence of a radial pulse, and sensation and movement in the digits. If there is no obvious neurovascular compromise, the remainder of the examination is based on the patient's history. The examination should follow the classic pattern of inspection, palpation, joint range of motion, muscle strength testing, and sensory and neurologic examination. It is helpful to understand the nerves commonly involved, their function, and the corresponding areas of the body at risk of compression or entrapment. Figures 1 and 2 show typical distributions of nerves in the upper extremity.⁷

Common Nerve Injuries and Entrapment Syndromes of the Upper Extremity

Tables 2 through 4 outline the evaluation process and differential diagnosis of nerve injuries to the upper extremity.^5,6

SHOULDER AND ARM

Axillary Nerve: Quadrilateral Space Syndrome. The axillary nerve is vulnerable to trauma as it passes through the quadrilateral space. Injury can occur from shoulder dislocation; upward pressure (e.g., from improper crutch use); repetitive overload activities (e.g., pitching a ball, swimming); and arthroscopy or rotator cuff repair. The typical symptom is arm fatigue with overhead activity or throwing. There may be associated paresthesias of the lateral and posterior upper arm. Examination reveals weak lateral abduction and external rotation of the arm.

Brachial Plexus Nerve: Stinger. A brachial plexus injury (i.e., stinger) is common in persons who play football, but it also occurs with other collision sports. The classic presentation is acute onset of paresthesias in the upper arm. A key characteristic is a circumferential rather than dermatomal pattern of paresthesias. Symptoms typically last seconds to minutes. Motor symptoms may be present initially or develop later.

A brachial plexus injury must be differentiated from a cervical spine injury. The initial examination should focus on the neck, with palpation of the cervical vertebrae to detect point tenderness and evaluation of neck range of motion. Any indication of a cervical spine injury mandates further emergent neurologic and radiologic evaluation. Point tenderness of the cervical vertebrae or pain with neck movement is a red flag for a cervical spine injury, in which case the patient should be immobilized. Bilateral symptoms or those involving upper and lower extremities are less likely to be from a brachial plexus injury.

If motor symptoms occur, the upper extremity muscle group exhibiting weakness correlates with the part of the brachial plexus that has been injured. Because motor symptoms may occur hours to days after the injury, repeated neurologic examinations are necessary—the patient should be reevaluated after 24 hours and then at least every few days for two weeks. If new symptoms or significant worsening of existing symptoms occurs, neuroimaging, electrodiagnostics, or surgical referral should be considered.⁸ Patients who have multiple occurrences of stingers should also have a more thorough workup, because they may have an underlying neck pathology that predisposes them to this injury.^9,10

Occurrence during participation in a sporting event raises the issue of return to play. If all symptoms resolve within 15 minutes and there is no concern for cervical spine injury, the player may return to the same event with at least one repeat examination during that event.¹¹

Long Thoracic Nerve. Injury to the long thoracic nerve occurs acutely from a blow to the shoulder, or with activities that involve chronic repetitive traction on the nerve (e.g., tennis, swimming, baseball). Presenting symptoms include diffuse shoulder or neck pain that worsens with overhead activities. Examination reveals scapular winging and weakness with forward elevation of the arm.

Spinal Accessory Nerve. Injury to the spinal accessory nerve can occur with trapezius trauma or shoulder dislocation. Radical neck dissection, carotid endarterectomy, and cervical node biopsy are iatrogenic sources of injury. Patients usually present with generalized shoulder pain and weakness. Examination of the shoulders reveals asymmetry. The affected side appears to sag and the patient is unable to shrug the shoulder toward the ear. Associated weakness of forward arm elevation above the horizontal plane is common. With chronic injury, the trapezius may atrophy.

Suprascapular Nerve. Injury to the suprascapular nerve is associated with repetitive overhead loading. The suprascapular nerve serves the supraspinatus and infraspinatus muscles. The infraspinatus may be the only muscle affected, depending on the site of injury. Loss of infraspinatus function presents as weak external rotation of the arm. Supraspinatus involvement additionally presents with weak arm elevation, which is most pronounced in the range of 90 to 180 degrees. Suprascapular nerve injury can result from other shoulder pathologies, specifically a glenoid labrum tear. Cyst formation at the suprascapular notch from a labral tear is not uncommon. The cyst compresses the suprascapular nerve, affecting the supraspinatus and infraspinatus muscles.¹² Suprascapular nerve injury and rotator cuff tear both lead to supraspinatus and infraspinatus weakness. Differentiating the two injuries may require magnetic resonance imaging (MRI).

FOREARM AND ELBOW

Median Nerve at the Elbow: Pronator Syndrome. The pronator teres muscle in the forearm can compress the median nerve, which may cause symptoms that mimic carpal tunnel syndrome. Symptoms are discomfort and aching in the forearm with activities requiring repetitive pronation of the forearm, especially with the elbow extended. Paresthesias in the thumb and first two digits may be present. Forearm sensation is normal, and sensation of the digits may also be normal. In pronator syndrome, there is sensory loss over the thenar eminence, which is not a finding of carpal tunnel syndrome. Results of the Tinel sign and Phalen maneuver at the wrist should be negative in patients with pronator syndrome.¹³

Radial Nerve at the Elbow: Radial Tunnel and Posterior Interosseous Nerve Syndromes. The radial nerve divides into a superficial branch (sensory only) and a deep branch (posterior interosseous nerve) at the lateral elbow. Forearm pain that is exacerbated by repetitive forearm pronation is the presenting symptom of radial tunnel syndrome, which involves injury to the superficial branch of the radial nerve. Symptoms of radial tunnel syndrome are almost identical to those of tennis elbow (i.e., lateral epicondylitis), and distinguishing the two can be difficult because physical examination maneuvers that aggravate radial tunnel syndrome may also be positive in patients with tennis elbow (e.g., supination against resistance with the elbow and wrist extended, and resisted extension of the middle finger).¹⁴ A differentiating factor is the point of maximal tenderness. In radial tunnel syndrome, this point is over the anterior radial neck; in tennis elbow, it is at the origin of the extensor carpi radialis brevis muscle.

The presence of any motor symptoms is more likely related to injury of the posterior interosseus nerve, which supplies the extensor muscles of the hand. Generalized hand weakness is the presenting symptom of posterior interosseus nerve syndrome. Examination reveals weakness of digit and wrist extension, although this is usually more prominent in the digits than in the wrist.

Ulnar Nerve at the Elbow: Cubital Tunnel Syndrome. The ulnar nerve at the elbow is very superficial and at risk of injury from acute contusion or chronic compression. Compression can be from an external or internal source. As the elbow flexes, the cubital tunnel volume decreases, causing internal compression. Cubital tunnel syndrome may cause paresthesias of the fourth and fifth digits. There may be elbow pain radiating to the hand, and symptoms may be worse with prolonged or repetitive elbow flexion. Paresthesias precede clinical examination findings of sensory loss. Weakness may occur, but is a late symptom. When present, motor findings are weak digit abduction, weak thumb abduction, and weak thumb-index finger pinch. Power grip is ultimately affected.

HAND AND WRIST

Median Nerve at the Wrist: Carpal Tunnel Syndrome. Carpal tunnel syndrome is the most common nerve entrapment injury.¹⁵ Early symptoms are paresthesias of the thumb, index digit, and long digit. Some patients also have forearm pain. The most helpful physical examination findings are hypalgesia (positive likelihood ratio of 3.1) and abnormality in a Katz hand diagram.¹⁶ Although commonly used in patients with carpal tunnel syndrome, Tinel sign and Phalen maneuver are less accurate.¹⁶ The sensory examination is normal initially, although late findings include sensory loss in the median nerve distribution, weak thumb abduction, and thenar atrophy. Electrodiagnostic testing can be useful and quantitates severity of entrapment, although false negatives and false positives may occur.^16,17

Radial Nerve at the Wrist: Handcuff Neuropathy. The superficial branch of the radial nerve crosses the volar wrist on top of the flexor retinaculum of the carpal tunnel. It is vulnerable to compression by anything wound tightly around the wrist. Historically, this is an area easily injured by tight handcuffs, thus the name “handcuff neuropathy.” The injury leads to numbness on the back of the hand, mostly on the radial side. Examination may reveal decreased sensation to soft touch and pinprick over the dorsoradial hand, dorsal thumb, and index digit. Motor function is typically intact.

Ulnar Nerve at the Wrist: Cyclist's Palsy. Injury of the ulnar nerve at the wrist is common in cyclists because the ulnar nerve gets compressed against the handlebar during cycling, resulting in “cyclist's palsy.” This type of nerve injury occurs with other activities involving prolonged pressure on the volar wrist (e.g., jackhammer use). Symptoms are paresthesias in the fourth and fifth digits. Digit weakness is uncommon because the motor portion of the nerve at the wrist is less superficial. Unless the activity is prolonged or chronic, results of the sensory examination are normal and numbness will resolve within a few hours after stopping the activity.

Diagnostic Testing

IMAGING

Plain radiography is primarily useful for identifying other diagnoses, such as fracture or cervical spondyloarthropathy. MRI is rarely needed for initial evaluation of a typical nerve injury, although it may be helpful for specific nerves (Table 5).¹⁸

Chronic nerve injury can lead to denervation changes in muscle. These changes may be visible on MRI as abnormal signal patterns. A normal MRI finding does not rule out nerve injury. Newer techniques, such as gadofluorine M–enhanced MRI, may ultimately be able to assess nerve regeneration.¹⁹ Ultrasonography is a less expensive modality to define anatomic entrapment, but its use is limited by lack of standardization of technique and interpretation.²⁰

ELECTRODIAGNOSTIC TESTING

Electrodiagnostic testing consists of nerve conduction studies and electromyography (EMG). Nerve conduction studies assess the integrity of sensory and motor nerves. Areas of nerve injury or demyelination appear as slowing of conduction velocity along the nerve segment in question. EMG records the electrical activity of a muscle from a needle placed into the muscle, looking for signs of denervation.^21,22 The combination of nerve conduction studies and EMG can help distinguish peripheral from central nerve injuries. Electrodiagnostic testing is commonly used to evaluate for carpal tunnel syndrome and cubital tunnel syndrome. Nerve conduction studies have been shown to confirm carpal tunnel syndrome with a sensitivity of 85 percent and a specificity of 95 percent.²³ Nerve conduction studies also may help confirm the diagnosis in patients who have a history or physical examination findings that are atypical of carpal tunnel syndrome. For most patients who have a typical presentation, nerve conduction studies do not change the diagnosis or management.^24,25

Treatment

The initial management of most nerve injuries is nonsurgical. The main components of treatment are relative rest and protection of the injured area. Anti-inflammatory medications are often added, although it is unknown if they aid healing. Mobility of associated joints should be maintained at full range of motion, and effort should be made to increase the strength of any supporting or accessory muscles. Specifics of conservative therapy and indications for surgical referral are shown in Table 6.^{13,15,25–46}

Systematic reviews of carpal tunnel syndrome have found short-term benefit from local corticosteroid injection, splinting, oral corticosteroids, ultrasound, yoga, and carpal bone mobilization.²⁹ Symptom relief from local injection has not been shown to last longer than one month, and there is no demonstrated benefit from a second injection.³⁰ Clinical outcome from local corticosteroid injection is similar to that from splinting combined with anti-inflammatory medication.²⁹ Vitamin B₆, ergonomic keyboards, diuretics, and nonsteroidal anti-inflammatory drugs have not been shown to be beneficial.^29,30 Patient characteristics that predict a poor response to nonsurgical therapy include age older than 50 years, symptom duration longer than 10 months, history of trigger digit, constant paresthesias, and Phalen maneuver that is positive in less than 30 seconds.⁴⁷ Surgical treatment likely has better outcomes than splinting, but it is unclear if surgical treatment is better than corticosteroid injection.⁴⁸