Mononeuropathies

Anatomy

The median nerve is formed in the axilla by branches of the lateral and medial cords of the brachial plexus ( Figure 14.2 ). In the forearm, the median nerve innervates the pronator teres (C6–C7), flexor carpi radialis (C6–C7), palmaris longus (C7–T1) and flexor digitorum superficialis (C7–C8). The anterior interosseous nerve, a primarily motor nerve, separates from the main trunk of the median nerve in the upper forearm and innervates the lateral head of the flexor digitorum profundus (C7–C8), flexor pollicis longus (C7–C8), and pronator quadratus (C7–C8). In the lower forearm, the median nerve gives off the palmar cutaneous branch. The main trunk then enters the wrist and travels through the carpal tunnel. Distal to the carpal tunnel, the median nerve divides into sensory and motor terminal branches. The motor branch supplies the first and second lumbricals (C8–T1) in the palm; in addition, a recurrent thenar motor branch supplies the abductor pollicis brevis (C8–T1), opponens pollicis (C8–T1), and superficial head of the flexor pollicis brevis (C8–T1). The terminal sensory branches supply sensation to the thumb, index, and middle fingers, and the lateral aspect of the ring finger.

Clinical Evaluation

In infants and young children, mononeuropathies may become apparent only when an observant parent or pediatrician notices focal weakness, atrophy, or an abnormal limb position. Children aged less than 5 years often have difficulty describing discomfort from median neuropathies. Many authors have reported a paucity of complaints of numbness, tingling, or nocturnal pain, and absence of Tinel’s and Phalen’s signs in children. Diagnosis is especially difficult in children with cognitive impairment, in whom it becomes additionally important to recognize subtle symptoms such as decreased sweating, clumsiness and tremor, nocturnal waking, gnawing of fingers and apparent insensitivity to pain, withdrawal of hands from touch of others, alterations in grasp or playing pattern, and increasing difficulties with fine motor tasks, which may be associated with classic signs of pulp atrophy, wasting of the thenar eminence, and weakness of thumb abduction and opposition.

Examination should include a careful search for atrophy and fasciculations. The hand may take on an abnormal appearance when there is atrophy of the thenar eminence (the “simian hand”) and median-innervated finger flexors (benediction hand). To evaluate median motor function, the examiner should evaluate the patient’s performance of the following maneuvers, without and against resistance: pronation of the forearm, flexion and abduction of the wrist, flexion of the fingers at the proximal and distal interphalangeal joints, extension of the index finger at the proximal interphalangeal joint, flexion of the distal phalanx of the thumb, abduction of the thumb at a right angle to the palm, and opposition of the thumb to the base of the small finger.

The sensory examination includes testing of light touch and pinprick sensation of the fingers and hand. The sensory abnormalities in proximal median neuropathies typically involve the lateral ring finger, the lateral aspect of the palm, and the palmar aspects of the thumb, index, and middle fingers. Sensory loss is generally incomplete. The sensory nerve examination is normal in lesions restricted to the anterior interosseous nerve. Median mononeuropathies at the wrist (e.g. carpal tunnel syndrome) spare palmar sensation, as the palmar cutaneous branch does not traverse the carpal tunnel. Occasionally, Phalen’s maneuver (wrist flexion test) and Tinel’s test (nerve tap test) provide additional help with localization, but these tests are neither specific nor sensitive.

In older children, the clinical evaluation of focal median nerve injuries is like that of adults. Symptoms of chronic mild or intermittent median nerve dysfunction are usually entirely sensory in nature and include paresthesias of the lateral hand, thumb, and index, middle, and ring fingers, which may radiate into the forearm, upper arm, and shoulder. With distal median mononeuropathies, symptoms often worsen with certain positions (e.g. wrist flexion) and activities (e.g. sleep, use of a computer keyboard).

The differential diagnosis of median mononeuropathies includes C8–T1 spinal segment or root lesions causing thenar muscle weakness and atrophy, C6–C7 root lesions for sensory symptoms, and brachial plexopathy (e.g. thoracic outlet syndrome). Thenar hypoplasia can also be present as a congenital defect in isolation (Cavanagh's syndrome) or in association with cardiac (Holt-Oram syndrome) or ocular lesions (Okihiro's syndrome).

Proximal Median Neuropathies

Reported causes of pediatric proximal median mononeuropathies are shown in Box 14.1 . Trauma is the most common cause of proximal median nerve injuries in children. Fractures of the supracondylar humerus, mid-radius, and radioulnar joint can cause median nerve compression, entrapment, or laceration. Most of these injuries involve the main trunk of the median nerve, but the anterior interosseous nerve can be traumatized in isolation with supracondylar fractures. Traumatic median nerve injuries may also result from elbow dislocations, lacerations, blunt soft tissue injury, and arterial or venous puncture, or during nerve blocks for regional anesthesia.

Less common are median neuropathies caused by congenital soft tissue anomalies such as constriction bands, or entrapment by the ligament of Struthers, pronator teres, or bicipital aponeurosis. The ligament of Struthers, a fibrous band extending from a small supracondylar spur to the medial epicondyle of the humerus, forms the roof of a tunnel through which the median nerve and brachial artery pass. These spurs are relatively common but the ligament of Struthers is a rare cause of median nerve entrapment. In pronator syndrome, the median nerve is compressed by the hypertrophied heads or thickened tendinous bands of the pronator teres muscle. Because this compression is distal to the origin of the motor branch innervating the pronator teres, this muscle is usually not involved, distinguishing this syndrome from more proximal median lesions (e.g. ligament of Struthers entrapment). Congenital or acquired soft tissue lesions can also cause compressive proximal or distal median neuropathies.

Anterior interosseous neuropathy has been reported in several children, either spontaneously due to brachial neuritis or after supracondylar fractures of the humerus.

Distal Median Neuropathies

The causes of distal median mononeuropathy are shown in Box 14.2 . In children, compressive median neuropathy at the wrist—carpal tunnel syndrome (CTS)—is rare, and generally associated with a predisposing disorder. In a large epidemiologic study conducted over a 2-year period in Wisconsin, the diagnosis of probable or definite CTS was made in 309 patients. Of these, only 7 (2.3%) were children under age 17 years, establishing an incidence rate of 0.26 per 1000 person-years.

Pediatric CTS is most often associated with lysosomal storage disorders (specifically, the mucopolysaccharidoses and mucolipidoses), in which it can be seen as early as 2 years of age and may be associated with trigger finger. Infantile CTS is also occasionally familial. In distinction, idiopathic CTS is usually reported in older children and adolescents, in whom this is a diagnosis of exclusion.

Predisposing factors are common in pediatric CTS and include congenital canal stenosis (e.g. familial CTS) or other anatomic anomalies, wrist trauma or injuries, thickening of the flexor retinaculum (e.g. mucopolysaccharidoses, mucolipidoses, trigger finger), repetitive hand and wrist movements or abnormal posturing, inflammatory conditions, and hereditary neuropathy with liability to pressure palsies (HNPP).

In addition to CTS, pediatric distal median mononeuropathies may result from congenital soft tissue anomalies, or from focal compression from hematoma, casting, or burns.

Evaluation

The EMG evaluation of the median nerve in infants includes measurement of a sensory nerve action potential (SNAP) from either the index or middle finger (the latter is preferable in babies because it allows longer interelectrode distance), the compound muscle action potential (CMAP) from the thenar muscles, median motor conduction velocity across the forearm segment, comparative ulnar sensory and motor conduction studies, and needle examination of the minimal number of muscles necessary.

In older children with mild symptoms, the examination should also include a more sensitive test for CTS, including either comparative transcarpal studies of either the median and ulnar mixed nerve action potential (MNAP) (palmar studies) peak latencies, or the median-to-second lumbrical and ulnar-to-interosseous distal latencies. In mild CTS, one may find prolonged latencies of the median SNAP and median-to-second lumbrical CMAP. Prolongation of both the median CMAP distal latency and attenuation of the SNAP amplitude indicates moderate disease. Severe CTS is associated with an absent median SNAP and attenuated thenar CMAP, and needle examination abnormalities indicating both ongoing and chronic changes of denervation and reinnervation in the abductor pollicis brevis. The distal latencies are normal in proximal median neuropathies, but conduction velocities across the forearm segment may be reduced with demyelinating injuries, and needle examination abnormalities often extend into median-innervated forearm muscles with axonal or mixed injuries.

In congenital thenar hypoplasia (e.g. Cavanagh’s syndrome), the median SNAP amplitude and peak latency are normal, the CMAP is of low amplitude or absent, and thenar motor unit action potentials (MUAPs) are reduced in number without associated changes of denervation.

The clinical findings and EMG results may dictate the need for additional studies to evaluate median mononeuropathies. Plain X-rays of the hand may be indicated in suspected cases of congenital thenar hypotrophy, in order to assess for hypoplastic changes of hand bones. X-rays of the distal humerus are recommended in suspected cases of ligament of Struthers entrapment, to identify the bony spur often seen in that disorder. Nerve ultrasound or magnetic resonance imaging (MRI) may offer more precise anatomic assessment of suspected soft tissue infiltrative or compressive lesions, especially with symptoms of slowly progressive nerve dysfunction or with focal tenderness or palpable swelling. Metabolic studies for the mucopolysaccharidoses and mucolipidoses are indicated in cases of CTS associated with dysmorphic features, organomegaly, and other systemic findings. Unexplained multiple or repeated mononeuropathies should prompt a chromosomal microarray or genetic testing to exclude the chromosome 17p11 deletion seen in hereditary neuropathy with tendency to pressure palsies.

Treatment and Prognosis

Traumatic median neuropathies require prompt attention. As with all peripheral nerve injuries, the main considerations in cases of acute median nerve trauma include nerve continuity, injury type (demyelinating, axonal, or mixed), and prognosis for meaningful recovery. Within 9 to 11 days following acute axonal injury, Wallerian degeneration is complete. EMG studies are recommended at this time in cases with severe weakness or when nerve function cannot be assessed clinically due to other factors related to the trauma (e.g. immobilization, casting). In neurapraxia (focal demyelinating injury), the distal median SNAP and CMAP are preserved, but stimulation proximal to the site of injury evokes an attenuated or absent response (partial or complete conduction block). In mild or moderate axonotmesis (axonal injury), the distal SNAP and CMAP are attenuated or absent, and after 2 to 3 weeks electromyographic examination will show fibrillations and positive waves in median-innervated muscles distal to the site of injury. In severe axonotmesis or neurotmesis, the distal median SNAP and CMAP are absent. Because nerve continuity may be uncertain in these cases, early surgical exploration (and if necessary, repair) is usually recommended. Quite often, EMG findings indicate both demyelinating and axonal features (mixed nerve injury) in traumatic nerve injuries. In cases with preserved continuity and slow return of function, periodic EMG evaluation may be helpful in following nerve regeneration, collateral sprouting, and muscle reinnervation.

Pediatric median neuropathies generally have a good prognosis. Treatment of mild idiopathic and activity-related CTS is focused on conservative measures such as avoidance of compromising hand positions and activities. (See Case Example 14.1 .) Median nerve decompression is required in children with continuing or progressive symptoms refractory to conservative measures, or where the EMG study indicates axon loss. Surgical decompression is indicated in distal median neuropathies associated with inborn errors of metabolism and compression secondary to congenital or acquired soft tissue or bone lesions. This surgery includes division of the flexor retinaculum and release of the median nerve, with or without external neurolysis and/or synovectomy. Surgical decompression usually affords some improvement in symptoms and function, with ultimate recovery related to the degree of preoperative axonal loss.

Anatomy

The ulnar nerve is derived from the medial cord of the brachial plexus (C8–T1) ( Figure 14.3 ). At the elbow, it travels first through the epicondylar groove, and then the cubital tunnel as it enters the forearm. Here the ulnar nerve innervates the flexor carpi ulnaris (C8–T1) and the medial head of flexor digitorum profundus (C8–T1). Prior to reaching the hand, the ulnar nerve gives off the palmar and dorsal cutaneous branches in the lower forearm. At the wrist, the nerve enters the Guyon canal and then bifurcates into the superficial and deep branches. The superficial branch innervates the palmaris brevis muscle (C8–T1), and then becomes the terminal sensory nerves, which supply sensation to the fifth digit and medial aspect of the ring finger. The deep branch supplies the hypothenar muscles (C8–T1), including the abductor digiti minimi, opponens digiti minimi, and flexor digiti minimi, and then curves along the palm providing motor branches to the third and fourth lumbricals (C8–T1), the 4 dorsal and 3 palmar interossei (C8–T1), adductor pollicis (C8–T1), and the deep head of the flexor pollicis brevis (C8–T1).

Clinical Evaluation

Mild or intermittent ulnar nerve dysfunction is often associated only with sensory symptoms, which involve the medial aspect of the hand and ring finger, and the entire small finger. Sensory symptoms or signs involving the dorsomedial hand point to dysfunction proximal to the dorsal ulnar cutaneous branch, usually at the elbow or cubital tunnel. Specific arm positions or maneuvers may reproduce sensory symptoms due to proximal ulnar lesions. For example, symptoms produced by extreme elbow flexion point to dysfunction at the level of the elbow or cubital tunnel. In some patients with cubital tunnel entrapment, symptoms are reproduced by forceful flexion and medial deviation of the wrist. Tenderness or a positive Tinel’s sign at the elbow or wrist may also help to localize the level of injury. Weakness of the flexor carpi ulnaris and flexor digitorum profundus of digit 5 also point to ulnar nerve involvement at the cubital tunnel or elbow; however, preserved function of these muscles is also seen with proximal lesions.

More severe ulnar lesions are associated with weakness and atrophy of ulnar-innervated muscles of the hand and, occasionally, the forearm, depending on localization. Weakness without atrophy is indicative of a demyelinating or early axon-loss injury. Chronic axonal injury and muscle atrophy may produce the so-called claw-hand deformity wherein the fourth and fifth fingers are hyperextended at the metacarpophalangeal joints, due to weakness of the interossei and ulnar lumbricals, and flexed at the interphalangeal joints due to the action of the unopposed flexor digitorum superficialis.

The differential diagnosis of ulnar mononeuropathies includes C8–T1 spinal segment or root lesions, lower trunk or medial cord brachial plexopathy, and mononeuropathy multiplex (e.g. HNPP).

Etiology

The many reported causes of pediatric ulnar mononeuropathy are listed in Box 14.3 . Trauma is the most common cause of ulnar mononeuropathies in children. Traumatic proximal ulnar neuropathies result from fractures (supracondylar, medial epicondylar or forearm). (See Case Example 14.2 .) Distal radial fractures can result in ulnar neuropathies via blunt injury, entrapment, compression, or laceration. The ulnar nerve may also be damaged as the result of surgery to repair an elbow or forearm fracture. Finally, delayed (or “tardy”) ulnar nerve palsy is occasionally described in children following elbow trauma, presumably due to posttraumatic bony changes and fibrosis. The ulnar nerve can also be injured by intraneural injections during nerve blocks for regional anesthesia, elbow lacerations, and puncture wounds.

Case Example 14.2

An 11-year-old girl developed left hand weakness and numbness 4 months after falling off her bicycle and fracturing her proximal radius and ulna. The upper extremity neurologic examination was remarkable for moderately severe weakness and atrophy of the ulnar hand muscles. The ulnar forearm and median hand muscles were normal. There was reduced light touch sensation along the medial hand (including the dorsal surface) and fingers 4 (with splitting) and 5.

Nerve conduction studies demonstrated absent ulnar digital and dorsal cutaneous SNAPs on the affected side. The left ulnar/hypothenar and ulnar/first dorsal interosseous motor nerve conduction studies ( Figure 14.4 ) revealed low-amplitude CMAPs with normal distal latencies and conduction velocities. Contralateral ulnar and ipsilateral median nerve studies were normal. The needle examination revealed fibrillations and positive waves, with reduced recruitment of MUAPs from the left first dorsal interosseous and abductor digiti minimi. The MUAPs were of increased amplitude and duration and fired at increased frequencies. Needle examination of the left flexor carpi ulnaris and abductor pollicis brevis was normal.

Comment

The EMG findings were consistent with a severe but incomplete axon-loss ulnar mononeuropathy, localized proximal to the dorsal cutaneous sensory branch (upper forearm or elbow), probably due to blunt nerve injury. One year following the injury, hand muscle strength had normalized and the only residual symptom was continuing numbness of the little finger.

Ulnar nerve entrapment within the cubital tunnel is occasionally reported in childhood, in some cases possibly related to overuse injury. Other causes of ulnar nerve entrapment include a persistent epitrochleoanconeus muscle and congenital constriction bands.

Compressive injuries to the ulnar nerve may develop intraoperatively or as a result of pressure in sleep, from wheelchair rests, or from bicycle handlebars. Less common causes of ulnar neuropathy are listed in Box 14.3 .

Evaluation

The ulnar nerve EMG evaluation requires measurement of an ulnar SNAP from the little finger, ulnar CMAP from the hypothenar muscles, and motor conduction velocities across the forearm and elbow segments; comparative median sensory and motor nerve conduction studies; and needle examination of the first dorsal interosseous, abductor digiti minimi, and flexor carpi ulnaris muscles. Ulnar motor nerve conduction studies recorded from the first dorsal interosseous muscle are occasionally required as a more sensitive test for demyelinating injuries at the wrist or elbow segments. Addition of the dorsal ulnar cutaneous SNAP may help in differentiating between ulnar lesions at the wrist and elbow. Study of the medial antebrachial cutaneous SNAP is helpful in identifying brachial plexus involvement in selected cases.

Proximal ulnar neuropathies at the elbow are associated with focal motor conduction block, or conduction slowing when there is a primary demyelinating process. If there is an axonal component to the nerve injury, the distal SNAP and CMAP responses are either attenuated or absent, with variable changes of denervation and reinnervation on needle examination of ulnar hand and forearm muscles. Mixed injuries are associated with a combination of demyelinating and axonal features.

Treatment and Prognosis

The management of pediatric ulnar focal neuropathies variously includes nerve decompression and repair of fractures following trauma, surgical nerve repair and grafting following nerve lacerations and severe traumatic injuries, resection of compressive masses and tumors, nerve decompression in cubital tunnel syndrome, and nerve transposition and decompression with progressive lesions localized at the elbow segment. Prognosis for full recovery is better in those with atraumatic lesions.

Anatomy

The radial nerve is derived from the posterior cord of the brachial plexus (C5–C8) ( Figure 14.5 ). The nerve descends in the upper arm between the long and medial heads of the triceps, posterior to the axillary artery. Proximal to the spiral groove, the radial nerve gives off the posterior cutaneous nerve of the arm, motor branches to the triceps (C6–C8) and anconeus (C6–C8), and the posterior cutaneous nerve of the forearm. At the spiral groove, the nerve travels from the medial to the posterolateral aspect of the lower arm. Distal to the groove, the radial nerve innervates the brachioradialis (C5–C6) and extensor carpi radialis longus (C5–C6), then bifurcates into superficial and deep branches. The superficial branch descends the forearm under the brachioradialis prior to emerging in the distal forearm as the superficial sensory branch, supplying sensation to the dorsomedial hand and first web space. The deep branch continues as the posterior interosseous nerve and enters the supinator muscle through an opening termed the arcade of Frohse. Within the extensor compartment of the forearm, the posterior interosseous nerve innervates the supinator (C6–C7), extensor carpi radialis brevis (C5–C7), extensor digitorum (C7–C8), extensor digiti minimi (C7–C8), extensor carpi ulnaris (C7–C8), abductor pollicis longus (C7–C8), extensor pollicis longus (C7–C8), extensor pollicis brevis (C7–C8), and extensor indicis (C7–C8).

Clinical Evaluation

Radial nerve lesions are typically localized to the axilla, spiral groove segment, or restricted to the terminal motor (posterior interosseous nerve) or superficial sensory branches. Injuries in the axilla affect all sensory and motor branches of the radial nerve, causing weakness of arm extension, wrist extension (wrist drop), and finger/thumb extension; hypoactive triceps and brachioradialis reflexes; and sensory loss and paresthesias involving the posterolateral surface of the arm, forearm, hand, and fingers 1–4. Spiral groove segment lesions typically spare the motor branches supplying the triceps and anconeus muscles, and the posterior cutaneous nerve of the arm. Weakness resulting from radial nerve damage in the spiral groove usually affects the brachioradialis, wrist extensors, supinator, and finger and thumb extensors. Lesions restricted to the posterior interosseous nerve cause weakness without sensory loss, as this radial nerve branch is purely motor. Children present with finger and thumb drop without associated wrist drop, due to sparing of the extensor carpi radialis longus. Radial deviation of the wrist is noted, however, due to weakness of the extensor carpi ulnaris. Lesions of the superficial cutaneous sensory branch are associated with sensory loss or paresthesias affecting the dorsolateral hand (especially the first web space), and the dorsum of fingers 1 to 4.

The differential diagnosis of radial neuropathies includes C6–C8 spinal segment or root lesions, middle trunk or posterior cord brachial plexopathy, and mononeuropathy multiplex (e.g. HNPP and inflammatory nerve disease).