Peripheral Nerves

2 Peripheral Nerves

This chapter discusses the functional anatomy and clinical correlation of the peripheral nerves of the upper and lower extremities. The discussion is limited to those nerves that are susceptible to “entrapment” neuropathies in which the involved nerve is lesioned by extrinsic compression. For each nerve described, there is a discussion of its origin, course, and motor and sensory innervation, as well as the clinical syndrome or syndromes with which the nerve is most closely associated.

A significant number of these syndromes may be amenable to surgical treatment. Therefore, accurate and timely clinical diagnosis is imperative. Important nerves not covered in this chapter, because they are not associated with common entrapment syndromes, are noted at the beginning of the two major sections.

Upper Extremity

The nerves of the upper extremity discussed in the following sections include the radial, the median, and the ulnar nerves. Omitted from the discussion are (1) the axillary nerve, which supplies the deltoid muscle and the skin overlying the muscle, and (2) the musculocutane-ous nerve, which supplies the biceps and the brachialis muscles and the skin overlying the radial aspect of the forearm.

Radial Nerve

Anatomy

See Fig. 2.1.

The radial nerve receives contributions from the C5–C8 spinal nerves. These contributions pass through the upper, middle, and lower trunks and posterior cord of the brachial plexus. The radial nerve supplies the extensor muscles of the arm and forearm as well as the skin covering them.

Radial Nerve in the Upper Arm

As it winds around the humerus, or just proximal to this section, the radial nerve supplies the triceps muscle. After its course in the spiral groove, it then supplies the brachioradialis and the extensor carpi radialis longus and brevis muscles. The nerve then bifurcates into a superficial (sensory) branch and a deep (motor) branch.

Radial Nerve in the Forearm

The superficial branch passes distally into the hand, where it supplies the skin of the radial aspect of the dorsum of the hand and the dorsum of the first four fingers. The sensory autonomous zone of the radial nerve is the skin over the first interosseous space.

The deep branch of the radial nerve passes deep through the fibrous arch of the supinator muscle (the arcade of Frohse ) to enter the posterior compartment of the forearm. The nerve continues in this compartment as the purely motor posterior interosseous nerve, which innervates the remaining wrist and finger extensors. These include the following: (1) supinator, a forearm supinator; (2) extensor digitorum, an extensor of the second through the fifth metacarpophalangeal joints; (3) extensor digiti minimi, an extensor of the fifth metacarpophalangeal joint; (4) extensor carpi ulnaris, an ulnar extensor of the wrist; (5) abductor pollicis longus, an abductor of the carpometacarpal joint of the thumb; (6) extensor pol-licis longus, an extensor of the interphalangeal joint of the thumb; (7) extensor pollicis brevis, an extensor of the metacarpophalangeal joint of the thumb; and (10) extensor indicis, an extensor of the second finger.

Fig. 2.1 Radial nerve anatomy.

Clinical Syndromes

Two of the most common clinical syndromes of the radial nerve are radial nerve palsy and the posterior interos-seous nerve syndrome.

Radial Nerve Palsy

See Fig. 2.2.

This syndrome may be caused by a humeral fracture or a lesion due to prolonged pressure on the nerve. The term radial nerve palsy refers to the latter mechanism of injury, also known as Saturday night palsy because it is classically associated with a drunkard who falls asleep with his arm hyperabducted across a park bench. The site of compression in either case is in the region of the spiral groove. The syndrome consists of a wrist drop, inability to extend the fingers, weakness of the supinator muscle, and sensory loss involving the radial nerve-innervated areas of the forearm and hand. Wrist drop is the most impressive and typical sign. Weakness of supination is only partial because supination may be accomplished with either biceps or supinator. Note that the triceps is preserved in these lesions because the branches of the radial nerve that innervate the triceps originate proximal to the spiral groove.

Fig. 2.2 Radial nerve palsy. Functions or responses marked with an “X” are impaired or absent.

Posterior Interosseous Nerve Syndrome

See Fig. 2.3.

The posterior interosseous nerve (PIN) syndrome is the most common syndrome caused by compression at the arcade of Frohse. As the PIN passes under the arcade of Frohse, a fibrous arch at the origin of the supinator muscle, the nerve may be pathologically constricted. The cardinal features of this syndrome are an inability to extend the fingers at the metacarpophalangeal joint, the absence of wrist drop, and normal sensation. Because the finger extensors at the interphalangeal joint are median and ul-nar innervated, the patient is able to extend the fingers at this joint. Branches to the supinator muscle are given off proximal to the nerve entering the arcade of Frohse, causing the supinator muscle to be spared. Although no wrist drop is present because the extensor carpi radialis is preserved, the extensor carpi ulnaris is a PIN-innervated muscle, and thus any attempt to extend the wrist results in a radial deviation of the hand. Because the PIN is a pure motor nerve, sensation in this syndrome is entirely normal.

Fig. 2.3 Posterior interosseous nerve syndrome. Functions or responses marked with an “X” are impaired or absent.

Median Nerve

Anatomy

See Fig. 2.4.

The median nerve receives contributions from the C6 to T1 spinal nerves. These pass through the upper, middle, and lower trunks and the lateral and medial cords of the brachial plexus. There are no median nerve branches that originate proximal to the elbow.

Median Nerve at the Elbow

At the elbow, the median nerve lies behind the bicipital aponeurosis (lacertus fibrosus), providing supply to the following muscles: (1) pronator teres, a forearm pronator; (2) flexor carpi radialis, a radial wrist flexor; (3) palmaris longus, a wrist flexor; and (4) flexor digitorum superficialis, a flexor at the interphalangeal joint for the second, third, fourth, and fifth fingers.

Median Nerve in the Forearm

From beneath the lacertus fibrosus, the nerve then passes into the forearm between the two heads of the median-innervated pronator teres muscle. As it passes deep to the pronator teres muscle, the median nerve gives off the anterior interosseous nerve. The anterior interosseous nerve is a purely motor branch, which supplies the (1) flexor pollicis longus, (2) pronator quadratus, and (3) flexor digitorum profundus I and II.

Median Nerve in the Hand

The median nerve then continues deep to the flexor reti-naculum through the so-called carpal tunnel to innervate the LOAF muscles of the hand. These include the (1) lum-bricals I and II, (2) opponens pollicis, (3) abductor pollicis brevis, and (4) flexor pollicis brevis.

Sensory branches also originate from the median nerve as it emerges from the carpal tunnel. These palmar digital nerves supply the skin of the palmar aspect of the thumb, second, third, and half of the fourth fingers; the radial aspect of the palm; and the dorsal aspect of the distal and middle phalanges of the second, third, and half of the fourth fingers.

A palmar cutaneous branch originates from the median nerve just proximal to the carpal tunnel, where it crosses the wrist to enter the hand superficial to the flexor retinaculum. It supplies the skin over the median eminence and the proximal palm on the radial aspect of the hand.

Fig. 2.4 Median nerve anatomy.

Clinical Syndromes

Three major entrapment syndromes involving the median nerve and its branches are described: (1) the prona-tor teres syndrome, (2) the anterior interosseous nerve syndrome, and (3) the carpal tunnel syndrome.

Pronator Teres Syndrome

See Fig. 2.5.

The pronator teres syndrome results from entrapment of the median nerve as it passes between the two heads of the pronator teres muscle and under the fibrous arch of the flexor digitorum superficialis. Compression may be caused by (1) a thickened lacertus fibrosus (an aponeurosis that overlies the median nerve just proximal to the passage of the nerve between the two heads of the pronator teres), (2) a hypertrophied pronator teres muscle, or (3) a tight fibrous band of the flexor digitorum superficialis.

The syndrome is characterized by pain in the forearm. In addition, weakness in the hand grip and numbness and tingling in the index finger and thumb are characteristically present. The symptoms are similar to those of carpal tunnel syndrome (see p. 64), but nocturnal exacerbation of pain is conspicuously absent. In advanced cases, the hand assumes a “benediction attitude” due to impairment of flexion in the radial three digits. Findings on muscle testing will vary depending on the degree of compression, but often there is no measurable weakness in the median nerve-innervated muscles.

Fig. 2.5 Pronator teres syndrome. Functions or responses marked with an “X” are impaired or absent.

Fig. 2.6 Anterior interosseous nerve syndrome. Functions or responses marked with an “X” are impaired or absent.

Anterior Interosseous Nerve Syndrome

See Figs. 2.6 and 2.7.

The anterior interosseous nerve syndrome is most commonly due to a constricting band causing an entrapment neuropathy near the origin of the nerve. Clinically, it is characterized by weakness in two muscles: (1) the flexor digitorum profundus I and II and (2) the flexor pol-licis longus. Its most singular feature, due to weakness in these muscles, is an abnormal “pinch attitude” of the hand. This attitude is characterized by an extension or hyperextension of the terminal phalanges of the thumb and index finger when the thumb and index finger are opposed. Weakness in the pronator quadratus muscle is usually clinically insignificant because of simultaneous contraction of the more powerful pronator teres muscle during pronation motion of the forearm. Sensation in the anterior interosseous nerve syndrome is entirely normal.

Fig. 2.7 Abnormal pinch attitude.

Carpal Tunnel Syndrome

See Fig. 2.8.

Carpal tunnel syndrome is caused by compression of the median nerve as it passes through the carpal tunnel. The carpal tunnel is formed by the transverse carpal ligament, or flexor retinaculum, which stretches transversely across the concavity of the carpal bones.

Clinically, the syndrome is characterized by both sensory and motor symptoms of the hand.

The most common presenting symptom of the syndrome is pain and paresthesias in the wrist and hand, which classically worsen at night, awakening the patient from sleep. These “positive” sensory symptoms are typically associated with “negative” symptoms, involving sensory loss in the distribution of the palmar digital branches of the median nerve (i.e., the radial palm, the palmar aspect of the first three and a half fingers, and the dorsal aspect of the terminal phalanges of the second, third, and half of the fourth fingers). Sensory loss is easiest to discern along the volar tips of the index and middle fingers, which are the autonomous areas for median nerve distribution.

Although pain is most frequently limited to the hand, it may involve the forearm, the elbow, or the shoulder, and thus this syndrome must be considered in the differential diagnosis of any obscure arm pain. Two historical clues that favor carpal tunnel syndrome are radiation of arm pain from distal to proximal and a tendency for the patient to rub or shake the hand to alleviate the pain because pain of more proximal origin such as the cervical spine or the thoracic outlet is usually exacerbated by motion.

Motor involvement, which usually occurs late in the course of the disease process, comprises weakness and atrophy in the four LOAF muscles of the hand: (1) the lumbricals, (2) the opponens pollicis, (3) the abductor pollicis brevis, and (4) the flexor pollicis brevis. Examination may thus reveal weakness in the abduction, opposition, and flexion of the thumb. It should be emphasized, however, that patients with carpal tunnel syndrome most commonly present with sensory complaints, and only rarely do they present with the complaint of weakness or muscle atrophy.

Paradoxically, those patients with pain may demonstrate no weakness, whereas a smaller group of patients may demonstrate weakness but do not complain of pain. In testing for weakness, the abductor pollicis brevis is the easiest of the thenar muscles to test in a reliable way.

Finally, Phalen’s test may be used to corroborate the diagnosis. It is done by asking the patient to forcibly dor-siflex the affected hand for 60 seconds. A positive test will reproduce the patient’s symptoms, although a false-positive test will occur in normal individuals if this position is maintained for too long.