The median nerve is formed from the union of portions of the lateral and medial cord in the brachial plexus. The lateral cord is derived from the C5–C6 nerve root fibers from the upper trunk and C7 nerve root fibers from the middle trunk and serves predominantly sensory and proximal motor functions. The medial cord is derived from C8–T1 nerve root fibers through the lower trunk and serves exclusively motor functions. The nerve does not innervate any muscles in the upper arm, which can present challenges for precise localization in proximal medial neuropathies. In the forearm the nerve innervates the pronator teres (C6–C7), flexor carpi radialis (C6–C7), palmaris longus (C7–T1), and flexor digitorum superficialis (C7–C8). In the forearm the anterior interosseous nerve branches off the main stem of the median nerve, providing exclusively motor innervation to the lateral head of the flexor digitorum profundus (C7–C8), flexor pollicis longus (C7–C8), and pronator quadratus (C7–C8). The remainder of the median nerve enters the wrist through the carpal tunnel, with the palmar cutaneous nerve branching off proximal to the carpal tunnel. It then provides sensation to the palmar surface of the first three digits and half of the fourth digit along with the dorsum of their distal phalanges, as well as motor input to the first two lumbricals (C8–T1), the abductor pollicis brevis (C8–T1), the opponens pollicis (C8–T1), and the superficial head of the flexor pollicis brevis (C8–T1).

Traumatic causes, typically from elbow fractures, lacerations or medical procedures, are the leading culprits for proximal medial neuropathies. Entrapment from the fibromuscular bands such as lacertum fibrosus, the ligament of Struthers, bicipital aponeurosis, hypertrophied heads of pronator teres muscle and the sublimis bridge of the flexor digitorum superficialis, as well as compression from soft tissue or bone tumors have also been reported [6]. Additionally, the anterior interosseous branch can be selectively affected from idiopathic brachial plexitis [7]. Distal median neuropathies (a.k.a. carpal tunnel syndrome) are rare in children. Metabolic (e.g., mucopolysaccharidoses), genetic (e.g., familial carpal tunnel syndrome or hereditary neuropathy with liability to pressure palsies) and mechanical (e.g., sport trauma, arthritis, tenosynovitis, macrodactyly, hemophilia, tumors) disorders constitute the main etiologies for carpal tunnel syndrome in children [8–10].

In infants and young children or in older children with cognitive difficulties related to inborn errors of metabolism, sensory complaints are hard to tease out. When present, they conform to the palmar surface of the first three and a half digits, and in proximal cases, also to the thenar eminence. Subtle symptoms such as clumsiness, nocturnal waking, and nibbling of the fingers should be noted. Examination will typically reveal atrophy and weakness in the thenar eminence (“simian hand”), and in proximal lesions, also of the median-innervated finger flexors (“benediction hand”) [4]. Tinel’s and/or Phalen’s signs may be present, though notoriously fraught by variable sensitivity and specificity, especially in childhood [10]. In anterior interosseous neuropathies, the inability to make a round “O” with the thumb and index finger is a classic sign (Table 20.1).

Nerve conduction studies should include SNAP recording either from one of the median-innervated digits (typically the second, though in infants the middle finger is sometimes used), CMAP recording from the thenar eminence, and needle examination of distal and proximal median innervated muscles. In mild carpal tunnel syndrome cases, the only abnormalities detected may be sensory findings, starting with peak latency prolongation, or median to ulnar or median to radial comparison studies. In more severe cases, amplitude reduction of the median sensory study and similar abnormalities in the median motor studies are present. In young children, normal values for distal latencies cannot be used due to the difficulty of standardizing distances between the stimulating and the recording electrodes, thus nerve conduction velocities are important values to calculate, when applicable. For proximal median neuropathies, sensory and motor median amplitudes may be reduced with stimulation above the area of compression, in conjunction with normal distal latencies and reduced conduction velocity at the forearm. In contrast, nerve conduction studies are typically normal in pure anterior interosseous neuropathies. Needle examination can be very helpful in localizing the lesion. In demyelinating lesions affecting the motor fibers, only reduced recruitment is seen, while in cases involving axonal damage, signs of denervation and/or reinnervation should be observed. The possibility of congenital thenar hypoplasia should be remembered. For this benign diagnosis, electrophysiologic testing only shows a reduced CMAP in the abductor pollicis brevis with normal sensory NCS and needle exam [11]. When sensory symptoms predominate, the differential diagnosis also includes a middle trunk brachial plexopathy and C6–C7 radiculopathy. When motor complaints dominate the clinical picture, the differential diagnosis of median neuropathy also includes a medial cord/lower trunk plexopathy (e.g., thoracic outlet syndrome) and C8–T1 radiculopathy (Table 20.3). Thus, testing of the ulnar and radial SNAPs, as well the ulnar CMAP recording abductor digiti minimi is important, in addition to needle examination of non-median C6–C7 (e.g., triceps) and C8–T1 (e.g., first dorsal interosseous) muscles. Finally, NCS and needle EMG results should be evaluated with caution when a Martin-Gruber anastomosis is present, both in distal (e.g., positive dip in median CMAP studies with proximal stimulation along with artificially increased conduction velocity at the forearm), as well as in proximal (e.g., abnormalities may be detected in aberrantly innervated distal ulnar muscles) median neuropathies. The clinical and electrodiagnostic findings may require further investigations to pinpoint the underlying cause (e.g., imaging, metabolic or genetic screening, etc.).