The musculocutaneous nerve innervates the biceps brachii, brachialis, and coracobrachialis muscles. The main function of the biceps and brachialis muscles is to flex the forearm at the elbow (C5-C6). The median and the ulnar nerves innervate the muscles that flex the wrist and fingers. The median nerve innervates most of the flexors of the wrist except for the flexor carpi ulnaris. The flexor carpi ulnaris is innervated by the ulnar nerve. Flexion of the fifth and fourth fingers is carried out by muscles innervated by the ulnar nerve. Flexion of the second and middle fingers is carried out by the action of the median nerve. All intrinsic muscles of the hand, including the thumbs, are innervated by the ulnar nerve except for the abductor pollicis brevis, flexor pollicis brevis, opponent pollicis, and lumbricales I and II. These muscles are innervated by the median nerve. The radial nerve innervates the extensors of the forearms, wrist, and fingers (C5-C8). The radial nerve also innervates the brachioradialis muscle (C5-C6). The brachioradialis muscle flexes the forearm when the forearm is midposition between pronation and supination. The radial nerve innervates most of the muscles that supinate the forearm (C6-C7). The median nerve innervates the muscles that pronate the forearm (C6-C8).
The evaluation of a neonate with arm monoparesis follows the principles of neurological diagnosis: anatomical diagnosis followed by pathological diagnosis followed by etiological diagnosis. The anatomical diagnosis is made by determining if the weakness is due to an upper or lower motor neuron disorder. Pivotal to this distinction is the differentiation between spastic and flaccid arm weakness. Spastic arm monoparesis refers to a decreased frequency and strength of movements with increased muscle stretch reflexes and an exaggerated Moro reflex. Flaccid arm monoparesis refers to decreased frequency and strength of movement with decreased muscle stretch reflexes and diminished Moro response. Neonates with arm monoparesis, normal stretch muscle reflexes, and normal Moro reflex are likely to develop signs of spasticity. They should be evaluated as if they had spastic arm monoparesis.