Neuralgic amyotrophy is likely an immune-mediated disorder that affects peripheral nerves of the upper limb and is not restricted to elements of the brachial plexus. Unfortunately, the disorder is known by several misleading names that result sometimes in misdiagnosis (Table C16-1). Neuralgic amyotrophy has an estimated annual incidence of 1.64 cases per 100 000 population. It most often affects adults, peaking during their twenties. Males are affected twice as often as females. It is usually unilateral, but is sometimes bilateral and asymmetrical; it is occasionally recurrent. Most cases have no specific precipitating factors, but some appear few hours to weeks of an upper respiratory tract infection, a vaccination, childbirth, or an invasive diagnostic, therapeutic or surgical procedure.

Table C16-1 Terms Commonly Used to Describe the Syndrome of Neuralgic Amyotrophy (Listed Alphabetically)

Typically, the patient notices upper limb weakness, and sometimes wasting become apparent, during the first week as the pain starts to subside. Weakness of the shoulder girdle muscles, with or without scapular winging, is the most common since the upper plexus-innervated muscles are usually the most affected. The weakness is often restricted to multiple individual peripheral nerves and sometimes to a single nerve only (Table C16-2). Occasionally, the disorder afflicts the entire brachial plexus or exclusively the lower plexus leading to a near complete monoplegia or distal upper extremity weakness, respectively. Oddly, the main upper limb nerves, i.e., the median, ulnar, and radial nerves, are seldom exclusively affected. Occasionally, selective muscles are selectively denervated, presumably due to pathology of the motor branches. This includes the pronator teres, flexor pollicis longus and supraspinatus muscles (Figure C16-2). Sensory loss usually is mild but may be prominent in severe cases. Deep tendon reflexes are depressed or absent if the appropriate muscles are weakened significantly. Chest radiographs may reveal an elevated hemidiaphragm on the ipsilateral side, due to phrenic nerve palsy. Routine magnetic resonance imaging studies of the brachial plexus or upper limb usually are normal, apart from T2-weighted changes of denervated muscles. The diagnosis frequently is based on the clinical picture and is supported by EDX confirmation.

Table C16-2 Peripheral Nerves With High Predilection to Insult During Neuralgic Amyotrophy (Listed in Order of Frequency of Occurrence)

Long thoracic nerve

Suprascapular nerve

Axillary nerve

Musculocutaneous nerve

Anterior interosseous nerve

Phrenic nerve

Figure 16C-2 MRI of the forearm in a 37-year-old man who noted severe weakness of thumb flexion which was preceded by mild dull antecubital pain. Needle EMG showed prominent fibrillation potentials and no voluntary MUAPs in the flexor pollicis longus. In contrast, the median sensory and motor conduction studies, as well as needle EMG of all other muscles innervated by the median nerve, anterior interosseous nerve, and C8/T1 roots were normal. Note the high signal intensity in the flexor pollicis longus (white arrows), seen on sagittal (A), coronal (B), and axial (C) views of the forearm, consistent with denervation.

The long-term prognosis is overall good. Pain resolves within weeks. Muscle strength lags behind, but most patients recover almost completely within a few months. Some have a more protracted improvement with more prolonged pain or residual weakness and atrophy. Permanent weakness occurs when there is severe axonal loss, or when distal muscles are affected. Recurrence is rare (1–5%), usually occurring at highly irregular intervals over months to years.

There is no specific therapy for neuralgic amyotrophy. Strong analgesics are often necessary for pain control. Drugs that target neuropathic pain, such as anticonvulsants and tricyclics, are also useful. Corticosteroids often are prescribed in the acute phase, particularly in patients who have no contraindication to their use. Physical and occupational therapies are essential to maintain range of joint motion and to prevent contractures during the first months of illness.

The differential diagnosis of neuralgic amyotrophy is wide and depends upon the particular nerve(s) affected and the specific antecedent event. It includes rotator cuff tears, cervical radiculopathies, traumatic plexopathies, intraoperative nerve damage, and entrapment neuropathies. Pack palsy occasionally is mistaken for neuralgic amyotrophy, but the clinical circumstances and the fact that pain is not a component of its presentation help in distinguishing pack palsy from neuralgic amyotrophy. A familial disorder that is sometimes indistinguishable clinically and electrodiagnostically from the sporadic form of neuralgic amyotrophy occurs, and is inherited as an autosomal dominant trait. This disorder is often called hereditary neuralgic amyotrophy, or familial brachial plexus neuropathy and is linked to the distal long arm of chromosomes 17q25, with three mutations in the gene septin 9 identified so far. Certain features tend to help in establishing the diagnosis for these families (Table C16-3). Acute brachial plexopathy may occur after minor trauma in patients with another autosomal dominant disorder, hereditary neuropathy with liability to pressure palsy (HNPP). However, the plexopathy in HNPP is typically painless and resolves more rapidly. Also, the neurological examination as well as the nerve conduction studies in other limbs often reveal an underlying generalized demyelinating polyneuropathy. The diagnosis is confirmed by detecting deletion of the human peripheral myelin protein 22 (PMP22) gene, located on chromosome 17p11.2-12. Also, if done, pathologic studies of peripheral nerves reveal evidence of segmental demyelination and tomaculous or “sausage-like” formations.

Table C16-3 Distinguishing Features of Sporadic Neuralgic Amyotrophy and Hereditary Neuralgic Amyotrophy

Feature	Sporadic Neuralgic Amyotrophy	Hereditary Neuralgic Amyotrophy
Age	Adulthood	Onset is frequently in childhood
Sex	Male predominate	Males and females equally affected
Family history	Negative	Positive (dominant trait)
Recurrence	Rare (1–5%)	Common
Lower cranial nerve involvement	Exceedingly rare	Not uncommon
Associated findings	None	Dysmorphic features (cleft palate, canthal folds, syndactyly, etc.)

Electrodiagnosis

In assessing a patient with possible neuralgic amyotrophy, the electromyographer should have a firm grasp of the anatomy of the brachial plexus and its branches (Figure C16-3), as well as the peripheral nerves of the upper extremity (Table C16-4) and the myotomal chart of all muscles of the upper limb (see Figure C11–6). Three important anatomical facts must be emphasized in localizing lesions of the brachial plexus:

1. The median sensory fibers do not pass through the lower plexus. The thumb SNAP is innervated by C6 through the upper trunk, the index finger by C6 and C7 through the upper and middle trunks, and the middle finger by C7 through the middle trunk. All three SNAPs traverse the lateral cord to reach the median nerve. The lateral antebrachial SNAP is similar to the median SNAP to the thumb, originating in C6 and passing through the upper trunk and lateral cord to the musculocutaneous nerve. The radial SNAP originates from the C6 and C7 roots and reaches the posterior cord via the upper and middle trunks (Figure C16-4). All these SNAPs are normal in lower brachial plexus lesions (lower trunk or medial cord) while the ulnar SNAP originating from C8, and the medial antebrachial SNAP originating from T1, are usually abnormal.

2. The median motor fibers to the thenar muscles do not pass through the upper plexus. Thus, routine median motor conduction studies are normal in upper brachial plexus lesions while their sensory counterparts are abnormal.

3. In contrast to the median fibers to the hand, the ulnar motor fibers and ulnar sensory fibers do not separate while traversing the plexus. They pass through the lower trunk and medial cord and continue through the ulnar nerve to their targets in the forearm and hand.

Figure 16C-3 The brachial plexus. Trunks are named upper, middle, and lower. Cords are lateral (L), posterior (P), and median (M). Roots and trunks are supraclavicular, while cords and terminal peripheral nerves are infraclavicular.

(From Goodgold J. Anatomical correlates of clinical electromyography. Baltimore, MD: Williams and Wilkins, 1974, with permission.)

Table C16-4 Motor and Sensory Nerves Arising Directly from the Brachial Plexus (excluding the main terminal nerves)

Figure C16-4 Brachial plexus pathways for the sensory fibers assessed by median sensory nerve action potential (SNAP) (to the thumb (A), and index (B) and middle (C) fingers), radial SNAP (D), and lateral antebrachial SNAP (E). Solid lines represent predominant pathways and dashed lines represent possible additional pathways.

(From Ferrante MA, Wilbourn AJ. The utility of various sensory nerve conduction responses in assessing brachial plexopathies. Muscle Nerve 1995;18: 879–889, with permission.)

Brachial plexus lesions are divided into supraclavicular and infraclavicular plexopathies. Supraclavicular plexus lesions are further divided into the upper trunk, middle trunk, and lower trunk lesions. Infraclavicular lesions are divided into lateral cord, posterior cord and medial cord lesions. The electrodiagnosis of lesions of the brachial plexus, the largest and most complex structures of the peripheral nervous system, is more time consuming and requires performing multiple common and uncommon sensory and motor nerve conduction studies and sampling a large number of muscles with needle EMG. In lesions of the brachial plexus that are restricted to a specific trunk or cord, certain sensory and motor NCSs and muscles are likely to be abnormal based on the location of the injured fibers within the plexus (Tables C16-5 and C16-6).

Table C16-5 Nerve Conduction Studies and Muscles Commonly Affected in Upper Brachial Plexus Lesions

Sensory Conduction Studies	Motor Conduction Studies	Needle EMG
Lateral antebrachial cutaneous Median (recording thumb) Radial (recording base of thumb)

Axillary (recording deltoid)

Musculocutaneous (recording biceps)

Adapted with revisions from Wilbourn AJ. Assessment of the brachial plexus and the phrenic nerve. In: Johnson EW, Pease W, eds. Practical electromyog-raphy, 3rd ed. Baltimore, MD: Williams and Wilkins, 1997.

Table C16-6 Nerve Conduction Studies and Muscles Commonly Affected in Lower Brachial Plexus Lesions

Sensory Conduction Studies	Motor Conduction Studies	Needle EMG
Ulnar recording little finger Dorsal ulnar cutaneous Medial antebrachial cutaneous Medial brachial cutaneous	Ulnar recording hypothenar Ulnar recording 1st dorsal interosseous Median recording thenar Radial recording extensor digitorum communis	All hand intrinsics ^* Flexor pollicis longus Flexor carpi ulnaris Extensor indicis Flexor digitorum profundus Extensor digitorum communis