Case 16

Case 16

HISTORY AND PHYSICAL EXAMINATION

Acute, severe right upper extremity pain developed overnight in a 22-year-old man; it peaked in 2 days, resulted in multiple visits to the emergency room, and ultimately led to admission to a community hospital on the third day. The pain was deep and maximal around the right shoulder, with radiation to the arm and forearm, and did not worsen with coughing or sneezing. Three to four days after admission, while the patient was being treated with analgesics, he noted weakness of the right upper extremity. The weakness was maximal in the shoulder and arm, and much less severe in the hand. He had slight numbness in the hand and forearm, most particularly in the right thumb. He was given a 10-day course of oral prednisone and was referred to a tertiary care center.

Two weeks before the onset of pain, the patient had an upper respiratory tract infection with cough, nasal discharge, fever, and malaise. This was treated with antibiotics for one week. There was no history of trauma. He was otherwise in excellent health.

When the patient was seen 4 weeks after the onset of symptoms, he had minimal shoulder pain. Cranial nerve examination was normal. There was no Horner sign. Findings were limited to the right upper extremity, where there was complete loss of function of the right deltoid, spinati, biceps, and brachioradialis (Medical Research Council [MRC] 0/5). The right triceps was much less involved (4/5). There was mild diffuse weakness of all finger and wrist extensors, with severe weakness of the right thumb long flexion (flexor pollicis longus) and distal interphalangeal flexion of the index and middle fingers (flexor digitorum profundus). This resulted in a positive pincer or OK sign (Figure C16-1). Deep tendon reflexes of the right upper limb were absent. Sensation revealed mild sensory loss over the thumb, and lateral arm and forearm.

Figure 16C-1 Normal and abnormal OK sign (pincer sign). Normal sign on the left and abnormal sign on the right, as in lesions of the anterior interosseous nerve caused by weakness of the long flexors of the thumb and index fingers.

(From Mumenthaler M, Schliak H. Peripheral nerve lesions. Diagnosis and therapy. New York: Thieme Medical Publishers, 1990.)

An electrodiagnostic (EDX) study was performed.

Please now review the Nerve Conduction Studies and Needle EMG tables.

QUESTIONS

3. The routine motor nerve conduction study results in this disorder may be normal because:

A. The illness is not associated with significant axonal loss.

B. The lesion involves frequently the upper trunk of the brachial plexus.

C. The process is demyelinating.

D. All of the above.

EDX FINDINGS AND INTERPRETATION OF DATA

Abnormalities on EDX examination include:

1. Abnormal sensory nerve action potential (SNAP) amplitudes. As a rule, abnormal SNAPs point to lesions of the plexus or peripheral nerves, and are not consistent with root lesions (i.e., intraspinal canal lesions) because the latter interfere only with the proximal (central) axons of the dorsal root ganglia, leaving the distal (peripheral) axons intact. Although the SNAP abnormalities are diffuse, when one compares the right upper extremity to the left, the most abnormal SNAPs in order of decreasing severity are: lateral cutaneous of forearm (lateral antebrachial); median, recording the thumb and index; radial and median, recording the middle finger; and ulnar, recording the little finger. This suggests that the disorder is diffuse but has a preponderance to the upper plexus (upper trunk and lateral cord), where sensory fibers to the lateral cutaneous nerve of the forearm and median nerve are located.

2. Abnormal compound muscle action potential (CMAP) amplitudes. The CMAPs of routine motor nerve conduction studies (NCS), i.e., median and ulnar, are not as abnormal as the less commonly performed NCS, i.e., the axillary and musculocutaneous. This finding points to a pathological process that involves primarily the upper trunk of the brachial plexus or individual lesions of both the musculocutaneous and axillary nerves. The relatively lower median CMAP compared with the ulnar CMAP could be caused by a median nerve lesion or a fascicular lesion of the lower plexus (lower trunk or medial cord).

3. Needle EMG examination shows diffuse abnormalities in all muscles tested on the right, except the paraspinal, serratus anterior, and rhomboid muscles. This finding confirms that the lesion(s) is not at the level of the roots but is located more distally into the brachial plexus or peripheral nerves. Concurrent with diffuse involvement, certain muscles are exceptionally abnormal. These include:

• Severe denervation of the flexor pollicis longus, the pronator quadratus, and the median part of the flexor digitorum profundus, consistent with severe lesion of the anterior interosseous nerve.

• Total denervation of the supraspinatus, infraspinatus, deltoid, biceps, and brachioradialis consistent with severe lesion of the upper trunk of the brachial plexus. In this situation, it is impossible to confirm or exclude additional separate lesions of the suprascapular, axillary, or musculocutaneous nerves.

Case 16: Nerve Conduction Studies

Case 16: Needle EMG

The aforementioned findings point to a diffuse, likely multifocal process involving the neural elements of the upper extremity, with the most severe involvement to the upper trunk of the brachial plexus and the anterior interosseous nerve. The process is not the result of root(s) pathology (i.e., it is not due to intraspinal canal process), because of abnormal SNAPs and normal paraspinal muscles. The EDX findings are highly suggestive of neuralgic amyotrophy due to the predilection of lesions to the upper trunk and the anterior interosseous nerve.

DISCUSSION

Clinical Features

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)

As the most popular name implies, neuralgic amyotrophy is characterized by pain and weakness of the upper limb. The pain is usually abrupt in onset with a tendency to develop at night, sometimes awakening the patient from sleep. It is a severe deep boring shoulder pain, and maximal during the first few days of illness. The pain is not exacerbated by the Valsalva maneuver (e.g., cough, sneeze) which helps distinguishing it from a subacute cervical radiculopathy. Many patients visit the emergency department for pain control. In atypical cases, the pain is only modest and maximal over the antecubital fossa. The pain usually lasts for 7–10 days, gradually fades and becomes replaced by a dull ache.

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

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