Case 10

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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Case 10

HISTORY AND PHYSICAL EXAMINATION

An 82-year-old woman tripped and fell on her right shoulder. An anterior dislocation of the right shoulder was reduced on the same day; however, she continued to have pain and weakness in the right shoulder. Arthrography and magnetic resonance imaging (MRI) of the shoulder failed to reveal rotator cuff tear. She denied any numbness. She was otherwise in excellent health.

On examination 6 weeks later, she had significant restriction of passive range of movement of the right shoulder to approximately 90°. This movement was associated with pain. There was atrophy of the right deltoid and possibly the spinati. Motor examination of all shoulder girdle muscles, including assessment of active shoulder abduction, was limited by pain. However, the biceps, triceps, and brachioradialis seemed normal in strength. All forearm and hand muscle movements also were determined to be normal, as was sensation. Deep tendon reflexes were normal and the sensory examination revealed no clear abnormalities. The patient was referred to the electromyography (EMG) laboratory to explore the possibility of an injury of the brachial plexus or the axillary nerve.

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

QUESTIONS

1. In addition to the deltoid muscle, the axillary nerve innervates the:

A. Teres major muscle.
B. Teres minor muscle.
C. Rhomboideus major muscle.
D. Subscapularis muscle.
E. Brachialis muscle.

2. Most fibers within the affected nerve originate from:

A. The C5 and C6 roots via the upper trunk and the lateral cord.
B. The C6 and C7 roots via the upper and middle trunks and the posterior cord.
C. The C5 and C6 roots via the upper trunk and posterior cord.
D. The C6 and C7 roots via the upper and middle trunks and the lateral cord.

3. Common causes of this nerve palsy include all of the following except:

A. Shoulder dislocation.
B. Mid-humeral fracture.
C. Neuralgic amyotrophy.
D. Misplaced injection.

EDX FINDINGS AND INTERPRETATION OF DATA

Pertinent electrodiagnostic (EDX) findings include:

1. Very low-amplitude axillary compound muscle action potential (CMAP) on the right (0.88 mV versus 6.2 mV on the left) with slow distal latency (5.8 ms on the right versus 2.6 ms on the left).
2. Normal musculocutaneous CMAP.
3. Normal median sensory nerve action potentials (SNAPs), recording thumb, index, and middle fingers; radial SNAP; and lateral cutaneous forearm (lateral antebrachial) SNAP.
4. Fibrillation potentials and no voluntary motor unit action potentials in the right deltoid muscle only, without involvement of other upper plexus innervated muscles (supraspinatus, infraspinatus, biceps, brachioradialis, and pronator teres), posterior cord innervated muscles (triceps, brachioradialis, and extensor indicis), or cervical paraspinal muscles.

Case 10: Nerve Conduction Studies

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Case 10: Needle EMG

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These findings are consistent with an isolated severe injury to the right axillary nerve with no involvement of the upper trunk or posterior cord of the brachial plexus.

DISCUSSION

Applied Anatomy

The axillary nerve (also called the circumflex nerve) arises from the posterior cord of the brachial plexus near the shoulder joint. Its fibers originate from the C5 and C6 roots and travel through the upper trunk to the posterior cord. The nerve has a very short path. It courses posteriorly, inferior to the shoulder joint where it traverses the quadrilateral space, bounded superiorly by the teres minor muscle, inferiorly by the teres major muscle, medially by the long head of the triceps muscle, and laterally by the humeral neck. At this point, the axillary nerve gives branches to the teres minor muscle and then it curves around the neck of the humerus, under the deltoid muscle. There, it terminates into two branches, one posterior and the other, anterior; both innervate the deltoid muscle. The upper lateral cutaneous nerve of the arm, which innervates the skin overlying the deltoid muscle (Figure C10-1), originates from the posterior branch.

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