Case 13

Published on 03/03/2015 by admin

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Last modified 03/03/2015

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Case 13

HISTORY AND PHYSICAL EXAMINATION

A 50-year-old woman had chronic pain in the right shoulder and arm for at least 10 years. She also experienced intermittent numbness of the right upper extremity. Both symptoms worsened after any sustained activity of that extremity, such as writing or cleaning. The numbness was ill-defined but involved mostly the little finger and the medial forearm. The patient noted some weakness in hand grip. Four years previously, she underwent anterior C6–C7 cervical laminectomy with no improvement. Symptoms worsened during the last year and she was referred to the electromyography (EMG) laboratory for further diagnostic studies.

On examination, there was atrophy of the right thenar eminence only. There was mild weakness of the right hand, especially of thumb abduction and the interossei. Phalen sign was negative and Tinel signs were negative at the wrist and elbow. Sensory examination revealed relative hypesthesia of the little finger and medial forearm. Deep tendon reflexes were normal. Peripheral pulses were normal. There was no Horner sign.

A cervical spine radiograph showed a rudimentary cervical rib on the right (Figure C13-1). An EMG examination was performed.

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Figure C13-1 The patient’s cervical spine x-ray (anteroposterior view) revealing a rudimentary cervical rib on the right (arrow).

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

QUESTIONS

1. Clinical and electrodiagnostic (EDX) findings are consistent with:

A. Combined ulnar neuropathy and carpal tunnel syndrome.
B. C8/T1 radiculopathy.
C. Lower trunk brachial plexopathy.
D. Combined C8/T1 radiculopathy and carpal tunnel syndrome.

2. Findings consistent with this disorder include all of the following except:

A. Low-amplitude ulnar sensory nerve action potential (SNAP).
B. Low-amplitude median compound muscle action potential (CMAP).
C. Low-amplitude ulnar CMAP.
D. Low-amplitude median SNAP.
E. Denervation of thenar more than hypothenar muscles.

3. Which of the following is the least commonly encountered nerve conduction studies (NCS) finding associated with this disorder?

A. Focal slowing of ulnar motor conduction velocity in the Erb point-to-axilla segment.
B. Low-amplitude ulnar CMAP.
C. Low-amplitude median CMAP.
D. Low-amplitude ulnar SNAP.
E. Low-amplitude medial antebrachial SNAP.

EDX FINDINGS AND INTERPRETATION OF DATA

The significant EDX findings in this case include the following:

1. Relatively low-amplitude right ulnar SNAP, with normal ulnar sensory distal latency and conduction velocity. Because the sensory fibers contributing to the ulnar SNAP originate from the C8 dorsal root ganglion (DRG), this finding is compatible with a postganglionic C8 lesion (including an extraspinal C8 lesion, a lower trunk or medial cord brachial plexus lesion, or an ulnar nerve lesion). This abnormality is not consistent with a preganglionic (intraspinal root) lesion.
2. Very low-amplitude median CMAP with normal median motor distal latency and conduction velocity, combined with normal median SNAPs amplitudes and distal latencies. This is not consistent with carpal tunnel syndrome, in which the median SNAPs are usually more affected and there is usually focal slowing at the wrist.

• The combination of the aforementioned two findings on routine NCSs should raise the suspicion of a lower brachial plexus lesion (lower trunk or medial cord), resulting in axonal degeneration of the median motor fibers to the thenar muscles (which originate from T1 and C8 ventral roots) and of the ulnar postganglionic sensory fibers (which also originate from the C8 dorsal root ganglion). The preservation of median SNAPs is not consistent with a median nerve lesion.

3. Absent SNAP of the medial antebrachial cutaneous nerve (medial cutaneous nerve of the forearm), which branches directly from the medial cord. Its fibers originate mostly from the T1 DRGs. This abnormal NCS is a very important confirmatory evidence of lower trunk brachial plexus or extraspinal T1 lesion and excludes a selective ulnar nerve lesion.
4. Relatively low-amplitude right ulnar CMAP, recording first dorsal interosseous (when compared with the left), with normal ulnar motor distal latencies and conduction velocities. This is another supporting evidence of motor axon loss along the T1 and C8 fibers.
5. Denervation with reinnervation of muscles innervated by the T1 and C8 roots, including the median muscles (abductor pollicis brevis and flexor pollicis longus), the ulnar muscles (first dorsal interosseous, abductor digiti minimi, and flexor carpi ulnaris), and the radial muscles (extensor indicis proprius). Note that the findings are chronic (large motor unit action potentials, MUAPs) with slight ongoing denervation (fibrillation potentials), and most prominent in the abductor pollicis brevis.

Case 13: Nerve Conduction Studies

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Case 13: Needle EMG

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In summary, these findings are pathognomonic of a lesion affecting the lower trunk of the brachial plexus. The predilection of sensory and chronic motor axonal loss to the T1 root, as supported by a most severe axon loss in the abductor pollicis brevis/thenar muscles and the medial antebrachial cutaneous SNAP, are classical findings of neurogenic thoracic outlet syndrome (TOS). In most cases of neurogenic TOS, other lower trunk/C8 structures, such as the interossei and the abductor digiti minimi/hypothenar muscles and the ulnar SNAP, are affected to a much lesser extent.

DISCUSSION

Applied Anatomy

The brachial plexus is derived from the anterior primary rami of the C5 through T1 spinal roots. As is shown in Figure C13-2, these roots intertangle at multiple sites to form structures that usually are divided into five components: roots, trunks, divisions, cords, and peripheral (terminal) nerves. Because the divisions generally are located under the clavicle, some divide the brachial plexus into two regions: supraclavicular (roots and trunks) and infraclavicular (cords and peripheral nerves). Nerve fibers from the C5 and C6 anterior primary rami combine to form the upper trunk, C8 and T1 rami combine to form the lower trunk, and the C7 ramus continues as the middle trunk. Then, each trunk divides into two divisions (anterior and posterior). All three posterior divisions unite to form the posterior cord, the upper two anterior divisions merge to form the lateral cord, and the anterior division of the lower trunk continues as the medial cord.

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Figure C13-2

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