|
Year |
2002 |
|---|---|
|
Question Number |
236. |
|
Category |
Shoulder and Elbow |
Question
An 18 yr old man underwent an open shoulder capsular shift for recurrent instability. Post operatively, the patient had new onset weakness of forearm supination and elbow flexion. Nerve conduction velocity studies performed 3 weeks later show a normal evoked response distally. An electromyogram shows fibrillations and positive sharp waves. What is the most likely diagnosis?
1. Neurapraxia of the axillary nerve
2. Neurapraxia of the musculocutaneous nerve
3. Axonotmesis of the axillary nerve
4. Axonotmesis of the musculocutaneous nerve
5. neurotmesis of the musculocutaneous nerve
Figures
Answer (click for answer)
Preferred Answer: 4
First, the guy has loss of forearm supination and elbow flexion-which is a function of the biceps. The biceps is innervated by the musculocutaneous nerve. Therefore you can cross out 1 and 3. Now look at the nerve studies.
Nerve conduction velocity is usually only decreased when a widespread neuropathic process is occurring such as in diabetic polyneuropathy. The distal aspects of the nerve are not damaged in the case in the question as it is a post op complication due to iatrogenic injury to the nerve, most likely traction, cutting or suturing. It would therefore be expected to be normal in this case.
In an EMG, normal muscle fibers will show electrical silence at rest and a characteristic motor unit with contraction, (1 positive wave). Lesions only affecting the sensory component of a nerve will not show up on EMG. Fibrillations and sharp positive waves are characteristic of early denervation of LMN and imply axonal loss.
Neurapraxia is a reversible conduction block characterized by local ischemia and selective demylenation of the axon sheath. Demylination most commonly results from compression. Symptoms range from paresthesias in mild or early cases to paralysis in severe or late cases. Recovery usually occurs within days to weeks after removal of the cause.
Axonotmesis is a more severe injury with disruption of the axon and myelin sheath but leaving the epineurium intact. This is seen in compression, trauma, and metabolic derangements. With injury, there is a conduction block at the site, but distal conduction remains normal. Muscle fibers will begin to show signs of denervation recognizable with EMG 1-3 wks after the injury.
Neurotmesis is complete nerve division with disruption of the endoneurium, usually related to trauma. Conduction block is seen at the injury site initially, but preservation of the distal segment conduction continues.
The EMG results are implying damage to the axon, so the answer is axonotmesis of the musculocutaneous nerve.
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