Postoperative Shoulder Weakness: Musculocutaneous Nerve or Upper Trunk Plexopathy?
44-year-old man has weakness and numbness three weeks after arthroscopic shoulder stabilization performed in the beach-chair position. He cannot flex the elbow against resistance (3/5), supination is 4/5, and deltoid strength is 4/5; sensation is reduced over the lateral forearm. Biceps reflex is reduced. Musculocutaneous motor response is 2.0 mV (contralateral 8.0 mV), lateral antebrachial cutaneous SNAP is 2 microvolts (contralateral 12 microvolts), and median and ulnar studies are normal. Needle EMG shows 2+ fibrillation potentials in biceps, 1+ in brachialis, 1+ in deltoid, and no spontaneous activity in infraspinatus or triceps. He asks whether this is an isolated musculocutaneous nerve injury and whether surgery is needed.
1. Localize the lesion. Why is an isolated musculocutaneous neuropathy an incomplete explanation?
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The musculocutaneous findings are clear: low motor response, low lateral antebrachial cutaneous SNAP, and denervation in biceps and brachialis. However, deltoid is also weak and shows fibrillation potentials, which implicates the axillary nerve or a more proximal C5-C6 lesion. Both nerves arise from the upper trunk, so an upper trunk brachial plexopathy is more likely than an isolated musculocutaneous lesion. The normal infraspinatus and triceps argue against a complete upper trunk or posterior cord lesion, but fascicular or partial involvement can spare them. The beach-chair position and arthroscopic procedure support a position-related plexus injury. Isolated musculocutaneous neuropathy would not explain deltoid denervation.
2. What additional electrodiagnostic and imaging studies would you obtain, and which findings would favor a plexus lesion?
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I would add axillary motor and sensory studies if feasible, suprascapular nerve studies, and needle EMG of deltoid, supraspinatus, infraspinatus, rhomboids, serratus anterior, triceps, and cervical paraspinals. I would compare sides and study the median and ulnar nerves fully. Findings in rhomboids or serratus anterior would point to a more proximal plexus or root lesion; infraspinatus involvement would implicate the suprascapular nerve or upper trunk. MRI of the brachial plexus with contrast and shoulder imaging would assess hematoma, compression, or nerve injury. If the lesion is confined to musculocutaneous and axillary territories, an upper trunk lesion is most likely; if only musculocutaneous is abnormal, a fascicular injury is possible.
3. How would you counsel him about surgery and prognosis?
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I would explain that most position-related brachial plexopathies improve over months, and that EMG alone is not an indication for surgery. I would recommend physical therapy, pain management, and serial strength and sensory examinations. I would obtain MRI to exclude a compressive hematoma or structural lesion. Surgery or nerve exploration would be considered for a documented laceration, expanding hematoma, or failure to improve with a severe deficit after an appropriate interval. I would avoid pronouncing a precise nerve injury too early, because reinnervation and recovery can continue for 12-18 months.