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Five progressive clues
These are the case findings in their original reveal order, moving from the broader presentation to the most discriminating evidence.
Clue 1
A 24-year-old man arrives at an outpatient upper-extremity rehabilitation clinic six weeks after a high-speed motorcycle crash. He walks in unassisted, his right arm hanging limply at his side and tucked into his jacket pocket. An amateur guitarist, he is most distressed that he can no longer hold a chord or lift the neck of his instrument.
Clue 2
At the moment of impact he was thrown over the handlebars and landed on the top of his right shoulder, which was violently forced downward while his head was pushed hard toward the opposite side. The arm went numb and dead-weight instantly. Six weeks on there is no recovery; he describes a constant deep burning ache in the limb, and he cannot raise the arm from his side or bend the elbow. His neck CT the night of the crash showed no fracture.
Clue 3
On examination the arm rests adducted against the trunk, internally rotated, with the elbow straight and the forearm turned palm-backward. Shoulder abduction and elbow flexion are graded 0/5 and elbow extension is weak, yet grip strength and fine finger dexterity are fully preserved. Sensation is absent over the outer shoulder, the lateral forearm, and the thumb, and the biceps and brachioradialis reflexes cannot be elicited.
Clue 4
Nerve conduction studies show absent sensory responses recorded from the lateral forearm and thumb, while needle EMG reveals florid denervation, with fibrillations and positive sharp waves, in the deltoid, biceps, and brachioradialis. Critically, the cervical paraspinal muscles are entirely normal on EMG.
Diagnosis
Brachial Plexus Injury
Why the diagnosis fits
The instantaneous flaccid paralysis after a shoulder-depression, head-lateral-flexion traction injury, with weakness and sensory loss crossing several different peripheral-nerve territories yet confined to the C5-C6 myotomes and dermatomes (deltoid, biceps, brachioradialis; outer shoulder, lateral forearm, thumb), localizes the lesion to where those roots have already merged rather than to any single nerve. The decisive discriminator is the EMG: absent sensory nerve action potentials with completely normal cervical paraspinal muscles place the lesion distal to the dorsal root ganglion, the opposite of a cervical radiculopathy, which spares the sensory responses and denervates the paraspinals. An isolated peripheral nerve injury cannot explain deficits spanning the axillary, musculocutaneous, and radial territories simultaneously, and a rotator cuff tear would neither abolish reflexes nor cause dermatomal sensory loss.
Educational reference
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