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The effect of a proximal anesthetic nerve block on the SNAP, CMAP, and EMG?
In discussion with a fellow recently-graduated trainee, we were discussing anesthetic nerve blocks and the immediate effect on EDX. Hypothetically, if a person got their proximal median nerve blocked with lidocaine above the elbow, what would the effect be on the median antidromic sensory NCS to digit 2, the median motor NCS to APB, and the needle EMG of APB or pronator teres be?
I would assume, for the EMG, that you would have no spontaneous activity, no neurogenic changes, but (if truly blocked) you would have no activation of median-innervated muscles distal to the block, correct?
For the NCS, would a proximal median nerve block with lidocaine above the elbow appear like a severe post-ganglionic injury (like a brachial plexus injury), resulting in an absent SNAP? Or, conversely would the portion stimulated at the wrist and recorded at digit 2 be intact? Thoughts on CMAP?
Any clarification would be awesome. Thank you!
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