Electromyography (EMG) measures the electrical activity in muscle. Since specific muscles are attached to specific nerves, nerve function can be implied from the type of activity seen in the EMG recording.
Resting muscle with normal attached nerve is usually electically silent on standard EMG recordings. When the nerve is irritated or injured, it will fire spontaneously, causing reciprocal firing in the muscle. This manifests as motor unit firing ('spontaneous' single motor unit firing) that can occur in several patterns indicating increasing degrees of irritation or injury including: spikes (individual discharges), bursts (brief flurries of discharges), train activity (more persistent regularly repeating discharge patterns) and neurotonic discharges (persistent prolonged bursting).
Intraoperative EMG differs significantly from diagnostic EMG in several ways:
EMG recordings can be useful in any surgery where possible injury can occur to a peripheral or cranial nerve that has muscle innervations. They are particularly helpful in selective dorsal rhizotomy, tethered cord release and in assessing pedicle screw placement (triggered EMG from stimulation of the screw or screw hole and measurement of associated nerve root response). The latter helps in determining proximity of the screw to the nerve and the possibility of breach of the pedicle.
Intraoperative EMG is an invaluable tool for monitoring the integrity of peripheral and cranial nerve elements in the OR
In addition, it offers a simple non-invasive technique for evaluation of pedicle screw placement
R. O'Brien MD
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