EMG infers specific nerve function by recording electrical impulses in upper and lower extremity muscles. Continuous Free-Run EMG detects compound muscle action potentials immediately pursuant to associated nerve root irritation or damage. Through nerve root stimulation or other irritation due to, for example, incorrect pedicle screw placement, triggered EMG produces a Compound Muscle Action Potential, or CMAP. Both EMG types also may be used to monitor cranial nerves, with the most common nerves including the Recurrent Laryngeal nerve and the Facial nerve. During Facial nerve monitoring, EMG can often detect irritation or injury through readings from eye and mouth muscles.
EMG recordings may indicate severity of irritation or damage through recording patterns displaying spikes, bursts, train activity and neurotronic discharges. Particularly useful in evaluating cranial and peripheral nerves, EMG complements the SSEP and TCMEP modalities, which tend to concentrate on the central nervous system. EMG often is used during spinal instrumentation to evaluate pedicle screw integrity. This modality also assesses injury risk to specific nerves during surgeries on structures such as the parotid gland, spine and thyroid; or risk to muscles such as the sphincter muscles during tethered cord release.