Modalities – EMG

EMG- Electromyography

EMG is the study of the intrinsic electrical properties of skeletal muscles. EMG results reflect both the functioning connection between a nerve and its innervated muscle and the integrity of the muscle itself. Since specific muscles are attached to specific nerves, nerve function can be implied from the activity seen in the EMG recording. When muscle is at rest, it is typically electrically silent on EMG recordings. When a nerve is irritated, it causes the muscle that it innervates to fire spontaneously. When the muscle becomes active (fires), resultant EMG activity from the muscle is readily and easily discovered by intraoperative monitoring equipment.

This EMG activity can manifest itself in several different forms depending on the degree of irritation or injury to the nerve including: spikes (individual discharges), bursts (brief flurries of discharges), train activity (more persistent regularly repeating discharge patterns), and neurotonic discharges (persistent prolonged bursting). These EMG recordings can be particularly useful in certain types of operations. Surgeries that involve peripheral or cranial nerves having muscle innervations, selective dorsal rhizotomies, tethered cord releases, and assessing pedicle screw placement are the most common uses of EMG recordings.

For example, when assessing pedicle screw placement during thoracic or lumbar fusion, the pedicle screws are stimulated with triggered EMGs and the associated nerve roots are measured for a response. If a response is indicated by the monitoring technician, the proximity of the screw to a nerve root and the possibility of a breach of the pedicle should be examined by the operating physician.

In the area of nerve root surgery, the cost efficacy of monitoring was reported by Owen and Tamaki (1997). In that study, surgical outcome from 2 groups of matched patients was determined. In Group I, no intraoperative monitoring was used, while in Group II, routing EMG monitoring was administered. Results indicate that EMG monitoring significantly decreased short and long term costs associated with these deficits (Owen and Tamaki. Chapter 34 in The Adult Spine: Principles and Practice. 2nd edition, J.W. Frymoyer, Editor-in-Chief. Lippincott-Raven Publishers, Phil., 1997).

Final Word on EMG

Intraoperative EMG monitoring is a valuable asset in monitoring the integrity of peripheral and cranial nerves, and offers a simple non-invasive technique for determining pedicle screw placement.