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 a 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).

EMG

EMG showing train activity

Triggered EMG

Pedicle Screw Placement

Technique
Intraoperative EMG differs significantly from diagnostic EMG in several ways:

• Accompanying nerve conduction information is usually lacking.
• All recordings are done simultaneously, thus requiring multiple input channels, limiting the anatomic area that can be covered by specific equipment.
• Recording electrodes are usually placed much further apart and may be referenced to a distant part of the same muscle (so called ‘bipolar’ array) or to another muscle all together (active reference array). The latter allows for the broadest muscle coverage but less specificity to the muscle(s) involved. Diagnostic EMG covers one muscle at a time with relatively tiny recording fields.
• Recording electrodes in IONM are usually more superficial.
• IONM filter settings are usually narrower to screen extraneous noise.
• The use of so called ‘triggered’ EMG in which muscle response is measured after stimulation of a nervous element is more common.

Uses:
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.

Summary:

Upside:

• Very useful for assessment of peripheral nerve integrity.
• Complements recordings such as SSEP and TceMEPs that are more focused on central nervous system integrity.

Downside:

• ‘Active reference’ recordings increase anatomical coverage but may reduce specificity in determining nerve involvement.
• Sensitive to paralytic agents.

Last Word on IONM EMG:
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.

Selected References:

Balzer JR, Rose RD, Welch WC, Sclabassi RJ. Simultaneous somatosensory evoked potential and electromyographic recordings during lumbosacral decompression and instrumentation. Neurosurgery 1998;42(6):1318-25.

Weiss DS. Spinal cord and nerve root monitoring during surgical treatment of lumbar stenosis. Clin Orthop 2001;(384):82-100.

Lenke L et al. Triggered Electromyographic Threshold for Accuracy of Pedicle Screw Placement; An Animal Model and Clinical Correlation. Spine 1995;20:1585-1591

Tony Danesh-Clough, MB, ChB, Peter Taylor, PhD, Bruce Hodgson, FRACS and Mark Walton, PhD. The Use of Evoked EMG in Detecting Misplaced Thoracolumbar Pedicle Screws. Spine 2001;26 (12):1313-1316

David H Clements, MD, David E Morledge, PhD, William H. Martin, PhD and Randal R Betz, MD. Evoked and Spontanous Electromyography to Evaluate Lumbarsacral Pedicle Screw Placement. Spine 1996;21( 5): 600-604

Bruce V Darden II, Kenneth E Wood, Martha K Hatley, Jefferey H Owen and John Kostuik. Evaluation of Pedicle Screw Insertion Monitored by Intraoperative Evoked Electromyography. Journal of Spinal Disorders 1996;9 (1): 8-16

Benzel EC. Techniques, Complication, Avoidance and Management Intraoperative Electromyography Monitoring. Spine Surgery Volume 2 Chapter 95