Intraoperative Neurophysiological Monitoring 

Overview

This article provides an overview of the various neurophysiological monitoring techniques used intraoperatively.

Intraoperative neurophysiological monitoring has been utilized in attempts to minimize neurological morbidity from operative manipulations. The goal of such monitoring is to identify changes in brain, spinal cord, and peripheral nerve function prior to irreversible damage. Intraoperative monitoring also has been effective in localizing anatomical structures, including peripheral nerves and sensorimotor cortex, which helps guide the surgeon during dissection.

Evoked potential monitoring includes somatosensory evoked potentials (SSEP), brainstem auditory evoked potentials (BAEP), motor evoked potentials (MEP), and visual evoked potentials (VEP). Electromyography (EMG) also is used extensively during operative cases. Scalp electroencephalography (EEG) provides data for analysis in SSEP, BAEP, and VEP. Scalp EEG also can be used to monitor cerebral function during carotid or other vascular surgery. In addition, EEG recorded directly from the pial surface, or electrocorticography (ECoG), is used to help determine resection margins for epilepsy surgery, and to monitor for seizures during electrical stimulation of the brain carried out while mapping cortical function.

Electrophysiological monitoring in the operative milieu poses several specific challenges. These include (1) presence of electromagnetic interference, and (2) use of anesthetic agents that can alter recordings

history
In 1848, Du Bois-Reymond first demonstrated the action potential of nerves. He also is credited with describing the electrical activity of muscle, the first EMG. Electrical activity of the brain was described by Caton in 1875. Han Berger in 1928-29 was the first to report EEG tracings from human brains.

The first use of intraoperative EEG was by Foerster and Alternberger in 1935. In the late 1930s through the 1950s, Herbert Jasper and Wilder Penfield further developed this technique, using ECoG for localization and surgical treatment of epilepsy.^[1] They also performed careful mapping of cortical function by direct electrical stimulation.

Dawson recorded the first SSEP in 1947.^[2] Understanding of other evoked potentials, including those produced by motor activity and by visual and auditory stimulation, followed. In 1978, the first intraoperative use of BAEP was reported. Since then the following refinements in recording techniques have taken place:

• Improved differential amplifiers with development of transistors and integrated circuitry
• Reduced size of equipment to allow more widespread intraoperative use
• Signal-averaging techniques to allow recording of small signals on a large electrical background
• Computerized equipment with microprocessors to allow multiple channel recordings and various filtering capabilities as well as spectral analysis
• The image below depicts normal brainstem auditory evoked potentials