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
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
Electromyography
Purpose of Electromyography
EMG is the recording of electrical activity of muscle. Changes in EMG recordings are indirect indicators of function of the innervating nerve. Intraoperative uses have stressed localization and assurance of the integrity of peripheral nerves, including cranial nerves.
Technique
Multiple EMG needles typically are placed into the muscles to be examined. Practically any muscle can be monitored, including face, tongue, and sphincter musculature. EMG is recorded continually with a low noise amplifier. Recordings are displayed visually and usually also sent to a speaker to provide auditory feedback. Changes in muscle electrical activity then can be seen and heard. When a peripheral nerve is to be localized intraoperatively, a sterile stimulating probe is used during the operation. The image below illustrates two types of EMG needles
Interpretation
Spontaneous or induced EMG activity is monitored. Additionally, direct electrical stimulation of the nerve can help localize the neural structure. Note that spontaneous EMG activity does not assure the integrity of the peripheral nerve. If evoked EMG activity can be elicited consistently, integrity of the distal nerve and muscle can be assured.
Clinical Uses
Facial nerve/other cranial nerve monitoring
Cranial nerve monitoring is useful for surgical procedures in which the facial nerve is at risk, including posterior fossa surgery (eg, acoustic neuroma), vestibular neurectomy, surgery in the temporal bone, and parotid gland surgery. Trigeminal, glossopharyngeal, vagus, spinal accessory, and hypoglossal nerve functions can be monitored similarly by EMG. Electrical stimulation in the operative field can evaluate the integrity of peripheral nerves. Spontaneous EMG activity suggests manipulation in the vicinity of the cranial nerve.
Selective dorsal rhizotomy
Selective dorsal rhizotomy (SDR) is a procedure that is used to reduce debilitating spasticity in conditions such as cerebral palsy by selectively transecting spinal rootlets. Overactive excitatory influence on motor nerves is believed to be reduced by removing facilitory afferent input from muscle spindles. The procedure consists of stimulating spinal rootlets and monitoring EMG and motor function. Those rootlets that are associated with an abnormal motor response are sectioned selectively
Interpretation
Spontaneous or induced EMG activity is monitored. Additionally, direct electrical stimulation of the nerve can help localize the neural structure. Note that spontaneous EMG activity does not assure the integrity of the peripheral nerve. If evoked EMG activity can be elicited consistently, integrity of the distal nerve and muscle can be assured.
Clinical Uses
Facial nerve/other cranial nerve monitoring
Cranial nerve monitoring is useful for surgical procedures in which the facial nerve is at risk, including posterior fossa surgery (eg, acoustic neuroma), vestibular neurectomy, surgery in the temporal bone, and parotid gland surgery. Trigeminal, glossopharyngeal, vagus, spinal accessory, and hypoglossal nerve functions can be monitored similarly by EMG. Electrical stimulation in the operative field can evaluate the integrity of peripheral nerves. Spontaneous EMG activity suggests manipulation in the vicinity of the cranial nerve.
Selective dorsal rhizotomy
Selective dorsal rhizotomy (SDR) is a procedure that is used to reduce debilitating spasticity in conditions such as cerebral palsy by selectively transecting spinal rootlets. Overactive excitatory influence on motor nerves is believed to be reduced by removing facilitory afferent input from muscle spindles. The procedure consists of stimulating spinal rootlets and monitoring EMG and motor function. Those rootlets that are associated with an abnormal motor response are sectioned selectively
