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Neurophysiology: Intraoperative Monitoring (IOM)

Our department offers state-of- the art Intraoperative Neurophysiological Monitoring (IOM) 24/7 at University Medical Center with real-time remote monitoring by neurophysiologists via Internet networking resources, working with a team of dedicated technologists. A full complement of IOM modalities includes the following:

A. TRANSCRANIAL MOTOR EVOKED POTENTIALS (TCeMEP)

1. DESCRIPTION: TCeMEP involves controlled delivery of electrical current pulses across the cranium to the brain, with specific goal of stimulating cortical motor areas. It has been developed in the past decade to overcome the limitation of older cortical stimulation paradigms that were ineffective in the anesthetized patient. Typically, two stimulating electrodes (subdermal needle or surface "EEG" electrodes) are placed on the cranium. A brief (12 msec) train of 4-8 pulses (0.05 msec each) is then delivered. The resultant pyramidal cell discharges and corticospinal tract stimulation results in summation of EPSPs at the anterior horn cells increasing their resting membrane potentials to threshold, thereby evoking a muscle twitch. Recordings of resultant compound muscle action potentials (CMAP) are usually made from two muscles on each extremity, i.e., forearm and hand, foreleg and foot. Pulse trains are repeated only a few times to confirm reproducibility.

2. MAJOR PATHWAY MONITORED: Corticospinal tracts.

3. APPLICATIONS: The majority of TCeMEP recordings are performed to confirm viability of the spinal cord. During craniotomies, TCeMEP is used to monitor cortical, subcortical, and brainstem motor pathways, TCeMEP may also detect position-related impending central or peripheral nerve (ulnar nerve/brachial plexus) injury. For this reason, upper extremity responses are also obtained during thoracic or lumbar surgeries. In a variation of this technique, instead of recording myogenic activity, descending nerve volleys may be recording either from the spinal cord directly (with epidural electrodes place by the surgeon) or from peripheral nerve (surface or subdermal electrodes placed by technologist). This allows for neurophysiological monitoring with full neuromuscular blockade.

4. MAJOR ADVANTAGE: Monitors the corticospinal tracts. SSEPs monitor only posterior column function. Numerous reports have been published describing postoperative paralysis not detected with SSEP monitoring, likely due to vascular injury to the anterior portion of the spinal cord with sparing the posterior columns.
Another important advantage of TCeMEP is that averaging large numbers of stimuli is not necessary. One stimulus immediately produces one large amplitude response, permitting almost instantaneous feedback to the surgical team.

5. ANESTHESIA REQUIREMENTS: While total intravenous anesthesia (TIVA) and no post-induction neuromuscular blocking agent is ideal, and necessary in cases with preoperative compromise of corticospinal function, successful monitoring has been achieved with up to 0.5 MAC halogenated gas and carefully titrated neuromuscular blocking (continuous infusion of agent titrated to 2-3 out of four twitches). A bite block must be used to protect the tongue from masseter/temporalis contractions.

B. ELECTROMYOGRAPHY (EMG)

1. DESCRIPTION: EMG measure changes in transmembrane potentials measured from the surface of, or within, contracting muscles. Surface (self adhesive) leads, subdermal or intramuscular needle electrodes are used. Spontaneous EMG may be recorded continuously during surgery. Stimulus-evoked EMG involves assessing the response (compound muscle action potential or CMAPs, i.e., the temporally summed activity of simultaneously firing MUPs) to direct stimulation of a nerve or nerve root that is accessible in the surgical field, with a mono- or bipolar probe. Normal-appearing stimulus-evoked CMAPs demonstrate the integrity of the functional nerve-muscle unit. Continuous EMG is recorded from facial, limb, or axial muscles within and adjacent to the myotome of the root/nerve at risk. If root or nerve supplying monitored muscle is manipulated during the surgical procedure, the muscle discharges and resultant motor unit potentials (“MUPS”) are recorded. In the absence of permanent injury, these potentials quickly cease. Irritation/damage to the nerve/root produces more prolonged firing of the muscles, and ‘neurotonic discharges’ and/or ‘discharges trains’ occur. Complete transection of a nerve or nerve root does not produce these discharges, and it then becomes important to distinguish between muscle discharges with rare MUPS seen with intact nerves/roots and a ‘silent’ muscle resulting from a transected neural structure.
Stimulus-evoked EMG responses may additionally be obtained and require no change in the setup described above.

2. MAJOR STRUCTURES MONITORED: Integrity of anterior horn cell – nerve root – motor nerve functional unit.

3. APPLICATIONS: For two decades stimulus-evoked EMG has been used to guide dissection of acoustic neuromas. Response to stimulation identifies the path of the facial nerve within tumor tissue. In addition to protection of the facial nerve, all cranial nerves with a motor component may be monitored in a similar fashion. This has made the technique useful in ENT as well as neurosurgical applications. Parotid gland and other neck/shoulder resections are frequently monitored. Perineal EMG recording has application in resection of intradural lesions of the cauda equina as well as in tethered cord release procedures. Stimulus evoked EMG responses (CMAPs) are also monitored during craniotomy to guide resection of tumors adjacent to the motor strip.
Posterior lumbar, thoracic, and cervical fusions are monitored with both spontaneous and stimulated EMG techniques to detect nerve root injury. Note that dermatomal SSEP have proven unreliable under general anesthesia making them spuriously insensitive to root irritation. Spontaneous EMG is measured continuously during these cases, especially during placement of pedicle hole, markers, and screws, and during placement of intervertebral cages.
In addition to spontaneous EMG recording, pedicle screw stimulation for evaluation of proper placement of the pedicle screw has become a standard technique over the past several years. The screw is stimulated with the same hand-held stimulator (or alligator clip) used for nerve stimulation. A properly placed screw is surrounded by electrically insulating bone. In this case the muscle response threshold will be higher-typically greater than 8 mA. If the screw breaches the bone, or a crack in the pedicle occurs, muscle response at lower thresholds will be obtained. To detect problems before the screw is placed, stimulating clips may be attached to the “gear shift” (surgical awl) during creation of the pedicle hole. Additionally, pedicle hole pegs or markers may also be stimulated.

4. MAJOR ADVANTAGE: EMG monitoring is the only practical technique for nerve root monitoring. It provides early warning of impending injury, when surgical correction is possible. Stimulation and positive EMG response demonstrates integrity of the nerve or root. Pedicle screw stimulation threshold determination is extremely simple.

5. ANESTHESIA REQUIREMENTS: Stimulation or spontaneous EMG is unaffected by general anesthesia, but of course not possible with complete neuromuscular blocking. Generally, neuromuscular agents after the initial intubation bolus are avoided. If neuromuscular agents are deemed necessary by the surgical/anesthesia team, partial controlled paralysis may be obtained with continuous infusion of neuromuscular blocking agent. The degree of paralysis is titrated to 2-4 responses out of a train of four peripheral stimuli (routine “train of four” technique performed by the anesthesiologist and also by the IOM technologist).

C. SOMATOSENSORY EVOKED POTENTENIALS (SSEPs)

1. DESCRIPTION: This technique has been used since the 1970s for spinal cord monitoring. It involves continuously stimulating major peripheral nerves and recording responses from peripheral structures (like the proximal posterior tibial nerve in the popliteal fossa, or over the brachial plexus at Erb’s point), and central structures (cortical and subcortical responses are picked up from cerebral and nuchal electrodes). The responses represent conduction along only the fast conducting somatosensory nerve fibers that pass through the posterior columns (spinal cord), and lemniscal (brainstem) pathways. Since the responses are extremely small (in the microvolt range), the signal-to-noise ratio can be relatively large. This requires averaging of responses to several hundred stimuli over approximately one minute to achieve a reproducible response. SSEPs have been the standard technique for spinal cord monitoring as most extradural cord injuries cause disruption of the posterior columns. As noted, important exceptions to this assumption have been documented.

2. MAJOR PATHWAY MONITORED: Posterior columns and lemniscal pathways.

3. APPLICATIONS: Used frequently along with EEG during craniotomy to monitor cortical, subcortical, and brainstem motor pathways. It is also used to assess cerebral perfusion in cerebral vascular procedures such as endarterectomy and aneurysm or AVM clipping. Cortical strip SSEP recording is used to functionally identify the sensory/motor junction during craniotomy. SSEP has also been the technique to monitor spinal cord function during neurosurgical procedures such as resection of intra- or extra-medullary tumors and spinal orthopedic procedures, Although SSEP has also been used during spinal surgery of the lumbo-sacral region (below cord) to detect root injury, its sensitivity is extremely low for several reasons. Posterior tibial nerve spinal cord input is through multiple nerve roots (L4-S2). Single root disruption usually produces little change in the SSEPs. In addition, nerve root irritation without failure of conduction would not be detected, thus providing only post- injury information. The monitoring modality for monitoring nerve root function is EMG recording. Stimulated EMG is complimentary (see below).
Upper extremity SSEP provides supplementary monitoring of patient position-related nerve or brachial plexus injury, problems that have a significant incidence. For this reason, and also to provide a control for non-surgically related changes, upper extremity SSEPs are monitored during thoracic and lumbo-sacral procedures.

4. MAJOR ADVANTAGE: SSEPs may be obtained continuously during surgery.

5. DISADVANTAGES: Over three decades of SSEP monitoring, numerous reports have been published describing postoperative paralysis not detected with SSEP monitoring. Although infrequent, this would be expected due to the differential blood supply to the anterior and posterior portions of the spinal cord. As stated SSEPs monitors only posterior column function. Also, SSEP changes occur only with interruption of conduction and not with irritation (easily detected with EMG technique) and take a few minutes to obtain an interpretable average.

6. ANESTHESIA REQUIREMENTS: Cortical SSEPs are very sensitive to the typically used inhalation agents. Acceptable levels are limited to about 0.75 to 1.0 MAC. Nitrous oxide used in combination with the other gaseous agents produces a deterioration of the cortical SSEPs greater than the combined MAC value of the two agents. By contrast, cortical SSEPs are quite insensitive to IV agents, and these may be used without regard to the neurophysiological monitoring. It should be noted that the subcortical SSEPs are relatively resistant to the inhalation agents. The latter, however, are of low voltage, prone to interference and therefore not reliable in a significant number of patients. While neuromuscular blocking has no effect on SSEP responses per se, neuromuscular block significantly reduces myogenic noise, allowing quicker and more reliable SSEP information.

D. EEG

1. DESCRIPTION: Surface or subdermal electrodes are placed on the scalp. Stainless steel or platinum electrodes are used for direct cortical recording. The EEG is displayed and stored in the standard voltage-versus-time format. Numerous computer generated analyses and displays can be performed, such as the compressed spectral array (CSA). This is a display of EEG averaged over usually 1-4 seconds and plotted as power on the vertical axis versus different frequency components of the EEG trace on the horizontal axis.

2. MAJOR APPLICATION: Used during craniotomy to monitor cortical function, or, more specifically, to assess adequacy of perfusion. It is used, for example, to evaluate changes after temporary clamping of arterial branches feeding an aneurysm or AVM. It has long been used in extracranial surgery in a similar role, such as in carotid endarterectomy to evaluate the need for shunting. In recent years cortical SSEPs are used to complement EEG to increase sensitivity of monitoring.
EEG is also used to evaluate anesthesia levels. The most specific application in this category is in the titration of IV agents used to induce a burst-suppression EEG pattern confirming reduced cerebral metabolic demands during procedures involving compromised perfusion.

E. AUDITORY EVOKED POTENTIALS:

1. DESCRIPTION: For the stimulus, foam inserts are taped inside the auditory canal to deliver brief click. For recording, a single electrode at the scalp vertex is referred to electrodes on the earlobe or mastoid process. Responses are averaged as in other evoked potential modalities such as SSEP.

2. MAJOR PATHWAY MONITORED: The auditory pathway from the *the cranial nerve to the temporal primary auditory cortex is monitored. Transmission through major brainstem nuclei and lemniscal tracts produce five major waves. Stimulus-evoked brainstem potentials are most commonly utilized, although longer latency evoked potentials can be interpreted, i.e. thalamo-cortical responses.

3. APPLICATIONS: BAERs (brainstem auditory evoked responses) are used during major posterior fossa procedures to monitor brainstem function. In this regard they are complemented by median nerve SSEPs. A typical monitoring protocol employs ipsilateral (to the side of surgery) auditory and contralateral median nerve stimulation. BAERs are used in procedures that place the auditory nerve at risk, typically during resection of vestibular schwannomas (acoustic neuromas). In this regard a typical monitoring protocol would be BAER and free running and stimulated EMG of facial and other cranial nerve/muscle combinations.

4. ADVANTAGES: Although does require averaging, the responses are rapidly obtained due to the relatively high stimulus rate (up to 30/sec). No modification of anesthetic protocol is required.

5. ANESTHESIA REQUIREMENTS: Brainstem auditory evoked responses are essentially unaffected by anesthesia. This is also true for complementary EMG techniques. The only requirement for the latter is lack of neuromuscular blockade following the usual induction bolus.

F. PERIPHERAL NERVE TECHNIQUES (NERVE-NERVE RECORDING)

1. DESCRIPTION: During peripheral nerve repair or transposition, lesions affecting conduction can be identified and precisely located with the same nerve conduction techniques performed the out-patient EMG laboratory. In stimulated EMG, described above, a nerve is stimulated and the response of the muscle innervated by it is recorded. Nerve-nerve recording involves stimulation of a surgically-exposed nerve and recording from the same nerve at a distance. Sterile paired hook electrodes are held along the nerve. Generally the stimulation is moved proximally along the course of the nerve until a lesion is discovered by the response being lost, reduced in amplitude, increased in duration, or significantly delayed (an increase in latency greater than that accounted for by increased distance).

2. APPLICATIONS: Perhaps the most important application is in the exploration and repair of brachial plexus lesions. Whether the best chance of functional recovery is by resection and grafting, or by leaving a section of nerve intact to continue regeneration is largely determined by the response to stimulation. Structurally intact nerves may show full, partial, or no electrical continuity. The technique is also applied in ulnar or other nerve exploration, especially where clinical signs or preoperative studies suggest an atypical site of the lesion.

3. ANESTHESIA REQUIREMENTS: Peripheral nerve techniques are unaffected by general anesthesia and by neuromuscular blocking agents. A tourniquet, if used, must be released at least 20 minutes prior to testing. Local anesthesia should not be placed near the nerve to be tested.