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Intraoperative monitoring of electromyographic responses generated by cranial and peripheral motor nerves allows early detection of surgically induced nerve damage and assessment of level of nerve function intraoperatively. In these cases, the ability of a nerve to produce a response in the innervated muscle is used to assess the health of a cranial or peripheral nerve at risk during surgery. Recordings are made from surface electrodes (i.e., electroencephalographic or gold cup) or needle electrodes placed directly in the muscle of interest.
Electromyographic monitoring may be active or passive. During active monitoring, a cranial or peripheral nerve is stimulated electrically, and the evoked electromyographic (i.e., compound muscle action potential [CMAP]) response from the muscle is recorded. Stimulation of the nerve proximal to the operative area or tumor can be used to assess functional integrity of the nerve.[165] Nerve function may also be assessed by observing the intensity of nerve stimulus needed to evoke a muscle response and by the morphology of the CMAP. Nerve function may be monitored passively during surgery with continuous recording of all generated responses from innervated muscle groups. "Popcorn" electromyographic discharges are produced by simple, benign contact with the monitored nerve. Response trains are produced with more significant nerve irritation. Neurotonic discharges are produced by significant nerve irritation or nerve damage ( Fig. 38-20 ).[165] When these electromyographic responses reach a certain voltage threshold, they are usually converted into audible signals that provide immediate feedback to the surgeon and warn of impending nerve damage in real time. Real-time feedback is key, because the density and frequency of neurotonic discharges may correlate with the degree of postoperative nerve dysfunction, as demonstrated by data obtained from patients undergoing resection of acoustic tumors. [166]
Intraoperative monitoring of facial nerve function has been used in patients undergoing surgical procedures placing the facial nerve at risk, such as acoustic neuroma resection, microvascular decompression of the facial nerve for hemifacial spasm, resection of cerebellopontine angle meningiomas, removal of temporal bone neoplasms, resection of cerebellar hemangioblastomas, surgical treatment of glomus tympanicum, repair of traumatic facial nerve paralysis, and surgery risking the more peripheral facial nerve such as parotid tumor resection.[165] [167] Much of the experience with intraoperative facial nerve monitoring has been obtained from patients undergoing acoustic neuroma resection. Improved preservation of facial nerve function after acoustic neuroma resection by intraoperative facial nerve electromyographic monitoring has been demonstrated, especially in patients with medium- to large-size tumors.[166] The National Institutes of Health Consensus Conference on Acoustic Neuroma[168] recommended that facial nerve monitoring should be used in all patients undergoing surgical resection of an acoustic neuroma.
Figure 38-20
Schematic of facial nerve monitoring and typical responses
seen during surgery.
In patients undergoing microvascular decompression procedures for hemifacial spasm, two abnormal responses may be seen during electromyographic monitoring: autoexcitation and lateral spread response. Autoexcitation refers to late responses lasting 50 to 100 msec that follow the normal motor response to facial nerve stimulation. Lateral spread results when electrical stimulation of one branch of the facial nerve results in muscle response in the muscle innervated by that branch and in those innervated by other branches of the facial nerve. Both responses are abnormal and disappear intraoperatively when the facial nerve is adequately decompressed. During microvascular decompression for hemifacial spasm, intraoperative monitoring of the facial nerve provides warning of surgical damage to the nerve and information on the adequacy of the operative procedure to relieve the patient's symptoms intraoperatively.[169]
Intraoperative monitoring of the motor component of other cranial nerves has also been successfully performed. Electromyographic monitoring of the trigeminal nerve can be accomplished with electrodes placed over or in the temporalis or masseter muscles. Trigeminal nerve motor monitoring has been used during nerve section for tic douloureux to ensure preservation of the motor branch of the trigeminal nerve and in combination with facial nerve monitoring during resection of large posterior fossa lesions.[165] Using recording electrodes placed in or over the trapezius or sternocleidomastoid muscles, the spinal accessory nerve has been successfully monitored during resection of large meningiomas, glomus jugular tumors, and neck carcinomas.[165] Electromyographic monitoring of the hypoglossal nerve with needle electrodes placed in the tongue has been infrequently used for large posterior fossa lesions and clivus tumors.[165] Although electromyographic monitoring of the eye muscles can be performed using tiny hook wires for recording, it is rarely used.
Monitoring of peripheral motor nerves has been performed by placing needle electrodes in or over the muscles innervated by nerves that traverse the operative area and are at risk from the planned surgical procedure. Auditory feedback from electromyographic monitoring can warn the surgeon of unexpected surgical trespass of the nerve, help locate a nerve within the field (e.g., during untethering of the spinal cord), and localize the level of any conduction block or delay. Because radiculopathies have been reported to occur after spinal surgery, electromyographic monitoring of peripheral nerves has been used in patients undergoing spinal surgery to decrease the risk of nerve root injury during the procedure.[165]
In patients undergoing electromyographic monitoring of cranial or peripheral motor nerves, the choice of anesthetic technique is not important, but muscle relaxants should be avoided or limited during the period of monitoring. Although most clinicians avoid neuromuscular blocking agents completely during periods of electromyographic monitoring, adequate active monitoring of the facial nerve, which is relatively resistant to the effects of neuromuscular blocking agents, is possible during partial neuromuscular blockade (i.e., decrease of the CMAP by 50%). This level of blockade is associated with clinical weakness[170] ; however, the most commonly used monitors of neuromuscular paralysis are not adequate to accurately quantify this level of relaxation. The trials in which partial neuromuscular paralysis was used during facial nerve monitoring are small, and large-scale trials evaluating outcome have not been performed. Effects of partial neuromuscular blockade on passive facial nerve monitoring have not been studied. As a result, most neuroanesthesiologists continue to recommend complete avoidance of neuromuscular blocking agents after intubation until monitoring is completed.
Electromyographic monitoring has also been helpful during selective dorsal rhizotomy, usually performed in children with cerebral palsy for relief of spasticity. In this operation, sensory nerve roots are sectioned to reduce sensory input to the nervous system. The resultant pathologic spastic response is subsequently reduced. Sensory nerve rootlets are individually stimulated. In the normal patient, motor responses would be recorded only from muscle innervated by the corresponding motor nerve root. In patients with spasticity, the motor response produced by stimulation of a single sensory rootlet may generalize to other levels and muscle groups ipsilaterally and, in severe cases, to the opposite side. The most pathologic rootlets as indicated by electromyographic testing are sectioned. In some cases, volatile anesthetics and nitrous oxide may
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