The anesthesiologist must balance the condition of the patient , the surgical procedure, and the monitoring modalities to be used when selecting an anesthetic technique. Rare metabolic diseases such as mitochondrial myopathies may limit the choice of anesthetic agents that can be safely used. In these cases, the anesthesia team, surgeon, and neurophysiology team must formulate a plan that will provide as much information as possible to guide the procedure while protecting the patient from exposure to potentially hazardous anesthetic agents.

TIVA is the preferred anesthetic technique if TcMEPs are to be elicted. If TIVA is not a feasible option, 0.5 MAC of inhalation agent (desflurane) supplemented with remifentanil can be used in the absence of significant neurological compromise. Higher stimulation intensities may be required to elicit TcMEPs due to volatile agent effects at the level of the alpha motor neurons (αMNs) [1]. Increased stimulus intensities have attendant risks of excessive patient movement and increased risk of bite injuries such as tongue lacerations. The use of volatile agents is also associated with an increased rate of false-positive alerts compared with TIVA [2].

It is generally accepted that healthy young adults, with no pathology involving the pathways involved in IONM, can be anesthetized with either a TIVA or 0.5 MAC desflurane-based anesthetic during monitored (IONM) procedures. This is not the case for young children. The presence of low residual concentrations of sevoflurane, following an inhalation induction, may significantly impair the generation of TcMEPs in the lower extremities. Currently, unpublished data from the author’s institution indicates that volatile agent added over a TIVA, as is sometimes done to suppress patient movement, can dramatically reduce TcMEP amplitudes in the lower extremity muscles of young healthy adolescents. TIVA , with propofol and remifentanil, is the recommended primary anesthetic regimen when IONM is required in pediatric patients. The addition of ketamine (5–20 μg/kg/min) to the anesthetic may help provide improved hemodynamic stability, but care must be taken to terminate a ketamine infusion in a timely manner due to the pharmacokinetic properties of ketamine. Ketamine, propofol, and isoflurane have been shown to cause neurodegeneration in the brains of subhuman primates when used in high doses over a period of time [3]; however, the effects in humans are an area of controversy as these agents may protect from damage due to nociceptive inputs [4]. Some propofol sparing may also be achieved by using dexmedetomidine , but dexmedetomidine must be used judiciously as blood levels greater than 0.6 ng/mL depress the MEPs [1]. A low-dose infusion (0.2 μg/kg/h) without a loading bolus keeps the blood levels below the threshold. If electromyography (EMG) or TcMEPs are used as one of the monitoring modalities, the neuromuscular blocking agents (NMBs) should be avoided if possible. If used for intubation, the use of short-acting agents is preferred as baseline potentials may be acquired within 10–20 min of intubation in some cases. NMBs should be completely avoided, or completely reversed if cranial nerve EMG and/or cranial nerve TcMEPs are to be acquired in young children.

Concern about the ability to assess anesthetic depth with TIVA is cited as a primary reason for not using this technique. Recently published data define the EEG patterns associated with the initial loss of consciousness and maintenance of such loss in healthy volunteers receiving propofol infusions [5, 6], sevoflurane [7], and dexmedetomidine [8]. EEG patterns associated with surgical anesthesia conditions in infants from age 0 to 6 months using sevoflurane have also been published [9]. Similar patterns are also found when using fast-Fourier analysis (FFA ) of frontal montage EEG in patients in the same age groups receiving TIVA anesthetics (Fig. 43.1a–d). Although EEG data can be informative regarding cortical activity, it is not completely reliable as a predictor of movement. The successful use of TIVA relies on the suppression of nociceptive stimuli with sufficient narcotic dosing, as propofol has limited ability to prevent movement in dose ranges compatible with eliciting reliable TcMEPs in the lower extremities of young children. Using a combination of bilateral frontal montage EEG with fast Fourier transform, and EMG, it is possible to monitor both the cortical response to anesthetic dosing and EMG signs of incipient movement. Spontaneous EMG used to monitor the facial and lower cranial nerves has been found to be predictive of incipient patient movement during emergence from anesthesia [10, 11]. We have also found that spontaneous EMG activity in the small muscles of the hands can signify incipient movement as well.

The neurophysiology of brainstem auditory evoked responses and the techniques for acquiring them are discussed in Chap. 3, “Auditory Evoked Potentials .” Auditory evoked responses are typically divided into short, middle, and long latency responses. During surgery, only the short latency responses are resistant to the effects of anesthetic agents and are useful for assessing the integrity of the eighth cranial nerve and the ascending auditory pathways to the level of the inferior colliculus. The “resistance” of short-latency BAERs to anesthesia is age dependent; clinical experience indicates that identification of wave V in infants younger than 6 months who have been anesthetized with sevoflurane is much more difficult, due to low amplitude signals, than in infants anesthetized with propofol (Unpublished data). In the nonanesthetized state, BAERs can be recorded from infants at 25 to 27 weeks’ gestation. Term infants will have wave I latencies similar to those of adults; wave III and V latencies reach adult values by 18–36 months of age [47].

Monitoring the bulbocavernosus reflex has been proposed by Skinner as a means of preserving lower sacral nerve function during intradural and extradural surgeries at the level of the conus medullaris, cauda equina, sacral plexus, and the pudendal nerve [48] (see Chap. 8, “The Use of Reflex Responses for Intraoperative Monitoring” for details). Acquisition of the reflex requires use of a multipulse technique applied as a double train with a long intertrain interval. Alternatively, a double-tap technique can be used. In either case, the acquisition time must be sufficiently long to capture the response (200 ms window). The reflex is also sensitive to anesthetic technique in young children and infants; TIVA is the preferred anesthetic technique for acquisition of this reflex. Recording electrodes are placed on opposite sides of the anal sphincter. These same electrodes can be used to record TcMEPs from the anal sphincter. Loss of the BCR but not the anal sphincter TcMEP suggests damage to the efferent pathways in the reflex arc. We record BCRs during complex tethered cord releases, lipoma resections, and similar procedures. The presence of a preexisting major urodynamic abnormality usually makes recording the BCR impossible, whereas mild abnormalities are not incompatible with recording the BCR (Fig. 43.2).

A summary of common pediatric surgical procedures and the IONM modalities utilized during these procedures is presented in Table 43.3. A detailed discussion of some of the listed procedures follows.