Anesthesia and evoked responses in neurosurgery

Intraoperative evoked potential (EP) monitoring has become a routine part of operative neurosurgical procedures. The theoretical, technical, and clinical aspects of various EPs have been extensively characterized and significant clinical experience has been accumulated with this modality of neuromonitoring. Successful EP monitoring requires an adequate understanding of how anesthetic drugs and physiological variations affect EP signals and how to improve the sensitivity of neuromonitoring through appropriate drug selection and administration. Unlike intraoperative electroencephalography (EEG), EP signals are much smaller in amplitude (0.1–20 mcV) and indistinguishable from background noise. In order to extract the EP signal from the underlying EEG noise, multiple stimulations with summation and frequency filtering are necessary (Freye, 2005; Møller, 2011). EPs are highly sensitive to fluctuations in physiological parameters such as peripheral and core body temperature, arterial blood pressure, hematocrit etc. They are also susceptible to various general anesthetic agents and other drugs frequently given during surgery. The effects of general anesthetics on intraoperative EP depend on the mode of evoked response and the pharmacological characteristics of administered anesthetic drugs. Evoked responses that travel via polysynaptic pathways, such as visual EP are significantly more susceptible to the anesthesia and surgery when compared to EPs with fewer synapses in their pathway. In general, inhalational anesthetics are more potent suppressants of EP than intravenous agents (Banoub et al., 2003; Møller, 2011). Combinations of inhalational agents, as occurs with the addition of nitrous oxide, potentiate the suppressive effects of anesthesia even further. Despite their suppressive effects on EPs, inhalational anesthetics have obvious advantages for use during neurosurgery because they are easily titratable to provide stable anesthetic conditions. Lower doses of inhalational anesthetics (0.5–0.8 MAC depending on type of the evoked response) have been successfully applied during neurosurgical interventions and neurophysiological monitoring without compromising the quality of monitoring. Balanced general anesthesia with low doses of inhalational agents combined with low-dose constant infusions of remifentanil (0.05 mcg/kg/min), propofol (50 mcg/kg/min), or dexmedetomidine (0.003–0.005 mcg/kg/min) may be recommended when EP monitoring is anticipated. Such an approach will provide stable anesthesia and reduce the incidence of adverse events encountered occasionally during total intravenous anesthesia such as patient movement and awareness. Sevoflurane has low solubility compared with other inhalation anesthetics and thus is eliminated rapidly, minimizing its effects during monitoring later in the case (Sloan T, as cited in Fulkerson et al., 2011). Using sevoflurane as an induction agent, Fulkerson and colleagues were able to successfully monitor the intraoperative motor EPs in young children (less than 3 years) undergoing neurosurgical spinal procedures (Fulkerson et al., 2011). We believe that other inhalational anesthetics with low blood solubility (desflurane) may uneventfully be used for anesthesia induction and will be compatible with intraoperative neurophysiological monitoring. Anesthetics used for intravenous anesthesia, with a few exceptions, produce a dose-dependent suppressive effect on EP. Unlike the other intravenous hypnotics, etomidate, and ketamine tend to increase the SSEP amplitudes (Banoub et al., 2003). Further studies will be required to evaluate whether the use of various concentrations of these anesthetics during neurosurgical interventions suppress EP equally when different modalities of EP are being monitored. Opioids, in general, do not affect the quality of intraoperative EP monitoring. However, their mild suppressive effects are proportional to lipophilicity. When infused at higher doses, remifentanil causes a 20–80% decline in P37 peak amplitude of SSEP and a mild (<10%) increase in latency (Asouhidou et al., 2010). Midazolam and other benzodiazepines moderately suppress the intraoperative EP (Banoub et al., 2003), and their use, whenever possible, should be avoided. Benzodiazepine-induced EP suppression is less pronounced compared to inhalational agents. Dexmedetomidine, a relatively new hypnotic characterized by selective alpha-2 adrenergic antagonism, can be safely used to supplement general anesthesia during EP monitoring (Tobias et al., 2008). Intravenous lidocaine (1.5 mg/kg/h) is also a useful adjunct to general anesthesia with EP monitoring due to its ability to reduce anesthetic requirements, stabilize the cardiovascular parameters and decrease the incidence of patient movement during surgery (Sloan et al., 2014). Monitoring of motor EPs during surgery requires special caution, as they are more sensitive to anesthetics and muscle relaxants (Kunisawa et al., 2004; Lotto et al., 2004). Anesthetic conditions optimized for motor EP monitoring are suitable for SSEP registration as well