Intraoperative Neurophysiological Monitoring (IONM)

Anesthesia and IONM

Types of anesthesia. Anesthetics can have a profound influence over IONM recordings. TIVA (total intravenous anesthesia) is a cocktail of drugs that is given intravenously. One important component of TIVA is the drug propofol. Propofol, like other general anesthetics, acts by enhancing GABA-A neurotransmission via increased chloride conductance, and it may also act directly as an agonist and affect other neurotransmitter systems as well. Other intravenous agents, like ketamine, may increase inhibitory transmission indirectly by blocking the excitatory effects of glutamate. Overall, propofol affects IONM recordings to a lesser degree than other anesthetics. Inhalation (gaseous) anesthetics, such as sevoflurane and desflurane, also act by enhancing inhibitory neurotransmission but are often contraindicated for IONM. The affect of gas anesthesia on electrophysiological recordings is proportional to the number of synapses between the stimulation and recording site. Cortical SSEP and EEG recordings are particularly sensitive to gas because there are multiple synapses that contribute to these signals, whereas subcortical SSEPs, brainstem auditory evoked responses (BAERs), and cranial nerve MEPs, with fewer synaptic connections between the stimulation and recording sites, are less affected. Gas anesthesia does have a strong inhibitory effect on transcranial MEPs recorded from muscle tissue. The effect of gas on MEPs occurs at the anterior horn where upper motor neurons synapse with lower motor neurons. Although single-pulse MEPs are easily inhibited, the effect of gas can be overcome to some degree by using multi-pulse MEPs.

Muscle relaxants. Muscle relaxants are used clinically for intubation and surgical incisions. Common muscle relaxants include rocuronium (ROC) and succinylcholine (SUC). ROC is a steroid‐based non‐depolarizing muscle relaxant. ROC is a nicotinic receptor antagonist that has a duration of ~37-72 min with a standard dose. For intubation, a lower starting dose (~20 mg/kg) is optimal for recording of baseline signals, which typically happen shortly after intubation. SUC a nicotinic receptor agonist and a depolarizing neuromuscular blocker with a rapid onset and elimination. SUC causes desensitization because it is not hydrolyzed by acetylcholinesterase, which inhibits neurotransmission. Its duration of action is 6-10 min. In some cases SUC can cause hyperkalemia, variable increases in intracranial pressure, and intra‐ocular pressure. It is not recommended for denervation syndromes, muscular dystrophy, or malignant hyperthermia.

Muscle relaxants have an inhibitory effect on compound muscle action potentials recorded for transcranial MEPs, cranial nerve MEPs, and pedicle screw stimulation.

When using ROC, care must be taken with people who have myasthenia gravis or myasthenic syndrome, hepatic disease, neuromuscular disease, carcinomatosis, or severe cachexia, as the duration of action may be significantly increased. – Tran et al. (2017) cited Annals of Pharmacotherapy 2014; 48: 62– 76.