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 involved in the neural pathway. Cortical SSEP and EEG recordings are particularly sensitive to gas because there are multiple synapses that contribute to these signals, whereas subcortical SSEPs with fewer synaptic connections are less affected. Gas anesthesia also has a strong inhibitory effect on transcranial MEPs recorded from muscle tissue. The effect of gas on MEPs occurs at the anterior horn where the upper motor neurons synapse with lower motor neurons, as well as at the neuromuscular junction.

Muscle relaxants. Muscle relaxants are used clinically for the intubation of patients 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 baseline recordings, 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.

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.