Intraoperative Neurophysiological Monitoring (IONM)

Anesthesia and IONM

Anesthetics can have a profound influence over IONM signals. TIVA (total intravenous anesthesia) is a cocktail of drugs that is given intravenously. One important component of TIVA is the drug propofol. Propofol is a general anesthetic that acts as a positive allosteric modulator of the GABA-A receptor, and it may also act directly as an agonist and affect other neurotransmitter systems as well. Inhalation (gas) anesthesia is often contraindicated for IONM because it inhibits synaptic transmission and other neural processes. The affect of gas anesthesia on electrophysiological recordings is proportional to the number of synapses involved in the neural pathway. Cortical SSEP recordings are particularly sensitive to gas because there are several synapses that contribute to these measurements, whereas subcortical SSEPs are less affected. Sevoflurane and desflurane are common gas anesthetics, which have replaced isoflurane as the preferred gas anesthetic.

Muscle relaxants are needed for the 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 better for IONM baseline measurements that 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.