Inappropriate elevation of bispectral index and disruption of neurosurgery after irrigation-induced facial nerve irritation
Sheffield, UK
* E-mail: john.andrzejowski{at}sth.nhs.uk
Editor—We describe a case where saline-induced facial nerve irritation interfered with bispectral index monitoring and neurosurgery. Partial paralysis was used to overcome the problem. A healthy 52-yr-old female patient presented for translabyrinthine excision of a large acoustic neuroma. Anaesthesia consisted of remifentanil and propofol infusions, with an initial bolus of rocuronium to facilitate tracheal intubation and ventilation. Propofol infusion was titrated to maintain BIS between 40 and 50 (Bispectral Index version BIS-XP).
After 6 h of uneventful operating, remifentanil was running at 0.22 µg kg–1 min–1 and propofol at 2.5 µg ml–1 (calculated plasma concentration). The surgeon had to stop surgery as the facial nerve stimulator began to warn that the facial nerve had been irritated, or was at risk of damage. It was simultaneously noted that the BIS had increased from 44 to 75. Arterial pressure remained at 110 mm Hg systolic and heart rate was unchanged at 58 beats min–1. All infusions were found to be running satisfactorily, and there were no signs of patient movement or arousal. The ‘secondary variable’ display showed a large amount of EMG activity on the BIS monitor. Signal quality remained over 90% throughout. Facial nerve irritation was found to be due to surgical field irrigation with unwarmed (room temperature) saline. After 20 min, significant EMG activity persisted, preventing surgical progress. The facial nerve stimulator can still function with partial muscle paralysis so, to facilitate surgery, a subtherapeutic dose of neuromuscular blocking agent (rocuronium 15 mg) was administered. The noise from the facial nerve stimulator ceased 20 s later and the BIS number reverted to its previous steady levels (Fig. 1). Peripheral nerve stimulation confirmed that all four twitches on a ‘train of four’ stimulus were maintained, and the surgeon was still able to detect facial nerve stimulation, so surgery could proceed. There was no facial nerve weakness after operation.
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The BIS algorithm employs EMG signal to indicate light levels of hypnosis. The increase in EMG in this case was incorrectly interpreted by BIS as an arousal process; hence, the monitor displayed a high value for the processed EEG. The interference with BIS in this case arose not from an extraneous source, but from a physiological signal from within the patient. Irrigating fluids are routinely warmed to approximately 37°C since ‘cold’ irrigation is known to cause problems in neurosurgery including severe bradycardia. 1 2 The phenomenon of nerve ‘stunning’ after cold irrigation can also lead to localized vasoconstriction and secondary ischaemic injury to the nerve.3 The debate about whether or not neuromuscular blocking agents affect the value given by depth monitors goes on. 4 5 Episodes such as this can only help to inform this debate.
References
1 Arndt GA, Stock C. Bradycardia during cold ocular irrigation under general anesthesia: an example of the diving reflex. Can J Anesthesia (1993) 40:511–4.[Web of Science][Medline]
2 Sinha PK, Neema PK, Manikandan S, Unnikrishnan KP, Rathod RC. Bradycardia and sinus arrest following saline irrigation of the brain during epilepsy surgery. J Neurosurg Anesthesiol (2004) 16:160–3.[CrossRef][Web of Science][Medline]
3 Sampath P, Holliday M, Brem H, Niparko JK, Long DM. Facial nerve injury in acoustic neuroma (vestibular schwannoma) surgery: etiology prevention. Neurosurg Focus (1998) 5:a4.
4 Hans P, Giwer J, Brichant JF, Dewandre P-Y, Bonhomme V. Effect of an intubation dose of rocuronium on Spectral Entropy and Bispectral IndexTM responses to laryngoscopy during propofol anaesthesia. Br J Anaesth (2006) 97:842–7.
5 Bonhomme V, Hans P. Muscle relaxation depth of anaesthesia: where is the missing link? Br J Anaesth (2007) 99:456–60.
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