Effect of Irrigation on Multimodality Intraoperative Neuromonitoring during Skull Base Surgery

Introduction: Warm normal saline (NS) irrigation during skull base surgery often results in high-frequency cranial nerve discharge, reflected by protracted free-run electromyography (f-EMG) activity. This f-EMG activity, often involving the facial nerve, can be deleterious as it can be difficult to differentiate from injury potentials, and it interferes with the ability to test stimulus-evoked EMG or other important modalities such as brainstem auditory evoked potentials (BAER). These effects are harmful not only to the surgeon’s state of mind, but could also lead to premature withdrawal from further resection; or increase risk to hearing or facial nerve function. Given that lactated D Ringer’s solution (LR) is often preferred to abolish seizures in intra-axial tumor surgery, it was postulated that it could be less irritating than NS in skull base procedures involving the facial nerve.

Methods: This is a pilot study that included five patients who underwent posterior fossa surgery, and the facial nerve was sequentially exposed to both NS and LR irrigation. The amount of irrigation was not controlled for but rather the typical irrigation amount was utilized. Once irrigation was stopped, we recorded the duration of cranial nerve spontaneous firing as well as the frequency. This was first done for NS and then for LR in each patient. Additionally, both the EMG stimulation thresholds for the facial nerve as well as the BAER signal were analyzed under both conditions.

Results: During four acoustic neuroma resections and one microvascular decompression for hemifacial spasm, the application of NS irrigation resulted in protracted, high-frequency EMG discharge of the facial nerve ([Fig. 1A]; 9-minute 38-second duration with a frequency of ~140 Hz) as well as interfering with the ability to simultaneously record a BAER ([Fig. 1B]). This was performed multiple times resulting in a stereotypical increase in f-EMG. The effect was more pronounced in the case of hemifacial spasm, and it was also particularly accentuated in all four cases of acoustic neuroma during the final stages of dissecting the tumor from the facial nerve. In all five cases, application of LR during the time of f-EMG immediately abolished this activity, and subsequent irrigation did not elicit further such activity. Furthermore, it was found that the overall spontaneous firing of the nerve during manipulation was decreased. Nonetheless, the EMG stimulation threshold of the facial nerve at the brainstem was found to be the same with both NS and LR. All patients had intact facial nerve function postoperatively. At no time did the application of LR result in sustained, high-frequency f-EMG nor was the BAER interfered with during this time ([Fig. 2]).

Conclusions: Our preliminary findings suggest that using warm LR as an irrigant in posterior fossa surgery may be superior to NS as it may decrease the detrimental spontaneous f-EMG activity both during and after irrigation, but also during nerve dissection. While our results are limited to facial nerve EMG, these effects could likely be extrapolated to other cranial motor nerves. Future studies with randomization and larger numbers are warranted.

Fig. 1

Fig. 2

No conflict of interest has been declared by the author(s).

Publication History

Article published online:
05 February 2024