Intraoperative Neuromonitoring in Neurosurgery: Analysis of 307 Consecutive Cases

Aims: Intraoperative neuromonitoring (ION) aims to detect early neurophysiological changes due to reversible alterations of nervous system to prevent irreversible deficits. The present study aimed to determine the validity and reliability of ION based on motor-evoked potentials (MEPs).

Methods: Data were analyzed according to five categories: supratentorial tumors, supratentorial vascular, infratentorial, spinal pathologies, and epilepsy. Motor strength was evaluated with the Medical Research Council Scale the day before the surgery, 1, 3, and 5 days postsurgery, as well as 2, 3, and 6 months.

ION was conducted with the NimEclipse: MEPs were realized with transcranial electric stimulation (5 pulses, 350 Hz, 0.4 ms phase duration, max 180 mA; corkscrew electrodes at C1, C2, and 1 cm more anterior and lateral). MEPs could also be realized by direct cortical stimulation (strips, same parameters, max 14 mA). MEPs were gathered through subdermal electrodes (lower limbs: abductor halluces and anterior tibialis, gastrocnemius, and vastus medialis if required; upper limbs: lumbrical and brachioradialis, biceps, triceps, and thenar if required). The amplitude of MEPs was monitored; the alert criterion was a 50% decrease.

Results: Between 2009 and 2011, 307 consecutive patients underwent neurosurgery with MEPs in Geneva. MEPs were most often conducted for supratentorial tumor (32%), supratentorial vascular (29%), spinal (22%), infratentorial (14.5%), and epilepsy surgeries (2.5%).

Distribution of true-negatives (no new deficit and none alert), true-positives (alert and new deficit), false-positives (alert but no new deficit), and false-negatives (new deficit but none alert) are described in Table 1.

	% of 449 Contributive Limb MEPs	Comments
TN, true-negatives; TP, true positives; FP, false-positives; FN, false-negatives; MRI, magnetic resonance imaging; SMA; ION, intraoperative neuromonitoring.
TN	83.5
TP	7.8 (6.2% of patients, n = 19)	n = 16 recovery within max 3 mo
FP	2	Brain sagging, decrease in mean blood pressure, or strip displacement if direct cortical stimulation
FN	6.7 (4.9% of patients, n = 15)	n = 9 ischemic lesion, vasospasm, hematoma on postoperative MRI
		n = 2 SMA resection
		n = 4 ION technical reasons

Except 1% of patients (n = 3/307, 1 spinal deterioration and 2 primary motor deterioration), all patients fully recovered within 3 months. In addition, the surgeon was alarmed in 19% of supratentorial vascular surgeries at the time of temporary artery occlusion or in cases of perforant artery occlusion by an aneurysm clip.

Conclusion: In this group of 307 patients, MEPs contributed to prevent new permanent deficits, changing the intraoperative strategy in supratentorial tumor or epilepsy surgery, preventing damage to the primary motor cortex and to motor pathways; in vascular surgery, limiting duration of temporary clipping or indicating the replacement of clips; in infratentorial surgeries, preventing motor pathways; in spinal surgery by transient interruption of surgery.