- The authors report a novel technique used for continuous somatosensory evoked potentials (SSEP) monitoring via direct cortical recordings in 32 patients undergoing resection of fronto-parietal lesions
- Occurrence of SSEP changes influenced surgical management in a third of the patients; measures applied early, based on a new alarm criteria of 30% for decrease in SSEP amplitude (rather than the previously established 50%), resulted in avoidance of worsening sensory function
- Compared with preoperative MRI, SSEP recordings led to more accurate localization of the lesion, in relationship to sensory and motor regions, in 28% of cases
- Based on the results of SSEP testing, cortical electrical stimulation for motor mapping and/or monitoring could be limited and/or avoided altogether in half of the patients, thus decreasing the risk of intraoperative seizures
- Simultaneous SSEP and motor-evoked potentials (MEP) monitoring, performed in 31% of the patients, was associated with brief fluctuations in MEP, thought to reflect direct modulation of motor activity by thalamocortical input
Thalamocortical afferents (TCA) to the frontal and parietal regions carry information that is important not only for preserving somatosensorial perception but also for ensuring normal motor function. Even so, little attention has been paid to monitoring thalamocortical projections and their cortical relays during craniotomy.
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In Clinical Neurophysiology, Mirela V. Simon, MD, MSc, of the Department of Neurology and an expert in intraoperative neurophysiologic mapping and monitoring techniques; Brian V. Nahed, MD, MSc, associate director of the Neurosurgery Residency Program at Massachusetts General Hospital; and colleagues have described a new technique for TCA monitoring and mapping via direct cortical recordings.
Using the same subdural strip electrode employed to stimulate the primary motor cortex (M1), it was possible to reliably monitor median somatosensory evoked potentials (SSEP) in 31 of 32 consecutive patients who underwent resection of fronto-parietal tumors. The other patient had non-recordable baseline SSEP.
Posterior tibial SSEP monitoring was attempted in 19 of these patients who had lesions located more medially on the cerebral hemisphere; it was successful in 17 patients.
In 11 cases, changes in SSEP prompted changes in surgical management (e.g., delaying resection while irrigating and increasing the blood pressure, releasing brain retraction, adjusting the surgical plane, stopping resection) in an attempt to avoid damaging the thalamocortical tract:
- Seven patients showed drops in SSEP amplitudes of ≥50%; despite changes in management, they experienced worsened sensory function postoperatively. These results indicate that using the previously established alarm criteria of 50% reliably predicts (PPV 1, specificity 1) but does not prevent sensory deficit
- Three patients had management changed because of amplitude drops ≥30% and <50%; they had no permanent worsening of sensory function. This indicates that early intervention is essential for prevention of postoperative deficit
- One patient had postoperative sensory function worsened even though the median SSEP amplitude drop was <30% throughout the procedure. However, this patient had poor SSEP at baseline and preoperative arm sensory deficit, suggesting that in patients with already compromised sensory function, smaller decreases in SSEP amplitudes during monitoring may indicate clinically relevant worsening in sensory function
The paper reports in detail how amplitude, latency and morphology of SSEP predicted sensory deficit.
Compared with preoperative MRI, SSEP recordings led to more accurate localization of the lesion in nine of the 32 patients. From these, seven showed significant mass effect, and distorted peri Rolandic anatomy.
Based on SSEP results, cortical stimulation for motor mapping and/or monitoring was limited and/or considered unnecessary in 16 patients. Twelve of these patients still underwent identification of M1 by electrical stimulation; however, the frequency of cortical stimulations for motor monitoring could be significantly reduced. In the other four cases, M1 mapping was unnecessary because the lesion was located safely posterior to the primary somatosensory cortex.
Modulation of Activity in the Motor Cortex
In 10 patients, motor evoked potentials (MEP) and SEEP were simultaneously monitored using the same strip electrode. Nine of these patients exhibited short-lived, cyclic MEP fluctuations (i.e., facilitation and inhibition of varying degrees). Most fluctuations occurred in the muscle territory served by the peripheral nerve that was concomitantly stimulated.
This is the first evidence that TCA can modulate motor activity triggered by direct electrical stimulation of M1 under general anesthesia.
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