Intraoperative stimulation mapping of sensory branches of the trigeminal nerve using unconditioned reflexes: a case series

Introduction: Intraoperative stimulation mapping enables to determine the location of nerves in the surgical field when visualization is difficult, as well as to identify visualized nerves, thereby reducing the risk of postoperative iatrogenic complications. Motor nerve mapping is widely used in neurosurgical procedures but only a few attempts at intraoperative sensory nerve mapping have been reported to date. The introduction of an effective method for mapping the sensory branches/portions of the trigeminal nerve can reduce the risk of complications such as facial hypo- and hyperesthesia after operations in the posterior cranial fossa. Objective: To demonstrate the possibility of using unconditioned reflex motor reactions for stimulation mapping of the sensory branches/portions of the trigeminal nerve. Methods: Intraoperative neuromonitoring records were analyzed in three patients with trigeminal neuralgia who underwent nerve stimulation mapping during microvascular decompression. The using of an extended to 40 ms time window for recording myographic responses enabled to record both short-latency direct motor responses to stimulation of motor nerves and long-latency reflex motor responses to stimulation of the sensory branches of the trigeminal nerve. Results: It was shown that unconditioned reflex reactions of the orbicularis oculi muscle (stimulation of the V1 portion of the trigeminal nerve, blink reflex) and tongue muscles (stimulation of V3, trigemino hypoglossal reflex) were successfully induced in patients under total intravenous anesthesia with both monopolar and bipolar stimulation. One patient also showed a reaction of the orbicularis oris muscle (stimulation of V2, snout reflex). The technique used enabled to differentiate not only the motor and sensory roots of the trigeminal nerve, but also its different sensory portions separately. Conclusion: The intraoperative stimulation mapping of sensory branches/portions of the trigeminal nerve via unconditioned reflex motor responses can be performed using the equipment and anesthesia regimen typical for motor nerve mapping.