A novel method of neurophysiological brainstem mapping in neurosurgery

Background: Brainstem mapping with electrical stimulation allows functional identification of neural structures during resection of deep lesions. Single pulses or train of pulses are delivered to map cranial nerves and corticospinal tracts, respectively. New method: We introduce a hybrid stimulation technique for mapping the brainstem. The stimulus consists of an electrical single pulse followed by a short train of 3-5 pulses at 500 Hz, at an interval of 60-75 ms. The responses to this stimulation pattern are recorded from appropriate cranial and limb muscles. Results: Both the single pulse and the short train elicit electromyographic responses when motor fibers or motor nuclei of the cranial nerves are stimulated. Responses to the train but not to the preceding single pulse indicate activation of the descending motor tracts, in the mesencephalon and the pons. Conversely, in the medulla, limb responses to stimulation of the corticospinal tracts are elicited by a single pulse. Identification of the extra and intra-axial courses of the trigeminal motor and sensory fibers is possible by recording responses from the masseter and the tongue muscles. Comparison with existing method(s): To date, either a pulse or a train is delivered during brainstem mapping, switching from one to the other modality according to the expected target structure. This procedure can be timeconsuming and may even lead to false negative responses to the stimulation, eventually leading to inaccurate neurosurgical procedures. Conclusions: The novel hybrid pulse-train technique enhances the advantage of brainstem mapping procedure, minimizing pitfalls and improving patient safety.