Intraoperative Monitoring and Mapping of the Functional Integrity of the Brainstem

The risk of iatrogenic damage is very high in surgical interventions in or around the brainstem. However, surgical techniques and intraoperative neuromonitoring (ION) have evolved sufficiently to increase the likelihood of successful functional outcomes in many patients. We present a critical review of the methodologies available for intraoperative monitoring and mapping of the brainstem. There are three main groups of techniques that can be used to assess the functional integrity of the brainstem: 1) mapping, which provides rapid anatomical identification of neural structures using electrical stimulation with a hand-held probe, 2) monitoring, which provides real-time information about the functional integrity of the nervous tissue, and 3) techniques involving the examination of brainstem reflexes in the operating room, which allows for the evaluation of the reflex responses that are known to be crucial for most brainstem functions. These include the blink reflex, which is already in use, and other brainstem reflexes that are being explored, such as the masseter H-reflex. This is still under development but is likely to have important functional consequences. Today an abundant armory of ION methods is available for the monitoring and mapping of the functional integrity of the brainstem during surgery. ION methods are essential in surgery either in or around the brainstem; they facilitate the removal of lesions and contribute to notable improvements in the functional outcomes of patients.