Navigated Brainstem Monitoring in the Rhomboid Fossa

Monitoring of cranial nerve function during intrinsic brainstem surgery is crucial in order to avoid iatrogenic morbidity. The combination of neuronavigation and cranial nerve monitoring might offer a tool for the understanding of the sometimes complex anatomy of brainstem nuclei. In an intraoperative environment we have stimulated the surface of the rhomboid fossa in patients suffering from intrinsic brainstem lesions (cavernous malformations). EMG recording of the muscles innervated by the cranial nerve was performed (Nicolet, Germany, Viking IV). Once a nucleus was outlined by this technique, the exact point of stimulation was landmarked within a preoperatively acquired three-dimensional MRI data set by standard neuronavigation (BrainLab, Germany, VectorVision). Accordingly, a map of brainstem function could be established in the rhomboid fossas of 19 patients. On the basis of the mapped functional chart, the decision, as to where to incise the brainstem surface was facilitated. In our series, the excision of the intrinsic lesions was possible in all patients without permanent surgery-related morbidity. Therefore, we believe that this technique is an important tool to visualize, understand, and preserve the complex functional brainstem anatomy, which is often distorted in patients with intrinsic brainstem lesions.