Pericollicular surgical approaches to pons and medulla oblongata

Introduction: Safe surgical approaches into the floor of the IVth ventricle in brainstem lesions are based on detailed knowledge of rhomboid fossa anatomy and intraoperative localisation of superficial nuclei and fibers. Methods: For morphometric analysis 12 brainstem specimens were investigated in conventional axial and sagittal histological slices. Motor nuclei and fibers of the rhomboid fossa were measured in all 3 dimensions with consideration of shrinkage factors. During surgery electrical stimulation of superficial cranial nerve nuclei V-XII was performed in 40 patients with intraaxial lesions and lesions infiltrating the brainstem. Results: Morphometric analysis of rhomboid fossa anatomy defines a paramedian infracollicular approach of 9.2 mm located between the facial colliculus and the upper pole of the Nucleus N. hypoglossi. A paramedian supracollicular approach between facial colliculus and the fibers of the VIth cranial nerve within the medullary velum measures 13.8 mm. Morphometric data were transferred to the surgical field by direct electrical stimulation of cranial motor nuclei and fibers V-XII in a total of 40 patients. Localisation of facial colliculus and trigonum hypoglossi was achieved in all cases. No side effects were observed. Discussion: Morphometric investigation of histological anatomy defines two operative approaches into the rhomboid fossa. Morphometric data can be transferred to surgery using advanced neurophysiological methods. Direct electrical stimulation is a reliable, fast and safe method to localize functionally important landmarks, thus providing a safe approach for lesions within the rhomboid fossa.