No significant displacement of basal brain structures upon head movement: Kinematic MRI morphometry relevant to neuroendoscopy.

BACKGROUND An appreciation of normal intracranial anatomy allows optimal planning of trajectories necessary for safe and effective neuroendoscopy. Little information exists on displacement of the caudal brain relative to the skull upon head movement; this could have important implications for planning and performance of neuroendoscopic procedures. We used kinematic magnetic resonance imaging (MRI) studies to examine the morphometric displacement and changing anatomical relationships between the clivus and basal brain structures, intracranial vessels, and subarachnoid spaces. PATIENTS We retrospectively analyzed 15 patients undergoing sagittal T2 kinematic MRI of the head and neck in modest flexion and extension. The angle between a horizontal axial reference plane and a line between the opisthion and the hard palate defined the degree of flexion and extension. We then measured in flexion and extension (1) the cervicomedullary angle (CMA), (2) displacement of the ventral surface of the brainstem (i.e., depth of the prepontine and premedullary cisterns), (3) total sagittal area of the combined suprasellar and ventral brainstem cisterns, and (4) the basilar tip to tuber cinereum distance. RESULTS Relative to neutral head position, a mean extension angle of -15.8 degrees was achieved in all 15 patients, and a mean flexion angle of +9.9 degrees was achieved in 6 patients. The mean CMA was 146 degrees in flexion and 158 degrees in extension. The mean reduction in prepontine and premedullary cistern depth was 0.7 mm and 0.5 mm, respectively, upon flexion from extension. The combined area of suprasellar and ventral brainstem cisterns was minimally reduced from 402 mm2 in flexion to 399 mm(2) in extension. The basilar tip did not move significantly from its position in flexion to extension, 5.3 mm to 5.2 mm respectively from the tuber cinereum. CONCLUSION Kinematic MRI shows minimal brainstem-to-clivus displacement even within minor physiological changes in head flexion. Importantly, these movements are small and there is no significant shift in the position of the basilar tip in modest flexion or extension. These results should be useful for presurgical planning of optimal patient positioning during neuroendoscopic procedures such as third ventriculostomy and the expanded endonasal transsphenoidal approach to the retroclival space.