A preliminary MRI study of the geometry of brain displacement and level of consciousness with acute intracranial masses.

We determined the horizontal and vertical components of brain displacement on coronal MRIs in 10 patients with acute supratentorial masses. The vertical distances from the superior sagittal sinus to the pontomesencephalic junction (PMJ) and from the vertical midline of the brain to the PMJ did not differ from measurements in 30 patients without masses. Horizontal displacement of the 3rd ventricle (mean, 5.2 mm) exceeded vertical displacement of the PMJ (mean, 1.2 mm) in all but 1 patient. Total brain displacement near the incisura increased from 3 mm in awake patients to 6 to 13 mm in stuporous and comatose patients and remained predominantly horizontal. Uncal herniation, seen only in a patient who was imaged a day after the onset of coma, did not appear to be the immediate cause of brainstem compression but was accompanied by prominent lateral distortion above the tentorium. This preliminary study suggests that most patients with acute unilateral masses have upper brainstem distortion due predominantly to horizontal shift at or above the tentorium.