Magnetic resonance imaging measurements and clinical changes accompanying transtentorial and foramen magnum brain herniation

Abstract

Current Concept of brain herniaton have depended largely on correlating clinical sings and symptoms with indirect radiographic studies and the results of postmortem neuropathology. This article describes measurments on midsagittal magnetic resonance imaging (MRI) scans that distinctly define normal and abnormal rostral–caudal relationships between the diencephalic–mesencephalic junction and the plane of the tentorial incisura, herein termed the incisural line. We similarly provide quantitative MRI scan measurments relating the cerebellum and the plane of the foramen magnum, termed the foramen magnum line. Measurements from 156 midsagittal and 63 coronal MRI scans performed on 123 normal adults, placed the iter of the aqueduct 0.2 ± 0.8 mm (mean ± SD) below the incisural line and the cerebeller tonsils 0.1 ± 2.1 mm below the foramen magnum line. Defining 2 SD from thses norms as abnormal, 23 patients with intracranial mass or obstructive lesions showed 4 distinct patterns of brain herniation, i.e., upward or downward transtentorial shift with of without accompanying cerebellar tonsillar herination. Five patients with Posterior fossa masses demonstrated displacement of the iter above the incisura ranging from 1.6 to 6.3 mm. Eighteen patients with supratentorial masses demonstrated displacements of the iter ranging from 2.0 to 11.0 mm below the incisura. Two‐third of patients with upward and one‐half of those with downward transtentorial shift had concurrent tonsillar herination. In acute illnesses, MRI scan changes anticipated or confirmed clinical signs of brain herniation. In chronic cases, clinical and MRI scans correlated less well, with MRI sometimes revealing major degrees of anatomical herniation well in advance of clinical abnormalities.