Current concepts of brain herniation have depended largely on correlating clinical signs and symptoms with indirect radiographic studies and the results of postmortem neuropathology. This article describes measurements 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 measurements 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 cerebellar tonsils 0.1 +/- 2.1 mm below the foramen magnum line. Defining 2 SD from these 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 or without accompanying cerebellar tonsillar herniation. 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 displacement of the iter ranging from 2.0 to 11.0 mm below the incisura. Two-thirds of patients with upward and one-half of those with downward transtentorial shift had concurrent tonsillar herniation. 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.
Despite decades of investigation and discussion of the mechanisms involved in the pathophysiology of arachnoid cysts, fundamental issues concerning these entities remain poorly defined and controversial. Cine-mode magnetic resonance imaging (MRI) has shown two patterns of cerebrospinal fluid (CSF) flow within the cavity in patients harbouring arachnoid cysts. Some cysts present a harmonic flow with a patent flow entry zone. All these patients had intermittent, non-progressive and non-localizing symptoms requiring no surgery according to our criteria. The second pattern of CSF flow is more chaotic and is characterized by the presence of swirls throughout the entire cardiac cycle. This pattern is associated with a more disabling clinical picture. Some of these patients required surgical treatment. During surgery, an endoscope was used for inspection purposes revealing, above all, that arachnoid cysts always and variably communicate with the subarachnoid space. The CSF enters the cyst either through a patent flow entry zone or through minute perforations in areas more loosely packed of the arachnoid network that behave as a flexible mesh able to modify the area of flowing CSF. The slipstreams of CSF within arachnoid cysts may not be channelled properly leading to possible damage of the surrounding brain parenchyma.
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