Teaching Case

31-year-old male patient involved in a motor vehicle accident evaluated for head trauma.

CT shows a heterogeneous oval-shaped abnormality within the distal right transverse sinus (Fig. 1).
MRI demostrates a polyploid cortical herniation from the lateral inferior right temporal occipital cortex into the superior aspect of the right transverse sinus within a large arachnoid granulation, in keeping with an internal encephalocoele. (Fig. 2 and 3).
MR venogram shows a bifid right transverse sinus enclosing a large arachnoid granulation, an anatomic variant (Fig. 4).

Brain herniation into dural venous sinus, according to Battal et al [1], have a prevalence of 0.32% . Its pathophysiology and clinical significance is still controversial, being a recently described entity best depicted by MRI. It is yet unclear whether it represents true brain herniation into a dural defect or brain tissue getting into a pre-existing giant arachnoid granulation. In most cases it is not pathologically possible to identify arachnoid tissue next or around the herniation, but some authors postulate that arachnoid granulation predispose to its occurrence. Cohan et al [2] called it "occult encephaloceles" because they do not cross any dura o bony defects as classic encephaloceles do. The theory that brain herniations result from increased intracranial pressure is not widely accepted, since they have been found in patients with no signs of increased intracranial pressure.

There is no clear association between clinical symptoms and brain herniations or large arachnoid granulations. Although headaches and dizziness are mentioned occasionally, most described lesions have been incidental findings not related to any pathology [2].

From the imaging point of view, 3D T1- and T2-weighted sequences are most useful in characterising herniated brain, its continuity with adjacent brain parenchyma and CSF-containing compartments. These sequences allow the radiologist to identify the CSF outlining the brain parenchyma. The occipital squama, transverse sinus, lateral lacuna of the superior sagittal sinus, and straight sinus are the most frequent sites for their appearance, with cerebellar tissue being the most frequently involved in brain herniation [3].

On MR venography or venous phase DSA, round focal filling defects are identified, usually not obstructing the dural sinuses.

It is important to make the differentiation from dural sinus venous thrombosis, which usually reveal multiple and irregular filling defects, and from dural-based tumours (showing enhancement and diffusion restriction). Uncomplicated giant arachnoid granulation may also have similar appearance.

Brain herniation into a dural venous sinus most likely is an incidental findings with not enough supporting evidence of their clinical significance. Given the increasing amounts of MR studies being performed, they are more frequently found and should be differentiated from more ominous dural sinus pathology.