Subfalcine herniation, also known as midline shift or cingulate hernia, is the most common type of cerebral herniation. It is generally caused by unilateral frontal, parietal or temporal lobe disease that pushes ipsilateral cingulate gyrus beneath the free edge of the falx cerebri to the opposite side due to raised intracranial pressure 4. Subfalcine herniation frequently occurs anteriorly rather than posteriorly because posterior falx cerebri is more rigid than the anterior ones 4.
Radiographic features
CT
The easiest method of evaluating for subfalcine shift is a straight line drawn in the axial plane, at the level of the foramen of Monro, and measuring the distance between this line and the displaced septum pellucidum. The shift of the septum pellucidum from this midline can be measured in millimeters and compared over time to determine any change 4.
In more severe hernias, the displaced tissue may compress the corpus callosum and contralateral cingulate gyrus as well as the ipsilateral lateral ventricle, foramen of Monro, causing dilation of the contralateral lateral ventricle.
Subfalcine hernias occur anteriorly, as the anterior falx (although rigid) is displaced, secondary to mass effect. The posterior falx, on the other hand, is more rigid and will resist the displacement.
MRI
Findings are best visualized on coronal MR imaging 4. Unilateral mass effect from pathology in the frontal, parietal or temporal region, such as intracranial hemorrhage or tumor, causes displacement of the brain away from the mass.
Treatment and prognosis
In subfalcine herniation, the degree of midline shift correlates with the prognosis; less than 5 mm deviation has a good prognosis, whereas a shift of more than 15 mm is related to a poor outcome 4.
Complications
Subfalcine herniation compresses the ipsilateral ventricle and obstructs the foramen of Monro, causing contralateral hydrocephalus 4.
Anterior cerebral artery (ACA) territory infarction occurs as the cingulate sulcus extends under the falx dragging the ipsilateral branch of ACA with it, specifically the pericallosal artery. If this becomes compressed against the falx, distal anterior cerebral artery infarction can occur, the most common clinical manifestation being contralateral leg weakness 4.
Focal necrosis of the cingulate gyrus due to direct compression against the falx cerebri 4.
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