Subfalcine herniation

Subfalcine herniation, also known as midline shift or cingulate hernia, is the most common cerebral herniation pattern,. It is characterisedgenerally caused by displacementunilateral frontal, parietal or temporal lobe disease that creates a mass effect with medial direction of the brain (typically theipsilateral cingulate gyrus) beneath beneath the free edge of the falx cerebri due to raised intracranial pressure. 

Radiographic features

CT

The easiest method of evaluating for subfalcine shift is a straight line drawn in the expected locationaxial plane, at the level of theforamen of Monro, and measuring the distance between this line and the displaced septum pellucidum from the posterior most aspects to the falx on axial images. Shift of the septum pellucidum from this midline can be measured in millimetres and compared over time to determine any change.

Asymmetry atIn 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 on the contralateral anterior falxother hand is more rigid and will resist the displacement. There may be ipsilateral lateral ventricle compression with contralateral lateral ventricular dilation.

MRI

Findings are best visualised on coronal MR imaging. Unilateral mass effect from pathology in the frontal, parietal or temporal region, such as intracranial haemorrhage or tumour, causes displacement of the brain away from the mass. 

Treatment and prognosis

Complications

• contralateral hydrocephalus due to obstruction of the foramen of Monro
• anterior cerebral artery (ACA) territory infarct due due to compression of ACA branches, specifically the pericallosal artery. 
• focal necrosis of the cingulate gyrus due to direct compression against the falx cerebri. 

ACA infarction occurs as the cingulate sulcus extends under the falx dragging the ipsilateral anterior cerebral artery with it. If this becomes compressed against the falx occlusion can lead to a, distal anterior cerebral artery infarction and thuscan occur, the most common clinical symptom ofmanifestation being contralateral leg weakness.