Specific Imaging Findings

Subfalcine herniation is the most common type of acquired cerebral herniation with cingulate gyrus being displaced beneath the inferior free margin of the falx cerebri on imaging. The degree of midline shift can be quantified by measuring displacement of the septum pellucidum. Transtentorial herniation can be either descending or ascending, of which the former can be subdivided into lateral and central types. In lateral descending transtentorial herniation, the mesial temporal lobe is displaced inferomedially through the tentorial incisura, with the early imaging findings of effaced ipsilateral suprasellar cistern and widened ipsilateral ambient cistern. Central transtentorial herniation is caused by a central or bilateral mass leading to downward displacement of the brain, best seen as the inferiorly pushed pineal gland calcification, located much lower than the choroid plexus calcifications (for two or more 5-mm slices). In ascending transtentorial herniation the cerebellum is displaced upward through the tentorial hiatus. Imaging findings include effacement of the superior cerebellar cistern, protrusion of the vermis through the incisura, and brain-stem compression. Inferior displacement of the cerebellar tonsils through the foramen magnum is referred to as tonsillar herniation and is best depicted on sagittal images with the tonsils more than 5 mm below the foramen magnum. Hydrocephalus can occur with herniations as a result of obstructed CSF flow.

Pertinent Clinical Information

Cerebral herniations are frequently responsible for the presenting neurologic symptoms of the underlying pathologic processes. Subfalcine herniation can result in infarction of the anterior cerebral artery territory due to compression against the falx. With lateral transtentorial herniation, a lateralized, fixed and dilated ipsilateral or contralateral pupil due to cranial nerve III compression and occipital lobe infarcts due to posterior cerebral artery compression can occur. Central descending transtentorial herniation results in a fairly typical sequence of clinical signs as a result of brainstem compression occuring in an orderly rostral to caudal fashion. Ascending transtentorial herniation can compress the posterior cerebral or superior cerebellar arteries, resulting in infarctions or can cause hydrocephalus due to compression of the aqueduct.