Brain herniation may occur across the falx cerebri, tentorium cerebelli, or through openings such as foramen magnum and craniectomy defects. Any process that causes an elevation in the intracranial pressure can result in brain herniation. Space-occupying lesions (e.g., tumor, abscess, hematoma, and stroke), diffuse processes (e.g., edema, encephalitis, and subarachnoid hemorrhage), or increased cerebrospinal fluid (CSF) (e.g., hydrocephalus) are examples. Herniations of the brain are classified into five categories: (1) transtentorial, (2) subfalcine, (3) tonsillar, (4) alar, and (5) extracranial (Fig. 15-1).

Imaging findings related to herniation have a similar appearance on both computed tomography (CT) and magnetic resonance imaging (MRI) although they are better depicted on MRI due to multiplanar capability and superior soft tissue contrast. Each category of herniation may occur in isolation or conjunction with other categories.

Each category of herniation may be associated with a specific neurologic syndrome, described subsequently. Early, nonspecific signs of herniation reflect elevated intracranial pressure and include headache, nausea, vomiting, vision disturbance, papilledema, altered level of consciousness, and Cushing’s triad (alteration in respiration, bradycardia, and systemic hypertension). Late signs of herniation include decorticate and decerebrate posturing. Each category of herniation may rapidly progress to death, and because of this, it is imperative that the clinician be promptly notified of any imaging findings suggestive of herniation.