The posterior fossa represents the area of the cranial vault located below the tentorium cerebella. The latter arises from the superior crest of the petrous portion of the temporal bone and roofs over the posterior fossa. The cerebellum overlies the posterior aspect of the brain stem and extends laterally under the tentorium to fill most of the posterior fossa. The most caudal aspect of the posterior fossa is the foramen magnum, through which the medulla exits the cranium and becomes the spinal cord.

Mass lesions located in the posterior fossa mainly represent neoplasms, vascular lesions such as vascular malformations, and ischemic strokes with edema. Such mass lesions are classified according to their location within the posterior fossa (Table 207.1). The main areas where such lesions are found include the parenchyma (cerebellum and brain stem), the cerebellopontine angle, the fourth ventricle, and the foramen magnum. Because of the small anatomical space, these lesions can easily impinge upon the ventricular system (e.g., the fourth ventricle) and lead to obstructive hydrocephalus. Patients may present with headache, nausea, and vomiting, followed by altered sensorium. Posterior fossa lesions can also compress the brain stem, leading to various syndromes with multiple cranial nerve involvement with serious respiratory and cardiovascular consequences. Another important issue that needs to be recognized is that any occupying space in the posterior fossa has the potential to lead to cerebellar tonsillar herniation. The cerebellar tonsils are located in the most caudal and inferior aspect of the posterior lobe of the cerebellum. It can be easily appreciated that should there be tonsillar herniation, the foramen magnum becomes occluded and the cervicomedullary junction compressed. Respiratory failure and arrest with circulatory collapse ensues. Lastly, lesions that compress the upper brain stem may lead to the so-called upward herniation of the midbrain and the anterior cerebellar lobe clinically manifested by coma, hyperventilation, fixed pupils, and vertical gaze paralysis.