Because the skull is rigid after infancy, intracranial masses or swelling may increase intracranial pressure, sometimes causing protrusion (herniation) of brain tissue through one of the rigid intracranial barriers (tentorial notch, falx cerebri, foramen magnum). When intracranial pressure is increased sufficiently, regardless of the cause, Cushing reflex and other autonomic abnormalities can occur. Cushing reflex includes systolic hypertension with increased pulse pressure, irregular respirations, and bradycardia. Brain herniation is life threatening. Transtentorial herniation: The medial temporal lobe is squeezed by a unilateral mass across and under the tentlike tentorium that supports the temporal lobe. The herniating lobe compresses the following structures: Ipsilateral 3rd cranial nerve (often first) and posterior cerebral artery As herniation progresses, the ipsilateral cerebral peduncle In about 5% of patients, the contralateral 3rd cranial nerve and cerebral peduncle Eventually, the upper brain stem and the area in or around the thalamus Subfalcine herniation: The cingulate gyrus is pushed under the falx cerebri by an expanding mass high in a cerebral hemisphere. In this process, one or both anterior cerebral arteries become trapped, causing infarction of the paramedian cortex. As the infarcted area expands, patients are at risk of transtentorial herniation, central herniation, or both. Central herniation: Both temporal lobes herniate through the tentorial notch because of bilateral mass effects or diffuse brain edema. Ultimately, brain death occurs. Upward transtentorial herniation: This type can occur when an infratentorial mass (eg, tumor, cerebellar hemorrhage) compresses the brain stem, kinking it and causing patchy brain stem ischemia. The posterior 3rd ventricle becomes compressed. Upward herniation also distorts the mesencephalon vasculature, compresses the veins of Galen and Rosenthal, and causes superior cerebellar infarction due to occlusion of the superior cerebellar arteries. Tonsillar herniation: Usually, the cause is an expanding infratentorial mass (eg, cerebellar hemorrhage). The cerebellar tonsils, forced through the foramen magnum, compress the brain stem and obstruct CSF flow.