A cerebral edema is an abnormal accumulation of liquid in the brain tissue. The symptoms are generally severe headaches, nausea or vomiting, and visual impairment, together associated with intercranial pressure. Given that the elements of the brain are "enclosed" in the skull cavity, which is of bone and therefore not elastic, any increased in the liquid volume must be compensated by a reduction in another compound of the brain, or there is a resulting pressure on the structures that can lead to damage to the brain cells. To reduce a cerebral edema, one must investigate and treat the cause, while molecules such as mannitol or corticosteroids can help to reduce the pressure on the brain. There are many underlying causes for cerebral edema, such as significant cranial trauma, or high blood pressure due to a blood clot blocking a vessel, known as throbophlebitis.