Normally, the brain contains several areas of fluid called ventricles. Hydrocephalus occurs when a tumor blocks normal fluid drainage of the brain, causing the ventricles to become dilated. The buildup of fluid and pressure can damage the brain or cause symptoms including headache and visual changes. A temporary solution for hydrocephalus is to install an external ventricular drain (EVD) that drains fluid from the ventricles to outside the body. In some cases, the EVD may be removed after the plumbing is restored and the hydrocephalus resolves. If long-term drainage is required, the EVD may be converted to a device called a shunt that diverts ventricular fluid to safe locations inside the body. Sometimes an endoscopic third ventriculostomy may be performed to avoid a shunt.

Several tests help doctors to diagnose brain tumors. A brain scan with a computerized tomography (CT) or magnetic resonance image (MRI) may help to establish tumor size and location, as well as complications like bleeding or hydrocephalus. Doctors may obtain a biopsy, meaning a small sample of the tumor to examine in the laboratory. The laboratory examines tumors at the molecular level in order to diagnose the specific tumor subtype. Each subtype may require different treatment. A lumbar puncture, or ‘spinal tap,’ may reveal tumor cells in the fluid that surrounds the brain and spinal cord.

The most common tumor categories and subtypes are listed in Table 4.2, and described further below.