**3.3 Precautions**

There are several precautions to take when performing ETV when there is history of previous tumors, shunting procedure or thickened floor of third ventricle. The anatomy may be difficult to interpret as it can be altered by tumors e.g. a brainstem glioma. Floor of the third ventricle may be distorted and basilar artery may be displaced forward, minimizing the safe zone of the floor for penetration. The onset of hydrocephalus resulting from tumor obstruction may be relatively acute and the floor of the third ventricle may appear opaque and non-attenuated. The procedure will pose more challenges as the penetration of the floor will be difficult and invariably requires a sharper technique without visualization of the neurovascular structures underneath the membrane for penetration, which obviously increases the risk.
