An abbreviation for Endoscopic Third VentriculostomyA minimally invasive operation (“keyhole surgery”) performed upon the brain, through a single hole drilled in the skull. Through this is passed an instrument called an endoscope - sometimes referred to, in lay terms, as a “telescope”. A camera, attached to the endoscope, displays images on a television screen, enabling the operator to see the internal anatomy of this part of the brain. Instruments are passed down the working channels of the endoscope and are then used to burn an internal bypass channel, between one part of the cerebrospinal fluid channels and another, thereby by-passing a blockage that has been detected on magnetic resonance imaging and which is causing hydrocephalus..