Transcranial ultrasound has become a widely used technique mainly for measuring arterial blood flow velocities. More recently, transcranial ultrasound has been also used to assess brain tissue, brain perfusion and ventricular size. The major advantages of ultrasound are its non-invasive character, the possibility for bedside application, low costs and safety (no X-ray exposure). Some studies have tried to use transcranial ultrasound for indirect assessment of intracranial pressure. As target structures for transcranial ultrasound either the lateral ventricles, the third ventricle or the diameter of the optic nerve were used. All studies could demonstrate a correlation between the width of the lateral or third ventricle or the diameter of the optic nerve with intracranial pressure. To the investigators' knowledge, only one study assessed the change of the width of the lateral ventricles during clamping of EVD/LD.