As elaborated above only one study reported ultrasonographic changes of the width of the lateral ventricle during clamping of EVD/LD and indicated that ultrasound monitoring might be suitable. No firm data exists about the change of the width of the third ventricle, the diameter of the optic nerve or brain perfusion during clamping of the EVD/LD, although the width of these structures has been shown to depend on intracranial pressure. Ultrasonographic measurements of changes of the width of the third ventricle or the diameter of the optic nerve would have some advantages compared to the ultrasonographic assessments of the lateral ventricles. First, the width of the third ventricle can be measured easier and more reliable than the width of the lateral ventricles (better defined insonation plane and therefore higher repeatability of measurements) 3. Second, whereas assessments of the width of the side and third ventricles with ultrasound depend on the temporal bone windows (10 to 15% of patients have insufficient temporal bone windows), the measurement of the diameter of the optic nerve does not have this limitation and can therefore be performed in almost all patients. Hence, measurement of the diameter of the optic nerve would allow to overcome one major limitation of transcranial ultrasound. The latter limitation for transcranial ultrasound could also be minimized by the use of an ultrasound contrast agent (SonoVue®), but this was also not yet studied. The use of an ultrasound contrast agent would in addition allow to study changes of brain perfusion during clamping of EVD/LD.