Adding Dexmedetomidine to Mannitol to Attenuate the Increase of Intracranial Pressure

Inclusion Criteria: ASA I-II Patients scheduled for elective robot-assisted laparoscopic prostatectomy Exclusion Criteria: * • ASA physical status \> II * Cardiac patients * Cerebrovascular disease * Glaucoma * Hepatic failure * Renal failure * History of anaphylactic reaction to dexmedetomidine or mannitol * Hemodynamic instability * Cannot measure ONSD for example due to ophthalmological problems. * Any cause of preoperative increase in ICP for example intracranial space occupying lesions.