The aim of this clinical trial is to compare the effectiveness of two Serotonin (5- HT3) receptor antagonist, palonosetron and granisetron, administered along with dexamethasone as a preventive measure against early and delayed postoperative nausea and vomiting (PONV) in adult and adolescent patients with idiopathic scoliosis (IS) undergoing posterior spinal fusion (PSF) surgery under total intravenous anesthesia (TIVA). The main questions the study aims to answer are: How effective is palonosetron compared to granisetron, when both combined with dexamethasone, in preventing PONV after scoliosis surgery? Are there any differences in the need for rescue antiemetics, occurrence of adverse effects related to the study drugs, and patient satisfaction between the two treatment groups? Participants in the study will be randomly assigned to receive either palonosetron or granisetron in addition to dexamethasone as part of their anesthesia and antiemetic regimen. The incidence and/ or severity of nausea, vomiting and retching will be assessed at 1 hour, 4 hours, 12 hours, 24 hours and 48 hours after surgery.