Metoclopramide is associated with akathisia and dystonic extrapyramidal effects; akathisia is seen more frequently in patients older than 30 years, and dystonic extrapyramidal effects are seen more commonly in patients younger than 30 years. Diphenhydramine, benztropine mesylate, and trihexyphenidyl are commonly used prophylactically or therapeutically to pharmacologically antagonize EPRs.[7,16] While cogwheeling rigidity, acute dystonia, and tremor are responsive to anticholinergic medications, akathisia—the subjective sense of restlessness or inability to sit still—is best treated by the following:
- Switching to a lower potency neuroleptic for vomiting, if possible.
- Lowering the dose.
- Adding a benzodiazepine (e.g., lorazepam) or beta blocker (e.g., propranolol).
5-HT3 Receptor Antagonists
Four serotonin receptor antagonists—ondansetron, granisetron, dolasetron, and palonosetron—are available in the United States. Tropisetron, while not approved by the FDA, is available internationally. Agents in this class are thought to prevent N&V by preventing serotonin, which is released from enterochromaffin cells in the gastrointestinal (GI) mucosa, from initiating afferent transmission to the CNS via vagal and spinal sympathetic nerves. The 5-HT3 receptor antagonists may also block serotonin stimulation at the CTZ and other CNS structures.
Several studies have demonstrated that ondansetron produces an antiemetic response that equals or is superior to high doses of metoclopramide, but ondansetron has a superior toxicity profile compared with dopaminergic antagonist agents.[18,19,20,21,22][Level of evidence: I][23,24] Ondansetron (0.15 mg/kg) is given IV 15 to 30 minutes prior to chemotherapy and is repeated every 4 hours for two additional doses. Alternatively, for patients older than 18 years, a large multicenter study determined that a single 32-mg dose of ondansetron is more effective in treating cisplatin-induced N&V than a single 8-mg dose and is as effective as the standard regimen of three doses at 0.15 mg/kg given every 4 hours starting 30 minutes before chemotherapy.[Level of evidence: I] A single-center retrospective chart review has reported ondansetron-loading doses of 16 mg/m2 (maximum, 24 mg) IV to be safe in infants, children, and adolescents. However, data reported to the FDA raises concern about QT prolongation and potentially fatal arrhythmias with a single IV dose of 32 mg. Current drug labeling calls for a maximal single IV dose of 16 mg.