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Pharmacology of Opioids
Published in Pamela E. Macintyre, Stephan A. Schug, Acute Pain Management, 2021
Pamela E. Macintyre, Stephan A. Schug
Opioid receptors are found in the bowel wall (in the myenteric plexus), biliary tract, ureters, and bladder. In the GI tract they alter smooth muscle activity leading to delayed gastric emptying, reduction of bowel motility, and constipation. This inhibition is both locally (an effect on opioid receptors in the myenteric plexus of the bowel wall) and centrally mediated. While some decrease in bowel motility is inevitable, it is usually not necessary or appropriate to withhold opioids to facilitate the return of bowel function after surgery. However, after abdominal surgery, if patients complain of increasing pain that is colicky or “windy” in nature, it is worth explaining that this may be an early sign of improved gut motility and that treatment of that pain with opioids is not advised. Adequate fluid intake and mobilization should be encouraged and stool softeners and cathartics may be recommended (in the absence of contraindications) if opioids are to be given for more than a couple of days. Peripherally acting opioid antagonists, including naloxone, methylnaltrexone, naloxegol, and alvimopan, have been shown to reduce the effect of opioids on the bowel (Schwenk et al, 2017; Schug et al, 2020)—see Section 4.8.3.
Pain management
Published in Hemanshu Prabhakar, Charu Mahajan, Indu Kapoor, Essentials of Anesthesia for Neurotrauma, 2018
All surgical patients receiving opioids require close monitoring, especially immediately after surgery. Opioid use is often associated with adverse effects, such as somnolence, respiratory depression, gastrointestinal (GI) adverse effects such as constipation, nausea and vomiting, and itching. Although monitoring alone may be effective, some patients may require pharmacologic interventions. Ileus associated with opioid use may increase intraabdominal pressures, which can potentially increase ICP. Since tolerance to constipation does not occur over time, a standing regimen of stool softener and/or stimulant is recommended. More recently, newer agents, such as methylnaltrexone, naloxegol, and lubiprostone are available to treat opioid-induced constipation (Table 32.6).4 Patients who have had a craniotomy involving posterior fossa are particularly at risk of nausea and vomiting. Emesis can be associated with substantial elevations in ICP, therefore, postoperative nausea must be treated aggressively with agents such as ondansetron, metoclopramide, and promethazine.
Methylnaltrexone bromide for the treatment of opioid-induced constipation
Published in Expert Opinion on Pharmacotherapy, 2018
Shilan Mozaffari, Shekoufeh Nikfar, Mohammad Abdollahi
Peripheral mu-opioid receptors are responsible for OIC and activation of opioid receptors in the GI tract. The activation of these receptors results in reduced transit time and constipation. Several studies have established that antagonizing these peripheral receptors prevents the OIC. Peripheral mu-opioid receptor antagonists (PAMORAs) are successful options in OIC treatment [11]. Methylnaltrexone bromide blocks the mu-opioid receptor in the GI tract, without affecting the central opioid-induced analgesia [11,14]. It is the first PAMORA approved for the OIC in patients with advanced illness when laxatives were not efficient [1,11]. Another recently approved PAMORA is naloxegol. Naloxegol is the pegylated form of naloxone that blocks mu- and k-opioid receptors. Its structure restricts the blood-brain barrier (BBB) penetration [6]. Oxycodone-naloxone is another developed combination to reduce the opioid peripherally induced adverse effects [11]. Re-evaluating the use of opioid and its dose, switching to another opioid, and changing the route of administration are other options for the treatment of OIC [3,6].
Gastroparesis: pharmacotherapy and cardiac risk
Published in Scandinavian Journal of Gastroenterology, 2018
Per M. Hellström, Ahmad Al-Saffar
Stimulation of opioid-receptors delays gastric emptying and slows gastrointestinal transit. In 41%-81% of patients on chronic opioid treatment, constipation develops [45,46], which can worsen gastroparesis. Methylnaltrexone is a peripherally acting opioid μ-receptor antagonist for treatment of opioid-induced constipation, but may also decrease nausea and vomiting by improving gastric emptying [47]. Recently, naloxegol, a macromolecular peripherally acting μ-opioid antagonist was approved for treatment of opioid-induced constipation [48]. In addition to constipation, naloxegol and similar μ-opioid receptor blockers may also counteract general symptoms of opioid-induced bowel dysfunction [48]. Thus, improvement of gastric emptying, relief of nausea and reduction of the risk of regurgitation and pulmonary aspiration is a new approach to the use of opioid receptor antagonists. Side effects of naloxegol are usually seen in connection with the resolution of heavy opioid receptor stimulation in the periphery, such as gastrointestinal cramping [49]. A therapeutic daily dose of 25 mg naloxegol is generally safe with no significant effects on cardiac repolarization or QT-interval [50].