Vomiting

Vomiting is a defence reflex which involves the involuntary, forceful expulsion of the contents of one's stomach through the mouth and sometimes the nose

Vomiting can be caused by a variety of conditions, including:

• Uraemia (due to renal failure)
  • Presence of excess urea and creatine in the blood that would normally be excreted in the urine by the kidneys
  • Urea enzymes can decompose urea into ammonia which can stimulate the gastrointestinal mucosa and cause nausea and vomiting
• Gastroduodenal diseases
• Hepatic diseases
• Infectious diseases

Vomiting can also be drug-induced

There are certain neurotransmitters that can be triggered when someone takes opioids, which is why opiates cause nausea and vomiting. Other neurotransmitters include: acetylcholine, dopamine, histamine (H1 receptor), substance P (NK1 receptor) and serotonin (5-HT3 receptor). Therefore, these receptor antagonists are used as anti-emetics.

Drug-induced vomiting may be due to:

• Cancer chemotherapy and radiation therapy - cisplatin in particular is very emetogenic 
• Dopamine agonists
• Opiates
• Ipecacuanha - a Brazilian root which contains chemicals (emetine and cephaeline) that act as major alkaloids. They both act locally by irritating the gastric mucosa and centrally by stimulating the medullary CTZ to induce vomiting
• Alcohol

Vomiting can also be caused by:

• Post-operative emesis (PONV)
  • Emetogenic drugs commonly used in anaesthesia include nitrous oxide, physostigmine and opioids can stimulate the CTZ
• Motion sickness
  • A condition in which discordance exists between visually perceived movement and the vestibular systems sense of movement. For example, if motion is seen and not felt (e.g. virtual reality and films) or felt but not seen (e.g. car/sea/air sickness)
  •  visually perceived movement and the vestibular system's sense of movement. For example, if motion is seen and not felt (virtual reality and films) or felt but not seen (car/sea/air sickness)
• Pregnancy (morning sickness)

The area postrema is a medullary structure in the brain that controls vomiting.

The chemoreceptor trigger zone (CTZ) is located within the area postrema, which is on the fourth ventricle floor and is effectively outside of the blood-brain barrier. This makes it incredibly sensitive to toxin and pharmacological stimulation

It is triggered when it detects substances not meant to be in the blood. It works by receiving input from drugs or hormones and triggering other areas of the brain to induce vomiting. 

• Dopamine antagonists (neuroleptics - anti-psychotics)
  • Block dopamine receptors
  • Side effects: extra-pyramidal side effects
  • Examples: Metoclopramide, Droperidol, Domperidone
• 5-HT3 receptor antagonists
  • Block serotonin receptors in the CNS and the GI tract
  • Used to treat post-operative and cytotoxic drug nausea and vomiting
    
  • No extra-pyramidal side effects
  • Examples: Dolasetron, Granisetron, Ondansetron, Tropisetron, Palonosetron, Mirtazipine
• NK1 receptor antagonists
  • A substance P antagonist - it block NK1 receptors
  • Examples: Aprepitant, Casopitant, Rolapitant
• Steroids
  • Example: Dexamethasone

• Benzodiazepines
  • Examples: Midazolam, Lorazepam
• Cannabinoids
  • Used in patients with cachexia, cytotoxic nausea, and vomiting, or who are unresponsive to other agents. Causes euphoria and sedation
  • Side effects: changes in perception, dizziness, and loss of coordination
  • Examples: cannabis
• Anti-muscarinics (subset of anti-cholinergics)
  • Blocks the activity of the muscarinic acetylcholine receptor
  • Examples: scopolamine
• Anti-histamines
  • H1 histamine receptor antagonists
  • Used in motion sickness, morning sickness, and to combat opioid nausea
    
  • Examples: Dimenhydrinate, Cyclizine, Cinnarizine, Promethazine