Pathophysiology

Vomiting is controlled by a vomiting centre in the floor of the fourth ventricle of the brain, which receives signals from the chemoreceptor trigger zone (CTZ), hypothalamus, cerebral cortex and the vestibular apparatus. The role of the vomiting centre is to coordinate the complex process of vomiting.

Nausea is an unpleasant sensation accompanied by reduced gastric motility with increased contraction of the duodenum. Peripheral stimuli to the vomiting centre arise from the gut through the autonomic nervous system and the vagal nerve. Other stimuli include visceral stimulation (e.g. stomach after ingestion of exogenous chemical), vestibular stimulation (e.g. vertigo), sensory stimuli (e.g. foul smell), chemical stimuli (e.g. chemotherapy) and impulses from the chemoreceptor trigger zone directly (Figure 23.1).

The CTZ is a highly vascularised area of the brain designed to sample blood and CSF for chemicals that can trigger emesis, particularly following exposure to chemotherapy and opioid analgesia. There is considerable variation among individuals, which accounts for the variability in emetogenicity between different medications and different people.

Oncology practice

Not all cancer patients are affected by nausea and vomiting, but there are common patterns encountered in clinical practice, and a detailed medical history will aid diagnosis of the underlying cause.

• Iatrogenic: chemotherapy, opioid analgesia (constipation may contribute), radiotherapy-induced toxicity of the gut, liver or brain.
  
• Metabolic disturbances:
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