Nausea: Definition, Pathophysiology, and Clinical Context

Nausea is the subjective, unpleasant sensation of an urge to vomit, often preceding emesis but can occur independently. It arises from integration of visceral, vestibular, chemoreceptor, and cortical inputs in the brainstem.

Pathophysiology

• Chemoreceptor Trigger Zone (CTZ): Area postrema (outside BBB) — detects toxins, drugs (chemo, opioids), metabolic derangements (uremia, ketoacidosis).
• Vestibular Apparatus: Motion sickness, labyrinthitis — via CN VIII to vomiting center.
• Visceral Afferents: GI distension, inflammation, obstruction — via vagus/splanchnic nerves (5-HT₃, NK₁, dopamine D₂ receptors).
• Cortical/Limbic: Anxiety, anticipation, disgust, pain — higher centers modulate brainstem.

Common Causes

Clinical Assessment

• Onset, duration, triggers, relation to meals/meds/motion.
• Associated: vomiting, pain, fever, headache, vertigo, weight loss.
• Medication review (recent starts, dose changes).
• Pregnancy test (reproductive-age women).
• Basic labs: BMP, LFTs, lipase, β-hCG, CBC, TSH.

Antiemetic Classes (Receptor Targets)

• 5-HT₃ antagonists: ondansetron (chemo, post-op, gastroenteritis).
• Dopamine D₂ antagonists: metoclopramide (prokinetic), prochlorperazine, haloperidol.
• NK₁ antagonists: aprepitant (chemo delayed phase).
• Antihistamines/Anticholinergics: meclizine, scopolamine (vestibular).
• Corticosteroids: dexamethasone (chemo adjunct, raised ICP).
• Benzodiazepines: lorazepam (anticipatory nausea).
• Cannabinoids: dronabinol (refractory chemo nausea).

Example sentence: "A wave of nausea washed over me as the boat pitched in the choppy water." — illustrates vestibular-triggered nausea.

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Category	Examples
GI	Gastroenteritis, GERD, gastroparesis, obstruction, pancreatitis, hepatitis, constipation
Drugs	Chemo (highly emetogenic: cisplatin), opioids, antibiotics, NSAIDs, digoxin, iron
Metabolic	Pregnancy (hCG), uremia, DKA, hyponatremia, hypercalcemia, thyroid storm
Neurologic	Migraine, increased ICP, vestibular neuritis, concussion
Psychiatric	Anxiety, eating disorders, somatic symptom disorder