Nausea is a sensation of unease in the stomach, with an involuntary urge to vomit. In some, it may precede vomiting, but many may feel nauseous without vomiting. Nausea is a common aspect of many conditions and illnesses, as well as medications and treatments, e.g. chemotherapy, antibiotics, colds and flus, narcotic pain medications. Common causes of nausea include low blood sugar, gastroenteritis (stomach infection), food poisoning, morning sickness in the first trimester of pregnancy, migraine, motion sickness, dizziness and fainting.
Anxiety, depression and disgust (e.g. seeing or smelling something particularly nasty) may also cause nausea. There is also a rare condition known as “cyclic vomiting syndrome” (CVS), which may cause sudden, repeated attacks of nausea and vomiting.There are a huge number of conditions, both on and off this list, that cause nausea. Persistent nausea and vomiting can lead to a dramatically reduced quality of life, dehydration, a loss of appetite, weight loss and fatigue. Medications used to prevent and treat nausea are called antiemetics. The most commonly prescribed antiemetics include promethazine (a strong sedative with weak psychoactive effects), metoclopramide (Primperan, Reglan) and ondansetron (Zofran).
There are four pathways that trigger nausea. Antiemetics tend to work by working on the receptors and transmitters that trigger or stimulate these pathways.
- Central nervous system (CNS): Stimuli can affect areas of the CNS including the cerebral cortex and the limbic system. These areas are activated by elevated intracranial pressure, irritation of the meninges (i.e. blood or infection), and extreme emotional triggers such as anxiety.
- Chemoreceptor trigger zone (CTZ): Located in the area postrema in the floor of the fourth ventricle within the brain. This area is outside the blood brain barrier, and SO IS readily exposed to substances circulating through the blood and cerebrospinal fluid. Common triggers include metabolic abnormalities, toxins, and medications. Activation is mediated by dopamine (D2) receptors, serotonin (5HT3) receptors, and neurokinin receptors (NK1). TRPV1 (vanilloid) receptors may also play a role in nausea.
- Vestibular system: Activated by disturbances to the vestibular apparatus in the inner ear. These include movements that cause motion sickness and dizziness. Histamine (H1) receptors and acetylcholine (ACh) receptors trigger this pathway.
- Peripheral Pathways: These pathways are triggered via chemoreceptors and mechanoreceptors in the gastrointestinal tract, as well as other organs such as the heart and kidneys. Common activators of these pathways include toxins present in the gastrointestinal lumen, and distension of the gastrointestinal lumen from blockage or dysmotility of the bowels. Signals from these pathways travel via multiple neural tracts including the vagus, glossopharyngeal, splanchnic, and sympathetic nerves.
Signals from any of these pathways travel up through to the brainstem, activate several structures within the brain, including the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and central pattern generator.