Vomiting is a process by which the body expels substances that may irritate the stomach and the intestines. This explanation is a very simplistic and our reality is that we still aren’t very sure of all the reasons and mechanisms of vomiting from the many causes.
There are many causes for vomiting. They may be classified as follows:
Intestinal obstruction – tumours, adhesions, hernias etc
Reduced/Abnormal motility – Diabetic gastroparesis
Infection – acute peritonitis, appendicitis, gastroenteritis (bacterial and viral), food poisoning etc.
Gastric irritants -alcohol, pain killer intake, antibiotics etc
Others – pancreatitis, hepatitis, kidney stone disease, heart attacks etc
Vestibular (related to the inner ear)
Acute viral labrynthitis
Infections -meningitis and encephalitis
Increased pressure- tumours, strokes, intra-cranial bleeding
Drugs and toxins
Uraemia (kidney failure)
Mechanics of Vomiting
The process starts with increased salivation and nausea (the uncomfortable feeling of vomiting). The stomach starts contracting upwards (reverse peristalsis), the wind pipe is shut by the epiglottis reducing the chances of the vomited food from entering the lungs, the tummy muscle contract forcefully, the chest is held fixed in mid inspiration, the increased intra-abdominal pressure ejects the stomach’s contents out through the mouth and the nose.
Neural control of Vomiting
The neural pathways that bring about vomiting all lie within the brainstem. The brainstem consists of 3 areas, the midbrain, the pons and the medulla. Behind the brain stem is the cerebellum (the little brain) and above the brainstem is the Cerebrum. Between the cerebral hemispheres and the brain stem is the amygdala (which controls emotions).
There are 4 pathways to causing vomiting.
1. The stimulus from the throat (via the Glossopharyngeal Nerve) and the stimuli from the stomach and other abdominal organs (via the Vagus nerve) are carried to the Nucleus Tractus solitaries (NTS) which can not only set off the vomiting reflex but also acts by stimulating the brainstem vomiting centre. This pathway works via the serotonin (5HT3) pathway.
2. Stimuli from the inner ear and the vestibular system directly activates the Brainstem vomiting centre and brings about its effect. Histamine and muscarinic/cholinergic stimulation mediate this pathway.
3. The higher brain centres such as the Amygdala (emotion) and other centres that perceive stimuli such as pain and bad smells stimulate the brainstem vomiting centre directly to cause vomiting.
4. There exists a Blood brain barrier which protects the brain from toxins in the blood. The Chemo Receptor trigger zone is one such area that lies outside the blood brain barrier. Drugs such as opiates and chemotherapy medicines work on this area, to stimulate both the NTS and the brainstem vomiting centre to induce vomiting. These stimuli work through serotonin, NK1 and dopamine pathways.
Why should vomiting be treated promptly?
Repeated vomiting can cause dehydration, loss of stomach acid (and therefore alkalosis), loss of sodium and hyponatraemia, aspiration (when what is vomited enters the lungs) and pneumonia, rupture of the lower end of the gullet/esophagus and blood vomiting (Mallory -Weiss tears) etc.
How is vomiting treated?
Knowing which pathway is involved will suggest the best medicines to stop vomiting.
Stimuli from the pharynx (throat) stomach and other abdominal contents work through serotonin. Therefore serotonin antagonists such as Ondansetrol (Emeset), Granisetron, Dolosetron and Palonosetron will help.
Stimuli from the ear (due to vertigo, motion sickness) or from the cerebellum (due to reduced blood flow) will cause vomiting mediated via histamine receptors. This vomiting would respond well to medicines that have an antihistamine function such as Meclizine, Dimenhydrinate etc.
Stimuli from chemotherapy which stimulates the Chemoreceptor trigger zone via the Neurokinin (NK-1) receptors would respond to treatment with NK antagonists such as Aprepitant, Fosaprepitant, Rolapitant and Netupitant.
Second line antiemetics are:
1. Steroids – Dexamethasone works in ways that aren’t clear.It is definitely useful as a powerful anti-vomitng especially a second line and for vomiting after surgery.
2. Dopamine antagonists such as Prochlorperazine (Avomine) and Promethazine (Phenergan), Chlorpromazine, Haloperidol and Olanzapine (with additional serotonin blocking effects)
3. Cannabinoids also work by unknown mechanisms.
4. A combination of vitamin B6 and Doxylamine (Doxinate 24) is used for vomiting of Pregnancy.
So after reading this article and if you should have severe vomiting, you will be able to ask your doctor for better medicines for yourself!