Past exam of the medicine course of the University of Oxford / 1867 / Second examination / Pathology / 4 / Solution

Vomiting is a coordinated nervous motor act that can be initiated from the digestive system, circulating emetic substances, the vestibular system, or higher and lower brain pathways. A cause need not lie in the stomach.

Local causes include irritant food or poisons, gastritis, gastroenteritis, gastric outlet obstruction and intestinal obstruction. Chemical irritation, inflammation or distension activates visceral sensory pathways, notably afferents of the vagus nerve, which relay to the medulla oblongata. In obstruction, retention and distension provide continuing stimulation; merely seeing food return does not imply excessive gastric contractile strength. Peritonitis and painful disease elsewhere in the abdomen can also recruit visceral afferent pathways.^[ref]

Circulating emetic agents, including some drugs and metabolic products accumulating in kidney failure, act partly through the area postrema, a chemosensitive region with unusually direct access to substances in the blood. Gastrointestinal and blood-borne inputs converge through the nucleus tractus solitarius and connected medullary networks. It is more accurate to describe interacting groups of neurons than one isolated “vomiting centre”.^[ref]

Other important groups are motion sickness, where information from the vestibular system conflicts with other sensory information; migraine and intracranial disease, including raised intracranial pressure from a brain tumour or intracranial haemorrhage; and higher-centre influences such as a repulsive sight, smell, fear or learned association. Pregnancy-associated vomiting belongs in the differential, with endocrine and central contributions rather than an explanation that it is simply mechanical pressure by the uterus upon the stomach. These causes engage different inputs to an overlapping final motor programme.^[ref]

The act itself requires several coordinated changes. The proximal stomach and lower oesophageal sphincter relax; retrograde activity can bring contents of the upper intestine into the stomach; the diaphragm and abdominal muscles generate a pressure gradient that expels the contents. During expulsion the upper passage opens while coordinated laryngeal closure helps protect the airway. Retching and actual expulsion are related stages, and neither should be confused with effortless regurgitation.^[ref]

This mechanism explains the diversity of causes: many sensory and chemical inputs can recruit the same coordinated output. A pressure recording and observation of gastrointestinal movements during an emetic response can distinguish the patterned act from a simple spontaneous return of contents; they would not, without further evidence, identify the original cause in an individual.