Pathophysiology of vomiting

Vomiting is a reflux of food and fluid from the stomach. It is a reflex act that involves prodromal signs of nausea, which may include anxiety, salivation, lip licking and restlessness.

Prior to vomiting, reverse peristalsis moves intestinal contents into the stomach and the animal begins to retch. There are then gastric antral and retrograde antral contractions with relaxation of the body of the stomach. The gastro-oesophageal and pharyngo-oesophageal sphincters relax. The driving force of vomiting is the contraction of the abdominal muscles and the diaphragmatic muscles.

Vomiting is a reflex act resulting from stimulation of the vomiting centre in the brain stem (Fig 10.1). The vomiting centre receives afferent input from peripheral receptors in the viscera from the chemoreceptor trigger zone (CRTZ) in the floor of the fourth ventricle of the brain, from the vestibular apparatus and from the higher centres of the brain such as the cerebral cortex. The peripheral visceral receptors are located throughout organs in the body, especially in the duodenum, sometimes referred to as ‘the organ of vomition or nausea’. Distension or irritation of the intestinal mucosa may stimulate the vomiting centre and inflammation of other organs such as the pancreas can also result in vomiting. Afferent nerve fibres run from these organs in the vagal and sympathetic nerves.

Fig 10.1 The vomiting centre

The CRTZ is on the floor of the fourth ventricle and mediates vomiting from toxins such as those present in uraemia and also vomiting associated with some drugs, e.g. apomorphine or xylazine.

The vestibular apparatus mediates vomiting due to motion sickness or vestibular disease. Stimulation of the higher centres such as the cerebral cortex from acute stress may result in vomiting, although this may be more common in human beings than in animals.

Diagnosis of vomiting

Several questions need to be answered early in the approach to vomiting cases. First, is it vomiting or regurgitation? Second, is the vomiting due to gastrointestinal (GI) disorders or due to disorders of other body systems? The signalment, history and physical examination often provide clues to these answers.

Signalment and history

The patient’s ‘signalment’ is the age, breed and sex. Knowing these factors often increases (or decreases) the index of suspicion for some disorders and helps the clinician order the differential diagnoses list.

The history should include the progression of the vomiting, the frequency, the timing with regards to eating and the time of day the vomiting most often occurs. Vomiting more than 12 hours after eating indicates delayed gastric emptying. Vomiting bile early in the morning can be part of the ‘bilious vomiting syndrome’ in dogs. A description of the contents of the vomitus, e.g. bile, foam, digested food, bright red blood or digested blood ‘coffee grounds’, is useful.

Careful questioning can usually allow the veterinary surgeon to determine if the patient is vomiting or regurgitating, although some cases do show both signs. Vomiting is the forceful ejection of gastric contents from the stomach, while regurgitation is the expulsion of ingesta from the oesophagus or pharynx. Regurgitation is described as a passive event; however, gagging may be present which can result in retching. With regurgitation, the animal should not show nausea, but drooling can be present if swallowing is impaired. Regurgitated food usually does not contain bile and should not have an acidic pH (unless refluxed from the stomach into the oesophagus). It is more common for animals to ‘re-eat’ regurgitated food than vomited food. Regurgitated food often appears undigested and may have the shape of the oesophagus. Regurgitation may take place immediately after eating, but can occur hours after eating, especially in cats where the normal oesophageal motility is slow.

Vomiting is usually accompanied by signs of nausea and a forceful ejection of food, with an abdominal effort present. The pH of vomitus may be acid, but can be alkaline, especially if duodenal contents are present. Expectoration – the ejection of material from the airway and laryngopharyngeal area – is also a forceful act and must be differentiated from vomiting and regurgitation. Expectoration and regurgitation are what cats do to remove hairballs from the laryngopharynx and oesophagus.

Environmental history should provide information regarding access to toxins such as lead, houseplants, cleaners, floor finishes, deodorants and organophosphates. The history of medication administered is important as many drugs can cause vomiting, e.g. non-steroidal anti-inflammatory drugs, corticosteroids, digitalis and some antibiotics. Asking questions regarding playing with toys, sticks, stones and string are important for possible foreign body ingestion, especially in young animals and retrievers.

During the history taking, particular attention should be paid to the diet, including information about scavenging, treats, snacks and food provided by neighbours or children. When questioned carefully, many owners provide evidence of a dietary influence on the patient’s vomiting. A review of all body systems is always necessary. Anything that incites or exacerbates the signs should be noted, as well as abnormal posturing that may reflect abdominal pain.