Pathophysiology of Vomiting

Vomiting is a clinical sign and not a diagnosis. It is a protective reflex initiated by stimulation of the vomiting centre. The emetic mechanism consists of two anatomically and functionally separate units, a vomiting center located in the medulla oblongata and a chemoreceptor trigger zone (CTZ) located at the base of the 4th ventricle. The vomiting center integrates input from four different locations:

1.  The cerebral cortex causes vomiting associated with anxiety, pain and other behavioural influences.

2.  The vestibular nucleus is responsible for vomiting due to vestibulitis and motion sickness.

3.  The chemoreceptor trigger zone receives input from blood-borne substances and the vestibular system.

4.  The abdominal visceral receptors send impulses via vagal and sympathetic nerves to the vomiting centre. These visceral receptors are sensitive to distention of the gastrointestinal or biliary tract, inflammation or irritation of the viscera, peritoneum or pharynx and hypertonicity of gastric and small intestinal content.

The CTZ cannot cause vomiting without an intact vomiting center and is primarily responsive to chemical stimuli such as uraemic and bacterial toxins, ketone bodies and drugs (digitalis, apomorphine, chemotherapeutic agents). Blood borne chemicals are able to exert a stimulus, because the CTZ has a less effective blood brain barrier.

History and Signalment

Vomiting can be caused by a vast number of conditions. Clinicians tend to concentrate on the gastrointestinal tract too early in the investigation, leading to erroneous diagnosis. Vomiting is the hallmark of gastric disease; however, many non-gastric diseases cause vomiting. A complete history and review of all body systems is necessary to determine an efficient work-up of the vomiting patient.

Ask the following questions in the history of the vomiting patient:

 Duration of the vomiting--is it recent and acute or chronic and intermittent; static or progressive?

 Any other associated signs--appetite change; diarrhoea; abdominal pain; weight change?

 Past history--vaccinations; worming; pancreatitis; garbage ingestion?

 Dietary history--type; recent changes?

 Drug history--non-steroidal anti-inflammatory drugs; cardiac drugs; antibiotics e.g., erythromycin?

 Environment--free roaming; scavenging; travel history?

An exhaustive history is a very important first step to make a correct diagnosis. From the history, the following features of vomiting might be helpful in localization and diagnosis of the inciting cause:

Table 1. Features of vomiting.

Table 2. Features of dysphagia, regurgitation and vomiting.

Young unvaccinated pups are more prone to infections such as parvovirus, while older dogs with haematemesis are more likely to suffer from gastric neoplasia. Young Labradors or bull terriers are much more likely to ingest foreign bodies than older dogs.

Clinical Examination

A complete physical examination is of great importance in the vomiting patient. The mucous membranes are evaluated for evidence of blood loss, dehydration, shock and sepsis. In cats it is important to remember that jaundice is sometimes first recognized on the caudal hard palate. An oral examination will reveal the much cited string foreign body wrapped around the tongue. Next, the cervical area must be palpated for thyroid nodules. Cardiac auscultation can reveal gallop rhythms, or tachy- and bradycardia. A careful assessment is made for abdominal pain that could be either generalized (e.g., GI ulceration and peritonitis--although the author has seen a few cases of peritonitis in which abdominal pain was minimal) or localized (e.g., pyelonephritis, hepatic disease). Other abdominal factors to evaluate include abnormal organ sizes and shapes e.g., hepatomegaly, small or enlarged kidneys, thickened bowel loops or lymphadenopathy. A rectal examination can be attempted to assess stool colour, presence of diarrhoea or foreign material, and especially for possible anal sac adenocarcinomas that can cause vomition secondary to paraneoplastic hypercalcaemia.

Diagnosis

Vomiting in some patients requires an extensive work-up, but an organized approach will help to minimize the tests necessary for an early diagnosis. The most important initial considerations which are important in determining what tests to perform are: signalment; acute or chronic duration; frequency of vomiting; degree of clinical signs; other clinical signs (fever, shock, polyphagia); and, physical examination findings.

If the diagnosis is not apparent from the history or signalment then a minimum data base consisting of a full blood count, biochemistry profile, including T4 in cats over five years, complete urinalysis and faecal examination is essential. Survey abdominal radiography is indicated if thorough abdominal palpation is not possible or is suggestive of an abnormality. Further in-depth tests would be dictated by preliminary findings and/or response to therapy. These may include abdominal ultrasonography, contrast (both positive and negative) radiography, barium swallow with fluoroscopy and endoscopy. Endoscopy allows direct visualization of the gastric and duodenal mucosa and enables biopsies to be taken relatively atraumatically. Exploratory laparotomy is a valid and potentially rewarding procedure if other more sophisticated equipment is not available. It has the advantage of direct visualization of the whole abdominal contents and is indicated for the removal of intestinal foreign bodies, intussusception and the resection of neoplasms. It is important to always take biopsies even if the bowels look obviously normal. The full-thickness biopsies obtained via this procedure is often more diagnostic than endoscopic biopsies.

Differential Diagnoses for Chronic Vomiting

Here I use the algorithm of dividing the causes into intestinal and extra-intestinal and generating my DD list by using the DAMNIT system. You may however choose to think of the causes in simple anatomical terms, i.e., luminal, intramural or extra-mural causes. This aforementioned system will get you to DDs like extramural tumours or partial foreign bodies that do not readily come up with the DAMNIT system. You might equally choose to think of the transit of food through the gut and come up with DDs along the way. Then, sphincter and motility disorders will appear on your list, i.e., lower oesophageal sphincter incompetence.

Intestinal Causes

Gastric

 Developmental, i.e., pyloric hypertrophy

 Neoplastic, i.e., adenocarcinoma, esp. Siamese cats

 Infectious--Helicobacter-associated

 Inflammatory, i.e., all forms of gastritis

 Iatrogenic--gastric ulceration

Small Intestinal

 Macroparasites--verminosis

 Microparasites--viruses, bacteria

 Neoplastic--lymphoma

 Immune-mediated--IBD

 Ileus

 Foreign bodies

Large Intestinal

 Obstipation secondary to megacolon

 Colitis

Extra-Intestinal Causes

Metabolic/Endocrine

 Diabetic ketoacidosis

 Endotoxaemia/septicaemia

 Hyperthyroidism

 Hypercalcaemia

 Hypokalaemia

 Hypoadrenocorticism

Idiopathic

 Pancreatitis

 Peritonitis, including FIP

 Hepatobiliary disease

 Renal failure

 Pyelonephritis

Infectious

 Heartworm disease

 Cystitis

Toxins/Drugs

 NSAIDs, digoxin, chemotherapeutics etc.

References

1.  Simpson KW. Acute and chronic vomiting. BSAVA Manual of canine and feline gastroenterology 2nd edition, 2005.

2.  Twedt DC. Vomiting. Textbook of Veterinary Internal Medicine, Ettinger and Feldman (eds), 6th edition, 2005, p 132-136.