Nausea, vomiting: First treat the underlying cause

Nausea, vomiting: First treat the underlying cause

Anti-emetic and prokinetic drugs play a key part in management
Aug 01, 2008

Please provide a review of anti-emetic and prokinetic drugs — I am confused.

Dr. David C. Twedt gave an excellent lecture on anti-emetics, prokin-etics and antacids at the 2008 American College of Veterinary Medical Forum in San Antonio. Here are some relevant points:

Vomiting and its management

Nausea and vomiting are common complaints and anti-emetics may play an important part in their management, but the primary objective should always be first to identify the underlying cause and if possible treat that primary disorder.

Anti-emetics are indicated when vomiting is severe, resulting in dehydration, electrolyte loss and acid-base disturbances, and for the prevention of motion sickness. Additionally, anti-emetics should be used for prevention of aspiration pneumonia, vomiting associated with radiation or chemotherapy, management of nausea resulting in inadequate nutritional intake and to enhance animal comfort.

Anti-emetics work either peripherally, centrally or in both areas. Vomiting is best defined as a reflex act initiated by stimulation of the conceptualized "vomiting center" in the brain's medulla oblongata. Activation of that center occurs through a humoral pathway initiated via blood-borne substances or through various peri-pheral neural pathways leading to the vomiting center.

Neural stimulation of the center arises through either afferent vagal, sympathetic, vestibular and cerebrocortical pathways. Activation of peripheral receptors found throughout the body can stimulate these neural pathways. Particularly important are receptors located throughout the abdominal viscera.

Central nervous system (CNS) disease may directly stimulate the vomiting center, such as direct extension of inflammatory stimuli, hydrocephalus or space-occupying lesions. The vomiting center is stimulated indirectly via a humoral pathway by activating the chemoreceptor trigger zone (CRTZ) located in the area postrema at the base of the fourth ventricle, an area devoid of a blood-brain barrier that allows exposure to chemical stimuli found in the circulation.

Blood-borne substances stimulating the CRTZ include certain drugs, uremic toxins, electrolytes, osmolars and acid-base disorders, as well as a number of metabolic derangements. There is evidence in the dog that vestibular stimulation passes through the CRTZ before activating the vomiting center. Motion sickness, inflammation of the labyrinth or lesions in the cerebellum results in vomiting via this pathway.