Peripheral receptors
Peripheral receptors are located mainly in the gastrointestinal (GI) tract, particularly the duodenum, but also in the biliary tract, peritoneum and urinary organs. The receptors may be stimulated by distension, irritation, inflammation or changes in osmolarity. There are a few receptors in the lower bowel, which explains why patients with inflammatory lower bowel disease may occasionally vomit.
ASSESSMENT OF THE PATIENT REPORTED TO BE VOMITING
It’s important to differentiate vomiting from regurgitation, which involves the retrograde movement of food and fluid from the oesophagus, pharynx and oral cavity without initiation of reflex neural pathways other than the gag reflex.
It’s also important to differentiate vomiting from gastric reflux, which involves retrograde movement of food and fluid from the stomach into the oesophagus. This material may then travel some or all of the way to the pharynx and nasopharynx and may be inhaled, causing acid damage to mucosae it contacts. The severity ranges from subclinical to severe and life threatening. Many patients present for the respiratory consequences of ‘silent reflux’, but in this chapter we will focus on the presentation that can be confused with vomiting because material from the stomach comes out of the mouth.
It is also important to differentiate vomiting from coughing followed by gagging, which is often confused by owners with vomiting. Cat owners in particular may confuse vomiting and coughing in their pets.
Owners are often unable to differentiate vomiting, regurgitating, refluxing and gagging, and therefore it is important to ask specific questions to elicit appropriate information, for example, amount of effort involved, character of vomitus, etc. If still uncertain, the veterinarian may need to observe the animal. Even with veterinary observation, it is difficult and sometimes impossible to differentiate reflux and regurgitation without fluoroscopy. Without fluoroscopy, the concurrent problems need to be considered to reach a reasonable conclusion. Patients with nausea, vomiting and gastric dysmotility are predisposed to reflux, as are brachycephalic breeds.
Why is it important to differentiate vomiting from regurgitation, reflux and coughing?
The differential diagnoses, appropriate diagnostic tools and management strategies are completely different for patients who are truly vomiting compared with patients who are regurgitating, gagging or coughing. There is much in common in the treatment of vomiting and reflux because diseases that lead to vomiting may subsequently lead to reflux. It is important to appreciate this so that a gastric antacid can be prescribed to reduce the risk of reflux oesophagitis, which can be the reason a patient with an acute vomiting disorder does not recover as expected and which can contribute significantly to patient morbidity and mortality.
Patients who are vomiting (due to primary GI or secondary GI disease) may be treated symptomatically or investigated, depending on the case, using a variety of diagnostic tools, including clinical pathology, diagnostic imaging, endoscopy and exploratory laparotomy.
When regurgitation is the predominant clinical sign, it will usually be due to oesophageal disease (very occasionally pharyngeal) and usually carries a poor or guarded prognosis due to the type of lesion – for example, foreign body, stricture or megaoesophagus. The patient should not be treated symptomatically without diagnostic investigation to define the lesion where possible. In addition, the investigation of regurgitation essentially involves visualising the oesophagus (by endoscopy and/or diagnostic imaging tools) – it is rare for routine clinical pathology to be of diagnostic value in defining the type of lesion (megaoesophagus, foreign body etc.), although it may be of value once megaoesophagus is diagnosed in assessing possible metabolic causes.
Similarly, the patient who is gagging most likely has a lesion in the pharyngeal region or upper oesophagus, and visualising the lesion is the appropriate diagnostic path. Clearly, the animal that is coughing has respiratory or cardiac disease and requires an entirely different diagnostic approach.
Failure to define the problem appropriately can therefore potentially endanger the patient and may lead to wasted time and money and impair the veterinarian–client relationship and trust.
Clues to help differentiation of vomiting, regurgitation and reflux
The associated behaviour of patients who vomit differs from those who regurgitate or reflux. As discussed, vomiting is a neurologically coordinated activity with defined stages and physical manifestations. The patient will exhibit abdominal effort prior to bringing up material, and vomiting is often preceded by hypersalivation – manifested by licking of lips and repeated swallowing. The vomiting may be projectile.
In contrast, regurgitation and reflux are passive processes – there are no coordinated movements. Regurgitation is often induced or exacerbated by alterations in food consistency and exercise and facilitated by gravity when the head and neck are held down and extended. Patients who regurgitate will often gag as material accumulates in the pharynx. Reflux is often watery and low in volume but acidic, and patients may exhibit behaviour indicating local irritation.
The character of the vomitus may also give the clinician clues. While undigested food may be brought up by vomiting or regurgitation, if the food is partially digested and/or contains bile, the patient is vomiting and/or refluxing, not regurgitating. The pH of the vomitus is occasionally, but not always, useful. Acidic material strongly suggests vomiting or reflux, but pH‐neutral material may be the product of vomiting, reflux or regurgitation.
As mentioned, because the epiglottis does not close, regurgitating patients are at considerable risk of aspirating gastric contents. Thus, if an owner reports that his/her animal developed a cough at the same time it started ‘vomiting’, the clinician should be alert to the possibility that aspiration has occurred and that this is more likely to occur with regurgitation than vomiting.
There is a caveat, however, which should be kept in mind. Patients who have experienced serious vomiting of acidic gastric contents may develop a secondary oesophagitis and present with signs suggestive of both vomiting and regurgitation or reflux. Usually, vomiting will have been the first sign noted. Animals that ingest caustic or irritant material causing oesophagitis and gastritis may also present with signs of both vomiting and regurgitation.
Haematemesis
Patients may vomit fresh blood or digested blood. Digested blood has the appearance of coffee grounds and is often not recognised by the owner as blood (understandably). Owners are usually extremely concerned if fresh blood is observed in vomitus, though this may not be of great clinical consequence if it has resulted simply from the physical effect of intense vomiting rupturing a small superficial blood vessel.
Nausea
While most patients who vomit will also be nauseous, it is important to recognise, as mentioned earlier, that the neural pathways involved in nausea and vomiting are not the same. Indeed, the neural pathways involved in nausea are still not well understood.
Primary vs. secondary gastrointestinal disorders
