Nausea is an unpleasant subjective sensation often associated with the urge to vomit, and produces aversion and avoidance reactions in people.
Nausea is an important cause of suffering and the pathways associated with the development of nausea and vomiting in animals appear to have many similarities to those of humans.¹ Therefore it’s highly likely animals also experience nausea.
As is the case with pain assessment, the diagnosis of nausea will depend on the behavioural changes associated with this unpleasant sensation, in addition to any physiological responses.
Response to treatment can be used to confirm the presence of nausea. It’s important to note that nausea can commonly occur without any subsequent vomiting, whilst vomiting rarely occurs in the absence of an initial feeling of nausea.
Physiology of nausea
The peripheral pathways that can induce nausea are the same as those that can induce vomiting. It appears, however, that the sensation of nausea occurs at a lower threshold than that required to initiate the vomiting reflex which results in the expulsion of gastric contents.²
Retching and vomiting are subconscious reflexes that don’t require input from higher centres of the brain. In contrast, the sensation of nausea involves conscious perception in the cerebral hemispheres after receiving neural inputs from an area of the brainstem called the Nucleus Tractus Solitarius NTS.
The NTS in turn receives afferent inputs from several peripheral and central sources: the abdominal vagus nerve, the Chemoreceptor Trigger Zone CRT, the vestibular system and higher centres of the brain (see Figure 1).
The stomach and duodenum are innervated by mechanoreceptors that detect distension and chemoreceptors that will detect various noxious substances such as hypertonic solutions. These neuroreceptors send impulses via the abdominal vagus nerve to the NTS and cause the sensation of nausea.
Chemoreceptors in the CRT, an area of the medulla that has a relatively permeable blood brain barrier, can detect chemicals and hormones in the blood and CSF capable of initiating nausea and vomiting. The vestibular system is the source of motion sickness and higher brain centres appear to be involved in anticipatory nausea or the nausea and vomiting that can immediately follow overwhelming fear or a repulsive smell or sight.
Causes of nausea
Currently there is no objective way to measure the presence or intensity of nausea in a non-verbalising patient, therefore analogies with human medicine are inevitably used to identify situations where nausea may arise. A positive response in demeanour to the administration of an anti-nausea drug can be useful to help in its diagnosis.
Clinical signs
Studies have found that nausea and vomiting are associated with sequential autonomic changes(e.g. pallor, tachycardia, sweating) and an increase in the release of anti-diuretic hormone from the pituitary.²
In humans it is known that gastric rhythm increases just prior to the sensation of nausea, the intensity of the rhythm correlating to the intensity of nausea being experienced. As nausea subsides the rhythm slows back down to normal.²
The clinical signs associated with nausea in different species include those in Table 2.1,2,4.
Summary
Nausea is considered a highly unpleasant cause of suffering in humans and on-going nausea is well-known to reduce patients’ overall quality of life. Although different species can demonstrate varying levels of sensitivity to stimuli and some are unable to vomit, evidence suggests that animals process the sensory and affective aspects of nausea in a similar way to humans.
Therefore, if drugs known to cause nausea in people are being administered or animals have a disease process likely to be associated with nausea, then they should be given the benefit of the doubt and anti-nausea drugs provided.
- Holmes, A. M., Rudd, J. A., Tattersall, F.D. et al. (2009) Opportunities for the replacement of animals in the study of nausea and vomiting. British Journal of Pharmacology 157: 865-880.
- The European Emesis Council, Nausea Knowledge Statement 2009; London, UK.
- Ettinger, S. J. and Feldman,E.C. Textbookof VeterinaryInternalMedicine 4th Edition; W. B. Saunders.
- Fuller, F. and North, G. (2009) Nausea and Vomiting. EMS Magazine 18th November.
- Herrstedt, J. (2009) Antiemetics: an update and the MASCC guidelines applied in clinical practice. Nature Clinical Practice Oncology157: 865-880.
- Qin, X. Y.,Pilot,M.A., Thompson, H. and Scott, M. (1993) Effects of cholinoceptorand5hydroxytryptamine3 receptor antagonism on erythromycin-induced canine intestinal motility disruption and emesis. Br J Pharmacol. 108 (1): 44-49.
