The prevalence of anticipatory nausea and vomiting (emesis) (ANV) has varied, owing to changing definitions and assessment methods. However, anticipatory nausea appears to occur in approximately 29% of patients receiving chemotherapy (about one of three patients), while anticipatory vomiting appears to occur in 11% of patients (about one of ten patients). With the introduction of new pharmacologic agents (5-HT3 receptor antagonists), it was anticipated that the prevalence of ANV might decline; however, studies have shown mixed results. One study found a lower incidence of ANV, and three studies found comparable incidence rates.[2,4,5] It appears that the 5-HT3 agents reduce postchemotherapy vomiting but not postchemotherapy nausea,[2,5] and the resulting impact on ANV is unclear.
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Although other theoretical mechanisms have been proposed, ANV appears to be best explained by classical conditioning (also known as Pavlovian or respondent conditioning). In classical conditioning, a previously neutral stimulus (e.g., smells of the chemotherapy environment) elicits a conditioned response (e.g., ANV) after a number of prior pairings or learning trials. In cancer chemotherapy, the first few chemotherapy infusions are the learning trials. The chemotherapy drugs are the unconditioned stimuli that elicit postchemotherapy nausea and vomiting (N&V) (in some patients). The drugs are paired with a variety of other neutral, environmental stimuli (e.g., smells of the setting, oncology nurse, chemotherapy room). These previously neutral stimuli then become conditioned stimuli and elicit ANV in future chemotherapy cycles. ANV is not an indication of psychopathology but is rather a learned response that, in other life situations (e.g., food poisoning), results in adaptive avoidance. A variety of correlational studies provide empirical support for classical conditioning. For example, the prevalence of ANV prior to any chemotherapy is very rare, and few patients ever experience ANV without prior postchemotherapy nausea. Also, most studies have found (1) a higher probability of ANV with increasing numbers of chemotherapy infusions and (2) the intensity of ANV increasing as patients get closer to the actual time of their infusion. In one experimental study, it was shown that a novel beverage could become a conditioned stimulus to nausea when paired with several chemotherapy treatments.