This case provided two important clinical issues. First, HVS can occur after emergence from general anesthesia. This syndrome is a common disease as indicated in a study by Jones et al., estimating a prevalence of 9.5% in an adult population  and sometimes occurring during dental care [6, 7]. However, HVS after general anesthesia is less common because sedative drugs and opioids provide an anxiolytic and antiadrenergic action and relief of pain. Mizuno and colleagues  reported a case in which HVS developed before induction of and after emergence from general anesthesia. The authors considered that this syndrome could occur repeatedly under anxiety and stressful conditions even if a patient had no previous history of psychiatric disease. In this case, potential anxiety about tongue cancer induced HVS triggered by bladder distension. Anesthesiologists should consider that HVS can occur even after general anesthesia.
Although HVS is a common disease, HVS is a disorder with no widely accepted diagnostic criteria . Thus, it is diagnosed by the exclusion of alternative possibilities such as metabolic and cardiopulmonary diseases, asthma, pneumothorax, airway obstruction, and acute coronary syndrome. In this case, ECG, laryngofiberscopy, and arterial blood gas analysis ruled out these diseases and he was diagnosed with HVS. However, delirium or agitation also occurs after general anesthesia and differential diagnosis between them is difficult, and the overlap of delirium or agitation that are associated with hyperventilation should be kept in mind. Delirium is associated with increased mortality, prolonged hospital stay, and persistent cognitive dysfunction [10, 11]. Early diagnosis and treatment for altered mental status occurring in the postoperative period after emergence from general anesthesia are important.
Second, dexmedetomidine infusion was effective for HVS. Dexmedetomidine is an α-adrenergic agonist with dose-dependent α2-adrenoceptor selectivity. It seems that the mechanism of action of the drug is suitable to treat HVS. Pre- and post-synaptic activation of α2-adrenoceptor inhibits the release of norepinephrine and sympathetic activity and terminates the transmission of pain signals . These effects produce sedation, analgesia, and anxiolysis. Furthermore, α2-adrenoceptor inhibits the release of serotonin in the rostral raphe nuclei which is associated with arousal and fear which are associated with panic disorder or others . A large randomized control study revealed that dexmedetomidine reduced the incidence of postoperative delirium by 14% in patients aged over 65 years . In addition to analgesic and anxiolytic properties of dexmedetomidine, its anti-inflammatory action might have treated both HVS and delirium. Elevated biomarkers of inflammation have associated with the risk for developing delirium . Anti-inflammatory effects of dexmedetomidine were evident in both clinical and animal studies [16, 17]. Furthermore, dexmedetomidine has advantages over other sedating agents because of minimal suppression of respiratory function . In this case, the bladder might have been kept distended until the next morning and upper airway obstruction and respiratory depression after neck dissection were one of several concerns. Taking these factors into consideration, the drug may be considered to be a good choice for sedation for HVS after general anesthesia. Dexmedetomidine can be an option to treat and prevent an altered mental state occurring in the postoperative period after emergence from general anesthesia.