Consciousness consists of two components: arousal and awareness of the environment and self. Arousal refers to the behavioral continuum that occurs between sleep and wakefulness, while awareness to the content of consciousness (Laureys, Reference Laureys, Bayne, Cleeremans and Wilken2014). In normal physiological states, with the exception of REM sleep, the level of arousal is positively correlated with awareness (Laureys, Reference Laureys, Bayne, Cleeremans and Wilken2014) such that arousal is graded and measured by the degree to which it is diminished. This conceptualization appears to have led experts, including the DSM-5, to consider delirium as a condition to not associated with impairments in arousal (Schiff and Plum, Reference Schiff and Plum2000, American Psychiatric Association, 2013). However, arousal can be both diminished and heightened, not unlike the performance of an old-fashioned incandescent bulb at different voltages, giving barely sufficient light at low voltages and brightening up unbearably at high voltages (MacIsaac et al., Reference MacIsaac, Kanner and Anderson1999). Delirium is associated with both hypo-arousal and hyper-arousal (Han et al., Reference Han2017). A small but significant proportion of delirious older patients (10%; n = 155) in the emergency department in this study had normal arousal at the time of the short arousal assessment (Han et al., Reference Han2014).

Another important consideration in parsing delirium phenomenology is that arousal, awareness, and attention are hierarchically organized (Bhat and Rockwood, Reference Bhat and Rockwood2007). Impaired arousal precludes assessment of awareness and attention because of difficulty in engaging the patient in interview or formal cognitive testing. This in turn increases the likelihood of missed delirium diagnoses in people who are deemed “untestable.” These issues prompted both the European Delirium Association and the American Delirium Society to argue for the inclusion of arousal in DSM-5 delirium diagnostic criteria (European Delirium Association and American Delirium Society, 2014); a position further supported by the finding arousal may have prognostic significance for those with delirium (Han et al., Reference Han2017).

Arousal measurement is typically inferred and operationalized from observed spontaneous or stimulation-induced motor responses (Laureys, Reference Laureys, Bayne, Cleeremans and Wilken2014). This has been used fruitfully in developing the Richmond Agitation & Sedation Scale (RASS) for use in critical care units and subsequently modified for use in other hospital settings. The Observational Scale of Level of Arousal (OSLA) used in this study takes a more nuanced approach but at the cost of incorporating hypo- and hyper-arousal measures in the same direction (Tieges et al., Reference Tieges, McGrath, Hall and Maclullich2013).

Recent studies, building on previous exploratory studies (Chester et al., Reference Chester, Beth Harrington and Rudolph2012 and Tieges et al., 2013), have demonstrated the utility of measuring arousal in delirium diagnosis (Han et al., Reference Han2015; Morandi et al., Reference Morandi2016). In this context, the study by Richardson et al. (Reference Richardson2017) adds valuable evidence to the importance of measuring arousal in the diagnosis of delirium. This is important and welcome for several reasons. First, although the reference standard for delirium diagnosis is considered to be diagnosis by an expert in the field, there is considerable variability in the reference standard methods used in published research (Neufeld et al., Reference Neufeld2014). This study explicates DSM-5 criteria; once there is clarity about how a phenomenon is being measured then there can be discussion of the relative merits of the methods used allowing for sharpening of evaluation in time. Also of relevance is that this study does not use a standardized scale. Second, this study uses DSM-5 criteria that do not require the presence of impaired arousal for diagnosis, so the finding of impaired arousal showing good sensitivity (85%) and specificity (82%) to delirium diagnosis becomes more important. Third, it is unlikely that the measurement of disorder of a complex function such as consciousness can be reduced to just one domain. This study combines the use of structured bedside observation (OSLA) with a short test of attention (vigilance task); it's finding that this combined assessment had very high sensitivity and specificity to delirium diagnosis is an important advancement in improving recognition of delirium. Finally, this study carefully evaluates the utility simple tests both in people with only delirium and those with delirium superimposed on dementia. Delirium superimposed on dementia is a challenging clinical problem and a previous study has evaluated the utility of arousal assessment in this population (Morandi et al., Reference Morandi2016). This study adds to the evidence supporting the specificity of arousal assessment in delirium superimposed on dementia.

The study has its limitations, many of which are identified by the authors in the paper. In addition, the use of any error on orientation to time, place, and person as evidence of disturbance in awareness may be questionable and the lack of direction of impairment in arousal may not capture the entire range of impairments in arousal. Notwithstanding this, Richardson et al. (Reference Richardson2017) have established a standard against which future studies on delirium assessment will be measured against. This paper represents an important step forward in clarifying the confusion that surrounds delirium diagnosis. Further research that combines the nuanced approach of OSLA and directional measurement of hypo- and hyper-arousal of RASS and examines its utility in a larger sample would aid in improving understanding of delirium. In addition, it would also help better characterize fluctuations in mental state that are integral to delirium.