Subsyndromal delirium in old age: conceptual and methodological issues.

Delirium is a cognitive disorder characterized by acute onset, fluctuating course, altered level of consciousness, inattention, disorganized thinking, disorientation, memory impairment, and perceptual and motor disturbances (American Psychiatric Association, 2000; World Health Organization, 2010). It occurs in hyperactive, hypoactive, or mixed forms in up to 42% of older hospital inpatients (Siddiqi et al., 2006) and 70% of older long-term care residents (McCusker et al., 2011). In both settings, delirium is independently associated with poor outcomes (Siddiqi et al., 2006; McCusker et al., 2010; Witlox et al., 2010). Both DSM-IV-TR and ICD-10 diagnostic criteria for delirium require the coexistence of symptoms from multiple domains (American Psychiatric Association, 2000; World Health Organization, 2010). It is common, however, for older people to display one or more symptoms of delirium without having the full syndrome (Rockwood, 1993; Kiely et al., 2003). The occurrence of such symptoms has been labeled as subsyndromal delirium (SSD) (Levkoff et al., 1996). The existence of SSD is controversial. Some investigators claim that SSD may be a clinically important condition that falls on a continuum between no symptoms and full delirium, perhaps a marker for underlying medical conditions (e.g. infection, drug toxicity) not severe enough to cause full delirium (Levkoff et al., 1996; Cole et al., 2003). Others argue that distinguishing subsyndromal presentations from full delirium is not “clinically practical given the fluctuating course of delirium, as clinical manifestations may range from normality through subsyndromal to full syndromal delirium in a matter of hours” (Blazer and van Nieuwenhuizen, 2012). To date, evidence for the existence of SSD is inconclusive. On the one hand, two studies of the course of SSD have reported that episodes of SSD appear to occur independently of full delirium, last for 1–133 days, and often end in recovery but often recur (Tan et al., 2008; Cole et al., 2012a). On the other hand, a systematic review of the studies of SSD has reported significant unexplained heterogeneity in the results of studies of prevalence, incidence, and some risk factors that may undermine the credibility of the findings of these studies (Cole et al., 2012b). Only further research will advance knowledge and determine whether or not SSD is a clinically important condition. To advance knowledge, such research must wrestle with six conceptual and methodological issues and try to avoid teleological reasoning and research design. These issues include the following: the types of symptoms of delirium required for the diagnosis of SSD, the number of symptoms required for diagnosis, the relationship of the symptoms present to full delirium, the measurement of the symptoms of delirium, prevalent versus incident SSD, and the selection of patient populations. Each of these issues is outlined below.