Substance use disorder (SUD) related hospital presentations have become a leading health problem. Approximately 33% of people presenting to Australian hospitals have a SUD problem (Butler, Reeve, Viney, & Burns, 2016), which represents a three-fold increase over the past decade (Tran et al., 2020). Whilst SUDs are increasingly common amongst general hospital inpatients, they are often under-detected or not referred for specialist assessment and management by consultation-liaison psychiatry (CLP) (Diehl et al., 2009; Smith, Clarke, & Handrinos, 1995; Smothers, Yahr, & Ruhl, 2004). SUDs are highly comorbid with psychiatric disorders (Buckley & Brown, 2006), contribute to disengagement from medical treatment, and are associated with social disadvantage, medical complications, and increased health service utilization (Nordeck et al., 2018). Dedicated general hospital SUD liaison services have been developed to address these issues directly with the aim of reducing medical complications, healthcare costs, and rates of rehospitalization (Butler et al., 2016; Nordeck et al., 2018; Tadros et al., 2013).
Despite the extensive biopsychosocial problems associated with SUDs, few studies have examined the types, frequencies, and patterns of substance use amongst patients referred to CLP. Instead, studies have reported SUDs as a single diagnostic category or a single main psychiatric diagnosis rather than the patterns of comorbidity (Bourgeois, Wegelin, Servis, & Hales, 2005; Chandarana, Conlon, & Steinberg, 1988; Devasagayam & Clarke, 2016; Dilts, Mann, & Dilts, 2003; Lipowski & Wolston, 1981; Rothenhausler, Ehrentraut, & Kapfhammer, 2001). Previous CLP studies across different countries and hospital settings have found that the most common disorders of CLP practice have been Major Depression, Adjustment Disorder, and Delirium (Bourgeois et al., 2005; Clarke & Smith, 1995; Holmes, Handrinos, Theologus, & Salzberg, 2011; Huyse et al., 2001; Smith, Clarke, & Herrman, 1993; Wand, Wood, Macfarlane, & Hunt, 2016). The frequencies of SUDs were 1.5–12% (Clarke & Smith, 1995; Diefenbacher & Strain, 2002; Holmes et al., 2011; Krautgartner, Alexandrowicz, Benda, & Wancata, 2006; Loewenstein & Sharfstein, 1983; McKegney, McMahon, & King, 1983; Smith et al., 1993). However, these low rates of SUDs now appear inconsistent with the changing landscape of CLP practice towards a more contemporary emphasis on SUDs, which was the leading diagnostic category in two recent studies (Desai, Shah, Shah, Sharma, & Zankat, 2016; Lyne et al., 2010). Patters of multiple substance use are also infrequently examined amongst general hospital patients. However, multiple substance use may be linked with particular psychopathology, personality disorders, and trauma (Donald, Arunogiri, & Lubman, 2019; Khantzian, 1997; Lubman, Hall, Pennay, & Rao, 2011; Mills, Teesson, Ross, & Peters, 2006; Nesse & Berridge, 1997; Pagura et al., 2010).
Multiplicity of mental disorders is also an emerging measure of assessment and management complexity for patients within the CLP setting (Bourgeois et al., 2005). At least 25% of patients referred to CLP have multiple mental disorders (Bourgeois et al., 2005; Holmes et al., 2011; Loewenstein & Sharfstein, 1983; McKegney et al., 1983; Wood, Wand, & Hunt, 2015; Zimmerman & Mattia, 1999). However, there has been little investigation as to whether this multiplicity measure represents a different diagnostic construct for patients with or without SUD comorbidity. Patients who meet the criteria for at least one SUD and psychiatric disorder simultaneously are often defined in the psychiatric literature as having a ‘substance use comorbidity’ or ‘dual diagnosis’. Individuals with multiplicity of psychiatric disorders versus single or no psychiatric disorder may have similar problems to individuals with psychiatric disorder and SUD comorbidity, such as greater psychosocial impairment, treatment resistance, medical comorbidity, and rehospitalization (AIHW, 2012; Bourgeois et al., 2005; Wancata, Benda, Windhaber, & Nowotny, 2001; Zimmerman & Mattia, 1999). However, some studies lacked a clear definition whether the multiplicity of disorders measure involves the presence or absence of SUDs (Bourgeois et al., 2005). Hence, it is unclear whether the problems associated with multiple mental disorders are from comorbid SUDs or not.
The aim of the current study was to examine for differences in demographics, physical health, psychiatric variables, and health service utilization based on the number of diagnosed psychiatric disorders and the presence or absence of SUD comorbidity in a cohort of general hospital inpatients referred to CLP. It was also of interest to characterize the types, frequencies, and patterns of comorbidity between psychiatric disorders and SUDs.