NSMHWB was a national epidemiological survey of mental disorders that used similar methodology to the NCS. It aimed to answer three main questions:
- How many people meet DSM-IV and ICD-10 diagnostic criteria for the major mental disorders?
- How disabled are they by their mental disorders? and
- How many have seen a health professional for their mental disorder?
Data from the Australian survey of low prevalence disorders indicates that among people with psychotic illnesses, the prevalence of alcohol use disorders is 36% among men and 17% among women. The figures for drug use or dependence are 38% for men and 16% for women (Jablensky et al., 2000). Data from the adult survey indicates that 48% of females and 34% of males who met criteria for an alcohol use disorder also met criteria for another mental disorder in the previous 12 months (Teesson et al., 2000). Comorbidity between mental disorders and substance use disorders in the Australian population is not uncommon. However, such information does not tell us about whether such comorbidities are the most common or disabling in the community, nor about the prevalence of comorbidity as a general phenomenon.
Andrews et al, (Andrews, Slade, & Issakidis, 2002) used the Australian national survey data to show that even within a 12 month time frame, people with symptoms that met criteria for three or more disorders over the 12 months had ten times the risk of having a current disorder when compared with people who had had only one disorder in the past 12 months. In other words, similar to previous international studies, the Australian survey found that comorbidity is more frequent than expected based on the prevalence of individual disorders.
When patterns of associations were examined, within-disorder group associations were significantly larger than between-disorder group associations, a finding similar to that reported from the ECA and the NCS. Andrews et al (Andrews et al., 2002) extended the analysis to include clusters of personality disorder defined by ICD-10 and found a similar pattern of very strong associations between clusters within the personality disorder group. Again, anxiety disorders displayed strong associations with affective disorders and, similar to the ECA and the NCS, were sometimes stronger than those within the anxiety disorder group. It has often been argued that depressive disorders follow anxiety disorders and Kessler (1999) for example, again using data from the NCS, estimated that 10-15% of depression could be attributed to earlier social phobia. Obsessive compulsive disorder did not show elevated associations with the other anxiety disorders and there is continuing discussion as to whether it is best categorised as part of a separate group of disorders sometimes referred to as the obsessive compulsive spectrum disorders (Hollander & Wong, 1995). Patterns of association both between, and within, disorder groups have the potential to inform discussion of classification and aetiology of psychiatric disorders (see Bogenschutz & Nurnberg, 2000; Vella, Aragona, & Alliani, 2000).Top of page
Earlier in this chapter we presented findings from the ECA and the NCS that showed stronger associations between disorders within a six-month timeframe compared to over a person's lifetime. Andrews et al (2002) used data from the NSMHWB to compare associations between disorders within a 12 month and one-month timeframe and found a similar pattern. Eighty three per cent of odds ratios for current associations between pairs of disorders were higher than those for a twelve-month timeframe. We suggested that this replicated finding raises the possibility that the occurrence of one disorder can be affected by the presence of another disorder. That is, the presence of one disorder might generate symptoms in an individual that could meet criteria for another disorder, or be sufficient to convert a sub-threshold secondary disorder into one that met diagnostic criteria.
In summary, data from community surveys of the Australian population indicates that comorbidity as a general phenomenon is common. Like previous epidemiological surveys, the most common associations are between disorders from the same diagnostic groups and between anxiety and affective disorders. Comorbid associations between the common mental disorders and substance use disorders are less so.