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Interest in the comorbidity of mental disorders has gained momentum in the last decade. The 2 most common mental disorders in North America are major depression and substance dependence. Not surprisingly, these disorders tend to coexist, particularly in treatment-seeking patients (1–3). There remains some uncertainty regarding the true frequency of cooccurring major depression and SUDs in the general population. Studies conducted over the last 15 years have produced a range of comorbidity prevalence rates (1,4,5). The original NCS in the US found that 18% of individuals with a 12-month diagnosis of MDD had at least one SUD (6). Conversely, about 23% of persons with a 12-month SUD reported major depression during the same period. These estimates were slightly lower than reported from the ECA study conducted earlier (7). Comorbidity rates in the 2001 NCS-R fell between the NCS and ECA rates (6), although the rates for specific SUDs (that is, alcohol or drugs) have yet to be released. In the most recent US study, the NESARC (1), the prevalence of MDD in individuals with a 12-month SUD ranged from 8% for alcohol abuse to 40% for drug dependence. Among individuals with a 12-month MDD, the prevalence rate of SUDs ranged from 8% for alcohol abuse to 19% for any SUD. One explanation for the variability is the fact that the diagnostic criteria for MDD and the SUDs have changed over time. This would affect the prevalence rates for individual disorders and possibly amplify the effect for comorbidity. Second, epidemiologic surveys have used different instruments. Improvements in the CIDI, resulting in fewer false positives, are thought to account for the lower frequency of comorbidity in the NCS-R (4). Further, direct comparisons of diagnoses derived from various epidemiologic interviews have produced different SUD prevalences in the same sample (8,9). Third, surveys have differed in their management of overlapping symptoms. Many of the symptoms of intoxication and withdrawal can mimic the symptoms of major depression. Most household surveys do not include the necessary probe questions to determine whether symptoms are substance- induced or part of an independent disorder. The NESARC purportedly used a better interview (the AUDADIS) to disentangle overlapping symptoms present in both disorders. Investigators concluded that fewer than 1% of mood disorders are actually substance-induced (1). For most SUD cases, the mood disorder persisted during periods of abstinence or began before the onset of problematic substance use. Until the release of the CCHS 1.2, there were no national population data on comorbidity in Canada. Early epidemiologic studies were limited to Edmonton (10) and Ontario (11). The specific objectives of this analysis were as follows: 1. To estimate the 12-month prevalence of MDD within persons with a 12-month diagnosis of harmful alcohol abuse, alcohol dependence, and drug dependence. 2. To estimate the 12-month prevalence of alcohol dependence, harmful alcohol abuse, and drug dependence in persons with a 12-month and lifetime diagnosis of MDD. 3. To identify socioeconomic correlates of SUD–MDD comorbidity. 4. To determine the impact of comorbidity on reporting of suicidal thoughts. 5. To determine the impact of comorbidity on mental health service use. MethodsThe CCHS 1.2 Detailed descriptions of the CCHS 1.2 in terms of target population, sampling procedures, response rate, and psychiatric assessment are provided in a separate article in this issue (12). Briefly, the CCHS 1.2 was a cross-sectional survey of a nationally representative sample of individuals aged 15 years and over (13). The CCHS 1.2 data were collected by Statistics Canada between May and December 2002 (n = 36 984). Major Depression A Canadian adaptation of the WMH-CIDI (14) assessed DSM-IV–defined major depressive episodes. For the present analysis, our interest was in the lifetime and 12-month presentation of unipolar MDD. Hence, individuals with bipolar disorder are not included in the prevalence. Substance Use Disorders Twelve-month, but not lifetime, diagnoses for the SUDs were assessed in the CCHS 1.2. The WMH-CIDI’s assessment of alcohol dependence was based on DSM-III-R criteria (15). The alcohol dependence scale from the CIDI Short Form was tested extensively during the NCS (6,16). Alcohol dependence, as defined in the CCHS 1.2, represents a purported 85% predictive cut point, which corresponds to reporting at least 3 symptoms from the DSM criteria. Unfortunately, the CCHS 1.2 did not provide full coverage of DSM symptoms for alcohol abuse. Therefore, we developed a separate algorithm for defining harmful alcohol use on the basis of ICD-10 criteria (17). These criteria include the following: 1) alcohol use contributing to physical or psychological harm, leading to disability or adverse consequences; 2) the nature of the harm is identifiable; 3) the pattern of use persisted for at least 1 month; and 4) symptoms do not meet criteria for alcohol dependence. The CCHS 1.2 provided complete coverage of these symptoms. The illicit drug section of the CCHS 1.2 did not provide sufficient coverage of symptoms to define a category of harmful drug use but did cover all the DSM-IV items for drug dependence. The specific substances covered were cannabis, cocaine, amphetamines, MDMA, hallucinogens, solvents, heroin, and steroids, although our analysis focused on any drug dependence. Other Measures Other variables included in this analysis are demographics, suicidal thoughts in the past 12 months, and use of any health services, specifically for a problem concerning emotions, mental health, or use of alcohol and drugs in the past 12 months. Statistical Analysis All statistical analyses were carried out with SAS/STAT 8.02 (18); we used the sampling weights and bootstrap weights provided by Statistics Canada to account for the complex sampling procedure. Comorbidity estimates derived from subsamples of respondents who provided valid responses to major depression and the specific SUDs under investigation. For each analysis, we used the full sample available. The rate for missing data owing to nonresponse or refusal in the CCHS 1.2 was low (< 1%). Consistent with other epidemiologic studies in the area, comorbidity was defined as major depression and an SUD occurring within the same 12-month time period (1). ResultsThe individual prevalence rates for SUDs (12-month), MDD (12-month and lifetime), and associated comorbidity are provided in Table 1. Rates of MDD were 2 to 4 times higher in persons with an SUD, compared with the general population. The CCHS 1.2 results are displayed with comparison data from the original NCS (6) and NESARC (1) in Figures 1 and 2. Lifetime prevalence was available only for MDD. In persons with no history of MDD, the 12-month prevalence of alcohol or drug dependence was 2.9%, compared with 4.8% for persons with a lifetime MDD history. The lifetime MDD prevalence in persons with 12-month SUD diagnosis was 13.9% for harmful alcohol use, 16.3% for alcohol dependence, 21.2% for drug dependence, and 16.9% for alcohol or drug dependence. Note that the comorbidity estimates for the alcohol or drug dependence category represent a weighted proportion of the individual alcohol and drug dependence estimates (thus the value falls between the alcohol and drug dependence estimates, instead of being higher).
In logistic regression modelling, the 12-month prevalence of MDD was predicted by the demographic variables of age, sex, and marital status (income, education, and urban vs rural residence were not significant predictors). In the presence of these variables, an SUD increased the chances of MDD by factors ranging from 2.1 (harmful alcohol use) to 4.3 (drug dependence). No interactions were found between demographics and the specific SUDs in predicting MDD. These results are summarized in Table 2.
The impact of cooccurring SUD and MDD on reports of suicidal thoughts was similarly studied through logistic modelling. As shown in Table 3, MDD, alcohol dependence, and drug dependence all independently predict suicidal thoughts in the past 12 months, with MDD being the strongest predictor. The SUD–MDD interaction terms in the models were not significant; however, the fitted proportions from the reduced model indicated that the risk of suicidal thoughts was indeed higher in comorbid subjects. For example, individuals experiencing both MDD and alcohol dependence in the last 12 months were at least 20 times more likely to have suicidal ideation, compared with individuals with neither condition. The same trend emerged with drug dependence. Harmful alcohol use, however, was not associated with suicidal thoughts.
A similar pattern emerged with mental health service use. Alcohol dependence, drug dependence, and MDD all independently predicted greater use of mental health services in the last 12 months (ORs 3.1, 6.4, and 14.9, respectively), compared with persons without these disorders. Harmful alcohol use was not associated with greater mental health services, and no SUD-by-MDD interactions were found. DiscussionConsistent with other population surveys (1,4,6), persons with major depression are more likely to report an SUD than are persons with no depression. Further, risk of harmful alcohol use and alcohol dependence is 2 times greater for persons with major depression. Drug dependence was 4 times more likely in individuals with major depression and increased the chances of reporting suicidal thoughts by a factor of 5. The lifetime prevalence of major depression in persons with alcohol or drug dependence was about 17%, compared with about 11% in persons with no substance dependence disorder. No demographic correlates of MDD–SUD comorbidity emerged among those studied. Harmful alcohol use and substance dependence independently predicted MDD with no interaction with sex, age, or marital status. We found no significant interaction between SUD and MDD in predicting suicidal thoughts or mental health service use. Consistent with other research (19), the presence of both conditions during the same 12-month period increased the prevalence of suicidal thoughts in a multiplicative fashion. Estimating the exact frequency of suicidal thoughts associated with comorbidity is difficult because of the lack of statistical precision in the fitted proportions. Nevertheless, risk of suicidal thoughts in persons with both substance dependence and MDD may be dramatically higher than in persons with neither condition. A similar trend was observed with mental health service use. The clinical implication of these results is that comorbidity is associated with greater psychological distress than either condition alone. Fortunately, this appears to be directing individuals to seek professional assistance. Harmful alcohol use was neither associated with suicidal thoughts nor with mental health service use. The latter finding is not surprising given that persons with alcohol abuse disorders are not likely to seek treatment (20). Conversely, harmful alcohol use is associated with a twofold increase in MDD, suggesting that this is a vulnerable group for mental illness that is unlikely to be identified by mental health providers. Further, the prevalence of harmful alcohol use is more than 2 times higher than the prevalence of dependence, indicating a large group of at-risk individuals who are unlikely to seek treatment. Many of the comorbidity estimates in the CCHS 1.2 are noticeably lower than estimates reported in US surveys. For example, the rates of alcohol and drug dependence in persons with MDD were less than one-half the rates reported in the NCS and NESARC. We can only speculate on the reasons for these differences. The simplest explanation is that comorbidity is less common in Canada than in the US. Both 12-month and lifetime rates of major depression are lower in the CCHS 1.2, compared with the NCS and the NESARC (1,21). Conversely, the prevalence rates of the SUDs are similar to the NESARC findings (2.6% compared with 3.8% and 0.8 compared with 0.7% for alcohol and drug dependence, respectively). Socioeconomic differences between the US and Canada or between the study samples could account for the discrepant prevalence rates in MDD and its associated comorbidity. Another reason could be methodological. The CCHS 1.2 and the NESARC employed different diagnostic instruments (the CIDI and the AUDADIS). In direct comparisons, the AUDADIS has produced higher rates of alcohol dependence than the CIDI (8,9) in the same sample, most likely because of the stronger focus on alcohol in the AUDADIS. Finally, the CIDI used in the CCHS 1.2 relied on the DSM-III-R criteria for alcohol dependence, whereas the NCS-R and NESARC employed DSM-IV criteria. Although the absolute prevalence rates between Canada and the US diverge, the pattern of results is similar. The prevalence of major depression within alcohol and drug dependence is higher than the prevalence of alcohol and drug dependence within major depression. Further, drug dependence was associated with the highest rate of concurrent MDD across all surveys (1,6). The prevalence of MDD in individuals with drug dependence was almost twice the rate in individuals with alcohol dependence. Although cause and effect cannot be assumed in this association, it is possible that persons with more psychopathology gravitate toward harder drugs as a form of self-medication. Further analysis of the CCHS 1.2 may shed some light on the relation between drug dependence and major depression. Nevertheless, the elucidation of etiologic factors will be limited, given that the survey is cross-sectional. Notable limitations of this analysis are the absence of any lifetime prevalence estimates for the SUDs, the use of our own algorithm for defining harmful alcohol use, and the limited coverage of drugs in the assessment of dependence. The absence of lifetime questions concerning substance use precluded any examination of the temporal priority of MDD or SUD in age of onset. Other epidemiologic data suggest an earlier onset of major depression, compared with problematic substance use (4,6), but we are unable to corroborate this finding with Canadian estimates. Our definition of harmful alcohol use complies with the content of the ICD-10 diagnostic criteria, but it has not been validated. The symptom profile of harmful alcohol use is similar to alcohol abuse (17); however, direct comparisons of diagnoses deriving from the criteria have yielded poor concordance (8,9). Further, the interrater reliabilities for the alcohol abuse and harmful use categories have been historically poor (22). Finally, the coverage of the CCHS 1.2 drug module was limited to illicit drugs; hence, abuse of prescription narcotics and sedative drugs was not captured. This is unfortunate because Canada is the largest per capita consumer of codeine and the third largest per capita consumer of morphine in the world (23). Substance dependence has been shown to complicate the course of major depression, diminish treatment response, increase the chronicity of the mood disorder, and increase the level of psychological stress experienced by the individual (2,24). Risk of MDD and suicidal thoughts appears to increase with more severe involvement in substance use (lowest for harmful alcohol use, intermediate for alcohol dependence, and highest for drug dependence). Clinicians should routinely assess for SUDs in depression patients. Greater integration of mental health and addiction treatment services could improve the management of both disorders. DisclaimerSome of the data upon which the analyses contained in this paper derive from surveys conducted by Statistics Canada. The opinions expressed in this paper do not represent the opinions of Statistics Canada. Funding and SupportThis project was supported by an operating grant from the Canadian Institutes of Health Research. References1. Grant BF, Stinson FS, Dawson DA, Chou SP, Dufour MC, Compton W, and others. Prevalence and co-occurrence of substance use disorders and independent mood and anxiety disorders. Arch Gen Psychiatry 2004;61:807–16. 2. Galbaud du Fort G, Newman SC, Boothroyd LJ, Bland RC. Treatment seeking for depression: role of depressive symptoms and comorbid psychiatric diagnoses. J Affect Disord 1999;52:31–40. 3. RachBeisel J, Scott J, Dixon L. Co-occurring severe mental illness and substance use disorders: a review of recent research. Psychiatr Serv 1999;50:1427–34. 4. Kessler RC, Berglund P, Demler O, Jin R, Koretz D, Merikangas KR, and others. The epidemiology of major depressive disorder: results from the National Comorbidity Survey Replication (NCS-R). JAMA 2003;289:3095–105. 5. Schuckit MA, Tipp JE, Bucholz KK, Nurnberger JI Jr, Hesselbrock VM, Crowe RR, and others. The lifetime rates of three major mood disorders and four major anxiety disorders in alcoholics and controls. Addiction 1997;92:1289–304. 6. Kessler RC, Nelson CB, McGonagle KA, Edlund MJ, Frank RG, Leaf PJ. The epidemiology of co-occurring addictive and mental disorders: implications for prevention and service utilization. Am J Orthopsychiatry 1996;66:17–31. 7. Regier DA, Farmer ME, Rae DS, Locke BZ, Keith SJ, Judd LL, and others. Comorbidity of mental disorders with alcohol and other drug abuse. Results from the Epidemiologic Catchment Area (ECA) study. JAMA 1990;264:2511–8. 8. Cottler LB. Concordance of the DSM-IV alcohol and drug use disorder criteria and diagnoses as measured by AUDADIS-ADR, CIDI, and SCAN. Drug Alcohol Depend 1997;47:195–205. 9. Hasin D, Grant BF, Cottler L, Blaine J, Towle L, Ustun B, and others. Nosological comparisons of alcohol and drug diagnoses: a multisite, multi-instrument international study. Drug Alcohol Depend 1997;47:217–26. 10. Bland RC, Orn H, Newman SC. Lifetime prevalence of psychiatric disorders in Edmonton. Acta Psychiatr Scand 1988;77(S338):24–31. 11. Offord DR, Boyle MH, Campbell D, Goering P, Lin E, Wong M, and others. One-year prevalence of psychiatric disorder in Ontarians 15 to 64 years of age. Can J Psychiatry 1996;41:559–63. 12. Gravel R, Béland Y. The Canadian Community Health Survey: Mental Health and Well-Being. Can J Psychiatry 2005;50:573–9. 13. Statistics Canada. Canadian Community Health Survey, Cycle 1.2: Mental Health and Well-Being user guide. Ottawa (ON): Statistics Canada; 2004. 14. Kessler RC, Ustun TB. The World Mental Health (WMH) Survey Initiative Version of the World Health Organization (WHO) Composite International Diagnostic Interview (CIDI). Int J Methods Psychiatr Res 2004;13:83–121. 15. American Psychiatric Association. Diagnostic and statistical manual of mental disorders. 3rd edition-revised. Washington (DC): American Psychiatric Assoc; 1987. 16. Kessler RC, Andrews G, Mroczek D, Ustun B, Wittchen HU. The World Health Organization Composite International Diagnostic Interview-Short Form (CIDI-SF). Int J Methods Psychiatr Res 1998;7:171–85. 17. Hasin D. Classification of alcohol use disorders. Alcohol Res Health 2003;27:5–17. 18. SAS Institute. SAS/STAT user’s guide. Cary (NC): The SAS Institute; 1994. 19. Grant BF, Hasin D. Suicidal ideation among the United States drinking population: results from the National Longitudinal Alcohol Epidemiologic Survey. J Stud Alcohol 1997;60:422–9. 20. Babor TF, Higgins-Biddle JC. Brief intervention: for hazardous and harmful drinking–a manual for use in primary care. Geneva, Switzerland: WHO Department of Mental Health and Substance Dependence; 2001. 21. Patten SB, Wang J, Williams JVA, Currie SR, Beck CA, Maxwell CJ, and others. Descriptive epidemiology of major depression in Canada. Can J Psychiatry. Forthcoming. 22. Ustun B, Compton W, Mager D, Babor T, Baiyewu O, Chatterji S, and others. WHO study on the reliability and validity of the alcohol and drug use disorder instruments: overview of methods and results. Drug Alcohol Depend 1997;47:161–9. 23. Joranson DE, Ryan KM, Jorenby JP. Availability of opioid analgesics in Romania, Europe, and the world. Madison (WI): University of Wisconsin Pain and Policy Studies Group/WHO Collaborating Center for Policy and Communications in Cancer Care; 2003. 24. Hasin D, Liu X, Nunes E, McCloud S, Samet S, Endicott J. Effects of major depression on remission and relapse of substance dependence. Arch Gen Psychiatry 2002;59:375–80. Author(s)Manuscript received and accepted May 2005. 1. Adjunct Associate Professor, Departments of Psychiatry and Psychology, University of Calgary, Calgary, Alberta. 2. Associate Professor, Departments of Community Health Sciences and Psychiatry, University of Calgary, Calgary, Alberta. 3. Research Associate, Department of Psychiatry, University of Calgary, Calgary, Alberta. 4. Assistant Professor, Departments of Community Health Sciences and Psychiatry, University of Calgary, Calgary, Alberta. 5. Assistant Professor, Department of Psychiatry, University of Calgary, Calgary, Alberta. 6. Professor, Department of Psychiatry, University of Calgary, Calgary, Alberta. 7. Associate Professor, Department of Community Health Sciences, University of Calgary, Calgary, Alberta. Address for correspondence: Dr SR Currie, Addiction Centre, Foothills Medical Centre, 1403–29th St NW, Calgary, AB e-mail: scurrie@ucalgary.ca
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