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Epidemiological issues in comorbidity:lessons learnt from a pan-EuropeanISADORA projectA. Baldacchino a , N. Groussard-Escaffre b , C. Clancy c , C. Lack d

, K. Sieroslavrska e , C.-L. Hodges a , L.-B. Merinder f , T. Greacenb , M. Sorsa g , H. Laijarvi g & K. Baeck-Moller fa Centre for Addiction Research and Education Scotland (CARES),University of Dundee, Dundee, Scotland, UKb Laboratoire de Recherché de I'EPS Maison-Blanche, Paris, Francec Department of Mental Health, Middlesex University, London, UKd Department of Psychiatry, Cambridge University, Cambridge, UKe Institute of Psychiatry & Neurology, Warsaw, Polandf Psychiatric University Hospital, Aarhus, Denmarkg Department of Nursing Science, University of Tampere, Tampere,FinlandVersion of record first published: 02 Jun 2009.

To cite this article: A. Baldacchino , N. Groussard-Escaffre , C. Clancy , C. Lack , K. Sieroslavrska ,C.-L. Hodges , L.-B. Merinder , T. Greacen , M. Sorsa , H. Laijarvi & K. Baeck-Moller (2009):Epidemiological issues in comorbidity: lessons learnt from a pan-European ISADORA project, MentalHealth and Substance Use, 2:2, 88-100

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Epidemiological issues in comorbidity: lessons learnt from a

pan-European ISADORA project

A. Baldacchinoa*, N. Groussard-Escaffreb, C. Clancyc, C. Lackd, K. Sieroslavrskae,C.-L. Hodgesa, L.-B. Merinderf, T. Greacenb, M. Sorsag, H. Laijarvig and

K. Baeck-Mollerf

aCentre for Addiction Research and Education Scotland (CARES), University of Dundee,Dundee, Scotland, UK; bLaboratoire de Recherche de I’EPS Maison-Blanche, Paris, France;cDepartment of Mental Health, Middlesex University, London, UK; dDepartment of Psychiatry,Cambridge University, Cambridge, UK; eInstitute of Psychiatry & Neurology, Warsaw, Poland;fPsychiatric University Hospital, Aarhus, Denmark; gDepartment of Nursing Science, University

of Tampere, Tampere, Finland

(Accepted 19 March 2009)

Aims: This article sets out to identify the issues that are relevant to understandingthe current approaches used to determine the extent of the problems as a result ofco-existent substance misuse and mental health problems comorbidity1 in Europe.It is a fundamental prerequisite that it is based on robust epidemiologicalprocesses.Method: This article will describe an attempt to identify current data available in2002 on the prevalence of co-morbidity per 100,000 inhabitants in Poland(Warsaw), Denmark (Aarhus), Finland (Tampere), England & Wales (Londonand Cambridge), Scotland (Dundee) and France (Paris) as part of the ISADORAproject.Results: This exercise highlights methodological challenges in the epidemiology ofcomorbidity such as setting, subjects, intervention used and context, conceptual,units of contents, time window and accuracy of resolution and assessment methodused.Conclusion: These issues need to be resolved before representative informationcan be interpreted.

Keywords: comorbidity; dual diagnosis; epidemiology; European; mental health;substance misuse

Introduction

Epidemiological research in the field of comorbidity contributes in the understandingof differences between causation, association, relationships, consequences, and risk.Ultimately such accurate data will inform treatment interventions and servicedelivery, planning, and provision.

The Mental Health Action Plan for Europe, launched by the WHO RegionalOffice for Europe together with the European Commission in early 2005 (WHO,2005), acknowledged the issue of co-morbidity but this was, unfortunately, not given

*Corresponding author. Email: a.baldacchino@dundee.ac.uk

Mental Health and Substance Use: dual diagnosis

Vol. 2, No. 2, June 2009, 88–100

ISSN 1752-3281 print/ISSN 1752-3273 online

� 2009 Taylor & Francis

DOI: 10.1080/17523280902930130

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the appropriate focus for future action partly due to the lack of robust informationthat allows confident policies to be made.

The European Monitoring Centre for Drugs and Drug Addiction (EMCDDA)included a special section on co-morbidity in its annual report for 2004 and issued apolicy briefing on the subject in the 20 official EU languages and Norwegian(EMCDDA 2004a, 2004b) in order to help start a European process ofunderstanding this issue and learn from other countries. This is still in its embryonicstage and further incentives need to materialise in order to establish a Europeanagenda in co-morbidity.

Integrated Services Aimed at Dual Diagnosis and Optimal Recovery from Addiction(ISADORA) project

The ISADORA (Integrated Services Aimed at Dual Diagnosis and OptimalRecovery from Addiction) project was a pan-European project running over athree-year period from November 2002 to October 2005. The project was funded bythe European Commission under the research area ‘Generic Research on PublicHealth and Health Services and Health and Safety; Fighting drug related problems’.Seven sites across Europe collaborated on the project. The centres were:Establissement de Sante Maison-Blanche, Paris, France; University of Tampere,Finland; University of Dundee, Scotland; Institute of Psychiatry and Neurology,Warsaw, Poland; Middlesex University, London, England; Cambridge University,England; and Psychiatry in the County of Aarhus, Denmark.

The study is composed of six parts.

. A descriptive study of service options at the different European centres.

. A descriptive study of pathways through care for the comorbid cohortrecruited from acute psychiatric wards.

. A follow-up study evaluating social and clinical status and outcome of thesame cohort.

. An exploration of the views of staff and dual diagnosis clients concerningvulnerability factors and adequacy of service provision.

. A study of risk factors for the sample.

. Development of an educational programme for staff working with dualdiagnosis patients.

Another component of this project was to describe the actual information presenton the prevalence of co-morbid substance misuse and mental health problemswithin the same participant countries in order to help contextualise the abovemultipronged study. This article will describe the information collated.

Method

Identification of sources of information

It was recognised that partners at each ISADORA participant site wouldpotentially have different sources from which they can draw relevant information.Equally, we were aware that the policies and associated service provisions forindividuals accessing drug, alcohol, mental health and comorbidity services would

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also be different. This epidemiological exercise collated information from thefollowing.

. General population surveys – these surveys rely on the subjectivity of theresponse. They aim to measure the behaviour, attitudes, or opinions of thegeneral population with regard to the use or misuse of substances and presenceor absence of mental health problems. They are useful in yielding directmeasurement of the phenomenon; however, it is difficult to use them to identifyrelatively rare behaviour.

. Registries – national statistics enable us to estimate the use of harmfulsubstances. Other registries provide the type and extent of mental healthproblems (e.g. admissions to hospital, treatment-seeking opiate-dependentindividuals).

. Administrative statistics – these offer partial perspective to the phenomenon.They are focused to specifically identified and focused populations, and arenotorious in not identifying the ‘hidden population’.

. Qualitative studies – individual studies concern population subgroups that aredirectly identified by their subculture and not by the institution. These studieshelp describe types of behaviours but do not enable us to measure their scope.

. Grey literature from research databases.

. Indirect sources of information – the prevalence estimates are not concernedwith the problem of substance misuse or mental health problems but with, forexample, crime and homelessness, prison health surveys, health educationsurveys. The information is therefore an indirect product of a related butdifferent data set.

. Other sources – such as PhD dissertations – were also included.

Level and type of data collected

Since the information did not come from a standardised data set, it is clear thatdirect comparison of data between countries would be problematic. Allinformation collected included a description of the source, the year data refer to,the methods used, and any limitations/problems encountered. All data were viewedwith caution due to the numerous possible methodological problems present.

The terms ‘problematic drug and alcohol use and serious mental healthproblems’ were defined as in all ISADORA study stages: This was defined as theco-existence of at least one from F10 to F16, F18, F19 with at least one from F20,F23, F30 according to the International Classification Disorder (ICD-10) system(WHO, 1993). We do recognise that this definition excludes other conditions. AsCrome (1999) has suggested, comorbidity may be manifested as:

. substance use (even one dose) may lead to psychiatric symptoms orsyndromes;

. harmful use may produce psychiatric symptoms;

. dependence may produce psychological symptoms;

. intoxication from substances may produce psychological symptoms;

. withdrawal from substances may produce psychological symptoms;

. withdrawal from substances may lead to psychiatric syndromes;

. substance use may exacerbate pre-existing psychiatric disorder(s);

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. psychological morbidity not amounting to a ‘disorder’ may precipitatesubstance abuse;

. primary psychiatric disorder may lead to substance use disorder;

. primary psychiatric disorder may precipitate substance use disorder whichmay, in turn, lead to psychiatric syndromes.

The above ICD-10 definitions were used since it was felt by the ISADORAgroup that it was the accepted classification process used in all the participatingcountries health and social care system. This therefore allowed some standardisationwhen it came to recruiting and comparing individuals for the cohort aspect of thestudy.

Statistical information was gathered on the 2002 prevalence data on comorbidityin a population of 100,000 inhabitants. No statistical analytic modelling was used toidentify the potential prevalence rate. The mean prevalence of different studieslooking at the same population was taken and extrapolated to a 100,000 populationfigure.

Results

It was clear from the beginning of the survey that all the above descriptors wereconsidered as ambitious since the information was not available to complete. Inaddition, it was clear that some information available in 2000–01 and 2003–05 wasnot available in 2002. Some countries, such as Denmark and England, have moreinformation possibly due to more active data collection processes on substancemisuse and mental health-related problems. In other countries, such as France, datawere minimal or non-existent.

Denmark (county and city of Aarhus)

Denmark has a population of around 5.4 million people. Aarhus has a population ofaround 222,559 inhabitants. Data from the National Danish Psychiatric case registerfrom 2002 on comorbidity (Danmarks Statistik, 2002) demonstrates that 80,625adults were in inpatient or outpatient treatment in psychiatric services in Denmark.Of these, 9716 individuals were diagnosed with comorbid illness (11%). Of these,64% were male and 36% were female.

In relation to all inpatient admissions to Danish Psychiatric departments in 2002,28.7% were admissions of comorbid patients. The most prevalent age group was 35–40 years of age and the next largest group was 45–49 years of age. As a comparisonconcerning gender, the distribution of male/female patients presenting in emergencyrooms and admitted to a psychiatric ward as inpatients in Denmark in 2002 was 50/50, whereas males again dominated concerning entry to outpatient treatment incommunity psychiatry and concerning part hospitalisation (76%).

Schizophrenia and related illness (F20) was the most prevalent mental illness in2002 in the register data, followed by affective disorder (F30). Two other prevalentillness groups were nervous and stress-related disorders (F40), and personalitydisorders (F60). Alcohol was the most prevalent substance used, and over 50% of allcomorbid patients had an alcohol use diagnosis. A considerable group also hadmixed abuse (30%). In the main three diagnostic groups (F20, F30, and F60) alcoholis the most common substance used (39.1, 69.3 and 40.3%, respectively), and is

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highest in the group with affective disorder. In the F20 group (schizophrenia andrelated disorders), cannabis is the most common drug (15%), compared to the othertwo groups F30 (4.7%), and F60 (8.0%). Mixed abuse is also prominent in the F20group (24.1%), and more than in the affective group (11.1%). It is very common inthe F60 (personality disorder) group (24.6%).

Concerning inpatient admissions, under-registration of substance use is the casefor the data in the Psychiatric Case register. In Søberg Hansen’s study, it was foundthat only 13.7% of patients admitted to Danish Psychiatric departments in 2002 hada substance use diagnosis (Afdelingsleder, 2005). When patients due to be admittedwere interviewed in 12 Danish psychiatric departments, 52.3% of all patients withschizophrenia had a concurrent substance use problem. For affective disorders,10.8% were registered in the case register with a comorbid illness and 35.8% had acomorbid illness when interviewed at admission. These tendencies were true for alldiagnostic groups and the differences in under-registration between the diagnosticgroups were not shown to be statistically significant.

Concerning bed days, comorbid patients used 21.5% of the total number of beddays in psychiatric departments in 2002. Comorbid patients used 13.1% of alloutpatients’ visits in 2002 at Danish Psychiatric departments. As a result, peoplewith comorbidity are treated either in the psychiatric system or in the substanceabuse treatment system.

In the context of Danish psychiatry, people with ‘co-morbidity’ are usually definedas individuals who have a serious enduring mental illness (psychosis) and substanceuse problems. If between 30 and 50% of people with psychosis have a substance useproblem, then there is an estimated 10,000–15,000 individuals in Denmark withcomorbid conditions (Vendsborg, 2006). In some instances, ‘comorbidity’ is usedmore narrowly to define people with psychosis and injecting drug problems.The estimate will then be around 1000 comorbid individuals. When ‘comorbidity’ isused for people with substance abuse problems and any psychological/psychiatricdisorder, the estimation will total 100,000–200,000 of comorbid individuals. Table 1provides the number of people with mental disorders, substance abuse problems,and comorbidity in Denmark using the different definable thresholds (Vendsborg,2006).

The estimated prevalence of comorbidity in Denmark in 2002 using ISADORAdefinitions was 120 per 100,000 population compared to 161 per 100,000 population.The ratio between females and males is approximately 1:2. The most commonlydefined comorbidities are depression and alcohol, psychosis and alcohol, andpsychosis and substance misuse (Institute of Basic Psychiatric Research, 2002;Danmarks Statistik, 2002).

Table 1. Number of people with mental and/or substance abuse disorders in Denmark.

Mental disorderMental and substanceabuse disorders

Substance abusedisorder

Psychosis20,000–40,000

Psychosis and injectingopiate use 1000

Drug abuse20,000–30,000

Psychosis and substanceabuse 10,000–15,000

Non-psychosis400,000–800,000

Non-psychosis andabuse 100,000–200,000

Alcohol abuse200,000–400,000

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Poland (Mazowieckie Province and city of Warsaw)

Poland has a population of around 38.6 million people. Warsaw has a population ofaround 1.6 million inhabitants. An approach used by Swiatkiewicz (2006) is to use‘unspecific clients’ in specialist services as potentially comorbid cases. Every kind oftreatment service noted ‘unspecific cases’. Table 2 compares absolute numbers ofthese cases in 1998 and 2002. Depending on the units’ specialisation, diagnosistreated as unspecific is defined in different ways. For psychiatric units, unspecificmeans a client whose main diagnosis is alcohol- and/or drug-related. For alcoholunits, it means patient with drug-related and/or other psychiatric diagnoses, and fordrug treatment units the main code suggests alcohol-related and/or other psychiatricproblems (Swiatkiewicz & Zielinski 1998). The data of absolute numbers of‘unspecific’ cases in 1998 compared with 2002 are shown in Table 2 and are drawnfrom statistics from outpatient units.

In the 5-year period examined, the number of patients seeking treatment foralcohol problems and finally diagnosed as drug dependent or mentally ill rosealmost 7 times among all patients and more than 7 times among new outpatients.In all three types of treatment sectors, among the population who entered servicesfor the first time, the number of unspecific cases rose more spectacularly.New clients are usually younger than the average age of the treated population andtheir pattern of substance consumption is increasingly closer to current patterns inthe general population, e.g. a polydrug lifestyle. There is no other study to confirmor deny this dramatic increase in alcohol-, drug-, and mental health-relatedproblems within the psychiatric setting. Considering all the methodologicallimitations, the estimated prevalence of comorbidity for 2002 was 100 per100,000 population.

Finland (west Finland region, city of Tampere)

Finland has a population of around 5.2 million people. Tampere has a population ofaround 197,889 inhabitants. In Finland, data from the Hospital Discharge Registershow that the number of treatment periods associated with simultaneous drug-related and other mental health problems increased from 441 in 1987 to 2130 in 2001.Treatment periods for opiate use combined with psychiatric disorders have tripledsince 1996 (EMCDDA, 2004a).

According to hospital statistics, co-occurring drug-related and other psychiatricdiagnoses increased about fivefold (from 441 treatment episodes to 2130 episodes)during the period 1987–2002. The treatment episodes due to diagnoses of illicitopiate use and other psychiatric disorder(s) have tripled since 1996. Hospitalisations

Table 2. ‘Unspecific’ population in alcohol, drug and mental health facilities in Poland, 1998and 2002.

All outpatients New outpatients

1998 2002 1998 2002

Alcohol 3,298 21,819 1,615 13,187Drugs 2,781 9,624 610 5,283Mental health 15,529 38,331 5,337 13,291

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for comorbidity increased between 1987 and 2002 within every diagnosticcombination from 3- to 10-fold (Pirkola & Wahlbeck, 2004). The most significantincreases were the observed frequencies of diagnosed psychotic and affectivediagnosis combined with substance use diagnosis. The yearly number of clients withcomorbid substance use discharged increased from 441 to 2242 during the sameperiod (1987–2002). Considering all the methodological limitations, the prevalenceof comorbidity in 2002 is estimated at 42.6 per 100,000 population.

England (Cambridgeshire: Peterborough and Fenlands; and London: Barnet, Enfieldand Haringay)

England has an estimated population of 52 million. Table 3 provides more details onthe two ISADORA study sites.

Studies in the UK indicate that between 60 and 90% of substance misusers intreatment also have a mental health problem, and between 20 and 50% of those inmental health treatment have a substance misuse disorder. The OPCS NationalPsychiatric Morbidity Survey established a prevalence of drug misuse of 5% in thegeneral population and 10% in the acute psychiatric settings (Coulthard, Farrell,Singleton, & Meltzer, 2002).

A one-year prevalence rate of 32% with alcohol problems, and 16% with drugproblems were reported in 171 psychotic patients in South London (Menezes,Johnson, & Thornicrift, 1996). Within the same area, 10% of psychiatric patientswere found to have an alcohol problem, while 40% of patients with an alcoholproblem had comorbid psychiatric problems (Glass & Jackson, 1988). Of acutepsychiatric patients, 73.7% in London and 24% in Peterborough and Fenlandshad a history of substance misuse (alcohol, cannabis, and crack). In Cambridge-shire, the prevalence of comorbidity, as estimated through the ISADORAdefinitions, was approximately 18% for alcohol dependence, and 7% for drugdependence in the psychotic population. In Peterborough and Fenlands, 30% ofthe patients in the community alcohol team, 10% of patients in the communitydrug team, and 30% of the inpatient psychiatric population were thought to havecomorbid conditions. These are crude estimates and are likely to be an under-estimation.

A significantly higher proportion of Community Mental Health Team patientsfrom London centres reported problem drug use than those from Nottingham andSheffield (42 vs. 21%). Patients reporting problem drug use in the London centresalso reported past year use of a higher number of drug types than drug using patientsin either Nottingham or Sheffield (Weaver et al., 2003).

Duke, Pantelis, McPhillips and Barnes (2001) reported that in an epidemiologicalsurvey of South Westminster, London, where individuals with schizophrenia or

Table 3. Study sites in England.

Study areaStudy

population RegionRegionalpopulation

Cambridge Peterborough andpart of Fenland Area

200,000 Eastern 5.4 million?

Middlesex Barnet, Enfield and Haringey 205,000 London 1.7 million

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related psychoses were identified, 16% reported a lifetime history of non-alcoholsubstance misuse.

Among new clients entering drug treatment in the National TreatmentOutcome Research Study in England, females were twice as likely as males toreport anxiety (32 vs. 17%), depression (30 vs. 15%), paranoia (27 vs. 17%), andpsychoticism (33 vs. 20%) in the previous 90 days (Marsden, Gossop, Stewart,Rolfe, & Farrell, 2000). The British COSMIC study found that three-quarters ofclients in inner-city drug treatment centres suffered from one or more mentaldisorders: the most common being depression and/or anxiety disorder (68%),personality disorders (37%), and psychotic disorders (8%; Weaver, Charles,Madden, & Renton, 2002).

Fisher, Collins, Millson, Crome and Croft (2004) studied diagnosed psychiatricillness and substance misuse in 230 general practices in England and Wales from1993 to 1998. A comorbid case was defined as one with diagnosed substance misuse(not including alcohol and tobacco) and psychiatric illness within a calendar year. Apotentially chronic comorbid case was one that met this definition and in additionwas treated in subsequent years for either psychiatric illness or substance misuseconditions. The study demonstrated that the annual period prevalence increasedevery year from 1993 to 1998 in all psychiatric groups (e.g. psychosis, schizophrenia,paranoia, neurosis, and personality disorder). The estimated number of comorbidcases in England and Wales rose from 24,226 to 39,296. Working on the assumptionthat this increase was to continue at the same rate over the next 5 years, we canestimate that 53,268 patients would have been seen in 2003. There was an increase inall age groups (except 65–75 years) in men (79% increase) and women (44%increase), and the average age at primary diagnosis decreased from 38 to 34 years.There was a 147% increase in psychoses, 128% in schizophrenia, 144% in paranoia,62% in neuroses, and 55% in personality disorder. A very high proportion (80%) ofcomorbid cases were newly diagnosed in each study year, indicating a rapid increasein the size of the cohort.

The annual period prevalence of comorbid drug use rose during this time from18.7/100,000 patient years of exposure (PYE) to 36.6/100,000 PYE; the rate forcomorbid drug dependence increased from 26.1/100,000 PYE to 49.6/100,000 PYE.Although comorbid licit drug dependence rose from 10.6/100,000 PYE in 1993 to16.1/100,000 in 1995, it then fell to 10.8/100,000 PYE in 1998. Considering all themethodological limitations the estimated prevalence of comorbidity in 2002 was 76.1per 100,000 population.

Scotland (Tayside and Dundee)

Scotland has a population of around 5 million people. Tayside has a population ofaround 380,000 and Dundee around 100,000. At a national level, 21% of femalepsychiatric treatment population and 32% of the male population were comorbid.In the drug treatment population, 40% of males and 42% of females presentedwith mental health problems. The most common comorbid combinations atnational, regional, and local levels were alcohol and depression, alcohol andanxiety, and diazepam and anxiety (Scottish Advisory Committee on Drug/Alcohol Misuse (SACDAM), 2002). Considering all the methodological limita-tions, the estimated prevalence of comorbidity in 2002 was 91.86 per 100,000population.

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France (north central region of France and city of Paris)

France has a population of around 58.5 million people. Paris has a population ofaround 2.1 million. There was no information available that will allow an estimationof prevalence rate per 100,000 population.

Discussion

The aim of this initial epidemiological survey was to gather statistical information onthe 2002 prevalence of problematic drug and alcohol misuse, problematic andserious mental health problems, and comorbidity per 100,000 inhabitants withineach participant ISADORA countries.

Methodological issues in the epidemiology of comorbidity

This survey has highlighted a number of methodological difficulties. Comparisonsbetween different data sets are problematic for a number of reasons (Crawford,Crome, & Clancy, 2003; Todd et al., 2004; Wittchen, 1996). Considerable confusionand misleading information has arisen from many studies of comorbidity because ofloose definitions of the term (EMCDDA, 2004a; WHO, 1994). Moreover, confusionhas arisen because many research groups only vaguely describe specific diagnosticalgorithms. Complex sets of symptom, syndrome, and diagnostic exclusions – asemployed by the DSM-III-R (American Psychiatric Association (APA), 1987)DSM-IV (APA, 1994) and ICD-10 (WHO, 1993) – might all affect the resultingcomorbidity figures as well as their interpretation (Wittchen, 1996). Furtherproblems with defining comorbid populations is that, historically, mental healthand substance misuse services have evolved independently, using different terms andmodels to inform their service policies and objectives (Todd et al., 2004). The typeand number of diagnostic classes examined in a study can also influence comorbidityfindings. The greater the number of prevalent diagnoses considered in the analysis,the greater the probability of chance association (Wittchen, 1996).

The rates for comorbidity are also dependent on the time-scale of the assessmentfor each disorder. Some studies tend to limit the term comorbidity to pure cross-sectional syndromes and disorders, whilst others prefer a lifetime-ever approach(Wittchen, 1996).This aspect can be further complicated by the fact that diagnosismay change over time at an individual level (Todd et al., 2004).

The assessment method used to examine comorbidity can affect results.A comparison of the ICD-10 (WHO, 1993) and the Composite InternationalDiagnostic Interview (CIDI; Robins et al., 1988) suggests that two to three times asmany diagnoses as the clinician would assign in routine diagnostic assessment arerevealed by standardised instruments. This is particularly true for substance usedisorders (Francis, Widinger, & Fyer, 1990; Kessler et al., 1994).

The determination of prevalence rates depends on factors such as the competenceof the assessment team to elicit information and to interpret it. A detailed interviewlasting 1–2 h by a research nurse trained for the used assessment method will providemore accurate information than a brief interview with a key worker. Insufficientlytrained key workers tend to overestimate and/or underestimate drug misuse (Brown,1998). In addition, the clinical situation in which the assessment takes place isimportant. Many studies among substance abusers diagnose psychiatric disorders

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early in treatment or on entering treatment, when withdrawal symptoms or acuteintoxication may result in high levels of anxiety and depression (Verheul et al., 2000).

Studies may have comorbidity as their primary research area, or this may just bea secondary research question within a larger study. This will affect the type ofpopulation being studied. Research studies might exclude substance use patients andthose with severe and enduring mental illness. It is also necessary to distinguishbetween the different clinical populations studied. In general psychiatric services,alcohol and cannabis are more likely to be encountered as the comorbid dimension,whereas in addiction services, depressive, anxiety, and personality disorders aregoing to be the additional problems most commonly reported (Todd et al., 2004).

The identification of control subjects is equally important, since one can thencompare the epidemiological information from the study group with similarpopulations experiencing the same socio-cultural issues.

Research on ‘upstream’ factors has been called for by Abou-Saleh and Janca(2004), particularly those which operate at a population level, for example, housing,income, political systems and associated policies that impact on dual diagnosisservice delivery. Even with these difficulties it is clear that the guestimations ofcomorbidity range from 43 to 120 per 100,000 population.

Limitations of the study

These results highlight our current incapacity to identify the level of comorbidity inthe European community by population, age, morbidities, and region. We do notknow if these differences are due to methodological issues or actual populationvariations in morbidities or difference in systems providing services to mental healthand/or substance misuse-related problems. This will then bring us to hypothesisethat such figures could be a factual recognition that such problems do exist, butunable to state if they are over- or under-estimations. Again, the choice of definingcomorbidity must also be considered.

Strict comparisons between countries are impossible given the methodologicallimitations identified during this survey. Worse still, in some countries there are noteven the basic data collection processes available to identify comorbid treatmentevents. Thus, Table 4 can only be used as an aide memoire of prevalence levelsestimated.

Studies of substance misuse and mental health problems and associatedcomorbidity have focused primarily upon populations that are easily accessibleand/or treatment-seeking. Estimates of substance misuse have been made usingmortality and cross-sectional crime surveys, a capture/recapture approach, small-scale and non-representative surveys, or surveys of school children. Others have

Table 4. Alcohol, substance misuse, serious (psychotic) mental health problems andcomorbidity per 100,000 population at the national levels for the year 2002 across theISADORA sites.

Denmark Finland Poland England Scotland France

Estimated numberof population withco-morbidity

120 43 100 76 92 N/A

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utilised qualitative methods. Such approaches all provide important insights intoaddictive problems, but are of very limited value for investigating policy outcomes.Causes and consequences cannot be teased out without longitudinal data. Surveyscannot provide geographical detail and qualitative studies cannot be generalisable.Furthermore, issues relating to difficult to reach populations (e.g. the homeless andcomorbid individual) receive inadequate attention. There is a clear need forepidemiological studies of the prevalence of comorbidity and the development of aregularly updated, ‘live’ database of services available to the comorbid population inEurope.

Relevance in understanding better comorbid conditions

There are several reasons why it is important to have good community-based studiesof joint psychiatric and substance misuse morbidities (Abou-Saleh & Janca, 2004).In order to have policy that is effective, it is a fundamental prerequisite that it isbased on epidemiology and evaluation of the social and economic consequences ofdual diagnosis. It is also desirable for both planning health and social services tohave representative information for a geographical area. The possibility ofmonitoring the health of a population and trends in disease, as well as changes inpotential risk factors, can be facilitated through the repetition of community surveys.

The hypotheses about the aetiology of comorbidity can be generated and testedthrough the gathering of valid information on prevalence and associated factors ofpresumed causal importance. Longitudinal studies of the evolution of onsetsubstance use and other psychiatric disorders over time are needed, together withcross-sectional studies of the patterns of their inter-relationships using diagnosticinstruments with high reliability that can generate more specific DSM-IV-like (APA,1994) comorbid diagnoses.

Conclusion

Creating innovative and practical ways of collating a European data set on theepidemiology of comorbid mental health and substance misuse issues will help studyfurther the different patterns of comorbidity issues as a result of alcohol,nicotine, cannabis and/or psycho-stimulant use, misuse, harmful use and depen-dence, and associated mental health problems. It will start to help understand theprevalence, type, and impact of comorbidity present in different settings, e.g. prisonand varying medical (psychiatric, general medical, and general practice) populations.In addition, it will start to identify the relationships between comorbidity andcriminal behaviour. Moreover, cross-linkage of information will help in thediscussion of understanding the relationship between suicide and drug relateddeaths.

Acknowledgements

The ISADORA study was supported by the European Commission, the Fifth FrameworkProgramme, Cordis FP5 (Project QLG4-CT-2002-00911). We also wish to acknowledge allother researchers and administrators involved in the ISADORA project.

Note

1. Comorbidity and dual diagnosis will be used interchangeably.

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