Epidemiology of Mental Illness Research Paper

Originally, epidemiology and epidemiological research designs were developed to study chronic and infectious diseases, and have only been adapted slowly for use in mental and behavioral disorders since the 1950s, largely due to the initially controversial status of past diagnostic classification systems for mental disorders. The complex manifestations, poorly understood etiologies, and variability of the course of mental disorders, in addition to their previously low diagnostic reliability, were often difficult to capture in basic epidemiologic designs. This difficulty was particularly true for those involving one or two points in time (e.g., cohort studies). In addition, risk factors for mental disorders can be as difficult to conceptualize and assess as variables of relevance for mental disorders. Despite early difficulties, these problems have been partly overcome in recent years.

Strongly related to the introduction of more reliable and valid classification systems for mental disorders, based on explicit criteria and operationalized diagnoses in 1980 (APA 1980, 1994) and the increasing availability of structured and standardized diagnostic assessment instruments, the last two decades of the twentieth century witnessed an unprecedented progress in epidemiological research on mental disorders. The field has advanced in terms of number of studies, their scope, degree of methodological sophistication, and their linkages to allied disciplines such as psychology, neurobiology, and sociology (see Table 1). What started in the 1950s as a scientifically problematic and quite restricted area of ‘psychiatric epidemiology’ (Robins 1992) has now reached firmer ground and opened up new perspectives.

     Tab. 1

1. Descriptive Epidemiological Findings

Starting with the landmark Epidemiological Catchment Area (ECA) program in the early 1980s (Robins and Regier 1991), increasingly sophisticated largescale studies in the general population have made it evident that mental disorders are very frequent disorders of the brain, affecting almost every person over their life courses. These studies have also highlighted the correlates and the variability in the manifestations of disorders of emotion, cognition, and behavior, demonstrating that mental disorders are not as uniform as previously believed in terms of their risk factors, courses, outcome, associated disabilities, and impairments.

1.1 Pre alence and Correlates of Mental Disorders in the Community

Community surveys in the 1990s, such as the National Comorbidity Survey (NCS) (Kessler et al. 1994), the Health Examination and Interview Survey—Mental Health Supplement (Wittchen et al. 1999) and similar studies in other countries (WHO 2000) have estimated the lifetime rates for mood, anxiety, and substance disorders alone to range between 36.3 percent and 48.6 percent. Across studies point-(30-day) prevalence estimates vary between 10–17 percent, in spite of differences in coverage of diagnoses, design, and cultural setting. Relatively stable estimates were also found for specific disorders, such as panic disorder (lifetime estimates: 3–4 percent, point prevalence: 1–2 percent) as well as psychotic disorders (1–3 percent).

Similar convergent evidence has also become available from these studies with regard to sociodemographic correlates and impairment. For example: (a) The majority of people affected by a mental disorder usually report the onset of their condition in adolescence, largely due to a predominant early onset of anxiety and substance-use disorders (alcohol, drug abuse, and dependence), whereas depressive disorders occur at higher ages and over the whole lifespan; (b) Females are 2–3 times more frequently affected by various depressive and anxiety disorders; (c) Males more frequently develop substance-use and antisocial personality disorders; (d) Low social class has not been confirmed consistently as being associated with the onset of mental disorder in all countries; and (e) The presence of mental disorders has been found to be associated consistently with increased rates of disability and impairment days, which vary by type of disorder and comorbidity.

Data also reveal (WHO 2000) that only about 22 percent of cases in Canada and the USA, and only slightly more in the Netherlands (31.7 percent) and Germany (29.2 percent) receive any type of treatment. The vast majority in the three countries were cared for exclusively in the primary health care sector, with only few receiving treatment by mental health specialists, even though treatment was merely defined as ‘any treatment contact,’ irrespective of appropriateness in terms of type, dose, and duration of treatment. A further disturbing finding is the fact that the majority of patients delay many years after the first onset of their disorder before getting treatment (Olfson et al. 1998). It is not entirely clear to what degree these low treatment rates are due to patients’ poor helpseeking behavior, structural barriers in the health care system, or health service providers’ lack of recognition and diagnostic skills. However, the fact that low treatment rates are not confined to countries that do not cover health care and treatment cost by insurance plans suggests that there are many reasons for this problem. Another important key finding of studies is that, even in community samples, a high proportion of subjects suffer from more than one mental disorder. In the NCS (Kessler et al. 1994), the vast majority of people with one mental disorder had other comorbid conditions as well; 54 percent of the population with a lifetime mental disorder had three or more lifetime comorbid conditions, and among those with 12-month diagnoses 59 percent had three or more additional disorders. Most frequent patterns of comorbidity were among anxiety, affective, and substance-use disorders. The consistency of comorbidity findings across studies has made it clear that comorbidity is a fundamental characteristic of most mental disorders. Comorbidity has been shown to be neither simply an artifact of random association of frequent disorders, nor influenced by helpseeking behavior or methodological aspects of the studies (Wittchen 1996). Comorbidity has specific effects on the degree of suffering as well as the likelihood of seeking professional help. Further comorbidity has been demonstrated to have important etiological, pathogenetic, clinical, and psychopathological implications that have become a major research topic in mental health research (Wittchen et al. 1999a). Beyond the mere demonstration of the size of the problem of mental disorders, there is now increased national and international collaboration on the basis of more stringent methodologies and designs, which allow for powerful coordinated cross-national reanalyzes with regard to risk factors as well as worldwide studies such as the ongoing World Mental Health 2000 study (Kessler 1999). The significance of such endeavors can be highlighted by findings of The CrossNational Collaborative Group (1992) that demonstrated in powerful analyses that the rates of depressive disorders are increasing in each successively younger birth cohort in industrialized countries. In addition, age of first onset of depressive disorders had declined into early adolescence. This finding has prompted a series of studies that explore the reasons for a continuing increase in depression rates, and further led to a reconsideration of projections with regard to the burden of depressive disorders in the future.

1.2 Pre alence of Mental Disorders in Primary Care

Although large-scale comprehensive studies about the prevalence of mental disorders in primary care are lacking, there is considerable evidence from international collaborative primary care studies on selected anxiety and depressive disorders (Ustun and Sartorius 1995) that these disorders are highly prevalent in primary care settings around the world: point prevalence estimates for depressive disorder range between 8–11 percent, and those for anxiety disorders range between 4–10 percent. Such studies have also revealed—consistent with more recent studies (Wittchen 2001)—that mental disorders in primary care are usually poorly recognized and rarely treated. Only every second case is recognized, and of those recognized, only one-third receives some type of state of the art treatment.

1.3 The Burden of Mental Disorders

Along with the increased emphasis of epidemiological research on the role of general and diagnostic-specific disabilities and impairment, community studies have also been able to demonstrate the substantial burden that mental disorders have on the subjects’ lives, social functioning, and interpersonal environment. Based on available epidemiological data, the Global Burden of Disease study (Murray and Lopez 1996) showed that the burden of mental disorders has been underestimated greatly. Of the ten leading causes of disability worldwide in 1990, measured in years lived with a disability, five were psychiatric conditions: Unipolar depression, alcohol use, bipolar affective disorders, schizophrenia, and obsessive-compulsive disorder.

2. Unresolved Issues in Descriptive Epidemiology

Despite considerable progress, descriptive epidemiological knowledge is still limited, given in particular that only a few of all mental disorders have been investigated. Other areas with significant deficits are (a) mental disorders in children, adolescents, and the elderly; (b) the dimensional and subthreshold characterization of mental disorders; (c) a more comprehensive identification of risk factors; (d) the range of associated disabilities; and (e) the use of health services with emphasis on national and cultural variations.

2.1 Coverage of Disorders

Epidemiological research is still far away from having examined the full range of clinically significant psychopathological conditions. Noteworthy deficits exist, for example, with regard to somatoform disorders, sleep disorders, substance-related disorders beyond abuse and dependence, personality disorders, and some forms of childhood disorders. Each of these conditions merits considerable epidemiological attention from a public health, and even more from a pathogenetic, perspective, because they have different onset and course characteristics, and are frequently comorbid. For example, somatoform disorders (i.e., pain disorders, hypochondriasis, somatization), which were shown to be highly prevalent conditions that start early in life and constitute a major burden on the health care system, have rarely been included in past community surveys. Similarly, sleep disorders, which rank high as a principal reason for primary care consultations, have not been studied systematically in community surveys. The unclear nosological status of these conditions may be responsible for this neglect. Obstacles to research on personality disorders are of a methodological nature because there are not sufficiently reliable and time-efficient assessment tools available.

For childhood conditions, the challenge lies in consensus regarding appropriate choice of age developmental-stage-specific diagnostic assessments. Researchers must also concur on the degree to which multiple sources of information (i.e., parents and teachers) can be combined into one coherent strategy that mirrors the continuity from childhood to adolescent and adult mental disorders. Despite growing collaboration, there is still a remarkable division among investigators regarding epidemiological designs and methods used in childhood, adolescent, and adult mental disorder research. Inherent in this division is the issue of the developmental continuity of psychopathological features, which also touches partly on the ongoing controversy of dimensional versus categorical measures in epidemiological studies of mental disorders. Intensified research in this area is needed, especially because of evidence that most adult mental disorders begin in adolescence.

2.2 Subthreshold Conditions and Dimensional Approaches

With few exceptions, descriptive epidemiological evidence is based on a restricted range of threshold diagnoses, assessed with diagnostic instruments, such as the WHO Composite International Diagnostic Interview (CIDI) without sufficiently detailed consideration of available duration, persistence, and severity information. The exclusive reliance on categorical threshold diagnoses carries substantial risks of artifactual explanations (such as in comorbidity analyses) and fails to acknowledge the dimensional nature of most expressions of psychopathology. The DSM-IV (APA 1994), as the most frequently used classification system in research into mental disorders, makes only a few attempts to derive discrete categories that are mutually exclusive and lead to a single classification of an individual. In fact, the system was intended to stimulate further development of research on the thresholds for and boundaries among disorders (APA 1994). Available data with their primary reliance on categorical diagnostic decisions are not an optimal source for modifying diagnostic systems (Wittchen et al. 1999c).

2.3 Assessment of Disability and Need of Treatment

Over recent years, increasing recognition has emerged that diagnoses of mental disorders themselves cannot appropriately answer questions about need for care, service utilization, and treatment match. Such domains are thought to be determined largely by the individual’s functioning status and disability. Epidemiological studies from the last two decades of the twentieth century have not provided coherent and comprehensive information about these servicerelated issues, and do not allow for reliable characterizations of diagnosis-specific degrees of disability with regard to various social roles. Also, they do not provide sufficiently detailed data about helpseeking behavior, service utilization, and services needs (Regier et al. 1998). Recently, the World Health Organization has started collaborative, systematic, conceptual, and psychometric developmental work to design generic and diagnosis-specific assessment instruments modules to assess disability. These measures might also provide a better basis for need assessment in the area of treatment.

2.4 Assessment Instruments and Diagnostic Classification

Population studies and methods-related epidemiological work have been instrumental in the improvement of diagnostic classification systems for mental disorders. Reliable symptom and diagnostic assessment instruments of mental disorders have been created for use in epidemiology and clinical research. This work has not only significantly influenced the content and structure of clinical instruments: Structured Clinical Interview for DSM-IV (SCID) (First et al. 1997); Schedules for Clinical Assessment in Neuropsychiatry (SCAN) (Brugha et al. 1999b) and nonclinical tools: Composite International Diagnostic Instrument (CIDI) (WHO 1990), but also played an important role in the revision processes of diagnostic classification systems (DSM-IV and ICD-10).

Yet these conceptual models of mental disorders are not, and have never been, a paragon of elegance, nor have they resulted in sufficiently neat and crisp classification systems that match basic research findings, and clinical management and decisionmaking. The introduction of these operationalized and descriptive manuals have resulted in greater diagnostic reliability and consistency in the use of diagnostic terms around the world. In particular, they have been a key prerequisite for epidemiological progress. However, major problems (i.e., thresholds, overlap, and comorbidity), which remain a source of significant dissatisfaction and controversy, will require extensive future work.

At the center of this agenda is the need for convincing clinical and nosological validation in terms of prognostic value and stability, family and genetic findings, and laboratory findings for almost all mental disorders, allowing a sharper genotypical and phenotypical classification. Current diagnostic classification manuals (DSM-IV and ICD-10) deliberately do not contain mutually exclusive diagnostic categories in order to simulate research inquiries into diagnostic boundaries and thresholds—a valuable target for epidemiological research. Consensus is lacking on how to tailor appropriate psychopathological assessment instruments that are able to address such threshold issues appropriately. Further, despite the substantial scientific exploration and examination that went into instruments like the CIDI (WHO 1990) and the SCAN (Brugha et al. 1999a), some basic problems of reliability and validity inherent in the assessment of some mental disorder are yet unresolved.

At the center of discussion is no longer the traditional question of whether to go for categorical or dimensional, but rather to what degree and for which psychological conditions ‘clinical judgment and probing’ should be regarded as a mandatory core element. Empirical evidence needs to be gathered to determine in which diagnostic domains and clinical instruments are superior to fully standardized instruments, such as the CIDI, which try to identify explicitly the latent variables behind the vagueness of clinical judgment. Progress in the resolution of this issue will also offer ultimately more appropriate strategies in resolving the ‘gold standard’ question of the optimal strategy for validating epidemiological instruments (Brugha et al. 1999a; Wittchen et al. 1999c).

3. Longitudinal Studies and Causal–Analytic Epidemiology

Despite a slowly growing number of costly large-scale prospective–longitudinal studies (Grasbeck et al. 1998, Wittchen and Nelson 1998) that have become available, knowledge about natural course, longitudinal stability of symptoms, and comorbid associations, as well as vulnerability and risk factors for the onset and persistence of mental disorders is still quite meager. This deficiency is particularly true for children, adolescents, and young adults; Thus it remains difficult to characterize mental disorders reliably by patterns of course and incidence across the life span. Further the ‘causal risk factors’ status (Kraemer et al. 1997) has not yet been established for most putative risk factors of mental disorders. At this point it remains unclear what might be cause, consequence, or a mere correlate.

Prospective longitudinal studies such as the Dunedin and the Early Developmental Stages of Psychopathology study (Wittchen and Nelson 1998) can advance our etiological knowledge through the collection of information on early signs and risk factors, which have been gleaned from high-risk studies, and from studies on protective factors and processes related to the onset of mental disorders. From such studies a substantial body of evidence has already become available with regard to partly diagnosisspecific, partly general risk factors for many disorders. Well established risk factors include: A family history of mental disorders, the effect of threatening or stressful disruptions in the individuals’ immediate social environment, childhood loss, abuse, and trauma. There is also further evidence for more complex interactions: Familiar genetic factors may enhance the effects of loss events; patterns of symptom progression from preceding early anxiety disorders relate to the risk of developing secondary depression; and certain childhood temperamental traits increase the risk of subsequent mental disorders. What remains, however, is the large challenge of completing the complex vulnerability–stress interaction, and understanding how vulnerability and risk factors from formerly competing paradigms interact with each other in specific disorders or groups of mental disorders.

3.1 Linking Epidemiology Closer to Clinical and Basic Neurobiological Research

The limitations of convenience samples from clinical and other settings for etiological and pathogenic research are becoming recognized increasingly. Limitations include the risks of artifactual findings, the overand underestimation of effects, confoundation by comorbidity, and the impossibility of establishing causal risk factors for first onset of a disorder. Prospective–longitudinal studies and causal–analytic designs in representative population samples will be of key importance to overcome these limitations. A requirement for such designs is a comprehensive evaluation of the epidemiological triad of host (i.e., genetic variables or temperamental predispositions), agent (life stage transitions or life stress), and environment (social processes or environmental agents). A considerable challenge for such studies is the identification of how and when certain environmental factors potentiate or protect against genetic and biological vulnerability factors. Neuroscience, and in particular, genetic research, is likely to be essential for our future understanding of the etiology and pathogenesis of mental disorders, and ultimately a better genotypic classification. The complexity of longitudinal designs, and the time and costs involved, are substantial; however, findings promise a better understanding of the relevant developmental pathways of specific mental disorders and their interrelationships among each other (comorbidity) (see Table 2).

     Tab. 2

Family and other genetic studies have demonstrated convincingly that an individual’s genetic makeup is an important factor in determining vulnerability for almost all mental disorders (Merikangas 1999). Methods of genetic epidemiology for studying risk factors and etiology of familiar diseases appear to be the most promising ways to unravel the complex mechanisms through which genes exert their influence. The integration of population genetics and epidemiology is critical for determining the attributable risk of particular DNA markers for disease as well as how environmental conditions increase or reduce expressions of genetic vulnerability. Such studies might be directed at states or traits conferring susceptibility. In the near future it can be expected that such types of genetic epidemiologic studies will identify some of those genetic mechanisms that place individuals at increased risks for disorders such as substance dependence, anxiety, and depression.

4. Need E aluation and Its Implications on Inter entions in the General Population

Since the 1980s, quite comprehensive, interdisciplinary mental health systems of providers have emerged in most industrialized countries. However, given the size of the problem of mental disorders it is evident that no system could afford comprehensive treatments for all affected persons. Epidemiological data are a key prerequisite for identifying deficits and problems in health care systems, and offering guidance in service planning and resource allocation. At a time of increasing numbers of effective pharmacological and psychological treatments, competing provider models for mental disorders, and tighter health care budgets, epidemiology can be expected to gain further importance, especially in highlighting the efficacy of interventions. But certainly the available studies and data do not yet provide us with appropriate level of detail for this important task (Regier et al. 1998; Wittchen 2000) (see Table 3).

     Tab. 3

Core elements of need assessment are a reliable definition of a disorder, clearly defined associated disabilities, and existing effective interventions (taking into account both their limitations and modes of delivery). Nevertheless, despite the existence of many effective psychological and drug treatments shown to prevent disability, relapse, chronicity, and suffering for many mental disorders, few epidemiological studies are available that answer with sufficient detail core questions such as: How many anxiety or depressive disorders were treated by psychiatrists, psychotherapists or other types of providers; how many were treated by medication or some form of psychological treatment, or even treated at inor outpatient institutions; how many in need are treated appropriately; and how many remain untreated. Instead, mostly crude data regarding rates of service utilization and unmet service needs are available that emphasize predominantly the role of primary care physicians. Even though the quite limited role and poor recognition abilities of primary care physicians for mental disorders has been noted repeatedly throughout the world (Ustun and Sartorius 1995), it seems that traditional psychiatric epidemiology still seems to favor this model strongly. Neglected are more comprehensive service utilization and need assessment strategies for specialist treatment and other interventions. Another unfortunate deficit in this field is its strong emphasis on so-called ‘major mental disorders,’ such as psychotic disorders, as opposed to ‘minor and neurotic disorders.’ Although this oversimplified dichotomy has clearly outlived its usefulness and scientific justification since the 1980s, many health utilization and needs surveys still overemphasize major morbidity, neglecting what are actually the most prevalent and persisting ‘minor morbidity conditions,’ that cause by far the greatest financial burden of all disorders (Rice and Miller 1998). Current attempts that merely link diagnosis with measures of disability in order to improve need assessment (see above) might not solve this critical problem. Rather, they might result once again in an inappropriately strong emphasis on the most severely ill, neglecting those in earlier stages of their illness process who might profit most from modern treatment methods.

The challenge for epidemiology on a cross-national and international basis lies in a systematic comparison of such competing models. Development of appropriate assessment instruments for use in epidemiological studies to identify advantages and disadvantages of each of these perspectives, in terms of legal, cost, comprehensiveness, and effectiveness issues has been identified as a necessary first step (Jagger et al. 1998). Current perspectives on this issue seem to overemphasize two search strategies: the development of reliable and valid measures of disability (Regier et al. 1998), and the search for other ‘marker’ variables of those in greatest need or the ‘most severe.’ This perspective might fall too short. In search of improved approaches for comprehensive need assessment and evaluation, future epidemiological research should additionally emphasize: (a) a more comprehensive assessment of helpseeking behaviors that covers the full spectrum of professional providers in the respective country or region; (b) a wider coverage of types of interventions received, contingent on the availability of treatments in that country for that diagnosis; and (c) a detailed inquiry into perceived barriers to recognition and treatment.

5. Conclusion

In light of the ongoing rapid developments in neuroscience, clinical psychology, and psychiatry, as well as public health research, epidemiology can be expected to play an increasingly important role in basic, clinical, and public health research of mental disorders. The key challenge will be to understand how multiple risk and vulnerability factors interact over time and over the lifespan in producing a single or multiple mental disorder over brief or longer periods of time.

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