Transdiagnostic approaches to mental health problems: Current status and future directions.

Diagnostic and Transdiagnostic Approaches

Although attempts to classify mental health difficulties date back several thousand years, formalized diagnostic models only emerged properly from the biological and Linnaen botanical classification systems of the 19th century. Most prominently, Kraepelin’s Compendium der Psychiatrie in 1883 ( Compton & Guze, 1995 ) exerted a profound influence on the development of the emerging field of clinical psychiatry especially in the United States, laying the foundation for the publication of the first edition of the DSM in 1952 ( DSM-I , American Psychiatric Association, 1952 ). In 1980, the publication of the DSM–III outlined for the first time a thorough multiaxial diagnostic system with carefully operationalized criteria for a wide range of disorders, with no allegiance to any theoretical approach aside from a broad biomedical model. The DSM–III was hailed as a “paradigm shift” ( Blashfield, Keeley, Flanagan, & Miles, 2014 ) for diagnostic psychiatry, rescuing the profession “. . . from unreliability and the oblivion of irrelevancy” ( Frances, 2009 , p. 2). The current instantiation of the DSM —the DSM–5 – appeared in 2013 after a 14 year gestation and runs to 947 pages covering some 541 diagnostic categories (up from 106 in the DSM-I ; American Psychiatric Association, 2013 ).

The DSM and ICD have evolved into self-perpetuating systems that now govern and define all aspects of how we conceptualize mental health. They provide an organizing framework for virtually all core texts in psychiatry, clinical psychology, and abnormal psychology ( Cosgrove, Krimsky, Vijayaraghavan, & Schneider, 2006 ; Marecek & Hare-Mustin, 2009 ), they guide mental health training across the helping professions, and they define how we assess, manage and treat mental health problems worldwide. The diagnostic systems that they enshrine have created a form of “epistemic prison” ( Hyman, 2010 ) that constrains health insurance and pharmaceutical industry practices, is sanctioned and supported by government and legal policies, and dominates social and public discourse about mental health and illness, as reflected in art, literature and the visual media ( Ussher, 2010 ).

There are many factors underscoring this rise to dominance. Some are certainly sociopolitical ( Kawa & Giordano, 2012 ; Khoury, Langer, & Pagnini, 2014 ) with diagnoses offering a biomedical legitimacy to discourse about mental ill health that has a broad academic, professional, and social appeal. Others are more pragmatic as, without doubt, the diagnostic paradigm offers some clear benefits to clinical and research practice: It provides a lingua franca for describing clusters of symptoms that facilitates communication between users of services, clinicians and researchers; it sets out a common metric for research programs; and it provides an organizing principle for the development and evaluation of diagnosis-led assessment and treatment approaches ( Hayes & Hofmann, 2018 ). Finally, for some, the biomedical model at the heart of the diagnostic approach also brings a legitimacy to the suffering that is experienced, reducing stigma and deflecting pejorative judgments that mental ill health reflects some form of personal weakness on the part of the diagnosed.

Despite these advantages of the diagnostic paradigm, there is a gathering apprehension that the taxonomic approach instantiated in the DSM and ICD runs counter to the available clinical and research evidence and may hamper our understanding of mental ill health and consequently how we manage and treat mental distress ( Insel, 2014 ; Kotov et al., 2017 ). Here we touch briefly on seven areas of concern that have currency within this debate. We focus predominantly on so-called common mental health problems ( Craig & Boardman, 1997 ), captured by the various diagnoses of mood disorder, anxiety disorder, stressor-related disorders, and obsessive–compulsive disorders within the diagnostic manuals, but the arguments of course extend beyond these presentations.

The Symptom Space is Dimensional

Within the diagnostic manuals, symptoms are thresh-holded, imposing binary notions of “present” versus “absent” ( Regier, Kuhl, & Kupfer, 2013 ). Groupings of symptoms deemed to be present then comprise the different diagnoses albeit with guidance on severity qualifiers for “mild,” “moderate,” and “severe” manifestations of individual diagnoses. However, evidence overwhelmingly suggests that mental health symptoms are not all-or-none phenomena, but are better conceptualized along continuous dimensions within the population as opposed to these distinct categorical entities ( Brown, Campbell, Lehman, Grisham, & Mancill, 2001 ; Kessler, Chiu, Demler, Merikangas, & Walters, 2005 ). Indeed, there is a lack of compelling evidence for even a single symptom or disorder being a distinct category ( Haslam, Holland, & Kuppens, 2012 ). 1

This imposition of artificial categories onto a multidimensional space inevitably sacrifices much of the richness of the available clinical information, contributing to diagnostic instability with symptoms falling above or below imposed thresholds over time, as well as reduced interrater reliability as assessors struggle to elucidate whether marginal symptoms cross the designated thresholds ( Markon, Chmielewski, & Miller, 2011 ). Most importantly, many individuals experiencing psychological distress fall short of the criteria for any diagnosis, despite a manifest need for care ( Kotov et al., 2018 ).

Rampant Comorbidity and Poor Discrimination Between Supposedly Different Disorders

Comorbidity—when someone presents with a profile of problems that satisfy the criteria for more than one diagnosis at a time—is not a problem per se for the diagnostic model, with the notion of secondary diagnoses and/or complications of primary problems woven into the fabric of psychiatric taxonomies since the outset. However, of greater concern is that epidemiological findings reveal that comorbidity among psychiatric diagnoses is the rule rather than the exception, and single, uncomplicated clinical presentations are actually relatively scarce ( Kessler et al., 2005 ). Such comorbidity is associated with greater clinical severity and functional impairment ( Wittchen et al., 2011 ), higher rates of symptom chronicity ( Rapaport, Clary, Fayyad, & Endicott, 2005 ) and a greater detriment to overall quality of life ( Hofmeijer-Sevink et al., 2012 ). This “rampant” ( Clark, Cuthbert, Lewis-Fernández, Narrow, & Reed, 2017 ) diagnostic comorbidity suggests that the normative coexistence of psychiatric disorders must, to a considerable extent, be an artifact (e.g., Maj, 2005 ) arising from the structure of the categorical classification system itself, rather than the co-occurrence of genuinely separable syndromes ( van Loo & Romeijn, 2015 ).

Massive Heterogeneity Within Diagnoses

Formal diagnoses of different disorders typically comprise a number of criteria—clusters of conceptually similar symptoms that are heuristically grouped together. Most criteria contain more than one symptom and one or more of these symptoms would need to be present for the criterion to be met. Typically, the overall diagnosis then further depends on a specified number of criteria being satisfied. Even for diagnoses where criteria are not explicitly offered, diagnoses normatively require only a subset of symptoms from a larger set to be present. This polythetic checklist approach means that individuals receiving the same diagnosis can present with very different symptoms such that each diagnostic category incorporates built in heterogeneity.

To illustrate, within the DSM–5 major depressive disorder (MDD) diagnosis, two individuals meeting criteria for MDD could potentially have only one symptom in common from the nine listed in the Manual. Indeed, when we account for all of the subsymptoms and directional qualifiers, there are 16,400 different symptom profiles that all qualify as MDD ( Fried & Nesse, 2015 ). Such heterogeneity is orders of magnitude greater for complex criteria-based diagnoses such as posttraumatic stress disorder where there are 636,120 permutations that qualify for the diagnosis ( Galatzer-Levy & Bryant, 2013 ).

How does this play out in actual epidemiological data? In the Sequenced Treatment Alternatives to Relieve Depression (STARnD) data ( N = 3,703), Fried and Nesse (2015) identified 1,030 unique MDD symptom profiles, of which 864 (83.9%) were endorsed by fewer than six participants, with almost half of the profiles (501; 48.6%) endorsed by only a single individual. Indeed, the most common profile was only met by 67 people.

A primary function of any diagnostic system should be to facilitate our understanding of a complex problem space by organizing central, recurrent patterns into discrete categories. The data on heterogeneity cast doubt on whether this pragmatic aim of the psychiatric paradigm has even come close to being realized.

Incomplete Symptom Capture

A central question for the compilers of diagnostic compendia is which symptoms to include as prototypical to delineate a given disorder given that many symptoms of mental health problems also characterize everyday life; for example, tiredness, low mood, and so on and many disorders are associated with a multiplicity of signs and symptoms. The case of depression again provides a revealing illustration. If we look at established measurement tools for depression, there are some 280 different instruments developed in the last century, of which many are still in use ( Santor, Gregus, & Welch, 2006 ). These assessment instruments differ markedly in the signs and symptoms that they capture. For instance, Fried (2017) notes that across the seven most commonly used depression assessment tools, 52 distinct depression symptoms are measured (compared with the nine symptoms listed in the DSM–5 ), with 40% of those symptoms appearing in just one of the seven scales and only 12% appearing across all seven. Notwithstanding the fact that some of these 280 instruments may have weak clinical validity, the wide scope of clinical signs and symptoms covered suggests that there is no single set of cardinal symptoms that defines depression (and by extension other diagnostic categories) and consequently that the profile outlined in the diagnostic manuals may be overly narrow or rigid, failing to reasonably capture the range within the clinical data. 2

Phenotypic Plasticity Across Development and The Life Course

Mental health problems can morph across development and the life course such that individuals satisfy criteria either for different diagnoses or present differentially within the same diagnosis across time; for example, shifting between anxiety and unipolar depressive disorders ( Fichter, Quadflieg, Fischer, & Kohlboeck, 2010 ), within anxiety disorders ( Wittchen, Carter, Pfister, Montgomery, & Kessler, 2000 , 2008 ) or within depressive disorders ( Oquendo et al., 2004 ). Some of this phenotypic plasticity is a function of development, but the nature of the diagnostic approach itself arguably also contributes with its reliance on cross-sectional “snapshot” dichotomizations of what are in fact dimensional and dynamic symptom constructs that will wax and wane across time ( Bystritsky, Nierenberg, Feusner, & Rabinovich, 2012 ).

Diagnosis-Driven Clinical Intervention

A much-vaunted advantage of diagnostic taxonomies is facilitated clinical assessment, management and intervention. This has led, within the domain of psychological interventions which is our primary focus here, to the establishment of an evidence-base for a plethora of single-disorder-focused treatment approaches. These are then endorsed by diagnostically organized guidelines such as those compiled by the United Kingdom’s National Institute for Health and Care Excellence (NICE; Pilling, Whittington, Taylor, & Kendrick, 2011 ). Comorbid conditions are generally either glossed over, or minimally treated within these intervention packages and there little attention is paid to symptoms that fall outside of the diagnostic rubric. However, the majority of mental health treatments of all types actually appear to be effective across broad ranges of clinical populations, for example drugs such as selective serotonin reuptake inhibitors (SSRIs) and benzodiazepines, and psychological protocols such as cognitive-behavior therapy (CBT), or extinction-based approaches for anxiety-related difficulties. There is thus a mismatch between the clinical reality on the ground and the nature and scope of the recommended interventions. As a result, much of real-world clinical practice eschews the diagnosis-led treatment evidence base, preferring instead eclectic combinations of treatment elements tailored to the presentation and formulation of individual clients. This pragmatic approach enables goodness-of-fit matching of interventions to specific vulnerabilities and processes relevant to the individual, and provides a flexible treatment model that can be applied across a range of presentations including, critically, complex formulations, comorbidity, and subsyndromal or prodromal symptoms.

A Transdiagnostic Alternative

These diverse concerns about the diagnostic approach stem somewhat independently from research and scholarship across the three intellectual domains of classification and nosology, basic biopsychosocial research, and clinical science. Perhaps unsurprisingly, therefore, the alternative transdiagnostic approaches that abnegate the traditional psychiatric paradigm have also evolved and matured somewhat separately in each of these spheres, as we highlight below.

Within each of the three domains, the degree to which the diagnostic model is forsaken and consequently the strength of the transdiagnostic proposals vary. What we shall call here “soft” transdiagnostic approaches preserve the underlying diagnostic classification while seeking to elucidate processes or develop interventions that have relevance to one or more of the diagnoses as traditionally formulated. In contrast, more radical, “hard” transdiagnostic approaches dispense with the diagnostic system altogether, seeking to replace it with alternative frames of reference that characterize mental ill health in new ways.