Distress and fear disorders: an alternative empirically based taxonomy of the 'mood' and 'anxiety' disorders

The nosological organisation of DSM–IV and ICD–10does not capture the empirical structure of the mood and anxietydisorders. Instead, they form a broad group of ‘internalising’disorders with two subclasses: distress disorders and feardisorders. This empirical structure should form the basis forrevised taxonomies in DSM–V and ICD–11.

INTRODUCTION

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INTRODUCTION

As workgroups begin the task of revising the taxonomy of mentaldisorders and diagnostic criteria for DSM–V and ICD–11,the field has the opportunity to bring these classificationschemes in line with current empirical research. Even if theDSM–V Task Force adopts a conservative approach, revisingonly those elements for which there is strong empirical support,certain sections stand to be radically revised. Only if non-scientificconsiderations play an important part in the revision–or lack thereof–will these sections see minor rather thanmajor changes. We address here two such sections of DSM–IV:mood disorders and anxiety disorders.

DEVELOPMENT OF THE CURRENT TAXONOMY

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DEVELOPMENT OF THE CURRENT...

With the advent of DSM–III, a strong separation was madebetween ‘affective’ and ‘anxiety’ disorders,with hierarchical exclusion rules virtually dictating thatthe former trump the latter in cases in which both types ofdisorder were present. Research ignoring these rules foundno empirical basis for them, however, so they were eliminatedin DSM–III–R. Once these exclusion rules were relaxed,research reports on diagnostic comorbidity flooded the literature.Clark & Watson (1991) and Barlow and colleagues (e.g.Barlow et al, 1996) offered theoretical models to explain thesecomorbidity findings, proposing that anxiety and depressivedisorders were linked through a shared personality dimensionof negative emotionality (or neuroticism; N/NE), and distinguishedon the basis of unique factors–anhedonia or low positiveemotionality in depression and autonomic arousal in anxiety.

During the 1990s, the US National Comorbidity Survey data revealedthat major depressive disorder had very different comorbidityrates with various anxiety disorders, ranging from an oddsratio of 6 with generalised anxiety disorder to 4 with panicdisorder and 3 for simple and social phobia (Kessler et al, 1996). Results of genetic studies paralleled the US survey data inthat major depressive disorder and generalised anxiety disorderwere found to share a single genetic diathesis, which alsowas linked strongly to the N/NE personality trait (e.g. Kendler, 1996).In contrast, the genetic overlap of major depressive disorderand other anxiety disorders was lower (Kendler et al, 1995) or even negligible (Pauls et al, 1994). Moreover, a reviewof the voluminous comorbidity literature by Mineka et al (1998)revealed that, although either type of disorder conveyed anincreased risk for later development of the other, anxietydisorders were significantly more likely to appear first, andcases of pure depression were more rare than pure anxiety, raisingthe possibility that anxiety disorders represented a less severeform of a single spectrum.

These results led Mineka et al (1998) to propose an integrativehierarchical model of anxiety and depression with N/NE as acommon genetic substrate, and various specific factors differentiatingindividual disorders. Specifically, anhedonia/low positiveemotionality is conceptualised as the specific factor in depression,whereas autonomic arousal represents the specific componentin panic disorder (not anxiety disorders in general, as in theoriginal model of Clark & Watson, 1991). Other anxietydisorders such as phobias or obsessive–compulsive disorderalso are presumed to have their own (currently undetermined)specific factors. Fergusson et al (2006, this issue), using structural equation modelling on data from a 25-year longitudinalbirth cohort study, found evidence consistent with this model.Specifically, he demonstrated that a common factor (‘internalising,’on which we expand subsequently) explained both symptom comorbiditiesand continuity over time for major depressive disorder, generalisedanxiety disorder, phobias and panic disorder; at the same time,however, he found across-time continuity in disorder-specificcomponents of major depressive disorder and phobias. Althoughthis model explains many aspects of the data well, the exactnature of the additional specific factors (e.g. whether theyare only phenotypic or also have a genetic basis) remains unclear.

RECENT ADVANCES IN UNDERSTANDING THE STRUCTURE OF PSYCHOPATHOLOGY

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RECENT ADVANCES IN UNDERSTANDING...

An important related question is how the genetic and structuralfindings for anxiety and depression fit into the broader domainof psychopathology. The answer to this question has emergedover the past decade. During this period, six large-sampleindependent studies (Lahey et al, 2004; see Clark, 2005 forthe five others) have examined the structure of psychopathologyby studying diagnostic comorbidity patterns phenotypicallyand/or genotypically, each using a set of common mental disordersthat largely overlapped across studies. The results have revealeda remarkably consistent structure: a hierarchical model withtwo broad factors–externalising and internalising. Substancedependence, attention-deficit hyperactivity disorder, oppositionaldefiant disorder, and conduct disorder/antisocial personalitydisorder define the externalising factor. The internalisingfactor subsumes two highly related subfactors: ‘distress/misery’–comprising generalised anxiety disorder, overanxious disorder and depressive disorders – and ‘fear’, which includes simple and social phobias, separation anxietydisorder and panic disorder. Slade & Watson (2006) additionallyshowed that this structure fitted both DSM–IV and ICD–10 conceptualisations of these disorders, with neurasthenia representinga manifestation of distress/misery in the latter. Finally,it is noteworthy that this alternative hierarchical schemeconsistently captures the comorbidity data better than theDSM model, which separates these syndromes into ‘mood’and ‘anxiety’ disorders.

The recognition of this structure has engendered further questionsabout the nature of the internalising and externalising dimensionsthemselves. Based on an extensive review, Clark (2005) proposedthat both personality (e.g. N/NE) and psychopathology derivefrom innate general temperament dimensions, including negativeand positive temperament, which differentiate through developmentinto the full range of adult personality and also are the diathesesfrom which psychopathology develops in response to a sufficientlystressful environment. In this model, internalising emerges largely from negative temperament and externalising from temperamental disinhibition, alone or in combination with negative temperament.

IMPLICATIONS FOR DSM–V/ICD–11

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IMPLICATIONS FOR DSM-V/ICD-11

Moreover, this robust structure has two important implicationsfor DSM–V and ICD–11. First, the hard separationbetween mood disorders and anxiety disorders introduced inDSM–III, with particular diagnoses assigned to each group,is shown to be a pseudo-hierarchical, rational folk system.It now is abundantly clear that these two types of disordersare strongly related and should not be artificially separatedinto different diagnostic classes. Moreover, the current distinctionbetween mood disturbance (the defining element of the currentmood disorders) and anxiety/avoidance (the characteristic featuresof the current anxiety disorders) is unsound and does not providean optimal arrangement of these disorders (Watson, 2005). To the extent that the DSM and ICD purport to be empirical documents,the current folk taxonomy must be abandoned and replaced witha data-driven, scientifically supported taxonomy. Second, mentaldisorders are hierarchically arranged: that is, the evidenceestablishes that most disorders co-occur and are empiricallyrelated, but that some disorders are more highly comorbid than others. The taxonomic structures of official diagnostic manualsneed to reflect this fact.

What this would mean for DSM–V/ICD–11, for example,is that instead of grouping generalised anxiety disorder, panicdisorder, and so on together under the heading of ‘anxietydisorders’, as they are now in DSM–IV, generalisedanxiety disorder and overanxious disorder would be groupedwith major depressive disorder/dysthymia (in what Watson, 2005,labels the ‘distress disorders’) because they sharemore variance with these depressive disorders than with otheranxiety disorders. One clear advantage of such a hierarchicalstructure is that it reconciles the long-standing tension between‘lumpers’ (who value broad diagnostic categories)and ‘splitters’ (who argue for fine-grained diagnosticspecificity) by encompassing both at different levels of thediagnostic hierarchy. Thus, depending on the nature of theproblem at hand, clinicians and researchers can choose to focuson a few broad non-specific classes of psychopathology (e.g. distress disorders, externalising disorders), individual disorders,or some combination of the two. Note also that a hierarchicalmodel easily can be extended further to encompass subtypeswithin current disorders (e.g. subtypes of specific phobia;see Watson, 2005).

The primary immediate change would be organisational, with morehighly comorbid disorders placed together and those with lessoverlap falling farther apart in the hierarchical structure.However, although none of the current diagnoses necessarilywould disappear if the empirically revealed structure wereimplemented in DSM–V/ICD–11, it is likely that movingto a more thoroughly empirically based taxonomy eventuallywould result in more radical diagnostic revisions. In particular,data-based considerations eventually would create pressureto replace currently heterogeneous syndromes (such as manyof the current personality disorders) with more homogeneous diagnostic groups, or at least ones in which observed heterogeneityreflected more peripheral variation with little implicationfor differential treatment. For example, when relations betweenvarious personality and psychosocial variables and treatmentoutcome were examined in a sample of patients with recurrentmajor depression, it was the common, overlapping variance that carried the predictive weight (Clark et al, 2003).

There are likely to be pressures from various constituenciesto maintain the status quo, but their bases will be pragmaticrather than scientific. For example, directors of anxiety disordersclinics may resist revision for fear that the loss of generalisedanxiety disorder to the distress disorders will reduce theirclient base. Pharmaceutical companies may express concerns that extensive (translation: expensive) clinical trials will needto be conducted to examine the effectiveness of their current‘antidepressant’ drugs for generalised anxietydisorder. Even further, the fact that the distress and feardisorders are themselves collapsed together at a higher levelin the hierarchy has implications for the cross-effectivenessof ‘antidepressant’ and ‘anti-anxiety’drugs. Of course, practising clinicians have known for yearsthat there is no clear one-to-one correspondence between theformal DSM diagnoses they give their patients and the prescriptionsthey write for them, and the pervasive phenomenon of ‘comorbidity’is well known to those who are on the front lines of mental-disordertreatments. Thus, these pragmatic concerns should not hinder the development of an empirically adequate and clinically usefulpsychiatric classification scheme.

REFERENCES

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