Dimensional or categorical: different classifications and measures of anxietyand depression




Dan J. STEIN, MD, PhD
Dept of Psychiatry
University of Cape Town
Cape Town – SOUTH AFRICA

Dimensional or categorical: different classifications and measures of anxiety and depression


by D. J . Stein, South Africa



Psychiatric syndromes and symptoms are complex phenomena that can potentially be conceptualized and assessed both categorically and dimensionally. Both the Diagnostic and Statistical Manual of Mental Disorders (DSM) and the International Classification of Disease (ICD) systems rely extensively on a categorical approach, but also note the dimensional nature of syndromes and symptoms. Here, some of the relevant conceptual issues for anxiety and mood disorders are reviewed, and exemplars from the DSM and ICD revision process are used to illustrate these issues. Categorical diagnoses and specifiers are often useful, although they also have key limitations that should be recognized. Dimensional ratings allow more fine-grained conceptualization and assessment of symptom profiles and etiological factors; this may have some benefits, but also some costs. Categorical and dimensional approaches should be seen as complementary, andmay usefully be employed in tandem.

Medicographia. 2012;34:270-275 (see French abstract on page 275)



Psychiatric syndromes and symptoms are complex phenomena that can be conceptualized and assessed both categorically and dimensionally. The category of major depressive disorder, for example, is often used in clinical settings. In clinical settings, however, the profile and severity of symptoms in major depression varies a great deal. Such variation is even more apparent in community settings.1 Similarly, the diagnosis of anxiety disorders such as social anxiety disorder (SAD) is commonly made in clinical settings. But, again, the profile and severity of anxiety symptoms, including social phobia symptoms, ranges broadly in both clinical and community settings.2 Both the Diagnostic and Statistical Manual of Mental Disorders (DSM) and the International Classification of Disease (ICD) systems rely extensively on a categorical approach, but also note the dimensional nature of syndromes and symptoms. Thus in both systems, the main focus is on describing a range of categorical clinical entities. At the same time, both systems make use of a range of specifiers, such as “mild,” “moderate,” and “severe,” which introduce a dimensional aspect. DSM-IV explicitly notes that categories are fuzzy, and that it is difficult to draw clear boundaries between disorders and normality, and between different disorders.3 What are the advantages and disadvantages of categorical versus dimensional approaches to psychiatric syndromes and symptoms? Are these complementary or competing approaches? Here, some of the relevant conceptual issues for anxiety and mood disorders are reviewed, and exemplars from the DSM and ICD revision process are used to illustrate these issues.

Categorical approaches

A categorical approach to psychiatric disorders is frequently adopted in clinical practice, because the presence or absence of a particular condition crucially informs treatment decisions. Although symptoms are often measured dimensionally, categorical cut points are then used tomake a determination about treatment. The utility of categories is not limited to the clinic; psychiatric epidemiologists may, for example, find it useful to record details about a history of lifetime major depression, and intervention researchers may find it useful to determine categorical treatment response, even if statistical methods demonstrate that depressive symptoms fall on a continuum (ie, there is no underlying taxon).4

A categorical approach to conceptualizing and assessing psychiatric disorders has several major limitations that deserve recognition.A first problemis encouraging whatmay be termed “essentialism.”5,6 Major depression is not a “natural kind” in the same way that say gold or silver is.7,8 First, two individuals with major depression may have quite different symptom profiles and symptom severity. Second, two individuals with major depression may have quite different factors contributing to the pathogenesis of symptoms; these may differ both in type and extent (eg, one patient may have several minor genetic variants contributing to an episode; a second patient may have one major genetic variant that is responsible).

Nevertheless, the provision of categories such as “major depression,” defined in terms of operationalized criteria, means that such entities may be reified, and viewed as natural kinds. This may have significant negative consequences in clinical and research contexts. First, clinicians may focus their clinical assessment primarily on the operational criteria provided in the nosological systems rather than on many other kinds of symptoms and contexts which may also be clinically relevant. Second, clinicians may not only fail to appreciate the complexity of symptomatology, but they may also overlook the complexity of contributing factors, focusing for example, only on particular genetic and environmental risk factors at the expense of others.

The use of a categorical approach may lead, then, to a systematic under appreciation of the importance of variations in overt symptoms and in underlying mechanisms from individual to individual. One patient may have typical symptoms of a mild degree in one kind of situation, while another patient may have atypical symptoms of a moderate degree caused by a quite different range of factors. The type and extent of depressive and anxiety symptoms may differ across gender, developmental stage, and culture; such differences may be downplayed or ignored by using a single category, such as “generalized anxiety disorder,” to describe anxiety symptoms in each of these cases. Such dimensional variation may occur even in disorders where discontinuity in symptom measures indicates an underlying latent taxon (eg, schizotypy).4

Another important problem with the categorical approach is that when the DSM system is employed, many individuals are found to have more than one disorder. Such extensive comorbidity seems artifactual.9 Individuals with bothmajordepression and generalized anxiety may arguably, for example, be more accurately conceptualized as having a mixed anxiety depressive disorder.10 Diagnosing individuals with two comorbid disorders would seem to suggest that each involves different etiological mechanisms and requires different treatments, when a more parsimonious approach may be more accurate. (The entity of subthreshold mixed anxiety disorder remains, however, controversial.)11 A final problem with a categorical approach is particularly important in research settings; analyses of dimensional measures offer greater statistical power.

Dimensional approaches

Given these kinds of difficulties raised by the categorical approach, many clinicians and researchers advocate the use of a dimensional approach to conceptualizing and assessing psychiatric syndromes and symptoms. Given that nature cannot easily be carved at her joints,12 one argument is that an understanding of the psychobiology of psychiatric disorder requires an acceptance of the dimensional nature of symptoms.10 Dimensions can conceivably be used to record not only symptomprofiles, but also etiological contributors,8 including the full range of relevant types and extents. Indeed, there have been ongoing efforts to include a dimensional approach in DSM-5.13-15 Such dimensions may include continuous assessment of core symptoms, dimensional assessments that cut across different disorders, and spectrum constructs.16

At the same time, it should be noted that there are also potential problems with a dimensional approach. First, dimensional analysis is only useful when the association of predictors with dimensional scores is in fact constant throughout the relevant dimensional severity range.4 Second, the use of dimensions does not necessarily avoid essentialism and reification; instead of there being reification of a single entity, there is potentially reification of particular symptoms or causal dimensions. Third, while artifactual comorbidity may be diminished, there is potentially the problemof how to easily articulate complex patterns of overt symptoms and underlying mechanisms. Thus, for example, the use of just 3 dimensions requires 8 categories based on high and low cut-off scores to articulate constructs situated at the high and low ends of these dimensions. This creates difficulties for both clinicians and researchers, and raises issues of user acceptability.17





Table I
Table I. Differences between compulsive hoarding and obsessive-compulsive disorder.

Abbreviations: CBT, cognitive behavioral therapy; DSM-IV-TR, Diagnostic and Statistical Manual of Mental Disorders-IV-text revision; OCD, obsessive-compulsive disorder; OCPD, obsessive-compulsive personality disorder; PD, personality disorder; SRI, serotonin reuptake inhibitor.
Modified from reference 29: Mataix-Cols. Depress Anxiety. 2010;27:556-572. © 2010, Wiley-Liss, Inc.



Indeed, dimensional approaches are often translated back into categorical approaches via the use of cut points. Thus, in clinical settings, patients with scores above a particular cut point on a symptom severity rating scale may be viewed as having the relevant disorder, while those with scores below this cut point may be viewed as not having the condition. Fortunately, the research literature has provided a growing body of information on the relationship between coarse-grained, but user-friendly categorical measures, and more fine-grained, dimensional measures. Thus, for example, in the World Mental Health Surveys, tandem use of categorical and dimensional measures allowed validation of cut points for assessing constructs such as psychological distress.18-20 Similarly, in treatment studies, the tandem use of categorical and dimensional measures allows a rigorous evaluation of optimal cut points for determining treatment response.21

Exemplars

Is SAD a psychiatric disorder? Certainly, if one considers a patient with severe SAD symptoms and consequent symptoms of major depression, with associated marked distress and impairment, it seems clear that such an individual deserves to be diagnosed with a psychiatric disorder and to receive appropriate intervention. What about a patient who suffers from a single kind of performance anxiety, such as speaking in public, but who manages to avoid such situations, and who otherwise lives his or her life without distress or impairment? How about patients who are somewhat introverted and shy, but who find themselves in work situations that demand a high level of social interaction, where those who are more outgoing are rewarded with higher pay packages?22

From a categorical perspective, DSM-IV differentiates between those with and without SAD on the basis of the so-called clinical significance criterion.23 This criterion indicates that individuals with marked clinical distress, or significant impairment as a consequence of their symptoms, meet the criteria for suffering from a clinical disorder. DSM-IV also differentiates individuals who have generalized SAD; such individuals fear “most” social situations. The literature suggests that individuals with generalized SAD have more severe symptoms, greater impairment, and are more likely to have a family history of SAD.24 Both “SAD” and “generalized SAD” seem to be clinically useful categories.

From a dimensional perspective, however, some individuals with social anxiety symptoms fear only a few different social situations, and others fear many. Individuals with many social fears who do not quite meet the clinical significance criterion may arguably still have significant symptoms, and may be at significant risk for comorbidity. The question of when to treat is one that should be decided using considerations such as cost-effectiveness.1,7 Similarly, there is no specific cut point that differentiates those with generalized versus non-generalized social anxiety; instead there is a monotonic dose-response relationship between number of social anxiety symptoms and indicators such as increased SAD persistence, severity, and comorbidity.2 Nevertheless, the distinction between generalized and non-generalized may be important in other ways, such as in predicting treatment response.

Obsessive-compulsive disorder (OCD) provides another interesting exemplar. On the one hand, OCD seems a relatively homogenous neuropsychiatric entity, at least in comparison with disorders such as major depression or generalized anxiety disorder.25 On the other hand, there is growing evidence of the heterogeneity of OCD.26 For example, there is good evidence, based on meta-analysis of factor analytic studies of OCD symptom types, that OCD symptoms fall into a relatively limited number of key symptom dimensions.27 Also, there is evidence that OCD patients with and without comorbid tics may differ in important ways.28

From a categorical perspective, the construct of OCD has long been supported on the basis of considerations of both diagnostic validity and clinical utility. On the other hand, the tic specifier that has been newly proposed for DSM-5 provides an additional category which allows an even more fine-grained assessment of patients with OCD. Again, this specifier seems to have both diagnostic validity as well as clinical utility. Thus, patients with OCD as well as tics are more likely to have particular kinds of symptom profiles, they may have different underlying genetic profiles, and they may respond differentially to intervention.28

From a dimensional perspective, recording different OCD symptom dimensions may allow a more fine-grained level of assessment that is useful in both clinical and research settings.28 Thus, subjects with hoarding symptoms may have a different neurobiology and treatment response. Similarly, each of themajor symptomdimensions in OCD may have some level of specificity at a neurobiological level. The DSM-5 proposal, however, is to recognize hoarding disorder as a separate clinical entity (Table I),29 and to describe OCD symptom dimensions in the text of the OCD section rather than as formal specifiers.28 This reflects data that hoarding symptoms represent a unique diagnostic category, while ratings of other symptom dimensions in OCD are perhaps less clinically useful.28,29

Finally, there is the question of whether it is useful to assess obsessive-compulsive symptoms across a range of different anxiety disorders, and whether it is useful to have a separate chapter in the nosology on obsessive-compulsive spectrum disorders. Certainly, it has been suggested that the presence of obsessive-compulsive symptoms in disorders such as schizophrenia may have clinical utility.30 Furthermore, it has been argued that given overlaps in the phenomenology and psychobiology of several putative obsessive-compulsive spectrum disorders, it would enhance the scientific validity and clinical utility of the nosology to group such disorders together31 (Table II).32 The inclusion of a separate chapter on these conditions might help raise awareness of overlapping approaches to their diagnosis, assessment, and treatment. Several of these disorders may have similar dimensional specifiers, such as an insight specifier which ranges from good to poor (Table III).


Table II
Table II. Similarities between
obsessive-compulsive disorder
and selected obsessive-compulsive
spectrum disorders.

Abbreviation: OCD, obsessive-compulsive
disorder.
Reproduced from reference 32: Phillips.
Psychiatr Clin N Am. 2002;25:791-809.
© 2002, Elsevier Science (USA).



Table III
Table III. Classification of delusional and nondelusional forms of disorders in DSM-IV.

Abbreviations: DSM, Diagnostic and Statistical Manual of Mental Disorders; NOS, not otherwise specified; OCD, obsessive-compulsive disorder.
Reproduced from reference 32: Phillips. Psychiatr Clin N Am. 2002;25:791-809. © 2002, Elsevier Science (USA).



Table IV
Table IV. Cognitive deficits across anxiety disorders.

Abbreviations: GAD, generalized anxiety disorder; ID/ED, intradimensional-extradimensional set-shifting task; OCD, obsessive-compulsive disorder; PD, personality disorder; PTSD, posttraumatic stress disorder; SAD, society anxiety disorder; SSRT, stop signal reaction time test; WCST, Wisconsin card sort test.
Reproduced from reference 35: Stein et al. Depress Anxiety. 2010;27:495-506. © 2010, Wiley-Liss, Inc.



At the same time, it should be acknowledged that any particular metastructure has both strengths and weaknesses.33,34 Thus, for example, a separate chapter on obsessive-compulsive and related disorders may lead to underemphasis of the important overlaps between OCD and other anxiety disorders (Table IV).35 In addition, a separate chapter on obsessive-compulsive and related disorders runs the risk of downplaying important differences in the diagnosis, assessment, and treatment of different conditions that fall within this chapter. Placing obsessive-compulsive and related disorders immediately after the anxiety disorders in the DSM-5 metastructuremay help emphasize relationships between these conditions, and itmay be useful to emphasize in the DSM-5 text that there are important distinctions between each of the obsessive-compulsive and related disorders.34

Conclusion

Categorical and dimensional approaches to conceptualizing and assessing psychiatric syndromes and symptoms are complementary rather than mutually exclusive, with dimensional assessments often informing categorical treatment decisions. Furthermore, categorical ratings can be transformed into dimensional ones (eg, by summing the number of diagnostic criteria met) and vice versa (eg, by using cut points to determine whether a categorical diagnosis should be made). A categorical approach provides a clinically useful way to communicate rapidly the main features of a case, and is also valuable in particular research situations. A potential disadvantage of categorical approaches is that they may encourage reification and oversimplification of complex entities with multiple overt symptoms and underlying mechanisms. A dimensional perspective allows for a more fine-grained approach, but also has significant potential disadvantages. It is useful to employ categorical and dimensional approaches in tandem, in both clinical and research settings.4,10 _

Acknowledgment. Professor Stein is supported by the Medical Research Council of South Africa.

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