Prototypes, syndromes and dimensions of psychopathology: An open agenda for research

Abstract

The current process of revision of the DSM and ICD has generated requests to alter the criteria defining many individual disorders, to eliminate some and to add new “disorders”. However, all definitional changes have serious disadvantages: they are confusing to clinicians; they create a situation in which the relevance of all previous clinical and epidemiological research into disorders hitherto defined is uncertain; and they involve tedious and often costly changes in the content and wording of diagnostic interviews, as well as in the algorithms to generate diagnoses from clinical ratings. Most of psychiatry’s disease concepts are merely working hypotheses and their diagnostic criteria are provisional. Psychiatric disorders are complex psychobiological entities and both extremes – a totally unstructured approach to diagnosis and rigid operationalization – should be avoided. Defining a middle range of operational specificity, which would be optimal for stimulating critical thinking in both clinical practice and research, but also rigorous enough to enable meaningful communication and comparisons between results of different studies in different contexts, is a better solution. This is where, with certain caveats, the prototype-matching approach could fit the bill 1. Drew Westen’s proposals are to be welcomed for “biting the bullet” by bringing to attention an important alternative to current classificatory models, but the article leaves several key questions open for discussion. What are the caveats? First, the concept of a prototype, intuitively attractive to clinicians, remains ambiguous and poorly operationalized. Prototypes represent the central (“perceptually salient”) tendencies of categories 2. But could a prototype be synonymous, or partly overlapping, with a well-crafted narrative description of a “core” syndrome? In the complex psychiatric disorders, where aetiology is multifactorial, both research and everyday clinical practice could be considerably facilitated by a sharper delineation of the syndromal status of many current diagnostic categories. This provides a strong rationale for reinstating the syndrome (or its prototype template) as the basic unit of future versions of psychiatric classifications. Secondly, should the concise, “one paragraph” formulation of a prototype contain, wherever relevant, pointers to likely aetiology, pathophysiology or associated features, e.g. cognition, which are not part of the presenting clinical picture? According to Hempel 3, membership in a prototype is defined by correlated features, not the necessary presence of all defining features. Thirdly, how can a broad agreement be achieved on a universally “valid” prototype description of any particular disorder? Schizophrenia provides a relevant example. The description of the syndrome has undergone several metamorphoses since Bleuler’s 4 original distinction between basic symptoms (“loosening of associations”, uncoupling of affect from cognition, volitional ambivalence, autistic closure to reality) and accessory symptoms (delusions, hallucinations, catatonic phenomena). While the ICD-10 5 clinical diagnostic guidelines have retained a remote echo of Bleuler’s conceptualization (“a fundamental disturbance of personality…, involving its most basic functions which give the normal person a feeling of individuality, uniqueness and self-direction”), they attribute particular prominence to Schneider’s 6 first-rank symptoms. In contrast, the diagnostic criteria of DSM-IV 7 and of the draft DSM-5 require various combinations of “positive” and “negative” symptoms but do not attempt to provide any prototype or gestalt of the characteristic imprint of the disorder. Lastly, Westen’s claim that a single rating on a 5-point scale for assessing the extent to which an individual matches the prototype could generate, in addition to a categorical statement, a meaningful dimensional score raises doubts. Whether psychiatric disorders can be better described dimensionally or categorically remains an open question for research. However, a major problem for dimensional models of psychopathology is the absence of an established, empirically grounded metrics. Most existing scales of symptom severity are of a psychometrically low level of measurement: they are either nominal or ordinal, where one is simply able to state that a>b>c…>n on some property, arbitrarily assigning numbers which indicate rank order and nothing more 8. It seems unlikely that equal-interval scaling or ratio scales will ever be developed for complex configurations of psychopathology. Medical classifications are created with the primary purpose of meeting pragmatic needs related to diagnosis and treating people experiencing illness. Their secondary purposes is to assist in the generation of new knowledge relevant to those needs, though progress in medical research usually precedes, rather than follows, improvements in classification. The prototype matching approach has the potential of serving well both these purposes, provided that it is underpinned by sound decision rules and supported by evidence from multi-site field trials.