Diagnosis is the identification or recognition of a disorder on the basis of its characteristics. Making a diagnosis enables the clinician to refer to the base of knowledge that has accumulated with regard to the disorder. However, assigning a diagnosis does not mean that the etiology is known.

Classifying disorders is accomplished through categories (qualities) and dimensions (how much of a characteristic that person exhibits). A classification system helps clinicians match their client’s problems with the form of intervention that is most effective.

There are two main diagnostic systems: the Diagnostic Statistical Manual (DSM) and the International Classification of Diseases (ICD). The DSM is used mainly in the United States. The ICD is dominant in Europe. The most recent edition of the DSM is the DSM-V. The DSM is an evolving document produced by the American Psychiatric Association.

One of the fundamental difficulties in devising a classification of mental disorders is the lack of agreement among psychiatrists regarding the concepts upon which it should be based: diagnoses can rarely be verified objectively and the same or similar conditions are described under a confusing variety of names. This situation militates against the ready exchange of ideas and experiences and hampers progress.

The following elements are of particular importance for the definition of a mental disorder:

• Personal harm and suffering
• Abnormality (statistical, social, individual)
• Limitations or disabilities in what a person can perform
• Danger for others or the individual him/herself

In most instances more than one of these elements have to occur at the same time.

In order to standardize the description and interpretation of mental disorders, diagnosis and classification systems have been set up.

At present there exist two established classification systems for mental disorders:
The International Classification of Diseases (ICD-10) published by the World Health Organization (WHO) and the classification system of the American Psychiatric Association (APA), the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV).

Both classification systems converged strongly in their last revisions, therefore diagnoses are comparable for the most relevant points.

ICD-10

The International Classification of Diseases (ICD) is an international standard diagnostic classification for all general epidemiological and many health management purposes, published by WHO. It now exist in its tenth revision. Chapter V is relevant for mental and behavioural disorders.
The ICD-10-classification for mental disorders consists of 10 main groups:

In addition, there is a group of “unspecified mental disorders”.

For the chapter of mental disorders, every main group has the identification letter “F”. For each group exist more specific subcategories.

For more information see the ICD webpage of the WHO: http://www.who.int/classifications/icd/en/

DSM-IV

The classification system of the American Psychiatric Association (APA), the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), consists of five axes of disorders. Thereby it is suggested to the diagnostician not to focus only on one clinical disorder, but as well to consider other important aspects.

The five axes of DSM-IV are:

Axis I - Clinical Disorders (all mental disorders except Personality Disorders and Mental Retardation)

Axis II Personality Disorders and Mental Retardation

Axis IIISomatic (Non-Mental) Medical Conditions

Axis IV Psychosocial and Environmental Problems
(for example problems with primary support group)

Axis V Global Assessment of Functioning
(Psychological, social and job-related functions are evaluated on a
continuum between mental health and extreme mental disorder)

The main categories of clinical disorders (Axis I) according to DSM-IV are:

The classification of mental disorder: a simpler system for DSM - V and ICD - 11

How might the classifications become more rational?

In the past 15 years enormous progress has been made in understanding the genetics of mental disorders and the environmental factors that promote gene expression, in documenting abnormalities of brain function, in epidemiology and in gaining further insights into abnormal development. The aim of this article is to ask whether these advances might not impose some natural limits on the nature of the system, so that instead of becoming progressively more complex, a simpler classification might emerge.

The dimensional alternative

Multidimensional models have been around in psychiatry for many years. In the area of common mental disorders, scales such as the Symptom Checklist (SCL - 90; Derogatis 1976) provide a profile of scores on a number of scales thought relevant to these disorders. Modern equivalents are also available, such as the Psychiatric Diagnostic Screening Questionnaire (Zimmerman 2001). Both of these are self-report inventories, aimed at providing clinicians with a range of scores that may assist them in assessing the patient before them. The latter scale tends to use ‘top-down’ items derived from key symptoms in categorical DSM diagnostic criteria, and is aimed at traditional indicators relevant to screening tests, such as sensitivity and negative predictive value.

However, these are examples of pencil-and-paper tests that essentially try to present a system of categories in dimensional clothing. Those considering introducing dimensional measures to the DSM have more ambitious aims. At its simplest, they wish to produce simple, multi-point dimensional scales for widely distributed symptoms such as anxious mood, disturbed sleep, substance misuse, and suicidal thoughts and acts, and to have these rated for every patient seen.

A more ambitious alternative is to encourage clinicians to take account of the essentially dimensional nature of categorical diagnoses, so that cases of a particular disorder can be thought of as falling on a dimension ranging from no symptoms of that disorder present, through sub-threshold symptoms, to mild, moderate and severe degrees of a categorical diagnosis being present. The distinction between these grades of severity is mainly based on symptom counts. The ICD - 10 comes close to doing this already in the case of depressive episode, but the DSM takes an all-or-nothing, ‘you’re either depressed or you’re not’, approach. Even with relatively simple disorders such as depression, this fails to take account of the importance of the anxiety symptoms that commonly accompany depressive symptoms, so that a separate assessment may need to be made of these symptoms as well -  and one could easily continue and include other common symptom complexes, such as excessive concern with bodily functions, panic and obsessional symptoms.

With more complex disorders such as schizophrenia, numerous dimensions may need to be postulated to take account of the possible range of psychotic experience such as hallucinations, delusions, disorganisation, negative symptoms, impaired cognition, depression and mania. These dimensions would be in addition to the common symptoms which have to be rated for all disorders. If such dimensions were actually to form part of a future classification, the daily work of a clinician would be enormously increased for an arguable advantage, and the slide into endlessness would have begun in earnest.

There is clearly a distinction to be made between allowing what were simple, all-or-nothing categories to become dimensional concepts and attempts to capture the complexity of mental disorders with a huge, multidimensional net.

In practice of course, different clinicians need different sets of dimensions in order to make sense of their daily work. The set required by a hospital specialist or a general practitioner is not the same as that needed by an adult psychiatrist, and neither are the same as that needed by a child psychiatrist. This is not to suggest that there is an unmanageable number of possible dimensions -  merely that for any given clinician, the problem is finite.

Simple description of main problems, or multiple categories?

Karl Jaspers (1923) argued that there are three fundamental groups of mental disorders: known somatic disorders with psychic accompaniments and the major psychoses are examples of ‘disease entities’; but in addition to these there are the psychopathien or personality disorders, which comprise abnormal personalities and the neuroses. In this last group Jaspers argued that ‘there is no sharp line to be drawn between the types nor is there a decisive borderline between what is healthy and what is not. A diagnosis remains typological and multi-dimensional -  including a delineation of the kind of personality’ (Jaspers 1963 reprint: p. 611).

Jaspers seems to me to have got it almost exactly right. Clinical psychologists have for some time been tolerated as they take a ‘pick and mix’ approach to anxiety diagnoses, referring to such combinations as ‘agoraphobia with panic’, ‘generalised anxiety with social phobia’ or ‘specific phobias with panic’. Psychiatrists have been oddly reluctant to follow them, so that combinations such as ‘anxious depression’, ‘anxiety with somatic symptoms’, ‘depression with panic attacks’ or ‘somatic symptoms and pain problems’ are dealt with by diagnosing multiple categories. This approach assumes that several quite different disorders (comorbidity) have started simultaneously. A simple descriptive approach which notes the patient’s principal symptoms is a way of admitting that there is great overlap between common symptoms, and that combinations of the ‘pure’ categories are very common. For example, Lowe et al(2008) report that 6.6% of 2091 attenders in primary care clinics had a probable diagnosis of depression according to the PHQ - 9 test, but of these only 25.7% were above threshold for depression alone -  the remainder were also above threshold on tests for generalised anxiety and somatic symptoms.

The present proposals take account of the fact that superficially dissimilar disorders may have common aetiological roots, so that to some extent they may respond to similar therapeutic strategies. This is not to deny the undoubted differences between different disorders when seen in their pure form, unaccompanied by symptoms of other disorders. But a preparedness to also recognise that the range of a patient’s leading symptoms may go beyond the narrow confines of a single category may suggest different therapeutic approaches as well.

The present practice of rigid categories and counts of different ‘comorbid’ diagnoses in the same patient produces two major problems for nosologists: the assertion that the patient has the misfortune to have several different disorders present simultaneously, and the frequent use of the ‘not otherwise specified’ pseudo-category to take account of disorders that just fail to meet the diagnostic threshold. Both of these problems could be solved by the simple expedient of describing the patient’s main problems in simple descriptive terms.

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David Goldberg

DOI: 10.1192/apt.bp.109.007120 Published 3 January 2014