Towards a dimensional approach to common mental disorders in the ICD-11?

ICD Insight

One of the major advantages of the DSM and ICD systems is that they allow us to decide whether a patient has a mental disorder or not. That gives clarity to patients about what is happening to them, and it helps psychiatrists and other mental health professionals to choose which treatment is most appropriate and to communicate with other professionals in a common language. It has also helped psychiatry in general, because it has given a uniform description of mental disorders, which has led to an enormous increase in knowledge about these disorders and their treatment in the past decades. Without systems like the DSM and ICD, such enormous progress would not have been possible.

However, one of the main problems of such categorical thinking, especially in relation to common mental disorders like depression and anxiety disorders, is that there are no good thresholds for deciding whether a patient has the disorder or not. Most of these disorders can be better explained by continuous scales, ranging from no symptoms at one end to very severe symptoms at the other. Furthermore, some disorders, such as depression, are so broadly defined that the symptom patterns of patients show very large differences, with considerable clinical heterogeneity among patients. In addition, comorbidity among these disorders is so high that one may wonder whether these disorders are really separate entities, or just one cluster of symptoms with varying profiles. It has been estimated that more than 50% of patients with a mental illness suffer from a comorbid second depressive or anxiety disorder.

That common mental disorders can be better understood as continuous instead of categorical problems has been established best in depression. For example, it has been shown that subclinical depression in which patients have depressive symptoms without meeting criteria for major depression, also have decreased levels of health-related quality of life, increased use of health services and increased economic costs when compared with healthy controls (Cuijpers et al., 2004). When people with subclinical depression are compared with those who have a major depressive disorder, it is typically found that on an individual level health-related quality of life, health service use and economic costs are more affected in major depression than in subclinical depression. However, because the prevalence of subclinical depression is higher than that of major depression, the impact on economic costs, health care costs and even mortality is comparable with the impact of major depression on a population level (Cuijpers et al., 2013).

The DSM-5 has included a category of subclinical depression, but that does not really solve the problem. This just adds one more category, including new thresholds, to the continuum of depressive symptomatology, while in fact there are no clear thresholds just because it is a continuum. This also does not solve the problem of the high levels of comorbidity among depressive and anxiety disorders, or the problem of the large differences in symptom profiles in depressed patients.

A much better solution, which could be adopted by ICD-11, would be to use a dimensional approach to common mental disorders (Okasha, 2009; Widiger and Samueal, 2005; Watson, 2005). In such an approach the different dimensions of depression, anxiety, worrying, phobia, distress and others can be assessed. Depending on the severity on each of these dimensions a treatment or combination of treatments can be chosen. This dimensional approach could be used either instead of a categorical approach or in combination with it. When it is used instead of a categorical approach, several advantages of this approach will be lost. It will no longer be possible to decide whether a patient has a mental disorder or not, the communication with the patient and among professionals will be complicated, and the varying dimensions will complicate trials examining the effects considerably.

An alternative solution would be to retain a threshold for the disorders, but combine this with a dimensional approach. So, patients would still receive one or more diagnoses for common mental disorders, but at the same time the most important dimensions can be assessed. This may lead to a more precise diagnosis and more focused interventions, while retaining the advantages of the categorical model. A purely dimensional approach is probably better empirically, but a combined dimensional and categorical approach may be more practical and feasible in the current health care systems. The combined dimensional and categorical approach that was included in the DSM-5 for personality disorders is a good example of how this could be done in the ICD-11 for the common mental disorders.

It is a pity that the dimensional approach in addition or as an alternative to the current categorical approach was not pushed forward in the DSM-5. The ICD-11 offers a new opportunity, and I am sure we all hope that this is not also squandered.