Our model of depression

The North American DSM-IV classification system views depression as a single entity and differentiates conditions based on severity and duration.  Major depression – severe and present for more than two weeks, and dysthymia – moderate and present for more than two years.  The Black Dog Institute believes this severity-based approach is impractical both in theory and practice and is rarely used in other disciplines.

An alternative hierarchical model has been developed at the Black Dog Institute which assumes there are three subtypes to depression.  There are two distinct categories of essentially biological conditions (melancholic and psychotic depression) which have identifiable defining features, and a residual group of quite varying conditions (non-melancholic depression).  This residual group is therefore associated with varying presentations reflecting the contribution of life event stressors and personality style.

In all three subtypes there is a mood disorder component.  The key features include a depressed mood, decline in self esteem, self-criticism, the mood state is present for at least 2 weeks and causes social impairment.  The key feature which defines melancholic depression is observable psychomotor disturbance (PMD) – e.g. retardation and/or agitation together with a cognitive processing difficulty.  In psychotic depression, the PMD is generally more severe and combined with psychotic features such as delusions, hallucinations and/or over-valued ideas.

Based on research and clinical findings in response to treatment, we suggest that the non-melancholic depressions are principally underpinned by serotonergic dysfunction (affecting sleep, appetite, anxiety, irritability, and mood).  For the specific features (such as psychomotor change) evident in melancholic depression and the psychotic features in psychotic depression, there are additional noradrenergic and dopaminergic contributions respectively.

An understanding of the hierarchical model of depression helps to make logical and appropriate treatment decisions.

Hierarchical Model of Depression

The prevalence of each type of depressive disorder generally seen in general practice is:

• Non-melancholic – Common > 90 %
• Melancholic – Less common 2-10 %
• Psychotic – Rare 1-2 %

Model of Depression

Typically, two main profiles are seen in general practice:

• Depressions which are less common but nonetheless potentially fatal, such as psychotic, melancholic and bipolar depression.  On DSM-IV criteria, these are usually lumped together as ‘moderate’ or ‘severe’ Major Depression; and
• Depressions here termed ‘non-melancholic’ depression, are more common in the community and include a number of subtypes which reflect underlying personality styles.  On DSM-IV criteria, these are usually all lumped together as ‘mild’ or ‘moderate’ Major Depression.

It is important to note however, that non-melancholic depression as defined in our hierarchical model is a qualitative definition and can present at all levels of severity.

Useful References

Parker, G (2004), Dealing with Depression: A Common Sense Guide to Mood Disorders. (Second Edition) Allen & Unwin.  A comprehensive overview of depression from an Australian perspective.

Parker, G (2002), The Depressions. Australian Doctor (25th October 2002) provides a summary of the BDI approach to depression types and management approaches.