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Aetiology
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The aetiology of mood disorders is not fully understood but is clearly multi-factorial. It is probable that biological and genetic factors play an important role particularly in the aetiology of bipolar affective disorders and melancholic depression. The biological systems that are likely to be involved include monoamine systems, particularly 5-HT (5-hydroxytryptamine), and the hypothalamic-pituitary-adrenal axis.
Personality and psychosocial factors are probably more significant in dysthymic disorders.
Theoretical models often suggest an interaction between vulnerability, which is usually biological/genetic and precipitating factors, which may be a life event or biological (e.g. secondary to a viral illness, medication, hypothyroidism). The impact of the resulting stress will depend on social supports and the psychological capacity to cope.
Aetiological factors are summarised below under the headings:
- Biological: Genetic; Endocrine; Neurotransmitters
- Psychological: Psychodynamic; Cognitive; Behavioural
- Social: Predisposing; Precipitating
Biological.
Genetic:
In bipolar disorder twin studies show monozygotic concordance rates of ~70%; dizygotic rates of ~20%; monozygotic reared apart rates of ~66%.
Family studies in recurrent depressive disorder show an overall risk of ~7% for first degree relatives as compared to ~20% for first degree relatives of bipolars.
Endocrine:
Hypothalamic-pituitary adrenal axis - Many depressives have elevated plasma, CSF and urine cortisol. The diurnal variation of cortisol may be varied.
Thyroid function - The TSH response to TRH is impaired in some patients and in some with treatment resistant depression the addition of thyroxine may be beneficial. Hypo- and hyperthyroidism are recognised to cause alteration in mood.
Neurotransmitters:
In those individuals who develop an affective disorder a dysregulation of neurotransmitter systems is initiated with serotonin (5-HT) and noradrenaline the neurotransmitters most commonly implicated.
The disorders are not simply due to low levels of transmitters, but may be connected with alterations in the functioning of specific receptors.
Psychological.
Psychodynamic:
This emphasises the importance of loss, as in bereavement or separation, and also self-esteem and self-image. Psychoanalytic theory also views depression as a turning inward of aggression and hostility.
Cognitive:
This stresses the importance of cognitive distortions and errors that occur in depression. The cognitive theory of depression suggests that these are not just s result of the lowered mood but are instrumental in the origin and persistence of the disorder.
Behavioural:
Based on animal models where it has been noted that chronic stress can result in loss of ability to act and avoid the stress. These animals show behavioural and neurotransmitter system changes similar to that seen in depressed humans.
Social.
Predisposing:
Brown and Harris identified a number of factors that predisposed an individual to depression:
- lack of a confiding relationship;
- unemployment;
- 3 or more children under the age of 14 years at home;
- loss of mother before the age of 11 years
Precipitating:
In the 6 months after a life event (exit events - bereavement or separation; undesirable events - assault,redundancy etc.) the chance of an episode of depression is increased 5 to 6 times.
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