Aetiology

Genetic factors play an important role in the aetiology of bipolar disorder. There are increased rates of both bipolar and unipolar affective disorders among the families of people with bipolar disorder, and their first degree relatives have a 12% risk of developing a bipolar illness. Most twin studies have found concordance rates of around 60% for monozygotes and 20% for dizygotes. Numerous candidate genes have been identified and the inheritance of bipolar disorder is likely to be polygenic, with a strong gene–environment interaction.

There is a raised rate of adverse life events prior to manic episodes, although social factors appear to play less of a part in precipitating mania once several episodes have occurred. Disruption of biological rhythms, for example as a result of travelling across time zones, can sometimes precipitate mania and many people with bipolar disorder find that the frequency of manic relapse can be reduced if they maintain a regular sleep pattern. There is a raised rate of manic episodes in spring and early summer. Childbirth is a common precipitant of affective episodes in women with bipolar disorder (see ‘Puerperal psychosis’ section on p. 78).

Little is known of the neurochemical basis of mania. There is some limited evidence of increased monoamine transmission during manic episodes, involving dopamine, serotonin and noradrenaline. The effectiveness of antipsychotic drugs in the treatment of manic episodes suggests that an increase in dopaminergic activity may be involved.

Epidemiology

Bipolar disorder is much less common than unipolar depressive disorder, with a lifetime risk of around 1%. Women and men are at equal risk of developing bipolar disorder type I, but women are over-represented among type II cases. While some people are particularly creative and capable of high levels of achievement during periods of elevated mood, in general bipolar disorder is associated with high levels of functional impairment and is more common among people with low household incomes. Relatives of people with bipolar disorder are more likely to be high achievers than those of people with unipolar depression.

Course of illness

The mean age of onset is the late teens, whereas mean age of diagnosis is late twenties. The delay in diagnosis often results from manic episodes not being recognised as a manifestation of mood disorder. In other cases, early episodes are depressive in nature and it is not until a manic episode occurs that the bipolar nature of the condition becomes apparent.

Manic episodes typically last between 4 and 6 months, although this can vary considerably. Once someone has experienced a manic episode, it is likely that they will go on to have further affective episodes. The frequency and nature of these episodes varies greatly between individuals and some examples of relapse patterns in bipolar disorder are shown in Figure 1. The duration of remission between episodes tends to decrease over time. Clustering of episodes also occurs, with several episodes of mania and depression occurring close together, followed by a relatively long period of full recovery. ‘Rapid cycling’ can be viewed as an extreme form of clustering.

Fig. 1 Patterns of illness found in bipolar disorder.