In patients admitted to hospital between 1913 and 1940 and not treated by electroconvulsive therapy or modern psychotropic drugs, bipolar disorder clearly manifested earlier than unipolar depression;(8) this finding is confirmed by modern community studies. Bipolar disorder generally begins during adolescence but may manifest even earlier.
Epidemiological studies identify the age of onset of bipolar disorder as between 15 and 19 years (means), whereas studies of hospitalized patients date its onset in the early twenties or in the thirties. The age-of-onset curve is skewed, and therefore mean values are not representative. In a large Canadian general population study,(9) 95 per cent of manic cases manifested before the age of 26 (males) and 25 (females), and 95 per cent of major depressive episodes before the age of 55 (males) and 43 (females). There was a considerable time lag between the age at onset of the first impairing symptoms (15 years), diagnosis (19 years), first treatment (22 years) and hospitalization (26 years).
Bipolar I illness manifests earlier than bipolar II and psychotic bipolar disorder. Late-onset bipolar disorder is rare but does occur and may be associated with specific neuropathology.
Unlike bipolar disorder, depression may start at any time of life. There is no dichotomy but a continuum from early-onset to late-onset depression, with a systematic decrease in genetic vulnerability and an increase in precipitation by environmental factors. Late-onset depression is more often milder and chronic.
A two-peak distribution of the age of onset has sometimes been described for both bipolar disorder and depression, but there is no second peak linked with the menopause in women.
Prodromal hypomanic and depressive symptoms and subclinical syndromes frequently precede mania and major depression over many years; this is especially true for depression in old age (75 years and more). The first manic and depressive manifestations are commonly mild, brief, or uncharacteristic, and are only diagnosed in retrospect.
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