An understanding of the course of a disorder is fundamental for doctor and patient when deciding whether to start long-term prophylactic medication and, at a later stage, whether to stop a successful long-term treatment. Course is a crucial factor in estimating the social consequences, suicide risk, and mortality associated with mood disorders.
A distinction should be made between the natural history of a disorder, describing its spontaneous untreated course, and course as observed under treatment, whether of episodes or long term (both episodes and the intervals between them). Most mood disorders have a phasic course, characterized by multiple recurrences; a minority manifest as a single episode or have a chronic course.
The description of course includes the age of onset, episode length, recurrence of episodes, and residual symptoms between episodes and outcome (remission, chronicity, death). All these aspects are discussed in this chapter.
Our understanding of the course of affective disorders is limited for several reasons. The course of bipolar disorder and unipolar depression are markedly different; however, Kraepelin's(l) unification of bipolar disorder(2) with all affective disorders to form the single diagnosis 'manic depressive insanity' resulted in there being very little investigation of the natural history of the two subgroups (bipolar and depressive disorders) before the introduction of modern pharmacotherapy. Moreover, modern studies are carried out on treated patients, and the effect of drug-induced changes on the natural history of disorders is difficult to estimate. Another methodological limitation of modern studies of course is the selection of samples. Traditionally, samples have comprised hospitalized psychiatric patients, with a minority including psychiatric outpatients; studies following patients in primary care or in the community have been rare. Yet another methodological problem is that of memory artefacts. Although a retest study on the age of onset over a time span of 6 months showed good reliability, over the longer term patients significantly forget or misreport the previous history of their disorders, especially the precise age of onset and the number of earlier episodes, and there are no lifelong prospective studies of course in representative samples selected from the community or general practice to compensate for such distortions.
For all these reasons our understanding of the natural history and the course of mood disorders remains limited, especially in milder cases. Stability of the diagnoses of mood disorders
Ever since Kahlbaum(3) and Kraepelin(!) the course and outcome of mental disorders have played important roles as criteria and validators of psychiatric classification. Mood disorders can be roughly subclassified into bipolar disorder and depressive disorders. The two groups of disorders differ significantly as regards family history and course.(4,5 and 66
Distinguishing between bipolar disorder and unipolar depressive disorder is hampered by the fact that the diagnosis of unipolar depression is always uncertain. A long-term follow-up study over 27 years showed an annual rate of diagnostic change from depression to hypomania of about 1 per cent. The risk of such change seems not to fade but rather to be renewed with each fresh episode during a recurrent course. Therefore most studies conducted on unipolar depressive subjects include an unknown proportion of bipolar cases. For the same reason the exact ratio of bipolar to unipolar depressive subjects is unknown. Modern estimates range from 1:5 to 1:1. Thus many depressives may be hidden bipolar patients; these would include depressives with a positive family history of mania, those who have at some time manifested minor mood swings of hypomania (1-3 days), those who have shown so-called 'drug induced' hypomania, and depressive subjects with a hyperthymic or cyclothymic temperament or a cyclothymic mood disorder.
Today, bipolar disorder is conceptualized as a wide spectrum embracing all the milder forms of the disorder, ranging from cyclothymia and mild and brief hypomania, through hypomania, bipolar II disorder, and bipolar I disorder, to 'pure' mania. Bipolar II disorder shows good diagnostic stability over a 5-year period. (7) Nevertheless, over a patient's lifetime there may be frequent shifts across the diagnostic spectra of bipolar disorder and depressive disorders respectively. In the longer term the diagnosis of major depressive disorder remains stable in only 40 to 50 per cent of cases; in the remainder, diagnostic changes to dysthymia, recurrent brief depression, minor depression, and subthreshold depression are frequently observed, as well as recovery.
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