Most bipolar and depressive episodes are short, but a minority become chronic (lasting more than 2 years); the distribution of episode length is log normal, and therefore percentiles and not averages should be used as parameters. Using the data collected a century ago by Mendel (1.0) and Ziehen(H) on the natural length of episodes of mania and bipolar disorder, mainly among hospitalized patients, it is possible to compute a median length of 4 to 6 months for mania and 5 to 6 months for bipolar disorder. Wertham (12) also reports a median length of 4 to 6 months based on analysis of 2000 manic attacks. These figures do not differ from those obtained today despite a wide range of antimanic and antidepressant treatments. Among hospitalized patients episode length (median) was 4.2 months for bipolars and 5.4 months for major depressives;(13) 25 per cent of bipolar episodes lasted more than 7.3 months and 25 per cent of depressive episodes lasted more than 11 months. By contrast, in the community, where many untreated cases of depression occur, episode duration was considerably shorter; the 25th, 50th, and 75th percentiles were 4 weeks, 12 weeks, and 30 weeks respectively for 71 incident episode duration, and were 4 weeks, 8 weeks, and 16 weeks respectively for 28 recurrent episodes.(14) These subsamples were small and the data on recurrent episodes were not corrected for total number of episodes.
In 10 to 20 per cent of subjects, mood disorders take a chronic course (length over 24 months) without remission.(15) Electroconvulsive therapy can curtail depressive episodes, whereas antimanic and antidepressant drugs only alleviate the symptoms; this is why termination of treatment during an ongoing episode induces a rapid relapse.
It is important to note that the lengths of episodes of bipolar disorder and of depression have probably remained constant for over a century. Recurrence
Recurrence is typical of mood disorders. It can be characterized by the number of episodes, the length of intervals (measured from remission to the onset of a new episode), and the length of cycles (measured from the beginning of one episode to the beginning of the next). Statistically, a normal distribution of cycle length can be obtained by log n transformation. In prospective studies, the length of the interval is frequently used as a parameter for survival analyses of recurrence.
In both bipolar disorder and unipolar depression the time from the first to the second episode is on average much longer than that from the second to the third. This progressive shortening of cycles and free intervals then levels off and fluctuates around a certain (but still variable) individual limit. Most published data on interval length or cycle length are methodologically flawed because they have not been corrected for the number of episodes/cycles observed, an improvement suggested by Slater(16) as early as 1938. Nonetheless, multiple episodes obviously follow each other in more rapid succession than a few episodes distributed over a lifetime. Even after taking episode numbers into account, there is a clear intraindividual trend to a progressive shortening of cycle length, (1, ,18) dimming the prognosis for both bipolar disorder and unipolar depression. Cycle length tends to be shorter in late-onset than in early-onset mood disorders, increasing the risk of recurrence in the elderly.
Precipitating events play an important role in the onset of the first few affective episodes; thereafter recurrrence seems to become more autonomous with stressful events contributing little or nothing to the process.(19) Stressors may not only precipitate episodes but also increase a pre-existent vunerability, sensitizing the individual and thereby making him or her more vulnerable to further episodes (kindling effect(20)). In bipolar illness there is no difference in the quality or quantity of stressors precipitating depressive and manic episodes; a legacy or the loss of a relative can induce depression or hypomania.
Counts of single-episode cases of mood disorders depend strongly on whether or not mild and brief hypomania and depression are included and on the length of observation. Single-episode bipolar disorders probably do not occur, and single-episode unipolar depression is observed in only 10 to 15 per cent of cases.
The course of the mood disorders of the overwhelming majority of patients, whether psychiatric inpatients, outpatients, or general practitioner patients, is recurrent; even milder depressive episodes tend to be recurrent.(21)
The pattern of recurrence is irregular. Compared with normal subjects, the daily mood ratings of bipolar subjects were characterized as a low-dimensional chaotic process and true cyclicity was not apparent in the power spectra of either group (normal subjects or bipolars). (22)
Over a lifetime bipolar patients experience twice as many episodes as unipolar depressives, a difference which is not explained by the manifestation of manic episodes in addition to depression. The total number of episodes observed depends on the length of observation. In a 22- to 26-year follow-up study, bipolar patients experienced a median of 10 episodes, whereas depressive patients experienced four episodes. (13) On average, 0.44 episodes per year were observed in bipolar patients and 0.30 in depressive patients.
A follow-up study of manic subjects applying survival analysis demonstrated relapse and recurrence rates of 20 per cent by 6 months, 48 per cent by 1 year, and 81 per cent by 5 years; patients with mixed/cycling features had a 10 per cent higher recurrence rate than pure manics. In a recent large representative Danish record study (N = 20 350 first admissions) unipolar depressives had strikingly lower recurrence rates (hospitalizations) than bipolars, both correlating with the number of previous episodes/,18 The authors concluded: 'The course of severe unipolar and bipolar disorder seems to be progressive in nature despite the effect of treatment and irrespective of gender, age and type of disorder'.
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