Here is an excerpt from a manuscript I have written that includes a
summary of the epidemiology of cyclothymia.
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Akiskal (1983) notes that while cyclothymia differs from bipolar disorder
in several ways such as:  (1) a less developed symptom profile with less
severity; (2) the extremely rare occurrence of psychotic features; (3)
brief cycles; and (4) an irregular cycling pattern with few normal mood
periods, cyclothymics do have some striking similarities to bipolar
patients.  
 
These include: (1) a shared phenomenology of illness; (2) familial
prevalence similar to bipolars; (3) continued cycling in the absence of
chemotherapy; and (4) induced hypomania upon administration of
antidepressant drugs.  Although cyclothymia has been conceptualized as
a characterological disorder, Akiskal et. al. (1979) consider this
description to stem from the incorrect assumption that social
factors  including substance abuse, interpersonal conflict and episodic
promiscuity are primary rather than secondary to the affective
dysfunction.
 
Further, during the course of the illness, cyclothymics like bipolars often
experience intermittent difficulties with hypersomnia and hyposomnia,
fluctuation in self-esteem ranging from disparagement to grandiosity,
alternating intervals of mental confusion and indifference followed by
mental sharpness, uneven performance on the job and recurrent periods
of ntroversion or uninhibited social activity.
 
In addition to these similarities, Akiskal (1983) and Akiskal et al.
(1979) feel the most compelling evidence for classifying cyclothymia as
an affective disorder and linking it to bipolar illness has been the family 
studies, response to chemotherapy and the relapse rate following
treatment. 
 
Comparing the rate of bipolar illness among first degree relatives of
patients diagnosed with either cyclothymia or Bipolar I, 30% of the 
cyclothymic relatives and 26% of the  Bipolar I relatives also earned
diagnoses of bipolar illness.  Considering now the response rate to 
antidepressant chemotherapy, 60% of the cyclothymics while only 20%
of the non-affective personality disorder patients improved with 
administration of lithium. Additionally, hypomania was often induced by
the drug intervention.  Following the end of chemotherapy treatment,
36% of the cyclothymics began to experience more severe cycling while
only 5% of the non-affective personality disorder group went on to
develop the biphasic course of the illness.
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Basically, if you wish to go to the source, the work of Akiskal and the
work of Depue has been the most helpful to me in getting up to speed
with the study of cyclothymia.
 
Good Luck!
 
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=  David M. Fresco                                          =
=  Department of Psychology                                 =
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=  Internet:  fresco@unc.edu              `,/'(*)           =
=             fresco@med.unc.edu         (*) . ./"""        =
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