Letters to the Editor
Age at Onset of Bipolar II Disorder
Dear Editor:
Recently, 3 subgroups of bipolar I disorder (BD I), distinguished by different ages at onset, were found (1). Bipolar II disorder (BD II) has been found (according to Kolmogorov–Smirnov test) to have an age-at-onset distribution similar to that of BD I (2). Nevertheless, family studies (an important diagnostic validator) also support a distinction between the 2 disorders (3). The aim of the study reported here was to test whether BD II has 3 age-at-onset subgroups, as reported for BD I.
The study setting, patients, and interview methods are reported in detail elsewhere (4). The study setting was a private practice that has for many years studied BD II (see Benazzi F on PubMed/Medline). The study sample comprised 320 consecutively presenting outpatients (mean age of 41.6 years, SD 13.4) with BD II, 50.2% of whom had a family history of BD, and 68.4% of whom were women. Age at onset of the first major depressive episode (MDE) was assessed with the Structured Clinical Interview for DSM-IV-Clinician Version (5), often supplemented by interviews of family members or close friends.
With regard to onset-age of the first MDE, the mean (SD) age was 22.8 years (10.6), the median age was 20 years, and the age range was 4 to 67 years. Histogram and kernel density estimates were employed to study distribution of age at onset. A histogram results in disproportional representation of density at the centre and in the tails of the distribution, whereas kernel estimators are nonparametric histogram smoothers revealing multimodality. A histogram provides accurate pictures of categorical variables; univariate kernel density estimate is better to represent continuous variables (STATA 7 statistical software; 6). For this sample of patients with BD II, both histogram and kernel density estimate showed 3 age-at-onset subgroups: around age 19 years, around age 27 years, and around age 35 to 40 years (figures available on request from the author). Onset before age 20 years was present in 45.0% (144/320), onset between age 19 and 35 years in 42.8% (137/320), and onset after age 35 years in 12.2% (39/320).
Discussion
The similar results achieved from 2 different statistical approaches compared with the Bellivier and others study support the validity of these findings. Age-at-onset distribution in BD II was found to be similar to that in BD I, as reported by Bellivier and others (1). Because onset is an important diagnostic validator (2,7,8), finding 3 similar age-at-onset subgroups in BD I and BD II supports the hypothesis of a closer link between BD I and BD II. When more advanced statistical methods are used, the findings do not replicate the bimodality of onset that Kraepelin describes for manic-depressive insanity (8).
References
1. Bellivier F, Golmard J-L, Rietschel M, Schulze TG, Malafosse A, Preisig M, and others. Age at onset in bipolar I affective disorder: further evidence for three subgroups. Am J Psychiatry 2003;160:999–1001.
2. Benazzi F. A comparison of the age of onset of bipolar I and bipolar II outpatients. J Affect Disord 1999;54:249–53.
3. Coryell W. Bipolar II disorder: the importance of hypomania. In: Goldberg JF, Harrow M, editors. Bipolar disorders. Clinical course and outcome. Washington (DC): American Psychiatric Press; 1999. p 219–36.
4. Akiskal HS, Benazzi F. Family history validation of the bipolar nature of depressive mixed states.
J Affect Disord 2003;73:113–22.
5. First MB, Spitzer RL, Gibbon M, Williams JBW. Structured Clinical Interview for DSM-IV Axis I Disorders-Clinician Version (SCID-CV). Washington (DC): American Psychiatric Press; 1997.
6. Salgado-Ugarte IH, Shimizu M, Taniuchi T. Exploring the shape of univariate data using Kernel density estimators. STATA Technical Bulletin 1994;16:8–19.
7. McMahon FJ, Stine OC, Chase GA, Meyers DA, Simpson SG, DePaulo JR. Influence of clinical subtype, sex, and lineality on age at onset of major affective disorder in a family sample. Am J Psychiatry 1994;151:210–5.
8. Kraepelin E. Manic-depressive insanity and paranoia. Edinburgh (UK): E & S Livingstone; 1921.
Franco Benazzi, MD
Forli, Italy
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