Article

Age-at-onset in bipolar-I disorder: mixture analysis of 1369 cases identifies three distinct clinical sub-groups.

Biostatistics and Bioinformatics Unit, Wales School of Medicine, Cardiff University, Heath Park, Cardiff, CF14 4XN, United Kingdom.
Journal of Affective Disorders (Impact Factor: 3.76). 07/2009; 116(1-2):23-9. DOI: 10.1016/j.jad.2008.10.021
Source: PubMed

ABSTRACT To assess whether bipolar disorder type I segregates into three clinically distinct sub-groups defined by age-at-onset.
Clinical data were available on 1369 individuals with DSM-IV bipolar I disorder. Mixture analysis was performed on the age-at-onset (AAO) data to determine whether they were composed of more than one normal distribution. Individuals were allocated to groups according to the results of the mixture analysis. Categorical logistic regression was then used to investigate relationships between AAO and nine clinical characteristics.
The distribution of AAOs in our sample comprised a mixture of three normal distributions with means of 18.7 (SD=3.7), 28.3 (SD=5.5) and 43.3 (SD=9.1) years, with relative proportions of 0.47, 0.39 and 0.14 respectively. Individuals were allocated into three groups dependent on their AAO: < or = 22; 25-37; and > or = 40 years, producing a sample of 1225 individuals (144 with borderline values were excluded). Eight out of the nine clinical characteristics showed evidence for a statistical association with AAO group.
Systematic and non-systematic recruitment of participants. Some data relied on retrospective recall.
Our results provide further robust evidence to suggest that the AAO distribution of individuals affected with bipolar disorder is composed of three normal distributions. Substantial clinical heterogeneity between the three AAO groups may reflect genetic heterogeneity within bipolar I disorder. Future genetic studies should consider AAO grouping as potential sub-phenotypes.

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    ABSTRACT: Background Age-of-onset (AO) is increasingly used in molecular genetics of bipolar I disorder (BP-I) as a phenotypic specifier with the goal of reducing genetic heterogeneity. However, questions regarding the cut-off age for defining early onset (EO), as well as the number of onset groups characterizing BP-I have emerged over the last decade with no definite conclusion. The aims of this paper are: 1) to see whether a mixture of three distributions better describes the AO of BP-I than a mixture of two distributions in different independent samples; 2) to compare the morbid risk (MR) for BP-I and for major affective disorders and schizophrenia in first degree relatives of BP-I probands by proband onset group derived from commingling analysis, since the MR to relatives is a trait with strong genetic background. Methods We applied commingling (admixture) analysis to the AO of three BP-I samples from Romania (n=621), Germany (n=882), and Poland (n=354). Subsequently, the morbid risk (MR) for BP-I and for major psychoses (BP-I, BP-II, Mdd-UP, schizoaffective disorders, schizophrenia) was estimated in first degree relatives by proband AO-group derived from admixture analysis in the Romanian sample. Results In the three independent samples and in the combined sample two- and three-AO-group distributions fitted the empirical data equally well. The upper EO limit varied between 21 and 25 years from sample to sample. The MR for both BP-I and for all major psychoses was similar in first degree relatives of EO probands (AO≤21) and in relatives of intermediate-onset probands (AO=22–34). Significant MR differences appeared only when comparing the EO group to the late-onset (LO) group (AO>34). Similar to Mdd-UP and schizophrenia, a significant MR decrease in proband first degree relatives was visible after proband AO of 34 years. Under the three-AO-group classification the MR for both BP-I and all major psychoses in first degree relatives did not differ by relative sex in any proband AO-group. Under the two-AO-group classification female relatives of LO probands (AO>24) had a significantly higher MR for all major psychoses than male relatives, while there was no sex difference for the relatives of EO probands. Limitations MR was not computed in the German and Polish samples because family data were not available and 34% of the relatives of the Romanian probands were not available for direct interview. Conclusion Similar to other clinical traits, the MR for major psychoses to relatives failed to support a three-AO-group classification in BP-I suggesting that this is not more useful for the molecular analysis than a two-AO-group classification.
    Journal of Affective Disorders 10/2014; 168:197–204. DOI:10.1016/j.jad.2014.06.054 · 3.71 Impact Factor
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    Journal of Affective Disorders 10/2014; 168:197-204. DOI:10.1016/j.jad.2014.06.05 · 3.76 Impact Factor