Objectives Studies of twins in BD have been few.^1,2 Our clinical impression has been that patients with BD had a paucity of twin siblings. We studied the frequency of twin siblings among the BD patients in a controlled protocol.

Methods 1277 patients (773 males and 504 females with a mean age of 34.20 ± 9.59 years) attending a dedicated BD outpatient clinic were asked whether they had a twin sibling, living or dead. Inquiry into monozigosity was made by asking whether this twin (s) looked exactly like the proband as ‘two peas in a pod’.³ As controls the same information was sought among i. a group of patients with either rheumatoid arthritis (RA) or systemic lupus (SLE), composed of 180 patients with RA -150 males and 30 females with a mean age of 49.08 ± 14.05 years- and 79 patients with SLE -64 females and 15 males with a mean age of 34.80 ± 12.33 years- all attending a rheumatology outpatient clinic; and ii. among 7761 first year university students -3848 males and 3913 females with a mean age of 18.55 ± 1.38 years- during their university registration. Chi-square test (2_df) was used to compare the findings between the three groups.

Results Among the 7761 university students, 120 twins (1.5%) were identified. 92 were dizygotic (1.2%) and 28 (0.38%) monozygotic. Among the 259 patients with connective tissue disease there were 7 (2.7%), all dizygotic twins. Two twins belonged to the SLE and 5 to the RA patients. Among the 1277 patients with BD there were 10 twins (0.08%; x² _2df = 7.17, p = 0.03). Eight were dizygotic and 2 (0.15%; p > 0.12) were monozygotic. The monozygotic twins were discordant for BD in either case.

Conclusion The frequency of twin births among our controls was similar to that reported in general for twin-births.⁴ The rather marked decrease, 19 fold when compared to the group of university students and 34 fold when compared to the group of patients with RA or SLE, in the twin frequency among the BD patients could be due to the presence of in utero factors operative in the mothers of patients with BD, unfavourably affecting the successful initiation and/or completion of a twin pregnancy.

References

1. Hamuryudan V, et al. Arhtritis Rheum. 1991;34(8):1071-2

2. Gül A, et al. Br J Rheumatol. 1997;36(8):922–7

3. Cederlöf R, et al. Arch Environ Health 1967;14:401

4. Mac Gillivray I. Semin Perinatal. 1986;10(1):4–8