title: Correlation of intraocular pressures in pairs of monozygotic and dizygotic twins.
abstract: OBJECTIVE
  Studies of twins may help to establish the importance of hereditary and environmental determinants of chronic diseases. Intraocular pressure (IOP) is a risk factor for glaucoma. The authors hypothesized that IOPs would be more highly correlated between monozygotic (MZ) than between dizygotic (DZ) twins.
  
  
  METHODS
  This was a concurrent, case-control study. Volunteer pairs of twins were recruited at a twins' festival. Zygosity was determined by a questionnaire that correlates well with blood-typing classification. Intraocular pressure was measured with a Tonopen six times within 5 minutes (3 times per eye) and averaged.
  
  
  RESULTS
  Mean IOP was 14.4 +/- 2.7 mmHg for 61 pairs of MZ twins and 13.9 +/- 2.6 mmHg for 32 pairs of DZ twins. The difference in IOP between the members of a pair was abbreviated delta IOP. The authors found a significant difference in median delta IOP between the MZ and DZ groups (1.2 versus 2.3 mmHg; P = 0.0014). Intraocular pressures were more highly correlated when considering right versus left eyes (intraclass correlation, r1 = 0.853; P < 0.001) than when considering the members of pairs of MZ twins (r1 = 0.735; P < 0.001), which in turn were more highly correlated than pressures of the members of pairs of DZ twins (r1 = 0.407; P = 0.006).
  
  
  CONCLUSIONS
  The intraclass correlation for IOP was greater for MZ than for DZ twins. In addition, the median intrapair difference (median delta IOP) was less for MZ than for DZ twins. These results suggest that hereditary factors may play some role in the determination of IOP.
authors.affiliation: Department of Ophthalmology, Medical College of Wisconsin, Milwaukee 53226, USA.
authors.forename: J W
authors.initials: JW
authors.lastname: Kalenak
authors.forename: F
authors.initials: F
authors.lastname: Paydar
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D016022:Case-Control Studies; D002648:Child; D005260:Female; D006801:Humans; D007429:Intraocular Pressure / Q000235:genetics* / Q000502:physiology; D008297:Male; D008875:Middle Aged; D011795:Surveys and Questionnaires; D014065:Tonometry, Ocular; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic