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(Hypertension. 2009;53:e29.)
© 2009 American Heart Association, Inc.

Letters to the Editor

Response to Birth Factors and Retinal Vascular Caliber in a Twin Study

Centre for Eye Research Australia, University of Melbourne, Melbourne, Australia

Anne-Louise Ponsonby

Murdoch Children’s Research Institute, University of Melbourne, Melbourne, Australia, Menzies Institute, University of Tasmania, Tasmania, Australia

Alex W. Hewitt

Centre for Eye Research Australia, University of Melbourne, Melbourne, Australia

David A. Mackey

Centre for Eye Research Australia, University of Melbourne, Melbourne, Australia, Discipline of Medicine, University of Tasmania, Tasmania, Australia

An extract of the first 250 words of the full text is provided, because this article has no abstract.

We thank Cheung¹ for his interest in our article and, in particular, for highlighting the issues relating to the use of birth size as a sensitive marker of fetal growth restriction and its relationship with retinal vascular caliber.

The prevalence of prematurity is relatively high in twins compared with singletons, and, as such, we used birth weight for gestational score under the 10th percentile as a proxy for fetal growth restriction in our analysis. Although we could not adequately analyze birth weight relative to gestation in this study, because few twins had low birth weight for gestation and the majority of the twin pairs (67%) were concordant for birth weight by gestational scores, such analytic issues highlight the value of within-pair analyses. This method provides greater precision in controlling for gestation in a study of this size than other methods, such as adjusting for gestation in the regression analysis, because of collinearity between birth weight and gestation.

In addition, we observed that the association between gestation and retinal arteriolar caliber did not persist after adjusting for birth weight. Consistent with this finding, another study reported recently that low birth anthropometry, not prematurity, was linked to elevated blood pressure.² Clearly, additional support is required to confirm the associations identified in our relatively small study.

Finally, we thank Cheung¹ for his suggestion to include more direct measures of ocular magnification (eg, axial length) in the analysis rather than optic disc area. The reason that we included optic disc area in our models . . . [Full Text of this Article]