Abstract
PURPOSE: Twin studies, in the background of lack of twin registry in India, are a challenge. We report the response rates in tracing a hospital birth register-based cohort of twins using mail and door to door survey, ocular parameters influencing refractive errors, and their heritability. METHODS: Twins born from 1991 to 1995 in a tertiary hospital in the Vellore District of South India from a database were traced by mail and home survey from March 2011 to September 2013. Consenting live twin pairs who presented to the eye department were examined, including best corrected visual acuity, cycloplegic refraction, slit lamp examination, measurements of intraocular pressure (IOP) by Goldmann Applanation tonometry, axial length (AL) and anterior chamber depth (ACD), central corneal thickness (CCT), specular microscopy for endothelial count (EC), and a dilated fundus examination. Zygosity was determined by a validated zygosity questionnaire. Heritability was determined using the Falconers formula. RESULTS: Postal contact was made in 14%, and a home visit was successful in another 23% of the cohort. Of those presenting for examination, 51.3% were found to be monozygous. Heritability (h 2 ) of IOP was 0.34, that of ACD was 0.88, that of AL was 0.92, that of CCT was 0.96, and that of EC was 0.58. CONCLUSION: Tracing of twins using postal and home visits is challenging. Heritability for AL and CCT was found to be high, similar to that reported in other studies. This implies that a large proportion of variation found in AL and CCT is due to genetic causes as seen in twins from South India.