A COMPARISON OF NONCONTACT A-SCAN AND ULTRASOUND A-SCAN IN THE MEASUREMENT OF AXIAL LENGTH IN MYOPIC SUBJECTS

Abstract

Introduction: Myopia, which is also known as nearsightedness, is a common type of refractive error among the majority of the population. It is characterized by clear vision up close but blurry vision at a distance. Myopia is caused by an eyeball that is abnormally longer than normal and cornea that is too steeply curved. In both cases, images are focused in front of the retina instead of on it. With abnormal axial length of the eyeball, myopia has developed into a condition that threatens eyesight.

Methodology: In this study, we compared the axial length measurements between non-contact biometry, and ultrasound A-scan among young myopic adults in UKM Kuala Lumpur campus. A total of 90 healthy myopic subjects (14 male and 76 female) with mean age of 22.03 ± 1.14 years were enrolled in this cross-sectional study. Clinical investigations that included visual acuity, refraction and axial length measurement using optical and ultrasound biometry were carried out. Results: The mean spherical equivalent refractive error, axial length measured by optical biometry and ultrasound were found to be -3.04 ± 1.61 D, 24.74 ± 0.90 mm and 24.50 ± 0.86 mm respectively. Paired sample t-test showed that subjects’ axial length measured by ultrasound A-scan were significantly lower than optical biometry (p < 0.05). Negative and strong correlations were found between degree of myopia and axial length (r_s = -0.609, p < 0.001). Regression showed that axial length measurement accounted for a significant 33.5% of the degree of myopia, R² = 0.335, adjusted R² = 0.328, F = (1, 88) = 44.38, p < 0.01.

Conclusion: The results of this study demonstrated a significant difference in measurements of axial length using non-contact A-scan and ultrasound A-scan in myopic subjects, which must be considered during management of myopic patients.