Abstract
Purpose: :
The peripheral retina may play a role in myopia progression. On- and off-axis eye length measurements can be used to determine eye shape and if shape changes occur due to optical treatments that manipulate peripheral defocus. The repeatability of eye length measurements was determined centrally and at off-axis locations using a low-coherence interferometer.
Methods: :
Eye length was measured (left eye) in twenty-nine adults with a Haag-Streit Lenstar. Five measurements were made centrally, 10° temporally, and 10° nasally on the retina by the same examiner at two separate visits at the same time of day. For off-axis measurements, subjects viewed targets affixed to the instrument. To assess inter-session repeatability, the difference versus the mean of each pair of measurements was plotted to determine the bias (mean of the differences relative to zero) and 95% limits of agreement (LoA). The within-subject standard deviation (Sw) of five consecutive measurements at a visit was calculated to assess within-session repeatability. Due to measurement variability noted at one peripheral location, the Sw was also determined on a subset of 10 subjects using five measurements at one visit 30° nasally and 30° temporally on the retina.
Results: :
The mean ± SD age and spherical equivalent refractive error were 24.0 ± 1.4 years and -3.46 D ± 2.69 D, respectively. The mean ± SD eye lengths (at visit 1) of the left eye were: central 24.91 ± 1.10 mm, 10° nasal 24.87 ± 1.17 mm, and 10° temporal 24.77 ± 1.07 mm. There was no significant bias for central, 10° nasal, or 10° temporal measurements between visits (all p > 0.10). The 95% LoA were: ±0.05 mm central, ±0.14 mm 10° nasal, and ±0.06 mm 10° temporal. The Sw were: 0.025 mm central, 0.045 mm 10° nasal, and 0.028 mm 10° temporal. The Sw in the subset of subjects with 30° measurements were 0.023 mm (30° nasal) and 0.029 mm (30° temporal), and mean ± SD eye lengths were 24.44 ± 1.48 mm (30° nasal) and 24.01 ± 1.09 mm (30° temporal).
Conclusions: :
Repeatability was similar centrally and off-axis, with the exception of measurements 10° nasally on the retina as indicated by the larger 95% LoA between visits and larger Sw. The Sw for measurements centrally and 30° off-axis were similar, suggesting the reduced repeatability 10° nasally is anatomical in nature. The location of Lenstar A-scan retinal peak gates 10° nasally suggest that increased nerve fiber layer thickness near the optic nerve contributed to the reduced repeatability. Despite greater variability 10° nasally, the off-axis repeatability is still superior to the previously reported repeatability of on-axis ultrasonography.
Keywords: clinical research methodology • myopia • refractive error development