Axial Length Estimates May Correct OCT Magnification Errors

Using actual axial length (AL_act) measurements to correct optical coherence tomography (OCT) magnification errors is more accurate than using estimated axial length (AL_est) obtained from spherical equivalent (SE) and keratometry (K) measurements, according to research published in Eye and Vision. However, using AL_est to correct for magnification errors more accurately measures fundus parameters than uncorrected images, according to the report. 

Researchers conducted a retrospective, cross-sectional analysis of 1301 participants from the Raine Study Gen2-20 year follow-up (mean age, 20 years; 673 men). Patients underwent comprehensive ophthalmic exams with cycloplegic refraction and were divided into 2 datasets; 1 to create (n=650) and 1 to validate (n=651) an association between AL, SE, and K. Investigators developed a formula, applied it to 46 participants with available AL, SE, and K data, and compared the ability of AL_est and AL_act to correct transverse magnification error in OCT images measuring foveal avascular zone area (FAZA). 

An equation was created using multiple linear regression: AL_est = 2.102K−0.4125SE+7.268; R²=0.794. Overall, good agreement between AL_est and AL_act was noted among the cohorts. The mean difference between FAZA corrected with AL_est and AL_act was 0.002±0.015 mm² with the 95% limits of agreement (LoA) of −0.027 to 0.031 mm². The mean difference between uncorrected FAZA and FAZA measurements corrected with AL_act was − 0.005±0.030 mm² (95% LoA, −0.064−0.054 mm²).

“Our data suggest that AL_act and AL_est are comparable, especially when post-cycloplegic SE is used,” according to the researchers. “However, when applying AL in magnification error correction, AL_act is more accurate than AL_est and should be used preferentially in the clinical setting. In the absence of AL_act, clinicians should avoid relying on uncorrected measurements, and instead should use AL_est to correct for magnification error as this provides for a more accurate measurement of fundus parameters.”

Study limitations include the use of different instrumentation in determining K and SE within the cohorts, and an overrepresentation of participants who were of White ethnicity.