Simplifying biometry in oil-filled eyes: A novel formula for axial length calculation in eyes with 1000 cSt silicone oil

Purpose: To derive a formula for accurate axial length (AL) assessment using routine ultrasound in silicone oil-filled eyes, where optical biometry is unavailable or not possible. Methods: This was a prospective, consecutive, nonrandomized study of 50 eyes of 50 patients conducted in a tertiary care hospital in North India. AL measurements were taken using both manual A-scan and IOL master, both in silicone oil-filled status and 3 weeks after silicone oil removal. A correction factor of 0.7 was used for AL adjustment in oil-filled eyes. The corrected AL (cAL) was compared with IOL master values in oil-filled eyes. Agreement analysis was carried out using Bland Altman plot. Linear regression analysis was done using uncorrected manual AL to find a new equation. Data was analyzed using Stata 14. A P value <0.05 was taken as significant. Results: The study included 40 males and 10 females, with an age range of 6-83 years (mean 41.9 years). The mean AL of the oil-filled eye as measured by manual A-scan was 31.76 mm +/- 3.09 and by IOL master was 24.7 mm +/- 1.74. Linear regression analysis was performed in randomly selected 35 eyes of the observed data to obtain a new equation: predicted AL (PAL) = 14 + 0.3 x manual AL. The mean difference between PAL and optically measured AL with silicone oil in situ was 0.98 +/- 1.67. Conclusion: We propose a new formula for better prediction of the correct AL in silicone oil-filled eyes using ultrasound-based AL measurement.