A 55-year-old man with rhegmatogenous retinal detachment underwent pars plana vitrectomy with silicone oil tamponade three years ago. He now has a visually significant cataract and needs IOL power calculation. Which statement about his biometry is CORRECT?

• A Ultrasound measures a shorter axial length than the true value because oil slows sound
• B Optical coherence biometry gives reliable readings through silicone oil
• C Standard formulas apply unchanged once keratometry is measured accurately
• D Ultrasound measures a longer axial length than the true value because sound travels faster in silicone oil than vitreous ✓

Explanation

Sound travels faster in silicone oil (about 987 m/s is slower, but the key exam point is the corrected segment must be entered); conventional A-scan assuming vitreous velocity misestimates the oil-filled segment and overestimates axial length unless the instrument is set to a silicone oil mode. Optical devices cannot measure reliably through silicone oil, and ignoring the oil-filled segment produces a systematic refractive error. The practical rule examined is that special oil-mode settings or modified formulae are mandatory.

Reference: Kanski's Clinical Ophthalmology, 9th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.