A patient returns one week after cataract surgery with a large myopic refractive surprise despite an apparently correct target refraction. Biometry had been performed using applanation A-scan ultrasonography. What is the most likely explanation?

• A Applanation ultrasound artificially shortens the measured axial length, leading to selection of an IOL power that is too high ✓
• B Applanation ultrasound artificially lengthens the measured axial length, leading to selection of an IOL power that is too high
• C Applanation ultrasound artificially lengthens the measured axial length, leading to selection of an IOL power that is too low
• D Keratometry, not axial length, determines the sign of the refractive error after applanation biometry

Explanation

Applanation probes indent and compress the cornea, falsely shortening the measured axial length. B shorter axial length entered into the formula yields a higher IOL power than required, producing a myopic surprise. Immersion techniques and optical biometry avoid corneal compression and give longer, more accurate axial lengths. Keratometry errors affect the result too, but they do not explain the systematic bias specific to applanation technique.

Reference: Khurana's Comprehensive Ophthalmology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.