During A-scan biometry for intraocular lens calculation, the contact method is known to cause corneal compression. If a measurement is taken using the contact method and then repeated using the immersion method on the same eye, how will the axial length measurement change?

• A It will be shorter on immersion A-scan.
• B It will be longer on immersion A-scan. ✓
• C It will remain exactly the same on both methods.
• D It will be longer only if the cornea is edematous.

Explanation

In contact A-scan biometry, the ultrasound probe indents the cornea, causing corneal compression and a falsely shortened axial length. Immersion A-scan avoids this indentation, providing a more accurate, slightly longer measurement. If a falsely short axial length is used for intraocular lens power calculation, the resulting lens power will be too high, leading to a hyperopic refractive surprise after cataract surgery.

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

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Written and medically reviewed by the StethoPrep medical team.