Goldmann applanation tonometry performed on two patients gives identical readings of 21 mmHg. Patient X has central corneal thickness of 585 micrometres and patient Y has 495 micrometres. Which statement is correct?

• A Both patients truly have an IOP of 21 mmHg since applanation cancels corneal thickness
• B Patient X has a falsely low reading and patient Y a falsely high reading
• C Corneal thickness affects indentation tonometers but not applanation tonometers
• D Patient Y has a higher true IOP than measured, while patient X has a lower true IOP than measured ✓

Explanation

The Imbert-Fick principle assumes an infinitely thin, perfectly flexible cornea. A thicker than average cornea resists applanation, so the tonometer reads falsely high, meaning the true IOP in patient X is lower than 21. A thin cornea gives a falsely low reading, so patient Y's true IOP exceeds 21. This is why post-refractive surgery eyes with thin corneas are systematically underestimated, a classic examination trap.

Reference: Parson's Diseases of the Eye, 23rd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.