A patient undergoing evaluation for glaucoma has a central corneal thickness of 478 micrometres in both eyes. Goldmann applanation tonometry records an intraocular pressure of 20 mmHg bilaterally. Based on the corneal findings, the true intraocular pressure is most likely:

• A Exactly 20 mmHg, since applanation is independent of corneal properties
• B Lower than 20 mmHg, since a thin cornea causes overestimation by Goldmann tonometry
• C Higher than 20 mmHg, since a thin cornea causes underestimation by Goldmann tonometry ✓
• D Unmeasurable accurately, requiring pneumotonometry instead of any correction

Explanation

Goldmann applanation assumes a central corneal thickness of about 520 micrometres per the modified Imbert-Fick principle. C cornea thinner than this resists applanation less, so less force is needed and the device reads falsely low. The true pressure in this patient is therefore higher than the recorded 20 mmHg, and thick corneas produce the opposite error. Pneumotonometry is also affected by corneal thickness, so switching devices does not remove the error.

Reference: Kanski's Clinical Ophthalmology, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.