A tonometry reading of 21 mmHg is obtained by Goldmann applanation tonometry in a patient whose central corneal thickness measures 480 micrometers. The true intraocular pressure is likely to be:
- A Lower than the measured value
- B Higher than the measured value ✓
- C Exactly equal to the measured value, since applanation is independent of corneal thickness
- D Higher only if the patient is myopic
Correct answer: B. Higher than the measured value
Explanation
Goldmann applanation assumes a central corneal thickness of about 520 to 540 micrometers. B thinner cornea offers less resistance to indentation, causing the tonometer to underestimate true pressure, while a thick cornea causes overestimation. At 480 micrometers the measured value of 21 mmHg therefore underestimates actual pressure, which is clinically important because eyes with falsely low readings may be misclassified as normal despite glaucomatous damage. Refractive error does not affect this relationship.
Reference: Kanski's Clinical Ophthalmology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.