Tonometry and Keratometry Flashcards

A patient with keratoconus has a central corneal thickness of 430 µm measured by pachymetry. When Goldmann applanation tonometry (GAT) reads 14 mmHg, what is the most likely true IOP after applying a standard corneal thickness correction factor?

Answer: Approximately 17–18 mmHg

Thin corneas cause GAT to underestimate true IOP. The standard correction adds roughly 1 mmHg per 10 µm below the 545 µm population average. At 430 µm (115 µm below average), the correction is approximately +3 to +4 mmHg, bringing the estimated true IOP to about 17–18 mmHg. Keratoconus patients are at higher glaucoma risk, so underestimation is clinically significant.

During Goldmann applanation tonometry, the examiner notes the mire semicircles are unusually wide and blurry. Which fluorescein instillation error best explains this finding?

Answer: Too much fluorescein, causing overly wide, thick mires

Excessive fluorescein produces wide, thick mire rings because the large fluid meniscus inflates the apparent contact area, leading to falsely LOW IOP readings. Too little fluorescein causes thin, dim mires and falsely HIGH readings. The ideal mire width is approximately 0.25 mm, with inner edges just touching at the correct endpoint.

A keratometry reading yields principal meridian powers of 42.00 D @ 180° and 46.50 D @ 090°. What type of astigmatism is present, and what is the axis of the minus cylinder correction needed?

Answer: With-the-rule astigmatism; minus cylinder axis at 180°

With-the-rule (WTR) astigmatism means the vertical meridian (090°) is steeper (more powerful at 46.50 D) than the horizontal (180°). In minus cylinder notation, the cylinder axis is placed at the meridian of greatest power — the steeper meridian is 090°, so the minus cylinder axis is at 180°. Wait — the steepest meridian is 090°, so the minus cylinder axis is placed perpendicular to the steepest meridian, which is 180°. This is WTR because the steeper curve is vertical.

Non-contact tonometry (NCT) consistently measures IOP 4–6 mmHg HIGHER than Goldmann applanation in the same patient. Which corneal characteristic is most likely responsible?

Answer: Elevated corneal rigidity due to high central corneal thickness

Non-contact tonometry uses an air puff to indent the cornea and detects the applanation point optically. A thicker, more rigid cornea resists deformation more, requiring greater air pressure and resulting in artificially elevated NCT readings. Goldmann applanation is less affected because it directly contacts the cornea. High CCT is the classic cause of NCT overestimating true IOP relative to GAT.

When using a manual Javal-Schiøtz keratometer, the examiner cannot bring the mires into sharp focus despite adjusting the eyepiece. The most likely cause is:

Answer: Uncorrected refractive error of the examiner not compensated by the eyepiece

The Javal-Schiøtz keratometer has a focusing eyepiece that the examiner must adjust to their own refractive error before measuring the patient. If this diopter correction is not set, the examiner sees blurry mires regardless of the patient's cornea. Corneal irregularity distorts mire shape but does not prevent focus. PD settings are not a feature of standard keratometers.

A patient is being measured with Goldmann applanation tonometry immediately after a corneal refractive surgery (LASIK) that removed tissue from the central stroma. Compared to the pre-LASIK reading, the post-LASIK GAT measurement will most likely:

Answer: Underestimate IOP due to reduced central corneal thickness and altered corneal biomechanics

LASIK reduces central corneal thickness (CCT) and alters corneal biomechanics, making the cornea easier to applanate. This causes GAT to underestimate true IOP — sometimes by 4–8 mmHg depending on the ablation depth. This is clinically critical because post-LASIK glaucoma suspects may appear to have normal IOP when it is actually elevated. Corneal hysteresis is also reduced post-LASIK, compounding the error.