Tonometry and Keratometry Flashcards

A patient with a measured central corneal thickness (CCT) of 510 µm undergoes Goldmann applanation tonometry and reads 18 mmHg. Compared to a patient with a CCT of 545 µm and the same true IOP, which correction principle applies?

Answer: The thinner cornea causes GAT to underestimate IOP, so the corrected IOP is higher than 18 mmHg

Goldmann applanation tonometry assumes a standard CCT of ~545 µm. Thinner corneas require less force to applanate, causing GAT to underestimate the true IOP. With a CCT of 510 µm (35 µm below average), the actual IOP is higher than the 18 mmHg reading — roughly 1 mmHg per 10–15 µm deviation is a common correction estimate. This is clinically significant in glaucoma suspects.

During Goldmann applanation tonometry, the two fluorescein semicircles appear as shown: the inner edges of the mires are separated by approximately 1 prism diopter width and are irregular in shape. What is the MOST likely cause?

Answer: Corneal surface irregularity such as epithelial edema or superficial keratopathy

Irregular mire borders during GAT are a hallmark of corneal surface irregularities — epithelial edema, punctate keratopathy, scarring, or dry spots distort the fluorescein pattern and make the endpoint difficult to determine. Excessive fluorescein widens the rings (not irregularizes them). Lid squeezing elevates IOP but doesn't change mire shape. Contaminated prisms affect clarity but not this specific pattern of irregularity.

A keratometry reading on the right eye yields K1 = 44.25 D @ 180° and K2 = 46.75 D @ 90°. When fitting a spherical rigid gas-permeable (RGP) contact lens, the technician follows the 'fit-on-K' rule. What base curve should be selected, and what type of tear lens will result?

Answer: Base curve 44.25 D; a plus tear lens forms in the vertical meridian

Fitting 'on-K' for a spherical RGP means selecting the flatter K (44.25 D @ 180°). In the steeper vertical meridian (46.75 D), the cornea is steeper than the lens back surface, creating a pool of tears that forms a plus-powered tear lens. In the flat meridian the lens aligns with the cornea, producing a plano tear lens. The residual astigmatism equals the difference (2.50 D) and must be corrected by spectacles or a toric lens.

After photorefractive keratectomy (PRK), standard automated keratometry overestimates corneal power. Which of the following BEST explains this phenomenon?

Answer: PRK changes the anterior curvature but not the posterior cornea, invalidating the standard index of refraction (1.3375) used to calculate corneal power

Standard keratometers use a simplified refractive index of 1.3375 that accounts for both anterior and posterior corneal surfaces assuming a fixed ratio between them. PRK flattens the anterior surface without proportionally changing the posterior, disrupting this ratio. The calculated power using 1.3375 therefore overestimates actual corneal power. This 'keratometric error' is a central challenge in IOL power calculation after refractive surgery, requiring historical refraction or Scheimpflug-based methods.

A technician performs Schiotz tonometry on a patient and obtains a scale reading of 4.0 using the 5.5 g weight. The conversion table gives an IOP of approximately 24 mmHg. However, the patient has a history of high myopia and scleral staphyloma. Why should this reading be interpreted with particular caution?

Answer: Scleral rigidity is decreased in high myopia and staphyloma, causing Schiotz tonometry to underestimate true IOP

Schiotz tonometry is an indentation technique that displaces ocular volume. Its accuracy depends on ocular rigidity (the eye's resistance to deformation). Eyes with scleral staphyloma and high axial myopia have reduced scleral rigidity. A less rigid eye indents more easily, producing a higher scale reading for any given IOP — which the standard conversion table interprets as lower IOP. Thus Schiotz underestimates true IOP in low-rigidity eyes. This is a fundamental limitation that makes applanation tonometry preferable in such patients.

A manual keratometer is used to measure a patient's cornea. The technician notes that the reflected mires cannot be brought into sharp focus despite multiple attempts, and the mires appear doubled. Which of the following is the MOST likely explanation?

Answer: The patient has irregular corneal astigmatism, such as keratoconus, causing distorted and non-uniform mire reflection

In regular astigmatism, mires are clear but separated at different axis positions. In keratoconus or other irregular astigmatism conditions, the ectatic, non-spherical cone distorts the mire reflections — they appear blurred, irregular, or doubled and cannot be brought to a sharp endpoint regardless of instrument adjustment. This is a clinical red flag prompting corneal topography. Tear film instability causes fluctuating (not consistently doubled) mires. Exceeding the measurable range prevents the mires from aligning, but does not cause doubling.