Tonometry and Keratometry Flashcards

During Goldmann applanation tonometry, the fluorescein mires appear unusually thick and wide. What is the most likely effect on the recorded IOP?

Answer: Falsely low IOP, because excess tear film increases the surface tension pulling the prism toward the cornea

Excess fluorescein increases the attractive meniscus force between the applanation prism and the corneal tear film. This surface tension effectively 'pulls' the prism inward, so less externally applied force is needed to reach the split-mire endpoint — yielding a falsely low IOP reading. Conversely, too little fluorescein produces thin mires and a falsely high reading. Mire width is a direct quality indicator for measurement validity.

Goldmann applanation tonometry was calibrated for a corneal radius of curvature of 7.8 mm. A patient presents with a central corneal radius of 6.9 mm. What is the expected effect on the IOP measurement?

Answer: IOP will be falsely high; a steeper cornea is biomechanically stiffer and resists applanation more than the calibration standard

Goldmann calibration assumes an average corneal radius of 7.8 mm. A radius of 6.9 mm indicates a steeper-than-average cornea. Steeper corneas have greater intrinsic curvature resistance (bending stiffness), requiring more externally applied force to achieve the standard 3.06 mm applanation diameter — resulting in a falsely HIGH IOP reading. Flat corneas (radius > 7.8 mm) yield falsely low readings for the opposite reason.

A patient with prior myopic LASIK presents for cataract pre-op evaluation. Standard Javal-Schiotz keratometry is performed to calculate IOL power. Which outcome most accurately describes the expected error and its clinical consequence?

Answer: Keratometry will overestimate corneal power, causing an underpowered IOL and a hyperopic postoperative refraction

Standard keratometry samples an annular ring (~3 mm zone), which is on the shoulder of the LASIK ablation — not the flattest central zone. Furthermore, the assumed refractive index of 1.3375 encodes a fixed anterior-to-posterior corneal curvature ratio that is disrupted by LASIK (which flattens the anterior surface while leaving the posterior unchanged). Both factors cause keratometry to report a HIGHER corneal power than actually exists. Plugging this inflated K into standard IOL formulas yields a weaker IOL than needed, resulting in a hyperopic surprise — one of the most clinically significant errors in post-refractive cataract surgery.

Compared to Goldmann applanation tonometry as the gold standard, which statement most accurately characterizes the systematic bias of the Tono-Pen across the clinical IOP range?

Answer: It overestimates low pressures and underestimates high pressures, regressing toward the mean relative to Goldmann

The Tono-Pen exhibits a classic regression-toward-the-mean bias: it OVERESTIMATES IOP at low pressures (e.g., 30 mmHg) when compared to Goldmann. This is clinically important in glaucoma management — a falsely reassuring Tono-Pen reading of 22 mmHg may represent a true IOP of 28 mmHg in a hypertensive eye. The Tono-Pen is most accurate in the 10–20 mmHg range but should not replace Goldmann for definitive pressure management decisions.

While performing keratometry on a patient with suspected keratoconus, you record K readings of 45.00 D @ 180 / 47.50 D @ 090 with mildly distorted mires. What is the most critical limitation of relying on this keratometry data to characterize the cone?

Answer: Standard keratometry samples only an annular zone approximately 3.0–3.2 mm in diameter and may entirely miss a cone apex that is displaced centrally or inferiorly within that zone

A standard keratometer samples only the paracentral annular zone (~3 mm), measuring four points (two per meridian) and ignoring the central cornea entirely. Because the keratoconic cone apex frequently lies centrally or infero-nasally — well within the zone the keratometer skips — a standard K reading may appear relatively normal even in a significantly ectatic eye. This is why corneal topography (Placido disc or Scheimpflug imaging) that maps the full surface is mandatory for keratoconus diagnosis and progression monitoring. Relying on keratometry alone in suspected keratoconus risks missing or underestimating the true severity.

Schiotz indentation tonometry is performed on a 28-year-old patient with −14.00 D myopia, a 27 mm axial length, and thin sclera. Compared to simultaneous Goldmann applanation tonometry, the Schiotz reading is most likely to be:

Answer: Falsely low, because high myopia is associated with reduced ocular rigidity, allowing greater indentation per unit of true IOP

Schiotz tonometry measures the depth a weighted plunger indents the cornea; the reading is then converted to IOP using Friedenwald's conversion tables, which assume normal ocular rigidity. High myopia is strongly associated with REDUCED ocular rigidity (the globe is larger, thinner-walled, and more distensible). A low-rigidity eye deforms more easily under the plunger weight, producing greater indentation for a given true IOP — which the conversion tables interpret as a LOWER pressure than actually exists. Goldmann applanation tonometry is minimally affected by ocular rigidity (it applies only enough force to applanate 3.06 mm), making it far more reliable in high myopes. This is a classic reason Schiotz tonometry has been largely replaced in clinical practice.