What is the difference between WTR and ATR astigmatism?

With-the-rule (WTR) astigmatism has its steepest corneal meridian vertically oriented (near 90°) and is more common in children and young adults. Against-the-rule (ATR) astigmatism has its steepest meridian horizontally oriented (near 180°) and is more common in older adults. The 'rule' refers to the conventional axis of the correcting cylinder, which in WTR astigmatism is placed at 180° (with the steeper vertical meridian needing minus cylinder at 90° or plus cylinder at 180° depending on the sign convention used).

Why should you avoid doing keratometry right after contact lens wear?

Rigid contact lenses reshape the anterior corneal surface through prolonged mechanical contact. This corneal warpage temporarily changes the K readings compared to the true corneal curvature. For accurate contact lens fitting or surgical planning, patients should discontinue rigid lens wear for at least 2 weeks (some guidelines recommend longer for longer-term wearers) before keratometry. Soft lens wearers should remove lenses at least 24 hours before measurement.

Keratometry: CPO Exam Study Guide

Q: What do K readings measure and what are normal values?

K readings measure the curvature of the anterior corneal surface in two perpendicular meridians (the flattest and steepest meridians). They are expressed in diopters (D) or millimeters of radius (mm). Normal corneal curvature ranges from approximately 40.00 to 46.00 D (7.3-8.5 mm radius). Higher diopter values indicate a steeper (more curved) cornea; lower values indicate a flatter cornea.

What Is Keratometry?

Keratometry measures the curvature of the anterior corneal surface. The cornea provides about two-thirds of the eye's total refractive power, making its curvature highly relevant to refractive error, contact lens fitting, and surgical planning. As a CPO, you may perform keratometry or assist with interpreting K readings.

Keratometer Principles

The keratometer (also called an ophthalmometer) projects a target (mires) onto the corneal surface and measures the size of the reflected image. A steeper cornea reflects a smaller image; a flatter cornea reflects a larger image. By measuring images in two meridians 90° apart, the instrument determines the curvature of the flattest and steepest corneal meridians.

K Readings

Keratometry results are expressed as K readings:

K1 (flat K): the flatter (less curved) meridian, with its axis
K2 (steep K): the steeper (more curved) meridian, with its axis
Recorded as: K1 / K2 @ axis (e.g., 43.00 @ 180 / 44.50 @ 090)

K readings are expressed in diopters (D) or millimeters of radius (mm). Normal corneal curvature ranges from approximately 40.00 to 46.00 D (radius 7.3-8.5 mm). The relationship between the two units: Power (D) = 337.5 / Radius (mm).

Steeper corneas have higher diopter values and smaller radii. A 46.00 D cornea is steeper than a 43.00 D cornea. For contact lens fitting, a steeper cornea requires a steeper (smaller radius) base curve contact lens to match the corneal shape.

Corneal Astigmatism

When the K1 and K2 values differ, corneal astigmatism is present. The difference between the two K readings equals the corneal astigmatism magnitude:

Regular astigmatism: the two principal meridians are perpendicular (90° apart). Correctable with cylinder lenses.
With-the-rule (WTR) astigmatism: steeper meridian is vertical (near 90°); common in younger patients
Against-the-rule (ATR) astigmatism: steeper meridian is horizontal (near 180°); more common in older adults
Oblique astigmatism: steeper meridian is at an oblique axis
Irregular astigmatism: principal meridians not perpendicular; cannot be fully corrected with standard cylinder lenses. Associated with keratoconus, corneal scars, or post-surgical changes.

Clinical Applications

Application	How K Readings Are Used
Contact lens fitting	Base curve selection for GP lenses; corneal astigmatism assessment for toric lens need
Cataract/IOL surgery	Input for IOL power calculation formulas (along with axial length)
Refractive surgery screening	Identify irregular astigmatism, suspected keratoconus
Keratoconus monitoring	Serial K readings track progression of corneal steepening

When performing keratometry, ask the patient to blink once just before the measurement and then hold their blink. A fresh tear film provides a smooth surface for accurate mire reflection. A disrupted tear film creates irregular mires that lead to unreliable readings.

Keratometry Technique

Disinfect the chin rest and forehead rest
Position the patient with the chin on the rest, forehead against the bar
Align the instrument with the patient's right eye first
Focus the mires by adjusting the instrument position
Rotate the drum to align the mire targets as specified for your instrument
Take 3 readings per eye; the values should be consistent within 0.25 D
Record K1, K2, and their axes

Taking K readings immediately after contact lens wear. Rigid contact lenses can temporarily alter corneal curvature (corneal warpage), producing inaccurate K readings. Patients should remove soft lenses at least 24 hours and rigid lenses at least 2 weeks before keratometry for contact lens fitting or surgical planning.

Key Takeaways

Keratometry measures the curvature of the anterior corneal surface in two perpendicular meridians
K readings in diopters: higher value = steeper cornea; normal range 40-46 D
Difference between K1 and K2 = amount of corneal astigmatism
WTR astigmatism: steep at 90°; ATR astigmatism: steep at 180°
Blink before measurement for a fresh tear film; avoid measuring after contact lens wear
K readings are essential for IOL calculation, contact lens fitting, and refractive surgery screening