What is with-the-rule astigmatism?

With-the-rule (WTR) astigmatism occurs when the steepest corneal meridian is oriented vertically, between 60 and 120 degrees (most commonly at 90 degrees). It is the most common type of astigmatism and is more prevalent in younger patients. In minus cylinder notation, the cylinder axis is near 180 degrees, perpendicular to the steep meridian.

Why does astigmatism change from WTR to ATR with age?

The shift is thought to result from age-related changes in eyelid tension and corneal tissue elasticity. In younger patients, upper eyelid pressure compresses the vertical meridian, contributing to WTR astigmatism. As eyelid tension decreases with age, the horizontal meridian becomes relatively steeper, shifting the astigmatism toward ATR.

What is the LARS rule for toric contact lenses?

LARS stands for Left Add, Right Subtract. It is used to compensate for consistent toric lens rotation. If the lens consistently rotates to the left (counterclockwise from the practitioner's perspective), add the rotation amount to the ordered axis. If it rotates to the right (clockwise), subtract the rotation amount. This ensures the cylinder correction aligns with the patient's astigmatism axis despite the rotation.

With-the-Rule, Against-the-Rule, and Oblique Astigmatism Explained

Classifying Astigmatism by Orientation

When a patient has astigmatism, the cornea (or less commonly, the crystalline lens) has different curvatures in different meridians. Classifying the orientation of the steepest meridian helps predict how well different contact lens types will perform and anticipate fitting challenges. Three categories describe this orientation: with-the-rule, against-the-rule, and oblique.

With-the-Rule (WTR) Astigmatism

With-the-rule astigmatism occurs when the steepest corneal meridian is oriented vertically, typically between 60 and 120 degrees, with 90 degrees being the most common.

In practical terms, the cornea is shaped more like a football lying on its side: the vertical curvature is steeper than the horizontal curvature. The keratometry (K) readings will show a higher dioptric value at or near 90 degrees.

Key facts about WTR astigmatism:

It is the most common type of astigmatism overall
More prevalent in younger patients
In minus cylinder notation, the cylinder axis is near 180 degrees (the axis is always 90 degrees away from the steep meridian)
The upper eyelid naturally compresses the vertical meridian during blinking, which is thought to contribute to this orientation in younger populations

Against-the-Rule (ATR) Astigmatism

Against-the-rule astigmatism occurs when the steepest corneal meridian is oriented horizontally, typically between 150 and 30 degrees (wrapping around 180/0 degrees).

This is like a football standing upright: the horizontal curvature is steeper than the vertical. K readings show a higher dioptric value at or near 180 degrees.

Key facts about ATR astigmatism:

More common in older patients
The shift from WTR to ATR with age is thought to result from changes in eyelid tension and corneal tissue
In minus cylinder notation, the cylinder axis is near 90 degrees
Can be slightly more visually symptomatic than equivalent amounts of WTR astigmatism at the same magnitude

The cylinder axis in minus cylinder form is perpendicular to the steep meridian. WTR has the steep meridian at 90 degrees and the minus cylinder axis at 180 degrees. ATR has the steep meridian at 180 degrees and the minus cylinder axis at 90 degrees. This relationship is a frequent source of confusion on exams.

Oblique Astigmatism

Oblique astigmatism occurs when the steepest meridian falls outside the with-the-rule and against-the-rule ranges. The steep meridian is oriented between 30 and 60 degrees or between 120 and 150 degrees.

This is the least common orientation and tends to be more problematic for patients:

Often causes more visual distortion than equivalent amounts of WTR or ATR astigmatism
Patients are less likely to tolerate uncorrected oblique astigmatism
Toric contact lens fitting can be more challenging because standard stabilization designs are optimized for near-vertical or near-horizontal axes

Contact Lens Implications

Soft Toric Lens Rotation

Soft toric lenses must maintain a specific orientation on the eye to align the cylinder correction with the patient's astigmatism axis. These lenses use stabilization mechanisms (prism ballast, dual thin zones, or peri-ballast designs) to prevent rotation.

The eyelid-lens interaction affects rotation differently based on astigmatism orientation:

WTR: The cylinder axis is near 180 degrees. Most stabilization designs work well at this orientation since the thin zones align with the natural blink mechanics
ATR: The cylinder axis is near 90 degrees. Some lens designs may rotate slightly more at this axis, but modern stabilization generally manages well
Oblique: Axes at 30-60 or 120-150 degrees are often the most challenging. Lens rotation at oblique angles can significantly affect visual acuity because even small amounts of rotation at these axes create more noticeable axis error

When fitting toric lenses for oblique astigmatism, pay extra attention to rotational stability. Use the LARS rule (Left Add, Right Subtract) to adjust the axis ordering if the lens consistently rotates: if the lens rotates left (counterclockwise), add the rotation amount to the ordered axis. If right (clockwise), subtract it.

RGP Lens Considerations

For rigid gas permeable lenses, the orientation of astigmatism affects the tear lens pattern:

WTR: The tear lens has a toric shape that matches the common fitting patterns. RGP lenses typically perform well
ATR: The tear lens pattern may differ from expected, and lens-to-cornea alignment can change with blink mechanics
Higher amounts: Regardless of orientation, corneal astigmatism exceeding about 2.50 D may require a back-surface toric RGP for adequate fitting

Age-Related Considerations

Because astigmatism tends to shift from WTR toward ATR with age, long-term contact lens patients may need axis changes over time. A patient fitted with a toric lens at age 25 may need the axis adjusted at subsequent visits as the corneal curvature gradually shifts.

Confusing the steep meridian with the minus cylinder axis. They are perpendicular to each other. If keratometry shows the steep meridian at 90 degrees, the minus cylinder axis in the refraction will be near 180 degrees (assuming the astigmatism is primarily corneal). Always verify the relationship between K readings and refraction findings.

Key Takeaways

WTR: steep meridian vertical (60-120 degrees), more common in younger patients, minus cylinder axis near 180
ATR: steep meridian horizontal (150-30 degrees), more common in older patients, minus cylinder axis near 90
Oblique: steep meridian at 30-60 or 120-150 degrees, least common, most visually symptomatic
Astigmatism orientation shifts from WTR to ATR with age
Oblique astigmatism is the most challenging for toric contact lens fitting
The LARS rule helps compensate for consistent toric lens rotation