What is the difference between with-the-rule and against-the-rule astigmatism?

With-the-rule (WTR) astigmatism has the steepest corneal meridian oriented vertically, with minus cylinder axis near 180°. Against-the-rule (ATR) has the steepest meridian horizontally, with minus cylinder axis near 90°. WTR is more common in younger patients; ATR becomes more common with age.

Can irregular astigmatism be corrected with glasses?

Irregular astigmatism cannot be fully corrected with standard spectacle lenses because the meridians are not perpendicular or the power varies irregularly across the cornea. Rigid gas permeable contact lenses or scleral lenses work better because the tear film between the lens and cornea creates a smooth optical surface.

What does the axis number in an astigmatism prescription mean?

The axis number (1-180°) indicates the orientation of the meridian with ZERO cylinder power (the flat meridian). The full cylinder power acts 90° away from the axis. In minus cylinder form, the axis marks the least negative (flattest) meridian.

Astigmatism Types: Regular & Irregular for ABO

What Is Astigmatism?

Astigmatism occurs when the eye's refractive surfaces (primarily the cornea, sometimes the lens) have different curvatures in different meridians. Instead of being shaped like a basketball (spherical), an astigmatic cornea is shaped more like a football (toric), with one meridian steeper than the other. This causes light to focus at two different focal points rather than one, creating a blurred or distorted image.

Regular vs. Irregular Astigmatism

Feature	Regular Astigmatism	Irregular Astigmatism
Meridians	Two principal meridians at 90° to each other	Meridians not at 90°, or power varies within a meridian
Correction	Cylindrical or toric lenses	Rigid contact lenses, scleral lenses
Causes	Normal corneal/lenticular variation	Keratoconus, scarring, pterygium, post-surgical
Refraction	Clean endpoints with sphero-cylindrical Rx	Cannot be fully corrected with standard lenses

Types of Regular Astigmatism by Axis

With-the-Rule (WTR)

The vertical meridian is steepest. In minus cylinder form, the axis is near 180° (between 1° and 30° or 150° and 180°). This is the most common type in younger patients. Think of it as the cornea being shaped like an egg lying on its side.

Against-the-Rule (ATR)

The horizontal meridian is steepest. In minus cylinder form, the axis is near 90° (between 60° and 120°). This becomes more common with age as the horizontal meridian steepens from lid pressure changes and corneal tissue relaxation.

Oblique

The steepest meridian is between 30° and 60° or between 120° and 150° (neither vertical nor horizontal). Oblique astigmatism is less common and often more symptomatic because the visual system is less tolerant of blur at oblique orientations.

With-the-Rule has minus cylinder AXIS at 180 (plus cylinder axis at 90). Against-the-Rule has minus cylinder AXIS at 90 (plus cylinder axis at 180). The axis indicates the meridian of NO cylinder power, which is the FLATTEST meridian.

Classification by Focal Point Location

Based on where the two focal lines fall relative to the retina:

Type	Focal Line Positions	Spherical Equivalent
Simple myopic	One on retina, one in front	Minus
Simple hyperopic	One on retina, one behind	Plus
Compound myopic	Both in front of retina	Minus
Compound hyperopic	Both behind retina	Plus
Mixed	One in front, one behind retina	Near plano

Correcting Astigmatism

Regular astigmatism is corrected using cylindrical (toric) lenses that have different powers in different meridians:

Spectacle lenses: Sphero-cylindrical prescription (e.g., -2.00 -1.50 x 180)
Toric contact lenses: Must maintain proper orientation on the eye using stabilization methods (prism ballast, thin zones, truncation)
Rigid gas permeable lenses: The tear layer between the lens and cornea fills in the corneal irregularity, effectively neutralizing corneal astigmatism

When marking up a lens for edging, the cylinder axis must be precisely oriented to match the prescription. Even a 5° axis error on a -2.00 D cylinder can induce approximately 0.35 D of residual astigmatism in an unexpected direction, causing patient complaints. Use a lensometer to verify axis alignment carefully.

Confusing axis with the steep meridian. The axis notation marks the meridian of NO cylinder power (the flat meridian). The steep meridian is 90° away from the axis. For minus cylinder notation, the power at axis 180 means there is zero cylinder power along the horizontal, and the full cylinder power is along the vertical (90°).

Key Takeaways

Astigmatism results from different curvatures in different meridians, creating two focal points
Regular astigmatism has perpendicular principal meridians; irregular does not
WTR: steepest vertical (minus cyl axis near 180); ATR: steepest horizontal (minus cyl axis near 90)
The axis marks the flat (zero cylinder power) meridian, not the steep one
Corrected with cylindrical/toric lenses in spectacles or contacts