What is the difference between against the rule (ATR) and with the rule (WTR) astigmatism?

Difference Between Against-the-Rule and With-the-Rule Astigmatism

With-the-rule (WTR) astigmatism has the steepest corneal meridian oriented vertically (approximately 90 degrees), while against-the-rule (ATR) astigmatism has the steepest meridian oriented horizontally (approximately 180 degrees). 1

Anatomical Orientation

The fundamental distinction lies in the axis of maximum corneal curvature:

• WTR astigmatism: The steep meridian is vertical (near 90°), meaning the cornea is more curved in the vertical plane 1, 2
• ATR astigmatism: The steep meridian is horizontal (near 180°), meaning the cornea is more curved in the horizontal plane 1, 2

Age-Related Patterns

The prevalence and type of astigmatism shifts predictably across the lifespan:

• Young adults and children: WTR astigmatism predominates, with higher prevalence in younger populations 2, 3
• Older adults: The axis shifts from WTR to ATR astigmatism with advancing age 2, 3
• This age-related transition occurs due to alterations in corneal curvature, influenced by changes in eyelid position and tension, corneal stromal collagen fibrils, Descemet membrane, and extraocular muscle effects 3

Prevalence Considerations

In general populations requiring astigmatic correction:

• WTR astigmatism is slightly more common overall (32.9% vs 29.1% for ATR) 4
• Among eyes with astigmatism ≥0.75 D, WTR remains more prevalent (15.3% vs 14.5%) 4
• Approximately one-third of potential contact lens wearers require astigmatic correction 4

Clinical Implications for Visual Function

ATR astigmatism provides superior uncorrected near visual acuity compared to WTR astigmatism when both have similar degrees of myopic astigmatism. 5

• In pseudophakic patients with 1.00-1.50 D of simple myopic astigmatism, those with ATR had significantly better uncorrected near acuity (p < 0.001) 5
• Distance visual acuity showed no significant difference between WTR and ATR groups 5
• This makes low ATR myopic astigmatism preferable to WTR in certain post-cataract surgery scenarios 5

Correction Strategies

Both types are corrected using identical approaches, though patient tolerance may vary:

• Eyeglasses: Full cylindrical correction effectively treats both WTR and ATR astigmatism, though adults with previously uncorrected astigmatism may require gradual correction 2
• Contact lenses: Soft toric or rigid gas-permeable lenses correct low to moderate astigmatism (< 3.00 D) for both types 1
• Refractive surgery: Axis alignment and cyclotorsion compensation are critical regardless of astigmatism type, with reference marks placed while the patient is seated upright 2

Common Pitfalls

• Attempting immediate full correction in adults with previously uncorrected astigmatism of either type leads to poor tolerance; gradual correction improves acceptance 1, 2
• Failing to distinguish regular from irregular astigmatism: Regular astigmatism (both WTR and ATR) has uniform curvature in each meridian with principal meridians 90 degrees apart, while irregular astigmatism cannot be fully corrected with spherocylindrical lenses 1
• Ignoring bilateral symmetry patterns: Fellow eyes typically show mirror symmetry rather than direct symmetry in astigmatic axes (median difference 10° vs 20°, p < 10e-100) 6