## Definition of Regular Astigmatism **Key Point:** Regular astigmatism is characterized by unequal curvature of the cornea (or lens) in two perpendicular meridians, resulting in two principal meridians with different refractive powers. ## Optical Mechanism In regular astigmatism: - The cornea (or lens) has two principal meridians at right angles to each other - Each meridian has a different radius of curvature - This creates two focal lines instead of a single focal point - The image formed is blurred in both distance and near vision ## Classification of Regular Astigmatism | Type | Description | |------|-------------| | **With-the-rule (WTR)** | Vertical meridian more curved (steeper); horizontal meridian flatter | | **Against-the-rule (ATR)** | Horizontal meridian more curved (steeper); vertical meridian flatter | | **Oblique** | Principal meridians at 45° and 135° to the horizontal | ## Clinical Significance **High-Yield:** Regular astigmatism is the most common form and is fully correctable with cylindrical lenses or toric contact lenses. The correction axis must align with the principal meridians for optimal visual acuity. **Clinical Pearl:** With-the-rule astigmatism is more common in younger individuals, while against-the-rule astigmatism increases with age due to changes in corneal curvature. 
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