In astigmatism, there is unequal curvature of the cornea or lens in different meridians. What is the primary optical principle used to correct astigmatism?

Explanation

## Optical Correction of Astigmatism **Key Point:** Astigmatism is corrected using a **cylindrical lens** aligned along the principal meridian of the eye to equalize the refractive power in all meridians. ### Pathophysiology of Astigmatism Astigmatism occurs when: 1. The cornea or lens has different radii of curvature in different meridians 2. Light rays focus at two different points (focal lines) rather than one focal point 3. This results in blurred vision at all distances ### Cylindrical Lens Correction **Mechanism:** - A cylindrical lens has power in only one meridian (along its axis) - It has zero power perpendicular to its axis - Alignment is critical: the axis must match the meridian requiring correction - This allows independent correction of each meridian **High-Yield:** Astigmatic prescription includes **three parameters**: - Sphere (S): spherical power for myopia/hypermetropia - Cylinder (C): cylindrical power for astigmatism - Axis (A): direction of the cylinder in degrees (0–180°) ### Prescription Example - Notation: S −2.00 / C −1.50 × 90 - Means: −2.00 D sphere + −1.50 D cylinder at 90° axis ### Classification of Astigmatism | Type | Meridian 1 | Meridian 2 | Correction | |---|---|---|---| | **With-the-rule** | Steeper vertically | Flatter horizontally | Minus cylinder at 180° | | **Against-the-rule** | Flatter vertically | Steeper horizontally | Minus cylinder at 90° | | **Oblique** | Steeper at oblique axis | Flatter perpendicular | Cylinder at oblique axis | **Clinical Pearl:** Patients with astigmatism often experience blurred or distorted vision at all distances and may have asthenopic symptoms (eye strain, headaches).