A patient has astigmatism with the vertical meridian having a power of +48 D and the horizontal meridian having a power of +50 D. What is the magnitude and axis of the cylindrical correction required?

Explanation

## Calculating Cylindrical Correction **Key Point:** The cylindrical correction must equalize the power of the two principal meridians. The cylinder is prescribed for the weaker meridian to bring it to the power of the stronger meridian. ## Step-by-Step Calculation 1. **Identify the meridians:** - Vertical meridian (90°): +48 D (weaker) - Horizontal meridian (180°): +50 D (stronger) 2. **Calculate the difference:** - Difference = 50 D − 48 D = 2 D 3. **Determine cylinder power:** - The weaker meridian needs strengthening - Cylinder power = −2.00 D (negative cylinder adds power to the weaker meridian when placed perpendicular to it) 4. **Determine cylinder axis:** - The cylinder axis is placed along the weaker meridian - Weaker meridian is vertical (90°) - Cylinder axis = 90° ## Cylindrical Correction Principles | Principle | Application | |-----------|-------------| | **Cylinder axis** | Placed along the weaker meridian | | **Cylinder power sign** | Negative (−) to strengthen the weaker meridian | | **Magnitude** | Absolute difference between the two meridional powers | | **Result** | Both meridians end up with equal refractive power | **High-Yield:** In astigmatism correction, the cylinder axis always aligns with the meridian that needs correction (the weaker one). A negative cylinder adds power to that meridian. **Mnemonic:** **WEAK = AXIS** — The weaker meridian determines the cylinder axis.