A child has -4.00 DS / -2.00 DC × 90° in the right eye. The least myopic meridian (flattest) is at 90° and the most myopic (steepest) is at 180°. This is classified as:

• A Simple myopic astigmatism, with rule
• B Compound myopic astigmatism, against the rule ✓
• C Compound myopic astigmatism, with the rule
• D Mixed astigmatism, against the rule

Explanation

The prescription -4.00 DS / -2.00 DC × 90° means the cylinder axis is at 90°, so the cylinder acts in the 180° meridian (perpendicular to axis). Total powers: 180° meridian = -4.00 + (-2.00) = -6.00D; 90° meridian = -4.00D. Both meridians are myopic (compound myopic astigmatism). The steepest meridian is at 180° (horizontal). With-the-rule astigmatism = vertical meridian steepest (cylinder axis at 180°). Against-the-rule = horizontal meridian steepest (cylinder axis at 90°). Here, axis is at 90°, so horizontal meridian is steepest — against the rule. Against-the-rule astigmatism becomes more common with age.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.