Keratometry in a patient shows readings of 42 D at 90° and 46 D at 180°. What type and axis of astigmatism does this represent?

• A With-the-rule astigmatism; steeper meridian at 180°
• B Against-the-rule astigmatism; steeper meridian at 180° ✓
• C With-the-rule astigmatism; steeper meridian at 90°
• D Oblique astigmatism

Explanation

In keratometry, the steeper meridian (higher dioptric power) at 180° (horizontal axis) defines against-the-rule (ATR) astigmatism. In ATR astigmatism, the correcting minus cylinder axis is placed at 90°. With-the-rule astigmatism has the steeper meridian at 90° (vertical), which is the more common type in children and young adults. ATR astigmatism is more common with advancing age due to corneal shape changes. The 4 D of astigmatism (46 − 42) is significant and warrants correction.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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