A patient with keratoconus has keratometry readings of 52.00 D at 80° and 48.00 D at 170°. This degree of corneal astigmatism is best classified as:
- A Regular with-the-rule astigmatism
- B Regular against-the-rule astigmatism
- C Irregular astigmatism ✓
- D Regular oblique astigmatism
Correct answer: C. Irregular astigmatism
Explanation
In keratoconus, the corneal surface is irregular with asymmetric steepening, often inferiorly. The keratometry values of 52 D and 48 D with an oblique axis, combined with the clinical context of keratoconus, indicate irregular astigmatism. Regular astigmatism has two principal meridians 90° apart with a smooth, symmetric corneal surface. The high absolute K values (>47 D) and asymmetry are hallmark signs of keratoconus rather than simple regular astigmatism.
Reference: AAO BCSC Section 8: External Disease and Cornea, 2023-2024 ed.
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