A patient's refraction is: -3.00 DS / -2.00 DC × 180°. This means the correcting cylinder axis is at 180°. What is the power of the most curved meridian of this eye's cornea (the principal meridian with maximum refraction)?

• A The 180° meridian is most refractive (maximum negative power needed = -3.00 D)
• B Both meridians have equal refractive power since cylinder is spherical
• C The 90° meridian is most refractive (maximum negative power needed = -5.00 D total) ✓
• D The 45° meridian is most refractive since the prescription is in oblique astigmatism

Explanation

In minus cylinder notation, the cylinder axis indicates the meridian of LEAST power (the meridian the cylinder does NOT add power to). A -2.00 DC × 180° means the 180° meridian has only -3.00 D of correction needed, while the 90° meridian needs an additional -2.00 D of correction, making the total power in the 90° meridian = -3.00 + (-2.00) = -5.00 D. The eye is most myopic (most curved) in the 90° meridian — this is simple myopic astigmatism with the rule (against the rule would have the 180° more curved). The correcting lens axis is always perpendicular to the most powerful meridian in minus cylinder notation.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.