A Placido disc-based corneal topography map of a normal adult shows a red central area flanked by green-blue areas nasally and temporally along the horizontal meridian, resembling a symmetric hourglass. This classic 'bow tie' pattern indicates:
- A Keratoconus with inferior steepening
- B Contact lens-induced corneal warpage
- C Regular with-the-rule astigmatism ✓
- D Posterior polymorphous dystrophy
Correct answer: C. Regular with-the-rule astigmatism
Explanation
The normal cornea is prolate and slightly steeper vertically than horizontally, so with-the-rule astigmatism produces two steep semimeridians above and below the center, appearing as a vertical bow tie on the power map. Keratoconus instead shows asymmetric inferior paracentral steepening, often with skewed radial axes, and contact lens warpage produces irregular or flattened zones. Recognizing the physiological bow tie prevents mistaking a normal map for pathological steepening.
Reference: Basic and Clinical Science Course (BCSC), Section 3: Clinical Optics, 2023-2024 ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.