In A-scan biometry for IOL power calculation, which formula is most accurate for an extremely long axial length (>26 mm)?
- A Haigis formula with 3 optimized constants ✓
- B SRK-II
- C SRK-T
- D Holladay I
Explanation
The Haigis formula, particularly when optimized with three constants (a0, a1, a2), performs best for extreme axial lengths (very short <22 mm and very long >26 mm) because it separates anterior chamber depth prediction from axial length to a greater degree. The SRK-T performs reasonably for long eyes but may under- or over-correct in extremes. SRK-II is an older regression formula with lower accuracy at extremes. For highly myopic eyes, newer formulae like the Kane or Barrett Universal II are increasingly preferred, but Haigis with optimized constants remains the traditionally cited answer for long axial length.
Reference: Khurana Comprehensive Ophthalmology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.