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Knapp's law and unilateral axial high myopia. 1970
1The Wilmer Opthalmological Institute, The Johns Hopkins Hospital, Baltimore, Maryland, USA.
Binocular Vision & Strabismus Quarterly
|November 11, 1999
Summary
Knapp's Law, which predicts no aniseikonia in axial ametropia, is clinically inaccurate. Studies show significant aniseikonia despite axial length differences, suggesting retinal stretching causes perceptual aniseikonia.
Area of Science:
- Ophthalmology
- Optometry
- Visual Science
Background:
- Knapp's Law posits that correcting axial ametropia with lenses at the eye's anterior focus eliminates retinal image size disparity.
- Modern ocular biometry, including ultrasonic echography, allows for empirical testing of Knapp's Law.
Discussion:
- This study investigated Knapp's Law using a "Direct Comparison" Eikonometer and A-Scan ultrasonic echography in five patients with anisomyopia.
- Results revealed significant aniseikonia (10-60% angular minification) in myopic eyes, contradicting the law's prediction.
Key Insights:
- Axial length differences correlated with ametropia and the degree of aniseikonia found.
- Ultrasonic axial biometry measured axial length discrepancies ranging from 2 to 5.5 mm.
Outlook:
- Knapp's Law, while optically sound, appears clinically flawed in predicting aniseikonia.
- Retinal stretching in myopia may cause perceptual aniseikonia, even if retinal images are isometrically sized.
- Patients with myopia-induced perceptual aniseikonia may benefit from specialized corrections like contact lenses.