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Internal Astigmatism in Myopes and Non-myopes: Compensation or Constant?
Ruth E Manny1, Li Deng, Jane Gwiazda
1*OD, PhD, FAAO†PhD, FAAO‡OD, FAAOUniversity of Houston College of Optometry, Houston, Texas (REM, KDF); New England College of Optometry, Boston, Massachusetts (LD, JG, EW); Department of Preventive Medicine, Stony Brook University Health Sciences Center, Stony Brook, New York (LH); and Pennsylvania College of Optometry at Salus University, Philadelphia, Pennsylvania (MS).
Summary
Internal astigmatism (IA) may partially compensate for corneal astigmatism (CA) in low astigmatism eyes. However, IA does not actively reduce CA in myopes, despite variations by ethnicity.
Area of Science:
- Ophthalmology
- Optometry
- Vision Science
Background:
- Corneal astigmatism (CA) is a major source of refractive error.
- Internal astigmatism (IA) may play a role in compensating for CA.
- Understanding this relationship is crucial for myopia management.
Purpose of the Study:
- To assess the compensatory role of IA for CA in myopes and non-myopes.
- To identify predictors of high IA in young adult myopes.
- To investigate if IA actively changes with CA to reduce refractive astigmatism (RA).
Main Methods:
- Longitudinal study of 367 myopes and cross-sectional study of 204 non-myopes.
- Annual measurements of keratometry and cycloplegic autorefraction over 14 years.
- Calculation of IA as RA - CA and analysis of the compensation factor (IA/CA).
Main Results:
- IA magnitude differed between myopes and non-myopes.
- Compensation of CA by IA was less effective in myopes than non-myopes.
- IA did not show an active compensatory role as CA increased over time in myopes.
Conclusions:
- IA may offer some compensation for CA in eyes with low astigmatism.
- No evidence supports an active compensatory mechanism of IA for CA in myopes.
- Ethnicity influences IA magnitude over time.
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