Contributions of Eye Power and Optical Eye Length to Emmetropization during Lens Induced Myopia and Recovery in the Chick Eye

Abstract

Purpose: The refractive error induced by emmetropization to lens defocus is primarily due to changes in ocular length. However, during normal growth and diurnal mean ocular refraction (MOR) variation, we have shown that changes in MOR are due to both changes in optical length and optical power. We determine the contributions of eye power and optical length to emmetropization during myopia induction and recovery from lens induced myopia.

Methods: Two experimental data sets (20 chicks) with eye length (ultrasound) and MOR (retinoscopy or Hartmann-Shack) were considered. All were unilaterally treated with a -15 D goggle from the day of hatching to day 7. The goggle was removed and measurements continued up to day 15. Calculations of dioptric length and eye power were made as a function of age and treatment. We tested the assumption that the power in the goggled eye was the same as in the control eye.

Results: Changes in optical length and eye power for the control eye were similar to their behaviour during normal growth (ARVO 2012). MOR of the goggled eye showed a rapid, almost complete emmetropization to the -15 D goggle by day 7 primarily due to eye length changes. However, the power of the goggled eye decreased faster than the control. The dioptric length for the previously goggled eye was not significantly different between days 7 and 8 and days 8 and 9, but power decreased until day 8. Both decreased in the control eye. Beginning between days 7 and 8, the difference in power between the goggled and control eyes began to decrease significantly. After goggle removal, recovery from myopia was complete by day 9 with resulting dioptric length and eye power not significantly different from the control eye (paired t-test). After day 9, the goggled eye emmetropized similarly to the control eye.

Conclusions: Emmetropization to hyperopic defocus is mainly due to an increase in optical eye length, but power reduces somewhat. Eye power and optical length are not significantly different from the control eye within 2 days of goggle removal. During this recovery from myopia, optical length remains relatively constant in the goggled eye while power initially decreases. Differences in both eye power and optical length between goggled and control eyes decrease. Emmetropization following goggle removal is more rapid than to the goggle.

Keywords: 630 optical properties • 605 myopia • 511 emmetropization