Hypermetropia in dark reared chicks and the effect of lid suture.

Two experimental groups of domestic fowl chicks were reared in darkness. One group was normal (DR) and the second had unilateral lid closure (DRC). A control group was reared in normal illumination (LR). The optical components of the eye were examined by retinoscopy, keratometry and phacometry while physical measurements were made using ultrasonography and micrometry. The DR chicks developed a significant hyperopia (+3.11 D) compared to the LR chicks (+0.65 D), attributed to a significant decrease in corneal height and lens thickness. A significant increase in the anteroposterior axis of the DR chicks tends to reduce the dark induced hyperopia. Lid closure in the DRC chicks increases the hyperopic effect by +3.07 D due to additional corneal flattening. These results reinforce our proposal of the chick eye as a model for research in the various forms of ametropia.