Natural and imposed astigmatism and their relation to emmetropization in the chick

This study investigated the ocular response of young chicks to astigmatic errors imposed by spectacle lenses and as a related issue, we examined the nature and prevalence of astigmatism in young chicks, and its relation to corneal development and natural emmetropization. Normal hatchling chicks exhibited significant against-the-rule refractive astigmatism (approx. 8 D) of which 60-90% was corneal. Both types of astigmatism decreased in magnitude with normal corneal development as part of emmetropization. The apparent association with corneal growth is consistent with two further observations: (1) that smaller corneas, induced by constant light rearing, had higher than normal astigmatism (1.5 D greater at 15 days), (ii) that enlarged corneas, due to form deprivation, had reduced astigmatism (2.4 D less). When astigmatism was artificially imposed with (+/-10 DC spectacle lenses), altered ocular growth patterns were observed, although the changes were not consistent with the chicks having emmetropized to the imposed astigmatism. Irrespective of the axis setting used in positioning the lenses (45 degrees, 90 degrees, 180 degrees), eyes became hyperopic with +10 DC lenses (+8.8 +/- 1.3 D), and became slightly myopic with 10 DC lenses (-1.8 +/- 1.9 D). These refractive changes are consistent with the chicks having emmetropized to the more myopic meridian rather than the equivalent mean spherical error imposed (responses of control groups to +5 D and -5 D spherical lenses were +5.2 +/- 1.0 D and -5.1 +/- 0.8 D resp.). The same was true for chicks first prevented from accommodating by prior ciliary nerve section, except for one group wearing the 10 DC lens at 45 degrees axis where astigmatic changes consistent with partial compensation were seen, although this may represent an artefact of the surgery. These results argue against 'astigmatic emmetropization' as a normal phenomenon. Also consistent with this finding was the lack of significant astigmatic changes with accommodation-stimulating and inhibiting drugs (nicotine and vercuronium resp.), for normal chicks. These results imply that accommodation, while the most likely mechanism for astigmatic emmetropization, has little capacity to compensate for imposed astigmatic focussing errors.