Abstract
Purpose: :
We previously looked at the effects of lesions of the pterygopalatine ganglia (PGX) singly and in combination with ciliary ganglionectomy (CGX) or superior cervical ganglionectomy (SCGX) on the axial and choroidal responses to positive-lens-induced myopic defocus in chicks (Nickla & Schroedl, ARVO 08) and found that all three lesions inhibited the thickening of the choroid. We here use negative-lens-induced hyperopic defocus to see whether the same lesions affect the choroidal thinning response.
Methods: :
In 3-4 week old chicks, the PG was deafferented by lesioning the pre-ganglionic facial nerve VII, as described in Schroedl et al. (IOVS, 2006). Double lesions of the PG/SCG and PG/CG were also done. Eyes were measured using high-frequency A-scan ultrasonography at various intervals starting prior to the surgery. Several days later, -10 D lenses were worn for 5 days (PGX, n=7; PGX/CGX, n=5; PGX/SCGX, n=5). Age-matched chicks wearing -10 D lenses were the controls (n=9). In PGX, form deprivation was also examined (n=4), with age-matched controls (n=4).
Results: :
Unlike the results from eyes with myopic defocus, in which all three lesions reduced the choroidal thickening, none of these had any effect on choroidal thinning in response to hyperopic defocus (PGX: -42 µm, PGX/CGX: -47 µm; PGX/SCGX: -82 µm vs -93 µm for lens-wearing control eyes). Similar to the results from positive lenses, there were no effects on the rate of ocular elongation: all lesioned eyes grew as fast as non-lesioned controls (PGX: 634 µm; PGX/CGX: 640 µm, PGX/SCGX: 672 µm vs 628 µm for lens-wearing control eyes). In the PGX, choroids also thinned normally in response to form deprivation (-83 µm vs -76 µm for form deprivation controls).
Conclusions: :
No lesion affected choroidal thinning in response to lens-induced hyperopic defocus whereas all three lesions inhibited choroidal thickening in response to lens-induced myopic defocus. This argues that these bidirectional changes are not opposing elements of a single mechanism. Because ocular growth was unaffected by all lesions under all visual conditions, this suggests that there are separate pathways for the changes in choroidal thickness and in ocular length.
Keywords: choroid • emmetropization • myopia