Choroidal thickening plays a role in emmetropization

Increases in choroidal thickening are integral to ocular growth and vision, as demonstrated in bird models, and are not merely a focusing mechanism, according to one physician.

The choroid was found to be important in regulating the refractive state of the eye about 10 years ago when it was discovered in chicks that changes in the thickness of the choroid were a compensatory response that moved the retina toward the image plane. This occurs before the scleral changes that result in variations in the size of the eye, according to Debora Nickla, PhD, from the New England College of Optometry, Boston. She spoke at the annual meeting of the Association for Research in Vision and Ophthalmology.

Dr. Nickla hypothesized that transient daily increases in choroid thickness are part of the pathway mediating axial growth inhibition. Nitric oxide may be the underlying mechanism based on experiments using nitric oxide inhibitors. A series of experiments in birds supported her hypothesis.

"Preventing the development of myopia is associated with choroidal thickening linking the choroidal response to growth inhibition," Dr. Nickla said.

Apomorphine, which inhibits growth, resulted in increases in choroidal thickness and in choroidal nitric oxide.

"All evidence to date supports the hypothesis that the choroidal response plays a causal role in ocular growth regulation," she said.