ACTIVE EMMETROPIZATION - EVIDENCE FOR ITS EXISTENCE AND RAMIFICATIONSFOR CLINICAL-PRACTICE

There is increasing evidence from animal studies in support of the con cept of an active emmetropization mechanism which has potentially impo rtant clinical ramifications for the management of refractive errors, Recent research into refractive development and emmetropization is rev iewed, with emphasis given to work involving the chick, tree shrew and monkey, which represent the three most widely used animal models in t his field. The findings of this research are reviewed in a clinical co ntext. Compensatory eye growth responses to focusing errors imposed by lenses represent the most compelling evidence for active emmetropizat ion. These observations are complemented by other evidence showing rec overy from induced refractive errors such as form-deprivation myopia. Of the animals listed above, chicks show the most impressive emmetropi zation, being able to compensate fully (using choroidal and scleral me chanisms) to lens powers ranging from +15 D to -10 D. The range of len s powers eliciting appropriate compensatory responses is narrower in t he tree shrew and monkey, and the response patterns generally are also more complex to interpret, These data relate to young animals and tog ether indicate refractive plasticity during development, Extrapolation of these findings to humans predicts that natural emmetropization wil l be inhibited in neonates by early intervention with prescription len ses, and that refractive correction of myopia will lead to accelerated progression. This convincing evidence for active emmetropization warr ants due consideration in developing clinical management strategies fo r refractive errors. (C) 1997 The College of Optometrists. Published b y Elsevier Science Ltd.