Emmetropization

Emmetropization is the postnatal process by which the eye grows and its optical components adjust in a coordinated way so that images are focused on the retina with little or no refractive error, a state known as emmetropia. The mechanism is most active in infancy and early childhood but can continue to influence eye growth into later childhood and adolescence for some individuals.

Mechanistically, emmetropization involves signals generated by the retina in response to defocus. When images are focused behind the retina (hyperopic defocus), the eye tends to grow longer; when focus falls in front of the retina (myopic defocus), growth tends to slow. These signals coordinate changes in the choroid, which can thicken or thin to shift the retinal plane, and in the sclera, where extracellular matrix remodeling adjusts axial length. The net effect is a pursuit of optical equilibrium, guiding the eye toward emmetropia.

Genetic factors influence baseline refractive tendencies, while visual experience provides corrective cues. Environmental influences are notable: greater time outdoors is linked with a lower risk of myopia, whereas prolonged near work and sustained close-up tasks are associated with higher myopia risk, potentially by altering defocus signaling and growth trajectories.

Emmetropization is studied across humans and animal models, including chicks, tree shrews, and primates, to illuminate how early visual experience shapes eye growth. Although many children achieve emmetropia, deviations can occur, leading to refractive errors such as myopia or hyperopia. Understanding emmetropization informs prevention and management strategies for refractive development.