Peripheral defocus is the optical blur that occurs in the peripheral retina of the eye. It is an important factor in eye development and the progression of near-sightedness (myopia). In an uncorrected myopic eye, the central image focuses in front of the retina, while parts of the peripheral image may focus behind it, creating a relative hyperopic shift in the periphery. When light in the peripheral retina focuses behind the retina (hyperopic defocus), myopia is more likely to develop or worsen. Conversely, when peripheral light focuses in front of the retina (myopic defocus), eye growth tends to slow down.
The role of peripheral defocus in myopia was first proposed following a 1971 study of Dutch military pilots. This study found that pilots with eyes elongated along the visual axis relative to their equatorial width were at a greater risk of developing myopia. Researchers hypothesized that hyperopic defocus in the peripheral retina could be triggering the onset of myopia. Later studies revealed that an elongated eye shape is more likely a result of myopia rather than its cause, although peripheral hyperopic defocus remains a recognized factor in the continued progression of the condition.
Animal research highlights the peripheral retina's major role in regulating eye growth. Studies on rhesus monkeys demonstrated that even after the central fovea was destroyed, the eye still adjusted its refractive state. This indicates that peripheral regions independently guide emmetropization. Animal experiments further show that eye growth regulation is largely independent of the brain. Severing the optic nerve has little effect on growth responses driven by defocus, and covering half of the visual field with a translucent diffuser only causes overgrowth in that specific half of the eye.
When exposed to blur, the peripheral retina releases chemical mediators such as dopamine, retinoic acid, and nitric oxide. In animal models like chickens, the choroid responds to these signals within minutes to hours. It expands when myopic defocus is applied and thins under hyperopic defocus, which effectively repositions the retina closer to or further from the focal plane. Over time, these signals alter the composition of the scleral extracellular matrix, causing the eye to elongate and shift toward a more myopic state.
Various optical devices are designed to create a myopic peripheral defocus to slow the progression of myopia in children. Standard single-vision spectacles provide a uniform correction that focuses the central image clearly on the retina. However, this correction leaves the periphery of the image focused behind the retina. This creates a hyperopic defocus that can stimulate continued axial eye growth, a problem that worsens with stronger prescriptions. Specialized options to address this issue include: