Myopia, commonly known as nearsightedness, is a growing eye condition where distant objects appear blurry. It occurs when the eye focuses light in front of the retina instead of directly on it, often due to the eyeball growing too long. This condition affects a significant portion of the global population and is frequently diagnosed in childhood. Myopic defocus represents a strategy developed to manage the progression of myopia, particularly in children.
Understanding Myopia and Defocus
Traditional eyeglasses or contact lenses correct central vision but can inadvertently lead to “defocus” in the peripheral vision. Defocus refers to light rays not focusing perfectly on the retina. There are two main types: hyperopic defocus, where light focuses behind the retina, and myopic defocus, where light focuses in front of the retina. Traditional single-vision lenses can induce peripheral hyperopic defocus, which may encourage further eye elongation and myopia progression. This understanding is foundational to developing myopia management strategies.
How Myopic Defocus Works
The biological mechanism behind myopic defocus as a myopia management strategy centers on signaling the eye to slow its axial elongation. Instead of allowing peripheral light to focus behind the retina (hyperopic defocus), myopic defocus strategies introduce light rays that focus slightly in front of the peripheral retina. This intentional blur in the periphery acts as a “stop signal” for eye growth.
The retina, particularly the peripheral retina, plays a crucial role in detecting this specific type of defocus. When the peripheral retina receives myopic defocus, it triggers biochemical pathways that regulate eye growth and inhibit the excessive lengthening of the eyeball. Animal studies have shown that optically imposed myopic defocus can effectively slow axial elongation.
Clinical Applications
Myopic defocus is implemented in clinical settings through various specialized optical devices designed to provide simultaneous central clear vision and peripheral myopic defocus. One common method involves specialized spectacle lenses, such as Defocus Incorporated Multiple Segments (DIMS) technology lenses. These lenses feature a central optical zone for distance correction and a surrounding area with multiple segments that create a relative positive power, ensuring light focuses in front of the peripheral retina. Essilor’s Stellest lens, utilizing Highly Aspherical Lenslet Technology (HALT), also employs a similar principle, creating a “volume of myopic defocus” with numerous aspherical lenslets.
Another approach utilizes multifocal contact lenses specifically designed for myopia management. Lenses like MiSight daily disposable soft contact lenses have a central zone for clear distance vision and concentric treatment zones that induce myopic defocus in the periphery. Additionally, Orthokeratology (Ortho-K), while primarily reshaping the cornea overnight to provide clear daytime vision, also induces a peripheral myopic shift, contributing to its myopia control effects. Ortho-K lenses effectively create myopic defocus at various eccentricities.
Effectiveness and Importance
Research consistently demonstrates the effectiveness of myopic defocus strategies in slowing the progression of myopia, especially in children. Studies have shown a significant reduction in axial elongation and refractive error progression compared to traditional single-vision correction. For instance, DIMS spectacle lenses have shown approximately a 62% retardation in axial length elongation, and multifocal contact lenses have been found to reduce myopia progression by about 59% over three years.
Slowing myopia progression is important because higher degrees of myopia are associated with an increased risk of serious eye conditions in adulthood. These complications include retinal detachment, glaucoma, cataracts, and myopic maculopathy, which can lead to permanent vision impairment or loss. By managing myopia progression early through strategies like myopic defocus, the long-term ocular health of individuals can be protected, reducing the likelihood of developing these sight-threatening conditions.