Myopia, commonly known as nearsightedness, is a refractive error where distant objects appear blurry while close objects remain clear. This condition occurs when the eye focuses light in front of the retina instead of directly on it, typically because the eyeball is too long from front to back. Myopia exists on a spectrum of severity, ranging from slight blurriness to extreme levels that significantly compromise long-term eye health.
Understanding Myopia Severity: The Diopter Scale
Myopia is precisely measured using a unit called the diopter (D), which quantifies the power of the corrective lens needed to restore clear vision. The strength of the lens required to correct nearsightedness is always indicated by a negative sign, such as -3.00 D. The higher the numerical value of the diopter, the stronger the prescription, and the more severe the myopia.
Ophthalmologists and optometrists use the diopter value to classify myopia into categories. Low myopia generally ranges from -0.50 D up to -3.00 D, while moderate myopia typically falls between -3.00 D and -6.00 D. These classifications are based solely on the magnitude of the refractive error itself, without considering any underlying structural damage to the eye.
Defining High Myopia and Pathological Myopia
High myopia is conventionally defined as a refractive error of -6.00 D or greater. Some clinical classifications further divide this into severe myopia (often between -6.00 D and -9.00 D) and extreme myopia (exceeding -9.00 D). The highest reported levels of myopia can reach beyond -25 to -30 diopters in unusual cases.
A distinction exists between high myopia (a measure of refractive error) and pathological myopia, sometimes called degenerative myopia. Pathological myopia is a disease state characterized by excessive axial elongation, meaning the eyeball has stretched significantly. This stretching leads to structural changes in the back of the eye, such as myopic maculopathy and posterior staphyloma, which are the sources of vision loss.
The highest levels of myopia are associated with this pathological state, as extreme elongation causes the severe refractive error. While most pathological myopia occurs in eyes with high myopia, it is the structural damage from stretching—not the prescription number—that defines the highest risk to vision. This extreme condition is physically manifested by excessive axial length, often defined as an eye length greater than 26.5 millimeters.
Ocular Risks Associated with Extreme Myopia
The stretching of the eyeball in pathological myopia causes the layers of the eye to become thin and fragile, leading to specific vision-threatening complications. Myopic maculopathy is the most significant complication and a leading cause of irreversible vision impairment worldwide. This condition involves progressive, degenerative changes in the macula, the central part of the retina responsible for detailed vision, often manifesting as atrophy or tissue loss.
A serious risk is rhegmatogenous retinal detachment, where the stretched and thinned retina separates from the underlying tissue. The peripheral retina in highly myopic eyes is prone to developing tears or holes, allowing fluid to seep underneath and cause detachment. The risk of retinal detachment for a person with a prescription worse than -9.00 D is significantly higher compared to someone with low myopia.
Glaucoma is a heightened concern, as high axial myopia increases the susceptibility of the optic nerve to damage. The stretching of the eye can physically alter the optic nerve head, making it more vulnerable to damage even at normal intraocular pressure. High myopia can also lead to tiny breaks in the deepest layer of the retina called lacquer cracks. These cracks can trigger myopic choroidal neovascularization, which is the growth of abnormal, leaky blood vessels under the macula.
Advanced Management Strategies
Management for extreme myopia focuses primarily on monitoring and treating sight-threatening complications, not just vision correction. Regular, comprehensive eye exams with dilation are necessary to inspect the peripheral retina for signs of tears or degeneration. Specific monitoring tools, such as Optical Coherence Tomography (OCT) scans, map the retina’s thickness and structure, allowing doctors to catch early signs of maculopathy or traction.
For complications like myopic choroidal neovascularization, the treatment involves injections of anti-vascular endothelial growth factor (anti-VEGF) medications directly into the eye. These injections work to stop the growth and leakage of the abnormal blood vessels, preserving central vision. In cases where the retina is being pulled or detached due to extreme stretching, surgical procedures like pars plana vitrectomy may be performed to relieve the traction and reattach the retina.
Proactive care is also directed at preventing the progression of myopia during childhood, as every diopter of progression increases the lifetime risk of these severe complications. Strategies such as low-concentration atropine eye drops and specialized contact lenses are used to slow down the eye’s axial elongation. For adults, consistent monitoring and prompt intervention for any complication are the most effective strategies for maintaining vision.