Myopia, or nearsightedness, is a widespread vision problem where distant objects appear blurry. While common myopia is easily corrected and poses little risk, high myopia is a less common but significantly more serious form. High myopia is fundamentally a structural disease of the eye, carrying measurable, long-term risks of severe vision loss. These dangers arise from physical changes within the eye, transforming a simple focusing issue into a degenerative health concern.
Distinguishing High Myopia from Common Nearsightedness
Common nearsightedness (low to moderate myopia) is generally classified as a refractive error between -0.50 and -5.75 diopters (D). This indicates an eye that is slightly longer than average, causing light to focus in front of the retina. High myopia, by contrast, is defined by a refractive error of -6.00 D or worse, often involving an axial length—the measurement from the front to the back of the eyeball—of 26 millimeters or longer. This is not just a stronger prescription; it is often classified as a degenerative eye condition due to the severe structural changes caused by elongation.
The Structural Risk: Why the Eyeball Stretches
The primary danger of high myopia stems from axial elongation, where the eyeball grows excessively long from front to back. This growth is often progressive, continuing to lengthen beyond the normal developmental period. As the eye stretches, its outer layers, including the sclera and the choroid, are thinned, particularly at the posterior pole.
This mechanical stress forces the delicate internal structures, especially the retina, to cover a much larger surface area. The retina becomes stretched and significantly thinner, making it fragile and susceptible to damage. This thinning also affects the underlying choroid, the layer responsible for supplying blood and nutrients to the outer retina.
Major Vision-Threatening Complications
The structural weakening caused by axial elongation dramatically increases the risk of specific eye diseases that can lead to irreversible vision loss.
Myopic Maculopathy
Myopic maculopathy is a degenerative condition affecting the macula, the central part of the retina responsible for sharp, detailed vision. The stretching causes fine, linear breaks in the deeper layers, called lacquer cracks. These cracks can lead to choroidal neovascularization (CNV), where new, abnormal blood vessels grow beneath the macula. These fragile vessels leak fluid and blood, causing rapid and severe central vision loss.
Retinal Detachment
Retinal detachment occurs when the thinned retina tears and peels away from its underlying support tissue. High myopia can increase this risk by five to six times compared to low myopia, with the risk increasing linearly with the degree of nearsightedness. The stretching and degeneration of the vitreous gel inside the eye also contribute by pulling on the fragile retina. This condition is a medical emergency requiring immediate surgical intervention to prevent permanent blindness.
Glaucoma
High myopia significantly elevates the risk for open-angle glaucoma, a condition that damages the optic nerve. Individuals with high myopia are two to three times more likely to develop glaucoma than those with normal vision. The elongation of the eye may make the optic nerve head more susceptible to pressure damage, even at relatively normal eye pressure levels. Structural changes can also complicate the diagnosis, as the optic disc may appear tilted or abnormal.
Cataracts
High myopia is associated with the premature development of cataracts, which is the clouding of the eye’s natural lens. Highly myopic individuals are significantly more likely to require cataract surgery, sometimes decades earlier than the general population. This early onset is related to the metabolic and structural changes that occur in the elongated eye.
Essential Monitoring and Risk Mitigation
Given the substantial risks, individuals with high myopia require proactive and consistent eye care beyond simple vision correction. Comprehensive, dilated eye exams are essential, often recommended annually or more frequently, to allow a specialist to thoroughly inspect the entire retina. This allows for the early detection of peripheral retinal thinning, lattice degeneration, or early signs of maculopathy.
Patients should be vigilant for warning signs of acute complications, such as a sudden shower of new floaters, flashes of light, or a dark curtain moving across the field of vision. Immediate contact with an eye care professional is necessary if these symptoms occur, as timely intervention can be sight-saving. Risk mitigation strategies can include low-dose atropine eye drops or specialized contact lenses designed to slow the rate of axial elongation in children. Protective eyewear should also be worn during sports, as the elongated eye is more vulnerable to blunt trauma.