Myopia, commonly known as nearsightedness, is a prevalent refractive error where the eye focuses light in front of the retina instead of directly on it. This optical mismatch causes distant objects to appear blurry while close objects remain relatively clear. The degree of this focusing error is measured in diopters (D). A negative number on a prescription, such as -7.5 D, indicates the strength of the corrective lens needed to push the focal point back onto the retina.
Understanding Diopter Measurements and Severity
A prescription of -7.5 diopters places an individual firmly within the category of high myopia. Severity is classified into three levels based on corrective lens power. Mild myopia is generally considered to be up to -3.00 D, while moderate myopia spans from -3.00 D to -6.00 D. High myopia is defined as a refractive error greater than -6.00 D.
The high refractive error is usually caused by excessive elongation of the eyeball, a condition called axial myopia. While a typical adult eye measures about 23 to 24 millimeters in length, an eye with high myopia often exceeds 26 millimeters. This increased axial length stretches the internal structures of the eye, which creates the potential for significant long-term health complications. For reference, an elongation of just one millimeter can shift the prescription by approximately 2.00 to 2.50 diopters.
Ocular Health Risks Associated with High Myopia
The chronic stretching of the eye’s delicate tissues due to high axial length is the primary reason a -7.5 D prescription carries elevated health risks. The retina, which is the light-sensitive tissue lining the back of the eye, becomes significantly thinner and more fragile as it is spread over a larger surface area. This structural change increases the likelihood of a retinal tear or a full retinal detachment, which is a medical emergency that can lead to permanent vision loss. Patients with myopia greater than -5.0 D have a risk of retinal detachment that is up to ten times higher than the general population.
The optic nerve is also subjected to increased stress in highly myopic eyes, raising the risk of glaucoma. Studies suggest that for every diopter increase in myopia, the risk of developing open-angle glaucoma increases by about 20%. This condition damages the optic nerve, often due to elevated intraocular pressure, and can lead to a gradual loss of peripheral vision.
High myopia contributes to myopic maculopathy, a degenerative change in the macula responsible for sharp, detailed central vision. This condition can involve the formation of lacquer cracks, which are breaks in the deepest layers of the retina, or choroidal neovascularization, which is the growth of abnormal blood vessels beneath the macula. Furthermore, individuals with high myopia are more prone to developing cataracts, a clouding of the natural lens, at a younger age compared to those with lower prescriptions or no myopia.
Current Correction and Management Options
Correcting a -7.5 D refractive error requires specialized optical solutions to manage the thickness and distortions associated with strong lenses. Eyeglasses for this level of correction often necessitate the use of high-index plastic materials, which make the lenses thinner and lighter than standard plastic or glass. Strong lenses can still cause noticeable peripheral distortion and reduce the size of the image seen by the wearer.
Contact lenses offer a wider field of view and minimize image size distortion by sitting directly on the eye’s surface. Custom contact lens options, such as toric lenses for astigmatism or rigid gas permeable lenses, may be required to achieve the best visual acuity. These options provide a more natural visual experience for daily activities.
Refractive surgery also presents viable options, though standard procedures like LASIK and PRK may be near their safe limit for a -7.5 D correction. These laser procedures work by permanently reshaping the cornea to correct the refractive error, but they require sufficient corneal thickness, which is a major limiting factor for high prescriptions. For individuals with insufficient corneal tissue, an Implantable Collamer Lens (ICL) is often recommended, as it involves inserting a permanent corrective lens inside the eye without altering the corneal structure. Regardless of the chosen correction method, the long-term management of high myopia requires regular, comprehensive eye examinations, at least annually, to monitor the retina and optic nerve for any of the associated sight-threatening conditions.