An eye prescription measures a refractive error, which is the failure of the eye to focus light precisely on the retina. Eye care professionals quantify the focusing power needed for correction using a standardized global measurement system. This quantification is expressed in the Diopter (D), which serves as the universal unit of vision correction.
Understanding the Diopter Scale
The Diopter (D) is the unit used to measure the optical power of a lens, indicating its ability to bend or refract light rays. Mathematically, the diopter value is the reciprocal of the lens’s focal length measured in meters. A higher numerical value in diopters signifies a stronger lens and a greater degree of correction is needed for clear vision.
Prescriptions are differentiated by a simple sign convention. A negative sign (-) indicates a lens used to correct myopia (nearsightedness), which diverges light rays, pushing the focal point back onto the retina. Conversely, a positive sign (+) indicates a lens used to correct hyperopia (farsightedness), which converges light rays to pull the focal point forward onto the retina.
Defining Extremely High Prescriptions
There is no single absolute “highest” prescription number, but rather a point where correction becomes highly challenging and the risk to eye health increases significantly. High Myopia is clinically defined as a prescription of -6.00 D or worse. Prescriptions move into the range of extreme myopia when they exceed -10.00 D, with some cases reaching -20.00 D or even higher. Values near -30.00 D are exceedingly rare and present major practical challenges for correction.
The limits for plus prescriptions (hyperopia) are generally lower than for minus prescriptions. High hyperopia is often classified as +6.00 D or more, though prescriptions can be documented up to +20.00 D in rare congenital cases. This difference is rooted in the physical structure of the eye: the nearsighted eye physically stretches and can elongate significantly, whereas the farsighted eye is typically shorter, limiting the upper physical boundary for the refractive error.
Underlying Conditions Causing Extreme Refractive Error
The primary cause behind extremely high negative prescriptions is axial elongation, where the eyeball grows too long from front to back. For a normal eye, the axial length is between 22 and 25 millimeters, but in high myopia, this length often exceeds 26.0 millimeters, causing light to focus in front of the retina.
When this elongation becomes severe, it can lead to pathologic myopia, a more serious condition not defined by the prescription number alone. Pathologic myopia involves degenerative changes to the posterior segment of the eye, including the retina and choroid. This significantly increases the risk of complications like retinal detachment.
Another contributor to high prescriptions, especially involving irregular astigmatism, is advanced Keratoconus. This disorder causes the cornea to thin and bulge into a cone shape. This creates a highly irregular surface that drastically alters the eye’s focusing power.
Corrective Options for Maximum Prescriptions
For individuals with maximum prescriptions, standard glasses or contact lenses often become impractical due to discomfort, weight, or visual distortion. Glasses for these powers require high-index lenses, which bend light more efficiently, allowing the lens to be significantly thinner and lighter than conventional plastic. Even with these specialized materials, the edge thickness for minus prescriptions remains substantial.
For those who seek a more permanent solution, surgical options are available when traditional laser procedures like LASIK are deemed unsafe due to extreme power or thin corneas. The Implantable Collamer Lens (ICL) is a popular alternative, involving the insertion of a permanent, flexible lens inside the eye, often between the iris and the natural lens. This technique avoids the removal of corneal tissue required by LASIK and can correct myopia up to -20.00 D.
For older patients, Refractive Lens Exchange (RLE) may be recommended. This involves replacing the natural lens entirely with a custom-power artificial intraocular lens to achieve the necessary correction.