Determining how severe nearsightedness must be to constitute legal blindness involves meeting a specific, government-defined visual threshold. Legal blindness is a technical term used to determine eligibility for services and benefits, not a medical diagnosis of total sight loss. This threshold is defined by strict measurements of visual performance, which are sometimes met by individuals with extremely high degrees of nearsightedness.
Understanding Nearsightedness
Nearsightedness, medically termed myopia, is a common refractive error where distant objects appear blurry while close objects remain clear. This condition occurs because the eye is either too long or the cornea, the clear front surface of the eye, is too steeply curved. In either case, light entering the eye focuses in front of the retina instead of directly on its surface.
The severity of myopia is measured in diopters (D), which indicate the corrective lens power needed to refocus light correctly. Myopia is noted with a minus sign on a prescription; the higher the number, the stronger the correction required. Low myopia is typically classified as a prescription up to -3.00 D, while moderate myopia ranges from -3.00 D to -6.00 D.
The Official Definition of Legal Blindness
The United States government defines legal blindness using two primary, measurable criteria regarding the better-seeing eye, even with the best possible corrective lenses. The first standard relates to visual acuity, which is the sharpness of vision. A person is considered legally blind if their central visual acuity is 20/200 or worse.
The 20/200 measurement is based on the Snellen eye chart. The first number represents the testing distance (20 feet), and the second represents the distance at which a person with normal 20/20 vision could read that line (200 feet). The second criterion for legal blindness is a severe restriction of the visual field, specifically if the widest diameter of the field subtends an angle of no greater than 20 degrees.
Linking Myopia Severity to Legal Blindness
The refractive error itself, measured in diopters, does not qualify a person for legal blindness; rather, it is the resulting, uncorrectable visual acuity. High myopia, defined as a prescription of -6.00 D or greater, often leads to severe vision loss because it causes physical, degenerative changes to the eye’s structure. This high degree of nearsightedness is often called pathological or degenerative myopia because the excessive elongation of the eyeball stretches and thins the delicate tissues at the back of the eye, including the retina.
The vision loss that results in legal blindness is due to secondary complications, not the refractive error alone, which can be corrected by lenses. The stretching of the eye can lead to myopic maculopathy, which involves damage to the macula, the area responsible for sharp, central vision. A highly stretched retina is also at an increased risk of developing myopic choroidal neovascularization (CNV), where abnormal blood vessels grow under the retina. These degenerative conditions cause irreversible damage to the retina that prescription lenses cannot fix, resulting in a best-corrected visual acuity of 20/200 or worse.
The Process of Diagnosis and Certification
The determination of legal blindness is a precise process conducted by an eye care specialist, such as an ophthalmologist or optometrist. The first step involves a comprehensive eye examination to determine the individual’s best-corrected visual acuity. This measurement must be taken while the person is wearing the strongest possible corrective lenses.
If the visual acuity measurement is 20/200 or less, or if the visual field is 20 degrees or less, the patient meets the technical definition. The eye doctor then documents these findings on a specific certification form. This certification is necessary to verify eligibility for various governmental programs and services. The medical records confirming the uncorrectable nature of the vision loss are then submitted to the appropriate agencies to complete the certification process.