There are two types of blindness that I deal with; legal and complete. Diabetic retinopathy can cause both types while macular degeneration generally causes just legal blindness. Other types of vision loss are actually perceived; what I call “refractive” blindness and psychological blindness.
Legal blindness is defined as vision worse than 20/200, but it does not mean that there is total blindness. Peripheral vision is usually intact and central vision is commonly decreased. Legal blindness typically indicates that central vision is lost. Patients are usually able to function.
The macula, the functional center of the retina, provides our central vision. The rest of the retina serves to provide peripheral vision. Diseases such as macular degeneration and diabetic macular edema principally affect the macula, and, therefore, central vision. In fact, any macular disease, affects central, 20/20, vision.
Being legally blind, therefore, is based upon a person’s central vision. While the definition also includes the peripheral vision, central vision is the most important criteria for declaring someone “legally blind.” At this level, the central vision is so poor that reading may be impossible, even with glasses or most other visual aids. At this level of vision, a person has difficulties performing the activities of daily living. Patients that are legally blind may have difficulty performing everyday tasks and risk injury to themselves or others. For example, they may have difficulty turning on a stove or navigating safely around the house.
Complete blindness causes complete darkness. There is no light, color, shape or movement. Nothing.
Complete loss of vision can be caused by proliferative diabetic retinopathy, but not macular degeneration.
Remember, macular degeneration only affects the macula. Complete blindness, though rare, can be caused by any type of retinal detachment or damage to the entire optic nerve. Both of these can be complications of proliferative diabetic retinopathy. (There are diseases of the central nervous system that can cause complete blindness, such as brain tumors and stroke, but won’t be included today.)
Complete blindness is caused by disease of the entire retina, including the macula. Legal blindness is damage limited to just the macula.
What Does This Mean? Diabetes can affect the entire retina. Therefore, both central and peripheral vision can be affected and lead to complete, and absolute, loss of vision. Macular degeneration causes legal blindness only.
When we read about the leading causes of blindness, we end up talking about macular degeneration and diabetic retinopathy. We really should be saying that they are either the leading causes of legal blindness, or that macular degeneration is the leading cause of legal blindness and diabetic retinopathy is the leading cause of complete blindness.
Why? There are two types of blind.