A recent study from researchers at the Wilmer Eye Institute at Johns Hopkins University School of Medicine found that most Americans regard loss of eyesight as the worst ailment that could happen to them, surpassing such conditions as loss of limb, memory, hearing or speech, or having H.I.V./AIDS. Indeed, low vision ranks behind arthritis and heart disease as the third most common chronic cause of impaired functioning in people over 70, Dr. Eric A. Rosenberg of Weill Cornell Medical College and Laura C. Sperazza, a New York optometrist, wrote in American Family Physician.

Some 23.7 million American adults reported in 2015 that they are unable to see at all or have trouble seeing even with corrective lenses. This number is projected to perhaps double by 2050 based on the aging of the population and increasing prevalence of diseases that can cause vision loss. Yet, the Wilmer Eye Institute’s national study of 2,044 adults found that many Americans are unaware of the diseases and factors that can put their vision at risk and steps they might take to lower their risk.

Perhaps the single most valuable message to emerge from studies of vision loss is the importance of having a thorough eye checkup at least once every two years, if not annually. Many sight-robbing conditions can be effectively treated if detected early enough, in many cases limiting or eliminating the damage to eyesight.

Four eye diseases — age-related macular degeneration, diabetic retinopathy, glaucoma and cataracts — account for most cases of adult blindness and low vision among people in developed countries. Unlike many other ailments associated with aging, they cause no pain and often no early symptoms and thus do not automatically prompt a person to seek medical care. But a thorough checkup by an ophthalmologist can detect them in their earliest stages, followed by treatment that can slow or halt their progression or, in the case of cataracts, restore normal vision.

Macular degeneration, a leading cause of vision loss in Americans 60 and older, involves an irreversible loss of retinal cells that robs people of the central vision needed to read, watch a TV program or identify a face or object in front of them. There are two types, dry and wet. In the dry type, the light-sensitive cells in the macula, a structure near the center of the retina, gradually break down. In the wet type, abnormal blood vessels grow under the macula.