Drugstore reading glasses can't solve every vision problem

Q: My vision has been getting worse, and I definitely need reading glasses. Is there any reason not to get the inexpensive ones sold at the drugstore?

A If you're over 40, reading glasses can be a necessity. Many people end up buying several pairs at the drugstore. They are inexpensive and available in a wide variety of styles. (Not to mention that reading glasses tend to be easily misplaced.)

Why do we need reading glasses in the first place?

We see when light rays enter our eyes. The eye's lens bends and focuses the rays on the retina. This allows the eye to clearly focus on objects at different distances.

As we age, the lens loses some of its flexibility. As a result, it becomes harder to focus on close objects — to read, sew or knit, for example. This is known as presbyopia, which translates to “elder vision.”

It may seem strange that the words “elder vision” were used for a condition that typically starts in a person's 40s (or earlier). However, when the term was first used, the average life expectancy of a person in the United States was less than age 50.

You asked about off-the-rack reading glasses. These are essentially two magnifying lenses mounted in an eyeglass frame.

Like prescription lenses, they provide varying degrees of magnification or refraction (how much they bend and focus light rays). The strength of reading glasses increases in increments of .25, usually ranging from plus-1.00 to plus-2.50.

Drugstore reading glasses typically provide the same refraction for both eyes. That may be fine if you need the same refraction, but many people don't.

Another issue is that many people who need reading glasses for close vision also need glasses for distant vision. Drugstore reading glasses won't do that. They also won't correct for a common condition called astigmatism.

Of greater concern is the possibility that your reading difficulties are caused not by simple presbyopia. The lens isn't the only part of the eye that changes as we age.

As you get older, your risk for developing other, more serious conditions also increases. These include cataracts, glaucoma and macular degeneration.

That's why I really urge people to get professional advice when they start having problems reading. You might get lucky and drugstore reading glasses may do the trick. But there's a good chance that prescription glasses will give you better vision.

Also, if you have a more serious eye condition, it will be caught and treated early through an eye exam.

Finally, remember this: Once you're older than 40, you should be having regular eye exams with an ophthalmologist even if you're not having trouble reading.

I recall a new patient who once came to me. He was nearly completely blind from glaucoma. He had used drugstore reading glasses for 20 years, until they no longer helped. When the real problem, glaucoma, was diagnosed, it had already caused permanent loss of vision.

Regular eye exams are the cornerstone to good visual health, whether or not you need glasses.

• Dr. Komaroff is a physician and professor at Harvard Medical School. For questions, go to AskDoctorK.com.