what causes loss of depth perception

Presbyopia usually occurs beginning at around age 40, when people experience blurred near vision when reading, sewing or working at the computer.

You can't escape presbyopia, even if you've never had a vision problem before. Even people who are nearsighted will notice that their near vision blurs when they wear their usual eyeglasses or contact lenses to correct distance vision.

Presbyopia is widespread in the United States. According to U.S. Census Bureau data, over 135 million Americans were age 40 and older in 2008, and the country is growing older: The median age reached 36.8 in 2008, up 1.5 years since 2000. This growing number of older citizens generates a huge demand for eyewear, contact lenses and surgery that can help presbyopes deal with their failing near vision.

More than a billion people in the world were presbyopic as of 2005, according to the World Health Organization, and 517 million of these did not have adequate correction with eyeglasses. In developing countries, glasses are available in urban areas, but in rural regions they are unavailable or expensive. This is unfortunate, because good near vision is important for literacy and for performing close-up work.

Presbyopia Symptoms and Signs

When people develop presbyopia, they find they need to hold books, magazines, newspapers, menus and other reading materials at arm's length in order to focus properly. When they perform near work, such as embroidery or handwriting, they may develop headaches, eye strain or feel fatigued.

The eye's lens stiffens with age, so it is less able to focus when you view something up close. (Image: Varilux)

What Causes Presbyopia?

Presbyopia is caused by an age-related process. This differs from astigmatism. nearsightedness and farsightedness. which are related to the shape of the eyeball and are caused by genetic and environmental factors. Presbyopia generally is believed to stem from a gradual thickening and loss of flexibility of the natural lens inside your eye.

These age-related changes occur within the proteins in the lens, making the lens harder and less elastic over time. Age-related changes also take place in the muscle fibers surrounding the lens. With less elasticity, the eye has a harder time focusing up close. Other, less popular theories exist as well.

Presbyopia Treatment: Eyewear

Eyeglasses with bifocal or progressive addition lenses (PALs) are the most common correction for presbyopia. Bifocal means two points of focus: the main part of the spectacle lens contains a prescription for distance vision, while the lower portion of the lens holds the stronger near prescription for close work.

Progressive addition lenses are similar to bifocal lenses. but they offer a more gradual visual transition between the two prescriptions, with no visible line between them.

Reading glasses are another choice. Unlike bifocals and PALs, which most people wear all day, reading glasses typically are worn just during close work.

If you wear contact lenses, your eye doctor can prescribe reading glasses that you wear while your contacts are in. You may purchase readers over-the-counter at a retail store, or you can get higher-quality versions prescribed by your eye doctor.

Presbyopes also can opt for multifocal contact lenses. available in gas permeable or soft lens materials. Another type of

contact lens correction for presbyopia is monovision. in which one eye wears a distance prescription, and the other wears a prescription for near vision. The brain learns to favor one eye or the other for different tasks. But while some people are delighted with this solution, others complain of reduced visual acuity and some loss of depth perception with monovision.

Because the human lens continues to change as you grow older, your presbyopic prescription will need to be increased over time as well. You can expect your eye care practitioner to prescribe a stronger correction for near work as you need it.

Presbyopia Treatment: Surgery

Surgical options to treat presbyopia also are available. One example is Refractec Inc.'s conductive keratoplasty or NearVision CK treatment, which uses radio waves to create more curvature in the cornea for a higher "plus" prescription to improve near vision. The procedure is performed on one eye only for a monovision correction.

Presbyopia News

Why Do Women Need Reading

Glasses Earlier Than Men?

July 2012 — Are differences in eye anatomy and focusing ability responsible for women needing presbyopia correction earlier than men?

According to a new analysis of presbyopia prevalence studies, this is not the reason. Instead, it has to do with arm length or preferred reading distance.

On average, women's arms are shorter than men's, and when it becomes more and more difficult to read at arm's length, women must turn to presbyopia correction such as reading glasses sooner. Accordingly, they also need higher-powered reading glasses or bifocals than men of the same age.

In developing countries, this is a big issue since reading glasses are not widely available. Without them, it is very challenging for women who do close-up work to continue to make a living as they age.

A report of the study appeared in Investigative Ophthalmology & Visual Science in May.

Studies have shown that NearVision CK effectively treats presbyopia, but the correction is temporary and diminishes over time.

PresbyLASIK is a new presbyopia-correcting surgery now undergoing U.S. clinical trials. This innovative procedure uses an excimer laser to create a multifocal ablation directly on the eye's clear front surface (cornea). This enables vision at multiple distances.

LASIK also can be used to create monovision, in which one eye is corrected for near vision while the other eye is stronger for distance vision.

A highly experimental treatment being studied is the injection of an elastic gel into the capsular bag, the structure in the eye that contains the natural lens. In theory, the gel would replace the natural lens and serve as a new, more elastic lens.

Experiments also have centered on laser treatment of the eye's hardened lens to increase its flexibility and thereby improve focus.

With the recent introduction of presbyopia-correcting intraocular lenses. some people undergoing cataract surgery may be able to achieve clear vision at all distances.

Also, an elective procedure known as refractive lens exchange may enable you to replace your eye's clear but inflexible natural lens with an artificial presbyopia-correcting lens for multifocal vision.

Other surgical options for presbyopia are being researched as well.

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