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has some bad news for his adoring audience: He may be going blind. Beck told fans he went to the doctor because he was having trouble focusing his eyes and that the doctor "did all kinds of tests" before settling on a diagnosis of macular dystrophy.

So what exactly is macular dystrophy? Well, that depends, says Shantan Reddy, a retinal specialist at New York University School of Medicine. Reddy says there are up to 100 different types of macular dystrophy—sometimes individual cases can even be impossible to classify. "It's basically a hodgepodge of things that affect the macula that we routinely call macular dystrophy," Reddy says. Ophthalmologists then make more specific diagnoses using eye-imaging technologies.

The macula, a part of the inner eye that enables people to see fine details in the center of their vision, is a yellow-pigmented spot near the center of the retina—the layer of cells in the back of the eyeball that converts light into visual information. Damage to the macula generally impairs central vision.

Beck probably has a condition called vitelliform macular dystrophy, Reddy says, the symptoms for which often arise later in life. It's characterized by the appearance of a yellow, egg-yolk-colored lesion on the macula. The disorder progresses through phases, and in the end stage the center of the retina atrophies and "starts resembling something like macular degeneration," a more common degenerative condition that usually affects older adults, Reddy says.

There are two types of vitelliform macular dystrophy. The more severe version, called Best's disease, is inherited genetically—although symptoms may not become noticeable until adulthood. A more mild form is known as adult-onset vitelliform macular dystrophy, and often shows up in patients between the ages of 30 and 40. If Beck's doctor warned of severe symptoms, that might mean he has the more severe, genetically inherited Best's disease, Reddy says. But the only way to differentiate between the two forms is through an imaging test called electrooculogram, or EOG. "So if his doctor didn't do an EOG, he may not even know whether this is Best's disease or the adult-onset form," Reddy says.

Vitelliform macular dystrophy is extremely rare, and there is currently no treatment. Each case is different, but in general neither form progresses quickly, and total blindness is rarely an outcome. "They rarely go legally blind in both eyes. Usually they have some retained vision," Reddy says.

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