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(CBS NEWS) -- Rock and pop superstar of the 70s Linda Ronstadt says Parkinson's disease has robbed her of her ability to sing.Although Parkinson's is better known for causing tremors and troublewith movement, the disease frequently affects a person's voice as well,doctors say.

The 67-year-old singer, who is known for such hits as "You're No Good," "Blue Bayou" and "Don't Know Much," told AARP The Magazine lastFriday that she hadn't suspected she had the neurological disease foras many as eight years despite having a variety of symptoms, which shemistakenly attributed to other causes.

Then she lost her ability to sing.

"AndI couldn't figure out why," she told AARP. "I knew it was mechanical. Iknew it had to do with the muscles, but I thought it might have alsohad something to do with the tick disease that I had. And it didn'toccur to me to go to a neurologist... Then I had a shoulder operation,so I thought that's why my hands were trembling."

Up to 60,000 Americans are diagnosed with Parkinson's disease each year, according to the National Parkinson's Foundation, on top of the one million who currently have the condition.

Parkinson'sdisease as a degenerative neurological condition caused by the loss ofbrain cells that produce the neurotransmitter dopamine, which isessential for motor control and other important brain functions.

Theaverage age of diagnosis is around 60, and the disease worsens overtime to the point where it can cause significant disability.Parkinson's-related complications make up the fourteenth leading causeof death in the United States.

Common symptoms -- which typicallybegin between ages 50 and 60 -- include tremors or shaking, stiffmuscles, slow movements, difficulty walking, and loss of control offacial movements and throat muscles, WebMD reports.

Ronstadt told AARP that no one can sing with Parkinson's, "no matter how hard you try."

Parkinson'sdisease does indeed commonly affect a person's voice, explained Dr.Andrew Feigin, a professor of neurology and physician at the MovementDisorders Center at North Shore-LIJ's Cushing Neuroscience Institute inManhasset, N.Y., in an interview with CBSNews.com.

Most often, aperson may hear their voice drop in volume due to decreased size ofmovements in the vocal cords, a condition called hypophonia, explainedFeigin, who has no involvement in Ronstadt's care.

"It'sessentially a Parkinsonian symptom," he said, and added that people withthe disease may also develop a raspy, airy voice due to vocal cordproblems.

Ronstadtwas not aware her symptoms were signs of Parkinson's, according to herinterview, and Feigin says it's typical for some people eventuallydiagnosed with the disease to not be aware their symptoms are due to thebrain disorder.

"Its insidious in onset," he said. "It's quitepossible that someone has signs of the disease before it rises to thelevel where they think there's something wrong here."

Some less-obvious symptoms of Parkinson's thatmay occur before motor symptoms include loss of sense of smell,constipation, depression or behavior indicative of an REM sleep disorderin which people may act out their dreams.

Feigin urges peopleexperiencing problems like loss of facial expression, changes inhandwriting, problems with balance or obvious tremors to see a doctor.

Prognosismay vary, due to the unpredictable nature of the disease, butmedications can reduce tremors or improve coordination. One thing thatis lacking in Parkinson's treatment, however, is a drug that can preventthe disease from worsening.

"That's really an unfilled need," said Feigin. Parkinson's "progresses over years, not days or months."

Ronstadttold AARP she now uses poles to help her walk and a wheelchair whentraveling. She is planning to release a memoir in September. The bookdoes not address her diagnosis, according to the magazine.

The National Parkinson's Foundation has more information on the disease

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