Carly Simon reveals the common condition that masked her Parkinson's diagnosis

Singer Carly Simon has revealed that her Parkinson's disease was initially mistaken for arthritis, highlighting how the two conditions can share similar symptoms, particularly in the early stages.

"My health issues began with arthritis in both knees and one hip. I had all three joints replaced, and at first, I thought the difficulty I was having walking was simply part of the long recovery process," Simon said in a statement shared with multiple media outlets on July 27.

"Eventually, there were periods when I could not walk without considerable help. My family and I knew that something more was going on. After an extensive evaluation at Mayo Clinic, I was diagnosed with Parkinson's."

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Arthritis is far more common than Parkinson's disease, affecting nearly 60 million U.S. adults compared with about 1.1 million Americans living with Parkinson's, according to the Centers for Disease Control and Prevention.

Even so, about 90,000 Americans are diagnosed with Parkinson's disease each year, making it a relatively common neurological disorder despite being far less prevalent than arthritis.

Arthritis primarily affects the joints and connective tissues, causing pain, inflammation, stiffness and structural damage, per the CDC. The most common types are osteoarthritis (wear-and-tear of cartilage) and rheumatoid arthritis (an autoimmune disease).

Parkinson's disease is a progressive neurological disorder that mainly affects the brain and nervous system, according to the National Institute on Aging (NIA). Dopamine-producing nerve cells in the brain gradually die or become impaired, affecting movement.

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Although the conditions can share mobility problems, arthritis is typically driven by painful joint inflammation, while Parkinson's primarily affects movement through changes in the brain, according to Dr. Guy Schwartz, a neurologist with Stony Brook Medicine in New York.

"Parkinson's disease can sometimes be misdiagnosed as arthritis, and vice versa," Schwartz, who specializes in Parkinson's disease, told Fox News Digital. "The one main symptom that they share is difficulty with walking — some kind of impediment, limitation or disability."

While arthritis and Parkinson's are fundamentally different diseases, health experts say they can share the following symptoms that can make diagnosis difficult.

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The two conditions can also occur in the same person, Schwartz noted, since both become more common with age. "I've definitely been challenged by individuals who come to me with these symptoms," he said. "Sometimes the examination is confounded by significant arthritis."

While many people may connect Parkinson’s with tremors, early signs of the disease usually include stiffness, slowness and changes in gait. 

In older adults who have been diagnosed with arthritis or have had joint replacements, those same symptoms can easily be attributed to joint disease or surgical recovery.

Some Parkinson’s symptoms are not typically seen in arthritis cases. People who experience the following signs should consult a neurologist, according to the Parkinson's Foundation.

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While tremor is often considered a hallmark symptom, Schwartz cautioned that it isn't always present. "About a third of individuals with Parkinson's disease do not have a tremor," he said. "That's when the diagnosis becomes more challenging."

"In Parkinson's disease, dexterity becomes compromised because of slowness and stiffness," the expert added. "In arthritis, it's more of a mechanical issue, where the joints don't allow full range of motion."

The two conditions can be especially difficult to distinguish because both are largely diagnosed based on a patient's symptoms and physical examination rather than one definitive test, according to Schwartz.

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There is no single blood test or imaging scan that can definitively confirm Parkinson’s, as "very few things in medicine are 100%," he said.

However, newer diagnostic tools have emerged, including specialized imaging and skin biopsy testing. "We're still learning," the neurologist said, adding that a blood test may be on the horizon.

For now, diagnoses are made based on the following factors, per the National Institute of Neurological Disorders and Stroke (NINDS).

Schwartz emphasized that early diagnostic uncertainty is not uncommon. "It's not unusual for there to be misdiagnoses," he said. "Sometimes seeing a patient over time helps — eventually the disease declares itself, and then it's easier to make the diagnosis."

Anyone experiencing persistent walking difficulties, stiffness or other symptoms that don't seem fully explained by arthritis should discuss them with a healthcare provider, who can determine whether a neurological evaluation is appropriate.