An estimated one million Americans today are living with Parkinson’s disease (PD). According to the Parkinson's Foundation, it strikes more than 60,000 individuals annually, with men diagnosed 1.5 times more often than women. In the last several decades public awareness of PD has grown, as celebrities like boxing great Muhammad Ali and actor Michael J. Fox—who was diagnosed in 1991 at age 29—have chosen to address it publicly.
Early symptoms may be barely noticeable, experts say. But, with time, this central nervous system disorder progresses, causing a range of disabilities from increasingly visible tremors and stiff limbs to shaky balance and even slow, low or repetitive speech.
Researchers know that Parkinson's develops due to the death of nerve cells that coordinate muscle movement, located primarily in the mid-brain. Once too many of these cells are gone, the remaining ones can no longer produce enough of a vital chemical messenger called dopamine, which brain cells use to communicate.
Unlike Fox, most people who develop PD do so after age 60. But besides age, risk factors that contribute to disease development have not been as clear.
What we know about risk factors
Many studies have implicated environmental toxins, either alone or in interaction with our genes, but the results can be inconclusive, according to Jill Giordano Farmer, DO, MPH, director of the Parkinson’s Disease and Movement Disorders program at Drexel Neurosciences Institute in Philadelphia. In studying twins, for example, she says researchers have found differences in the development of PD based on their exposure to industrial solvents over time—those with greater exposure had higher rates of Parkinson's—but the connection requires further confirmation.
Study results about genetic risk factors also have proven inconclusive so far. Only about 15 percent of people with PD have a family history of the disease. And while mutations or errors in several genes have been identified, genetic factors, when present, are considered rare, she says. They carry a low risk of penetrance, or being passed from one generation to another.
“So, there really are no risk factors that are modifiable,” at least at this time, Dr. Farmer says. “We do not tell patients to avoid certain foods or to be on certain diets or to take certain supplements. The data just isn’t there.”
Instead, doctors advise patients to follow general rules of good health, like eating a healthy diet high and fruits and vegetables, as well as:
- Using barrier protection if working with chemicals. In her own practice, Farmer says, she always asks patients about exposure to Agent Orange, an herbicide widely used to defoliate trees during the Vietnam War; it's linked to some cancers and other diseases, including PD.
- Avoiding repetitive head trauma. A recent study involving military veterans found even a mild traumatic brain injury is associated with a higher risk of Parkinson’s disease later in life. Individually, the risk was seen as low, but the authors called the association between traumatic brain injuries and PD the "highest level of evidence” yet. The results, published in Neurology in April 2018, compared 165,000 vets who experienced a head injury to veterans of a similar age who hadn't.
Diagnosing the disorder
Early diagnosis of PD is still considered difficult, lacking blood or laboratory tests that are definitive. But based on medical history and a physical exam, Farmer says, doctors still can make fairly accurate diagnoses.
If patients have three out of four features on exam, she says, there’s a high probability of having the disease. The features are: slowness, rigidity, tremor and postural instability. Afterwards, doctors look to see how the patient responds to dopamine replacement, and “If there’s improvement in motor symptoms,” she says, “that gives us our answer.”
Meanwhile, as Farmer and other neurologists seek to manage these illnesses, scientists continue to work towards identifying biomarkers—or signposts of disease—that would allow for earlier intervention. As with cancer, the hope is to tailor treatments to each individual, to somehow stay ahead of worsening disease.
The future of treatment
Parkinson’s disease is not yet curable, but medical treatments, and in some cases, surgery, can bring symptom relief to patients, sometimes for many years.
“Symptomatically, anything that can best mimic continuous levels of dopamine,” offers the greatest benefit right now, says Farmer. That can mean either electronically (through deep brain stimulation), she says, or through dopamine-modulating drugs taken orally or pumped through the intestine or under the skin.
What holds the greatest promise, she says, involves research efforts to identify disease-modifying therapies that can stop or reverse the degeneration that occurs once Parkinson’s disease takes hold. “We’re not there yet,” she says, “but researchers are looking into these mechanisms.”
