The study estimated that 60,000 to 95,000 adults aged 45 or older are diagnosed with PD every year in North America, contrasting with previous estimates of 40,000 to 60,000 new cases per year.
The study found that PD cases in the United States tend to occur in geographic clusters, including clusters in Southern California, the South and the Midwest.
The study noted that estimates of PD incidence varied for numerous potential reasons, including diagnosis and case ascertainment methods, prevalence of protective markers or genetic risk factors, geographic location and exposure to toxins in the environment such as paraquat.
Study Finds Greatest Parkinson’s Disease Risk Factor Is Advanced Age
The study found that the greatest risk factor for developing PD is age, and that incidence of PD increased until patients’ 84th year of life. Two age trends were shown by the study past the 84th year of life: for three sets of data, PD incidence rates decreased slightly, and for two sets of data, incidence of PD continued increasing.
The study’s authors suggest that a reason for this could be that the data sets which showed a slight decrease of PD incidence past the 84th year of life relied on specific PD diagnostic criteria, and that as patients become older, it could be harder to identify mild cases of PD, especially in those with multiple comorbidities, and to tell the difference between normal aging and PD.
The study also found that incidence of PD was higher in males, and that the male to female ratio of PD incidence consistently increased with age across all data sources examined. The study found that this data supports the hypothesis that an intrinsic PD risk factor is biological sex.
The study proposes a working estimate of 62 per 100,000 person-years in those 45 years old or older regarding incidence rate of PD, which would correlate to 77,000 cases per year regarding 2012 population data and 86,000 cases per year regarding 2020 population data, assuming negligible differences in age distribution from 2010, the study’s standardization year.
The study notes that it has numerous limitations:
- Possible measurement error due to selection bias, miscoding, misclassification or confounding
- Estimates were mostly derived from data sourced from epidemiological studies, limiting the researchers’ ability to apply varying or new ascertainment and diagnostic criteria across datasets
- Incidence rates of PD for the current year might be higher than estimated because of better recognition of the symptoms of PD, lower prevalence of possible protective factors such as smoking, and a higher prevalence of risk markers, such as sedentary lifestyle, cognitive dysfunction, diabetes, or pesticides
- Incidence rates of PD from 2012 to the present may be reduced by reduced occupational and community exposure to environmental toxins, or lower case detection due to COVID-19-related changes in behavior regarding the seeking of healthcare
PD is a progressive neurodegenerative disorder which primarily affects the human motor system. There is no cure for PD, and there are no treatments which reverse, stop or slow its progression. Treatment of the disease is limited to treating its symptoms, and existing treatments tend to work less over time and can be accompanied by unwanted side effects.
The primary motor symptoms of PD are:
- Slow movement
- Problems walking
PD’s primary motor symptoms can result in secondary motor symptoms, such as:
- Freezing gait
- A quiet, monotonous, slurred voice
- A mask-like facial expression
- Handwriting which progressively shrinks
The primary pathological characteristic of PD is the death of cells in the basal ganglia of the brain. As much as 70% of the dopamine-producing neurons (dopaminergic neurons) in the brain’s substantia nigra pars compacta die by the end of the patient’s life.
Our brains need dopamine for control of our motor system, and dopaminergic neurons do not grow back once they die. So, once enough dopaminergic neurons die, our brains don’t get enough dopamine and the symptoms of PD are the result.
Parkinson’s Disease And Paraquat
The popular herbicide paraquat has been linked by numerous studies and meta-analyses to an increased risk of developing PD, including a 2009 study linking exposure to paraquat with twice the risk of developing PD, and a 2019 meta-analysis which noted “a statistically significant association” between PD and exposure to paraquat.
As a result, over 2,300 lawsuits have been filed and collected into a multidistrict litigation (MDL). These lawsuits claim that the plaintiffs developed PD as a result of their exposure to paraquat, that paraquat manufacturers knew or should have known that their products could cause PD but failed to warn about it, that products containing paraquat were defective by design because they could cause PD even when manufactured and used correctly, and that paraquat manufacturers were negligent by not testing their products enough and by not warning the public their products were dangerous.
Product manufacturers who know or should know their products are dangerous but fail to warn about it are strictly liable in the state of California for any injuries their products cause; see Burke v Almaden Vineyards, Inc.
Product manufacturers who manufacture and sell products which are defective by design are strictly liable for any injuries caused by their products; see Lucas v City of Visalia.
Those who injure others via negligence are liable for the injuries under California Civil Code 1714.
The lawsuits claim that paraquat causes PD because, even when it is used as instructed, it can enter the human body via inhalation, ingestion or absorption through the skin, and once it enters the body it eventually ends up in the brain, where it kills dopaminergic neurons by inducing oxidative stress. In fact, lawsuits claim that once paraquat enters the brain, it starts undergoing a cycle which will never stop as long as oxygen (which is plentiful in brain cells) is present, a cycle which produces oxidative stress. The lawsuits essentially claim that paraquat, once it enters the brain, will continue to kill dopaminergic neurons via oxidative stress until the victim dies.
First Jury Trial In Paraquat MDL Set For October 16, 2023
On December 12, 2022, U.S. District Judge Nancy J. Rosenstengel set the first jury trial in the paraquat MDL for 9:00 AM on October 16, 2023.
The original goal, established in June 2021, was to hold the first jury trial in November 2022. However, according to Judge Rosenstengel’s case management order, there are several reasons why the trial needed to be moved to a later date:
- The court failed to anticipate resistance coming from coordination with state court litigation that had settled after extended pre-trial proceedings on the eve of trial
- A hurricane
- COVID-19 infections
- Other problems
Judge Rosenstengel’s case management order also established several other deadlines regarding the MDL:
- The defendants’ expert reports need to be disclosed by January 20, 2023
- The plaintiffs need to complete depositions of the defendants’ experts by February 24, 2023
- The plaintiffs need to disclose rebuttal experts by March 3, 2023
- The defendants need to complete depositions of the plaintiffs’ rebuttal experts by March 20, 2023
- Daubert and summary judgment motions need to be filed by April 17, 2023
- Responses to summary judgment and Daubert motions need to be filed by May 17, 2023
- Replies to Daubert and summary judgment motions need to be filed by May 31, 2023
- A hearing on Daubert and summary judgment motions will start on June 26, 2023 at 9 AM