Certain Patient Groups Continue to Be Underrepresented in PD Studies in the Netherlands


With greater attention being paid to the heterogenous patient population, researchers questioned whether this has resulted in a more diverse group of patients included in Parkinson disease (PD) studies in the Netherlands.

Researchers are highlighting the need for better representation in studies for Parkinson disease (PD), highlighting findings from their recent study, which showed that females and other patient groups continue to be underrepresented in research.

Historically, clinical research has underrepresented certain patient groups despite the disease affecting a wide array of patients. With greater attention being paid to the heterogenous patient population, the researchers questioned whether this has resulted in a more diverse group of patients included in PD studies in the Netherlands. However, they found little differences in patient characteristics and outcome measures in studies over the last 2 decades.

Their findings recently were published in Clinical Parkinsonism & Related Disorders.

“The PD research community still has a lot to do in ensuring adequate representation and diversity in PD patients recruited into clinical research, assuming our results are generalizable of research conducted in other high-income countries,” noted the researchers. “It would be helpful to see if other centers or countries find similar results or do a better job in ensuring good representation. Further research should explore and evaluate how we can remove the barriers and implement facilitators of equitable access to the research experience for all patients with PD who wish to help the research endeavor.”

Data from 2003 through 2021 were analyzed from studies of patients at a single center. Across the studies, there were no changes in the proportion of female patients with PD over time (39%), even when correcting for age. Similarly, the mean age of study participants (66 years) did not change over time, even when correcting for the proportion of female patientss. The researchers noted that the mean age of patients reported in a medical claims data study of over 51,000 patients was 72 years.

Notably, the mean age of participants had no correlation with mean age at death despite the increasing number of older patients with the disease. Underrepresentation of older patients, explained the researchers, may be due to selection bias of the intervention studies, which may be a more attractive option for younger and fitter patients. These studies may actually exclude frailer or cognitively impaired patients, who tend to be older.

Of the 33 studies, 8 reported on the ethnicity of patients, and these findings showed that the proportion of people who were native Dutch did not change over time. The number of studies reporting on the ethnicity of patients did not differ throughout the period.

“Given the clinical heterogeneity of PD, it is important that studies not only investigate motor symptoms but also NMS [nonmotor symptoms]. We did observe an increase in studies that included NMS as an outcome measure in our center,” wrote the group. “This modest growth is explained in part by a greater awareness of NMS, but also by the retrospective character of the present study: Some NMS tools did not exist in 2003. However, even in the most recent time period, around a third of studies did not focus on NMS even though they are a key determinant of quality of life in people with PD. Furthermore, facilitating clinical research closer to real life of people with PD by, for example, continuous monitoring using wearable sensors may have added value.”

The group noted that focusing their study on 1 health care system allows for a small spatial parameter and better insight into the influence of the temporal parameter in specified outcomes. However, the group recognized that analyzing data from a single center limits the generalizability of their findings for patients in other countries.


Maas B, Bloem B, Ben-Shlomo Y, et al. Time trends in demographic characteristics of participants and outcome measures in Parkinson’s disease research: a 19-year single-center experience. Parkinsonism Relat Disord. Published online January 27, 2023. doi:10.1016/j.prdoa.2023.100185

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