For the first time, researchers have comprehensively defined the demographic and clinical characteristics of upper limb dystonia, a rare disorder that affects muscles in the arms, forearms, and hands. Powered by the Dystonia Coalition database, the Movement Disorders publication is the largest study of limb dystonia to date. The Dystonia Coalition is part of the Rare Diseases Clinical Research Network, a National Institutes of Health-funded network designed to advance medical research on rare diseases.
Dystonias are characterized by excessive pulling of muscles that can affect virtually any part of the body. As a result, patients experience twisting movements or abnormal postures. Most are categorized as focal dystonias, where a single part of the body is affected. Other types, including segmental, multifocal, and generalized dystonias, affect two or more parts of the body.
In patients with upper limb dystonia, symptoms can range from mild to severe, depending on which muscles are affected and how hard each one is pulling. Some may only experience stiffness or soreness in the arms, forearms, or hands—for example, writer’s cramps or difficulty playing a musical instrument. Others may experience unwanted or abnormal movements and postures.
While researchers know that people of all ages, sexes, and races can develop limb dystonias, the cause is still uncertain, and treatments are limited. Since there are no medical tests to detect dystonias, a diagnosis depends on experts—such as neurologists—recognizing the characteristic features.
Small, single-site studies have provided limited knowledge on these characteristics, but more extensive studies have proved challenging. Within the rare group of dystonia patients, there are even fewer who are only affected in the upper limbs. How can researchers access these patients to learn more about them?
With data from over 3,000 patients, the Dystonia Coalition database provides a unique opportunity to study the specific subgroup of upper limb dystonia. In a multi-center, international effort, the Coalition unites researchers and neurologists from 49 clinical centers in the United States, Canada, Australia, France, Germany, Italy, and the UK. Each member contributes resources from patients across the world to be combined in a standardized database.
“The large Dystonia Coalition database allows us to learn more about each patient’s experience with upper limb dystonia,” says corresponding author Scott Norris, MD, of the Washington University School of Medicine in St. Louis. “One longstanding problem for dystonia researchers is the large amount of variability in how dystonia affects each individual. Understanding the influence of these variables may allow doctors to better counsel and treat them.”
Using clinical and demographic data, researchers evaluated the characteristics of 367 participants with upper limb dystonia. Within this group, the team compared data across subcategories of focal onset, where a single part of the body is affected, and nonfocal onset, where multiple body regions are affected.
Among the participants, focal onset occurred in 80 percent, while 67 percent remained focal without dystonia spread. Most experienced task specificity, where dystonia only occurs during a certain task—particularly writer’s cramp in the dominant limb. In patients where dystonia onset occurred before age 21, focal onset with spread was more frequent. While focal onset occurred equally in both women and men, women were more likely to experience nonfocal onset.
What can these observations mean for patients? Researchers now know that upper limb dystonia distribution, focality, and task specificity relate to onset age and likelihood of regional spread. Moving forward, this knowledge could inform clinical counseling and design, execution, and interpretation of future studies.
“The results presented in this study could allow doctors to better communicate with patients regarding specific manifestations of limb dystonia, as well as describing the risk of future spread to other body regions,” says Norris. “For scientists interested in learning more about limb dystonia, our results allow them to ask better research questions and improve future study designs. All of these efforts advance our understanding of dystonia and enable us to improve diagnosis, prognosis, and treatment.”
The Rare Diseases Clinical Research Network (RDCRN) is a network of 20 individual clinical research consortia and a Data Management and Coordinating Center (DMCC) funded by the NIH and led by the National Center for Advancing Translational Sciences. The RDCRN is designed to advance medical research on rare diseases by providing support for clinical studies and facilitating collaboration, study enrollment, and data sharing.