Knowledge of Sub-Types Important to Understanding of the Prevalence of Myotonic Dystrophy

Whilst muscular dystrophies (MDs) are often described as being ‘rare', the impact these conditions can have on people's lives and society can be profound and demand attention. MDs are a group of diseases that involve progressive weakness and degeneration of the muscles caused by a genetic mutation. Myotonic dystrophy is one of the more common types of MDs and has 2 identified sub-types (DM1 and DM2) with differing genetic causes. Symptom onset can occur at a young age, but is more commonly observed in early adulthood. With no known cure, treatments focus on symptom management and delaying progression of the condition; consequently, the burden becomes lifelong and in many cases increases over time [1]. Yet, despite the wide burden, epidemiological studies of overall prevalence and sub-type using age-standardized estimates have been lacking. Indeed, whilst standardized methods of reporting have become widespread with other common conditions, such as stroke, there remains a lack of consistency in conditions such as MD. This may be reflected in the considerable variability found in current reported prevalence between 0.5 and 18.1 per 100,000 [2].In the current issue of Neuroepidemiology, Vanacore and colleagues present the findings of an epidemiological study of MD conducted in Rome, Italy [3]. The study identified cases through a database collating patient information across neuromuscular centers within the city. In this study, 435 people were confirmed to have a genetically confirmed diagnosis of MD. The study reports age-standardized prevalence of 9.65 per 100,000 for DM1 and 0.99 per 100,000 for DM2. It is important to note that the authors present their data as ‘minimum prevalence' in acknowledgment of the delays in obtaining a diagnosis, through misdiagnosis (particularly in the case of DM2) and as people may have the condition, but may not yet be symptomatic and therefore not diagnosed. The need for personal or family molecular testing to be eligible for this study may also reflect why prevalence is at the lower end from previous findings that have tended to rely on clinical diagnosis. The uptake of molecular testing in the study by Vanacore and colleagues seems high considering that many people chose not to be tested in many countries. The reporting of both genetically verified and clinical diagnosis may therefore be beneficial in future prevalence studies.The study highlights the value of merged clinic data sets and national disease registries to assist with determining prevalence of conditions such as MD. Increased value could also be gained from compiling national registries into international registries. For conditions, such as MD, that have a relatively low prevalence, international registries would generate increased numbers of patients and greater variance in factors that may affect progression of the condition. This in turn could generate significant advances in the unexplored factors influencing the population distribution of disorders and disease progression.