This is difficult to answer with certainty.
Dystonia is the third most common movement disorder after Parkinsonism and Essential Tremor. There are many disorders affecting the brain which have, as part of their symptoms and signs, dystonic movements affecting part of the body. For example in Parkinson’s disease there are a number of abnormal movements consistent with dystonia. When we talk about dystonia we are usually referring to a movement disorder is in which there are excessive muscle movements and abnormal postures without any other significant neurological signs (apart from a tremor).
The most common well-defined form of dystonia is adult onset dystonia. This can manifest as cervical dystonia, focal hand dystonia, blepharospasm, spasmodic dysphonia and oromandibular dystonia. Due to its different clinical presentations and poor recognition by physicians, adult onset dystonia is under-reported. In order to determine the frequency of adult onset dystonia it is necessary to carry out an epidemiological study. An epidemiological study means taking an area with a known population size and determining through a number of ways (health record searches, reporting by neurologists) the number of people in that area who have the disorder. For example recently (2015–2016) we conducted an epidemiological study of adult onset dystonia in Ireland (Republic of Ireland) and found that there were 592 individuals in Ireland with adult onset idiopathic isolated focal dystonia. The actual numbers of patients were: cervical dystonia 410 (69.2%); blepharospasm 102 (17.2%); focal hand dystonia 39 (6.6%); spasmodic dysphonia 18 (3.0%); musician’s dystonia 17 (2.9%) and oromandibular dystonia 6 (1.0%).
Overall the prevalence (the number of people with adult onset dystonia in the population) was 17.8 people per 100,000 population. In order to give you some perspective on this, the prevalence of multiple sclerosis is about 10 times that figure. This means that adult onset dystonia, the most common form of dystonia, is a relatively rare disorder; for that reason many doctors in their lifetime of practice will not see this disorder. Often they will not learn about it in medical school and often only get to know the disorder when they have patient with cervical dystonia (or some other type of adult onset dystonia). This is really the main reason for the delay in diagnosis; under recognition of dystonia by general practitioners.