What causes focal dystonia?
WHAT CAUSES ADULT-ONSET FOCAL DYSTONIA?
Present concepts of the cause(s) of focal dystonia
1) Adult-onset Focal Dystonia (for example Cervical dystonia) is a genetic disorder with markedly reduced penetrance.
As has been discussed elsewhere (see the section on cervical dystonia), for most patients with cervical dystonia (and the other focal dystonias), they are the only member of the family who have the disorder. This is called “Sporadic Cervical Dystonia”. If we enquire about and examine other family members, we will find that about 20-25% of patients with cervical dystonia have another family member with dystonia (this family member may be quite distant, for example a second cousin). When there is another family member affected this is called this “Familial Cervical Dystonia”.
However despite the fact that 75-80% of patients with cervical dystonia do not have another family member affected, we consider that all patients with cervical dystonia have a genetic disorder.
Why is this?
Most people (85%+) who inherit the gene(s) for cervical dystonia do not develop the disorder. This is called “reduced penetrance” ; the disordered gene is present but does not cause dystonia; this is a feature of many genetic disorders.
In cervical dystonia genetic penetrance is very markedly reduced to about 10-15%. This means that for every 8 to 10 people who inherit the gene for cervical dystonia, only one of them will develop cervical dystonia. That is why most patients do not have another affected family member.
The penetrance of the gene (the development of dystonia) depends upon many other factors including sex (women are more susceptible), age and environmental exposures (see the section on environmental factors).
2): Environmental factors affect penetrance (development of dystonia) and influence the nature of the dystonia which develops (expression of the particular type dystonia).
We have been interested in this aspect and have performed two research studies.
a): Environmental factors and cervical dystonia.
Patients with cervical dystonia, when compared to age and sex matched siblings, have an excess history of a): road traffic accidents with hospital attendance and b): Surgery of any type. These findings were published in 2015 in the Journal of Neurology, Neurosurgery and Psychiatry. It is not clear how this history of more frequent injuries relates to the development of cervical dystonia but it appears to be a fairly robust finding. Many of the preceding road traffic accidents were 15-20 years prior to the development of the cervical dystonia. There does not appear to be any one particular type of surgery as a preceding environmental factor.
b): Environmental factors and blepharospasm.
We analysed reports of the frequency of blepharospasm throughout the world from 15 different studies. This analysis showed that the frequency of blepharospasm was significantly related to latitude and, more significantly, annual sun exposure. For example in Sicily blepharospasm is more common than cervical dystonia; however in Northern European countries, including Ireland, cervical dystonia is much more common than blepharospasm. We hypothesise that excess sun exposure leads to excessive contraction of the muscles around the eye manifesting as excessive blinking and spasm of these muscles (the orbicularis oculi muscles). This would be in keeping with the concept of “overuse” which is seen in some task specific dystonias.
c): Environmental factors in musician’s dystonia and focal hand dystonia.
Musician’s dystonia: A small proportion of musicians develop abnormal movements of the fingers when playing an instrument after many years of practice and performance. For example a violin player might find problems with one or other fingers of the left hand when trying to play; the ring or little finger when pressing the strings may not contract (flex) when needed, affecting the quality of the performance. Similarly in patients presenting with focal hand dystonia (writer’s cramp) there is often a history of prolonged use of the dominant hand when writing. The risk of developing writer’s cramp increases with the number of hours spent writing daily. There is also evidence that a previous head injury with loss of consciousness increased the risk of developing writer’s cramp.
Summary: Environmental factors appear to increase the likelihood of developing a focal dystonia (car accidents and cervical dystonia; sun exposure and blepharospasm; hours of writing and focal hand dystonia). We hypothesise that adult onset dystonia is a genetic disorder, but the gene is poorly penetrant. Most people who have inherited the gene for adult onset dystonia do not develop the dystonia unless they are exposed to certain environmental factors. As is discussed elsewhere, we do not know yet the gene or genes which cause adult onset dystonia (a lot of research as directed in this area), however it would appear that one requires a combination of an inherited gene plus a particular environmental exposure in order to develop a particular adult onset dystonia. It is likely that there is a genetic predisposition to develop dystonia and the type of dystonia one develops depends upon the type of environmental exposure. In other words, there is probably not a specific gene for cervical dystonia and another difference gene for blepharospasm. It is more likely that there is a disorder in a particular gene which, depending upon the nature of the environmental exposure, can express itself as cervical dystonia or blepharospasm (or indeed focal hand dystonia).