Diagnosis of Laryngeal Dystonia (LD)
One of the most important challenges in the treatment of dysphonia (voice problems) is the correct diagnosis of what is causing the problem. Dysphonia can result from a number of conditions of which laryngeal dystonia (LD) is one. Another is muscle tension dysphonia (MTD) which is a functional problem with an emotional or behavioural rather than a neurological cause. There are a number of methods used to distinguish LD and MTD of which one is to listen to the voice - dystonia has been thought to have a spasmodic effect on the voice, causing voice breaks, while MTD has a more continuous effect.
The ability to diagnose these conditions is important for both treatment and research. Most cases of LD can be treated using botulinum toxin but usually this is not effective for MTD which is mostly treated using voice therapy. Also, for research, it is important to ensure all participants have the same diagnosis to avoid confusing the results. For these reasons, the Dystonia Coalition has run a project to develop a new diagnostic tool. The results were presented by Dr Christy Ludlow.
They initially identified that there is a lack of consistency in diagnosis with different doctors diagnosing the same symptoms in different ways. One possible reason is that people with LD develop MTD as a result of trying to cope with their dystonia during the long period pre-diagnosis making the conditions hard to distinguish. Another is that looking for “spasmodic” symptoms to identify LD is not always reliable – for instance, some patients who are known to have LD caused by a genetic form of dystonia do not have voice breaks.
Dr Buz Jinnah suggested the problem may be analogous to neck dystonia, where there is a spectrum of cases depending on how much movement they display – at one end of the spectrum are cases with a lot of tremor and at the other cases that are “tonic” which means the posture is abnormal but without movement. In between are cases that are a mixture of these two types. The same may apply in LD – so some cases may be “spasmodic” while others have a continuous effect on the voice – and this latter type may be very hard to distinguish from MTD.
The Coalition has used a process called DELPHI to develop an improved diagnostic tool. This tool has yet to be fully tested but initial results suggest that, while many cases can be identified as clearly LD or MTD, some cases remain difficult to diagnose. Unfortunately, there is no immediate solution to this. The hope is that brain imaging or other indicators such as temporal discrimination (see below) may provide other means of distinguishing dystonia from other conditions. Further work on this is ongoing.
Treatment of LD
Dr Andrew Blitzer presented information on using botulinum toxin to treat adductor LD (the type of dystonia that makes the voice sound strained or strangled). His results showed very good relief in the vast majority of his patients across the botulinum cycle – he emphasised that to get such good results it is important to be very flexible both in the injection gap and dose. He injects some patients as often as every 6 weeks with very small doses – others have much longer gaps. The size of the effective dose varies enormously between patients. It is not known why this is – one theory is that some people only require a very small dose as it creates a feedback loop with the central nervous system similar to the mechanism of the geste antagoniste.
For those with adductor LD who do not get good response from botulinum toxin a number of surgical options have been tried. Some people get very good results – but the results are mixed and it is not yet known how often the dystonia returns especially for the newer treatments.
Another concern that was highlighted was that, while both botulinum toxin and surgery can be helpful in treating abductor LD (the type that causes a whispery or breathy voice), they are not as effective as when treating adductor LD. It was agreed this was a priority for future research.
Causes of LD
Laurie Ozelius presented the latest findings on the role of genetics in causing LD. A number of genes have been identified that can cause dystonia and some of these cause LD as a symptom – however in cases of dystonia caused by the genes identified to date, LD rarely appears in isolation from dystonia affecting other parts of the body. In addition, the proportion of LD cases explained by such genes is tiny.
Sometimes more than one unexplained pure LD case (where LD is the only symptom) appears in the same family tree so there must be other genes causing LD that are yet to be identified. However, no family has yet been found with more than two pure LD cases – and this is a problem as, to isolate the responsible gene, researchers need several gene carriers in the same family. As often with genetic dystonia research the problem may be one of low “penetrance” - this means that only a small proportion of those who carry the gene actually develop symptoms of dystonia and, as a result, it is hard to find family trees with many cases.
A promising solution to this is to use a test called a temporal discrimination threshold. This is a simple cognitive test in which people with focal dystonia respond less quickly than a healthy control group. Helpfully, it appears that those who carry dystonia-causing genes but who have not developed symptoms of dystonia (this is called “non-manifesting”) also perform more slowly on this test – so it may be a quick and cost efficient method of identifying more carriers of the LD gene. This has yet to be proven but, if it is right, then the test may be able identify enough carriers of the LD causing gene to isolate it. Once the gene is identified, researchers can look what the gene does and explore the mechanisms that lead to LD with the hope this may lead in time to new treatments.
One question is, if many people carry the LD causing gene without developing LD, then what causes LD to appear some cases. Dr Caroline Tanner has used employment health insurance records to compare the employment and medical histories of people with LD with healthy controls. She found that people with LD were more likely to have worked in professions which require continual use of the voice and also to have been to doctor with a throat infection. More work needs to be done in this area – one possible route is to identify non-manifesting gene carriers using the temporal discrimination threshold and then compare their histories to those with LD.
Another approach to looking at causes of dystonia is to look for unusual activity in the brain using imaging technology such as MRI scanners. This work has found unusual activity in a number of brain areas: the basal ganglia, thalamus, cerebellum and cortex. These are all areas of the brain involved in the selection and implementation of movement. However, it remains a problem to distinguish unusual activity in the brain that is causing dystonia from unusual activity resulting from the brain trying to correct for dystonia caused in other areas.
Projects of the Dystonia Coalition
Key projects the coalition has been working on include:
- Setting up a Patient Registry. This is a registry of patients that can then be accessed by researchers looking to organise research projects. More than 3200 patients have signed up so far – if you interested in signing up you can access the registry in the “About dystonia” menu on our website.
- Developing a bio-repository. This is a more detailed database of information about patients and includes blood samples and videos as well as medical history. Information is provided through participating clinics.
- Developing a new rating scale to assess the severity of cervical dystonia. This is a development of the current scale (TWSTRS) to incorporate factors int currently taken into account such as tremor, retrocollis, antecollis and psychological impacts. The new scale performed well in initial tests and now needs to be validated for reliability.
- Providing grants for researchers to develop projects.
Twenty research papers have so far resulted from the work of the coalition.
Article reviewed November 2013
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