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Myoclonus-dystonia syndrome (MDS) is a rare movement disorder characterized by mild to moderate dystonia along with 'lightning-like' myoclonic jerks.
The estimated prevalence of MDS in Europe is 1/500,000.
Disease onset usually occurs in the first or second decade of life. Myoclonus is usually the presenting manifestation and is described as swift ''lightning-like'' jerks that can rarely appear at rest but that are usually triggered by complex motor tasks such as drawing and writing. These movements mainly affect the neck, arms and trunk but can also rarely be seen in the legs or the larynx. In two thirds of cases, dystonia is also experienced in the form of focal or cervical dystonia (see these terms), which may be only mild and does not exacerbate with time. Postural and other forms of tremor have sometimes been reported. MDS is often associated with depression, anxiety, panic attacks, obsessive-compulsive behavior and personality disorders and alcohol abuse. Isolated torticollis is seen in extremely rare cases.
The only known causative gene of MDS is the epsilon-sarcoglycan (SGCE) gene (7q21.3), encoding a transmembrane protein that is part of the dystrophin-associated glycoprotein complex found in skeletal and cardiac muscle. The epsilon-sarcoglycan protein is also abundant in monoaminergic neurons, cerebellar Purkinje cells, the cortex and the hippocampus of the brain. In one family with MDS, linkage to chromosome 18p has been reported (named DYT15), but the gene has not yet been identified.
Diagnosis is based on the presence of characteristic clinical symptoms. Neuroimaging studies are normal. Genetic molecular testing of SGCE can confirm the diagnosis.
Differential diagnosis includes cervical dystonia, Dopa-responsive dystonia, Tourette syndrome, familial cortical myoclonus, Wilson disease, spinocerebellar ataxia type 3 (SCA3) and type 14 (SCA14), ataxia with vitamin E deficiency, genetic disorders with myoclonus as a major component (e.g. Unverricht-Lundborg disease, Lafora disease) (see these terms) and other secondary forms of dystonia.
Prenatal testing is possible in families where a disease-causing mutation is identified.
MDS is inherited in an autosomal dominant manner. However, the SGCE gene is maternally imprinted, therefore in most cases (95%) a patient who inherits the mutation from their mother will remain healthy and only those that inherit the mutation from their father will develop MDS. De novo mutations also occur. Genetic counseling is recommended in those with a known mutation.
Management and treatment
Treatment plans are individualized to a patient's presenting symptoms. Benzodiazepines (clonazepam) and antiepileptic drugs (valproate, levetiracetam) are effective in relieving myoclonus and tremor, but patients should be carefully monitored. Similarly, alcohol frequently improves symptoms temporarily, but its long term use is not recommended. Injections of botulinum toxin can relieve focal and cervical dystonia. If these treatments fail or are insufficient, bilateral deep brain stimulation (DBS) of the internal globus pallidum (Gpi) and the central intermediate nucleus (VIM) of the thalamus have shown positive results in providing lasting relief from both myoclonus and dystonia. Gpi stimulation is often sufficient in treating MDS, and may be favored over VIM stimulation, which generally has very little effect on dystonia. In a staged surgical procedure, quadruple stimulation (VIM and Gpi) may also be considered in selected cases.
Patients with MDS have normal life-expectancy, but quality of life can be severely affected.