Symposium on Movement Disorders

Multiple system atrophy (MSA) is a degenerative disease manifesting any combination of parkinsonism, cerebellar, pyramidal, and autonomic (including urinary, sexual and anorectal) dysfunction. It is pathomorphologically defined by a typical distribution of neuron loss and gliosis, as well as by cytoplasmic oligodendroglial inclusions, but lacks a definitive clinical diagnostic test. As it may mimic many other disorders there is a need for specific and sensitive clinical diagnostic tools. Sphincter EMG, reflecting Onuf's nucleus degeneration, has been proposed as a helpful test. In patients with probable MSA abnormal sphincter EMG – as compared to control subjects – has been found in a majority of patients in most studies in all the different forms of the disease, including patients who as yet have no urological or anorectal problems. The prevalence of abnormalities in the early stages of MSA is as yet unclear. Patients with Parkinson's disease (PD) (even those with sacral dysfunction) as a rule do not show prominent sphincter EMG abnormalities in the early stage of the disease, but may do so in the long run. There is as yet no consensus which sphincter EMG parameters are most appropriately analysed: presence of abnormal spontaneous activity; prolonged duration or increased percentage of polyphasic motor unit potentials; increase in fibre density on Single Fibre EMG, etc. The sensitivity and specificity of sphincter EMG to distinguish MSA from PD have been reported as good by several authors, but this has recently been seriously questioned. Sphincter EMG seems to distinguish MSA from pure autonomic failure, and from cerebellar ataxias. EMG is generally less specific in female patients because of occasional sphincter denervation after complicated vaginal deliveries, and seems not to distinguish MSA from progressive supranuclear palsy. Further studies are needed before conclusions can be reached regarding the presence, extent, and nature of significant sphincter EMG differences between MSA and PD in different stages of the diseases. In the author’s opinion a clearly abnormal sphincter EMG in an appropriate clinical setting strengthens the diagnosis of MSA against pure autonomic failure and cerebellar ataxias, and also against Parkinson’s disease, but only in the first years of development of symptoms and signs.