The external anal or urethral sphincter electromyogram (EMG) is a useful investigation in patients with suspected MSA. Because of degeneration of Onuf's nucleus, both anal and urethral external sphincter muscles undergo denervation and re-innervation. Abnormality of the striated urethral sphincter EMG in MSA was first shown by Martinelli and Coccagna in 1978 (101). Subsequently, Kirby and colleagues (49) confirmed the presence of polyphasia and abnormal prolongation of individual motor units in MSA, and also examined the potential diagnostic role of sphincter EMG in patients with MSA and PD (102). Sixteen (62%) of 26 patients with probable MSA, and only 1 (8%) of 13 with probable PD had a pathological EMG result (sensitivity 0.62, specificity 0.92). This test also helps to identify patients in whom incontinence may develop or worsen following surgery. Anal sphincter EMG is generally better tolerated, and yields identical results (50). In at least 80% of patients with MSA, EMG of the external anal sphincter reveals signs of neuronal degeneration in Onuf's nucleus with spontaneous activity and increased polyphasia (103-105). However, these findings do not reliably differentiate between MSA and other forms of APD. An abnormal anal sphincter examination was present in 5 of 12 (41.6%) PSP patients (106). Furthermore, neurogenic changes of external anal sphincter muscle have also been demonstrated in advanced stages of PD by several investigators (107,108). Also chronic constipation, previous pelvic surgery, or vaginal deliveries can be confounding factors to induce nonspecific abnormalities (109). In summary, in patients with probable MSA, abnormal sphincter EMG, as compared to control subjects, has been found in the vast majority of patients, including those 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 PD as a rule do not show severe sphincter EMG abnormalities in the early stage of the disease, unless other causes for sphincter denervation are present. However, sphincter EMG does not distinguish MSA from PSP (110). For these reasons, this examination has not been included in the guidelines suggested by the Consensus Conference (8).
In general, the value of evoked potential studies in the diagnosis of MSA is limited. Magnetic evoked potentials are often, but not always, normal in MSA (111,112). Somatosensory, visual, and acoustic evoked potentials may show prolonged latencies in up to 40% of patients, but most patients show no abnormalities of central efferent and afferent neuronal pathways (112-114).
Some investigators (115,116) have suggested that both somatic anterior horn cells and peripheral nerves are commonly affected in MSA, and their involvement has therefore been regarded as part of the clinical spectrum of MSA. Abnormalities of nerve conduction studies seem to be more frequent in MSA-P (43%) compared to MSA-C (14%), suggesting that the peripheral nervous system is differentially affected in the motor presentations of this disorder (117).
Excessive auditory startle responses (ASRs) may also help differentiate MSA both from PD and other forms of APD (118). Exaggerated ASRs may reflect disinhibition of lower brainstem nuclei owing to the degenerative disorder. ASRs appear to be more disinhibited in MSA-P than MSA-C, and there is a lack of ASR habituation in MSA-C unlike MSA-P, suggesting involvement of different neural structures in the two MSA-subtypes (119).
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