F59. Spasmodic reflex myoclonus-an characteristic electrophysiological manifestation for the diagnosis of Stiff-person syndrome

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Introduction Stiff person syndrome (SPS) is a rare disease entity and the pivotal clinical features contain progressive course of fluctuating stiffness of the low back and lower limbs. Progressive encephalomyelitis with rigidity and myoclonus (PERM) is one of the unique variants of stiff person syndrome, and it was characterized with rigidity, stimulus-sensitive spasms, myoclonus, autonomic disturbance, and brainstem signs. These features resemble those of hyperekplexia. The accurate diagnosis of SPS is difficult but is crucial for the in time intervention with immunotherapy for life saving. In addition to the detection of responsible antibody, clinical electrophysiological manifestation, spasmodic reflex myoclonus, would be of great help in establishing the diagnosis. Methods Multichannel surface electromyogram (sEMG) and accelerometer recordings were conducted over the lumbar paraspinalis, rectus abdominalis, quadriceps, and hamstring muscles. Stimulus sensitive phenomenon was illustrated with accelerometer tapped over the pinprick. The signals were processed and digitized with CED power1401 with Spike2. The band pass for sEMG was set from 80 Hz to 1000 Hz. Results The results demonstrated that episodes of spontaneous onset of spasm of the low trunk and legs lasted about seconds to minutes. Stimuli with nose tip tapping, would elicit vigorous jerks with dissemination of long lasting muscle spasm of the neck, paraspinal muscle, rectus abdominalis muscle, to quadriceps and hamstring muscles in a descending spreading pattern. The unique electrophysiological manifestations illustrate the characteristic feature of spasmodic reflex myoclonus. The surface EMG recording also showed the phenomena of habituation, in which a trend of amplitude decrescendo from the first toward the third EMG responses were observed during consecutive auditory stimulus. The serum levels of anti-glutamic acid decarboxylase 65 antibodies was 164 IU. Conclusion The unique EMG pattern can help identify SPS. The clinical features of spasmodic reflex myoclonus are similar to hyperekplexia. However, there are some differences between the two phenomena. Unlike hyperekplexia, the spasmodic reflex myoclonus can be habituated after repetitive stimulation in the SPS patient. Furthermore, the initial myoclonic bursts of spasmodic reflex myoclonus were not present in hyperekplexia and the EMG burst duration was much shorter of the hyperekplexia as compared with that of spasmodic reflex myoclonus. These neurophysiological features discern spasmodic reflex myoclonus from hyperekplexia and are helpful in making diagnosis of SPS.