T48. Hypersensitivity of the persistent inward current

Abstract

Motor cortex controls movements but a sensitive mechanism called persistent inward current (PIC) at the spinal cord plays a major role in adjusting the strength. The PIC is subject to descending neuromodulatory systems but also peripheral sensory afferent activity. Hypersensitivity of PIC may cause spasms which are stimulation sensitive. We describe a 46-year old male patient who had frequent and severe but relatively short-lasting bouts of stimulus-sensitive spasms after a suspected myelitis 10 years earlier. Stiff person syndrome and channelopathy of the muscle membrane were closed out. The spasms were very therapy resistant and intrathecal baclofen therapy was planned. A six-channel surface EMG recording of the right upper extremity (muscles: opponens pollicis, abductor digiti minimi, flexor carpi radialis, extensor carpi radialis, biceps brachii, triceps brachii) and repeated serial electrical stimuli of the radial nerve (30 Hz, 10 mA, 50 stimuli) and the median nerve at wrist (30 Hz, 6 mA, 50 stimuli) and vibratory stimulation of muscles. H-reflex measurements of the median nerve and needle EMG recordings during spasms. The serial, but not single, electrical stimuli caused activation of all the recorded muscles with different delay times ranging from 250 to 2500 ms. We saw augmentation of the responses with shortening delay times with repeated serial stimuli. Vibratory stimulation of the abductor digiti minimi muscle also caused activation of all the muscles recorded but with little augmentation. Vibratory stimulus of the biceps brachii muscle caused a very strong contraction of all the muscles recorded. This stimulus could not be repeated because of the strong response. The post-stimulus contractions were confined to the right upper extremity. EMG recorded with needle during spasms resembled normal muscle activity. When measuring the median nerve H-reflex, getting an M-response increased the amplitude of H-reflex. With this patient we could reveal typical behavior of PIC hypersensitivity to peripheral stimuli. The delay times of stimulus-sensitive motor reactions of different muscles varied but they were relatively long indicating the involvement of a neuromodulatory system (G-proteins, intracellular cascades) instead of the ionotrophic activation. The responses were confined to one extremity. Serial stimuli of both A beta and A alpha sensory axons caused reactions. We saw augmentation of the responses and shortening of the delay times with repeated serial or vibratory stimuli. The motor contractions lasted for a few minutes at most and were mainly tonic. Measurements of H-reflex showed that even a small muscle afferent activation caused an increased PIC response. EMG during spasms resembled normal muscle activation. With these features hypersensitivity of PIC can be diagnosed and differentiated from other syndromes with sustained motor symptoms.