Excitability properties of motor axons in patients with spontaneous motor unit activity

Objectives-Measures of nerve excitability provide information about biophys ical properties of peripheral axons in disease states. One measure, the str ength duration time constant (tau (SD)), was previously reported to be prol onged in motor axons of patients with acquired neuromyotonia. The present s tudy used a new protocol that applies a more comprehensive and sensitive pa nel of measures of axonal excitability; to determine firstly whether change s in tau (SD) were present in a group of patients with evidence of spontane ous motor unit activity; and secondly, if such changes in tau (SD) were pre sent, whether other parameters of axonal excitability were affected, to cla rify the mechanism of the change in tau (SD). Methods-Eleven patients,vith both symptoms and EMG evidence of spontaneous motor unit activity were studied. Eight patients had autoimmune associated acquired neuromyotonia (aNMT) and three had the cramp fasciculation syndrom e. The protocol first measured stimulus-response behaviour using two stimul us durations (from which the distribution of strength-duration time constan ts was estimated), and then threshold tracking was used to determine thresh old electrotonus to 100 ms polarising currents, a current-threshold relatio n (indicating inward and outward rectification), and the recovery of excita bility after supramaximal activation. Results-The results were compared with previously published normal data. Th e value for tau (SD) of motor axons in the patient group was 0.43 (0.02) ms (mean (SEM)), identical with the control value. Most other indices of axon al excitability, including those dependent on fast potassium channels, were also found to be normal. When compared with age matched controls however, the patients with acquired neuromyotonia had significantly greater late sub excitability after an impulse, greater excitability overshoots after depola risation or hyperpolarisation, and more accommodation. Conclusions-No clear evidence for the mechanism of ectopic discharge in the se patients was obtained, probably because the activity was generated focal ly, and most often at the motor nerve terminals. The unexpected finding of increased excitability overshoots and accommodation compared with age match ed controls, suggests a relative up regulation of slow potassium conductanc e, possibly as a consequence of the continuous motor unit activity.