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.