Abnormal gating of axonal slow potassium current in cramp-fasciculation syndrome

•Cramp-fasciculation syndrome (CFS) can be caused by various etiologies.•Non-invasive nerve excitability testing identified abnormality in axonal slow K+ current (IKs).•Loss of voltage-dependency of axonal IKs may play a key role in axonal hyperexcitability.

ObjectiveCramp-fasciculation syndrome (CFS) is a heterogeneous condition with multiple underlying causes. Although dysfunction of slow K+ channels has been reported in patients with CFS, testing all potential candidates for this problem using conventional in vitro functional analysis would be prohibitively cost- and labor-intensive. However, relatively economical and non-invasive nerve-excitability testing can identify ion channel dysfunction in vivo when combined with numerical modeling.MethodsPatients with CFS underwent nerve conduction study, needle electromyography, and nerve excitability testing. Mathematical modeling of axonal properties was applied to identify the pathophysiology.ResultsFour patients had distinct electrophysiological findings (i.e., fasciculation potentials, doublet/multiplet motor unit potentials, and sustained F responses); excitability testing showed the following abnormalities: reduction of accommodation during prolonged depolarization, lack of late sub excitability after a supramaximal stimulation, and reduction of the strength-duration time constant. Mathematical modeling showed a loss of voltage-dependence of a slow K+ current. None of these patients had a mutation in the KCNQ2, 3, or 5 genes.ConclusionsThis study showed that patients with CFS might have abnormal kinetics in a slow K+ current.SignificanceNerve-excitability testing may aid the decision to start therapeutic intervention such as administration of slow K+ channel openers.