Idiopathic Limb Myokymia in Children: First Reported Cases Involving the Extensor Digitorum Brevis and Flexor Hallucis Brevis Muscles

ABSTRACT: Idiopathic limb myokymia is an uncommon presentation of peripheral nerve hyperexcitability, characterized by spontaneous, continuous, grouped motor unit discharges in the absence of an identifiable structural, metabolic, or autoimmune cause. Although peripheral nerve hyperexcitability is frequently associated with voltage-gated potassium channel complex antibodies, antibody-negative cases are well recognized and are thought to reflect functional membrane instability or ion channel dysfunction at the level of the distal motor axon. Pediatric presentations are rare, particularly when limited to intrinsic foot muscles. We report two cases of pediatric patients with isolated distal lower limb myokymia. A 13-year-old girl presented with persistent involuntary toe movements. Needle electromyography (EMG) demonstrated typical myokymic discharges, consisting of rhythmic or semi-rhythmic, grouped motor unit potentials firing at 2-60 Hz in the extensor digitorum brevis muscle. A 15-year-old boy exhibited continuous contractions of the right foot, localized to the flexor hallucis brevis muscle, with similar myokymic discharges confirmed on EMG. Comprehensive evaluation, including laboratory and neuroimaging studies, excluded secondary etiologies in both cases, supporting a diagnosis of exclusion. Both patients were treated with carbamazepine, a sodium channel blocker aimed at reducing axonal hyperexcitability, resulting in complete clinical resolution within 1 month, with no recurrence during follow-up. These cases emphasize that focal intrinsic foot muscle myokymia can occur in children without detectable autoimmune markers and likely reflects localized motor axon hyperexcitability. Recognition of characteristic EMG features is critical to differentiate myokymia from fasciculations, neuromyotonia, or other movement disorders and to guide appropriate therapy.