Stimulus-Induced Motor Afterdischarges in CASPR2 (Contactin-Associated Protein-Like 2)- Positive Peripheral Nerve Hyperexcitability Syndrome

Peripheral nerve hyperexcitability is an uncommon but treatable condition in neurology. Voltage-gated potassium channelopathies, especially contactin-associated protein-like 2 (CASPR2) antibody, are commonly implicated. We present the case of a 16-year-old boy with tremulousness of both feet and twitching of muscles all over the body for three months. Examination revealed irregular, arrhythmic, small amplitude twitching movements of the toes along with fasciculations in both thighs. Nerve conduction studies were within normal limits. F-wave studies showed a prolonged polyphasic large-amplitude discharge following the compound muscle action potential and obscuring the F waves. Electromyography showed extensive myokymic discharges. The serum autoimmune antibody profile showed strong positivity for CASPR2. He started lacosamide as a symptomatic treatment. In view of the good symptomatic response, further immunomodulation was deferred and he remains on follow-up. We present this case to highlight the role of motor afterdischarges as a diagnostic clue to peripheral nerve hyperexcitability and to review the literature on this interesting finding.

Automated summary

This article presents a case of peripheral nerve hyperexcitability in a 16-year-old boy who exhibited symptoms of muscle twitching and tremor. The diagnosis was confirmed through nerve conduction studies, electromyography, and serum antibody testing, which showed positivity for CASPR2 antibody. Symptomatic treatment with lacosamide was effective. Further immunomodulation was deferred due to the good response.