RESUMEN
We reported a 32-year-old man who was a sporadic case of myotonic syndrome with muscle stiffness or transient weakness of limbs upon initiating movements after rest. On examination, he showed painless myotonia with warm-up phenomenon, Hercules-like hypertrophic musculature and myotonic discharges in EMG. The clinical findings resembled to those of Becker disease rather than Thomsen disease. But electrodiagnosis suggested sodium channel myotonia instead of chloride channelopathy. Genetic testing detected a novel missense mutation (p.V1166A) in the SCN4A gene but not in the CLCN1 gene. Transient weakness upon initiating movements is usually observed in Becker disease but rare in Thomsen disease, which is not reported in sodium channel myotonia so far. He was probably the first case of sodium channel myotonia with transient weakness upon initiating movements, which was confirmed by 10â Hz repetitive nerve stimulation test as depolarization block.
Asunto(s)
Electrodiagnóstico , Movimiento/fisiología , Debilidad Muscular/diagnóstico , Miotonía Congénita/diagnóstico , Adulto , Electromiografía , Pruebas Genéticas , Humanos , Masculino , Debilidad Muscular/complicaciones , Debilidad Muscular/fisiopatología , Mutación Missense , Miotonía Congénita/complicaciones , Miotonía Congénita/genética , Miotonía Congénita/patología , Miotonía Congénita/fisiopatología , Canal de Sodio Activado por Voltaje NAV1.4/genética , Estimulación Eléctrica Transcutánea del NervioRESUMEN
OBJECTIVE: To clarify the features of decremental responses following repetitive nerve stimulation in patients with motor neuron diseases (MNDs), in comparison with myasthenia gravis (MG). METHODS: The subjects consisted of 48 MND, 39 generalized MG and 19 ocular MG patients. Six muscles, both proximal and distal muscles, were tested. RESULTS: Significant decrements (>5%) in at least one muscle were observed in 83% of the MND patients, and 74% and 47% of the generalized MG and ocular MG patients, respectively. Decrements were more frequently observed in the proximal muscles both in MND and MG patients (deltoid 76% and 62%, and trapezius 71% and 51% for MND and generalized MG, respectively), suggesting lower safety factors in neuromuscular transmission in those muscles. Decrements in the nasalis were rare in MND (8%) in comparison with generalized MG (54%). CONCLUSIONS: Decremental responses were frequently observed in MND patients. There were small differences between MND and MG regarding the distribution and other features of decrements, such as the degree of the U-shape or the responses to different stimulus frequencies and to brief exercise. SIGNIFICANCE: These results imply that the underlying mechanism regulating the decrements is common to MND and MG.