6MWT can identify type 3 SMA patients with neuromuscular junction dysfunction.

The aim of the study was to establish if the decrease in gait velocity on the 6 minute walk test relates to signs of neuromuscular junction dysfunction in spinal muscular atrophy type 3 patients. 6 minute walk test and low-rate repetitive nerve stimulation test were performed in fifteen ambulant patients with spinal muscular atrophy type 3 of age between 9 and 66 years. The 6 minute walk distance ranged between 66 and 575 m. The difference between the first and the 6th minute ranged between 0 and -69%. The low-rate repetitive nerve stimulation test measured in % of loss ranged between -31.7% to +4.2% to the axillary nerve. The correlation between 6 minute walk test changes and low-rate repetitive nerve stimulation test changes was 0.86. Our data suggest that the 6 minute walk test can identify fatigue in the ambulant type 3 patients who have a concurrent neuromuscular junction dysfunction. The identification of fatigue with a simple clinical test may help to target patients who may benefit from drugs that facilitate neuromuscular transmission.

Myotonia permanens with Nav1.4-G1306E displays varied phenotypes during course of life.

INTRODUCTION: Myotonia permanens due to Nav1.4-G1306E is a rare sodium channelopathy with potentially life-threatening respiratory complications. Our goal was to study phenotypic variability throughout life. METHODS: Clinical neurophysiology and genetic analysis were performed. Using existing functional expression data we determined the sodium window by integration. RESULTS: In 10 unrelated patients who were believed to have epilepsy, respiratory disease or Schwartz-Jampel syndrome, we made the same prima facie diagnosis and detected the same heterologous Nav1.4-G1306E channel mutation as for our first myotonia permanens patient published in 1993. Eight mutations were de-novo, two were inherited from the affected parent each. Seven patients improved with age, one had a benign phenotype from birth, and two died of respiratory complications. The clinical features age-dependently varied with severe neonatal episodic laryngospasm in childhood and myotonia throughout life. Weakness of varying degrees was present. The responses to cold, exercise and warm-up were different for lower than for upper extremities. Spontaneous membrane depolarization increased frequency and decreased size of action potentials; self-generated repolarization did the opposite. The overlapping of steady-state activation and inactivation curves generated a 3.1-fold window area for G1306E vs. normal channels. DISCUSSION: Residue G1306 Neonatal laryngospasm and unusual distribution of myotonia, muscle hypertrophy, and weakness encourage direct search for the G1306E mutation, a hotspot for de-novo mutations. Successful therapy with the sodium channel blocker flecainide is due to stabilization of the inactivated state and special effectiveness for enlarged window currents. Our G1306E collection is the first genetically clarified case series from newborn period to adulthood and therefore helpful for counselling.

Functional characterization of ClC-1 mutations from patients affected by recessive myotonia congenita presenting with different clinical phenotypes.

Myotonia congenita (MC) is caused by loss-of-function mutations of the muscle ClC-1 chloride channel. Clinical manifestations include the variable association of myotonia and transitory weakness. We recently described a cohort of recessive MC patients showing, at a low rate repetitive nerves stimulation protocol, different values of compound muscle action potential (CMAP) transitory depression, which is considered the neurophysiologic counterpart of transitory weakness. From among this cohort, we studied the chloride currents generated by G190S (associated with pronounced transitory depression), F167L (little or no transitory depression), and A531V (variable transitory depression) hClC-1 mutants in transfected HEK293 cells using patch-clamp. While F167L had no effect on chloride currents, G190S dramatically shifts the voltage dependence of channel activation and A531V reduces channel expression. Such variability in molecular mechanisms observed in the hClC-1 mutants may help to explain the different clinical and neurophysiologic manifestations of each ClCN1 mutation. In addition we examined five different mutations found in compound heterozygosis with F167L, including the novel P558S, and we identified additional molecular defects. Finally, the G190S mutation appeared to impair acetazolamide effects on chloride currents in vitro.