Objectives‐ To evaluate the following points about carpal tunnel syndrome (CTS): 1) characterization of a wide population; 2) sensitivity of electrodiagnostic tests, and particularly the contribution of disto‐proximal ratio test; 3) validity of a neurophysiological classification developed by us. Material and methods ‐ Prospective study in 500 hands with CTS symptoms. Neurophysiological “standard” tests were always performed: sensory nerve conduction velocity (SNCV) first‐ and third digit‐wrist and distal motor latency (DML). In “standard negative” hands disto‐proximal ratio technique (R) was performed. Neurophysiological classification: Extreme CTS (absence of median motor, sensory responses), Severe (absence of sensory response, abnormal DML), Moderate (abnormal SNCV, abnormal DML), Mild (abnormal SNCV, normal DML), Minimal (abnormal R or other segmental/comparative test, normal standard tests). Results‐ Sensibility of standard tests: 77%. R increased the diagnostic yield by 20%. CTS classification appeared reliable with significant differences between groups. Conclusion ‐ R is a useful test, the classification may be useful in clinical/therapeutical decisions.

SUMMARY1. Three families with a form of myotonia (muscle stiffness due to membrane hyperexcitability) clinically distinct from previously classified myotonias were examined. The severity of the disease greatly differed among the families.2. Three dominant point mutations were discovered at the same nucleotide position of the SCN4A gene encoding the adult skeletal muscle Na+ channel asubunit. They predict the substitution of either glutamic acid, valine or alanine for glycine1306, a highly conserved residue within the supposed inactivation gate.Additional SCN4A mutations were excluded.3. Electrophysiological studies were performed on biopsied muscle specimens obtained for each mutation. Patch clamp recordings on sarcolemmal blebs revealed an increase in the time constant of fast Na+ channel inactivation, Th, and in late channel openings as compared to normal controls. Th was increased from 1-2 to 1-6-2-1 ms and the average late currents from 04 to 1-6 % of the peak early current.4. Intracellular recordings on resealed fibre segments revealed an abnormal tetrodotoxin-sensitive steady-state inward current, and repetitive action potentials. Since K+ and Cl-conductances were normal, only the increase in the number of non-inactivating Na+ channels has to be responsible for the membrane hyperexcitability.5. Length, ramification and charge of the side-chains of the substitutions correlated well with the Na+ channel dysfunction and the severity of myotonia, with alanine as the most benign and glutamic acid as the substitution with a major steric effect.6. Our electrophysiological and molecular genetic studies strongly suggest that these Na+ channel mutations cause myotonia. The naturally occurring mutants allowed us to gain further insight into the mechanism of Na+ channel inactivation. ¶ To whom correspondence should be addressed.MS 2411

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.

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