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Ann Neurol. 2011 Feb;69(2):328-40. doi: 10.1002/ana.22238.

Refined exercise testing can aid DNA-based diagnosis in muscle channelopathies.

Author information

1
Medical Research Council Centre for Neuromuscular Diseases, National Hospital for Neurology and Neurosurgery, University College London, Institute of Neurology London, UK. veronic.tan@uclh.nhs.uk

Abstract

OBJECTIVE:

To improve the accuracy of genotype prediction and guide genetic testing in patients with muscle channelopathies we applied and refined specialized electrophysiological exercise test parameters.

METHODS:

We studied 56 genetically confirmed patients and 65 controls using needle electromyography, the long exercise test, and short exercise tests at room temperature, after cooling, and rewarming.

RESULTS:

Concordant amplitude-and-area decrements were more reliable than amplitude-only measurements when interpreting patterns of change during the short exercise tests. Concordant amplitude-and-area pattern I and pattern II decrements of >20% were 100% specific for paramyotonia congenita and myotonia congenita, respectively. When decrements at room temperature and after cooling were <20%, a repeat short exercise test after rewarming was useful in patients with myotonia congenita. Area measurements and rewarming distinguished true temperature sensitivity from amplitude reduction due to cold-induced slowing of muscle fiber conduction. In patients with negative short exercise tests, symptomatic eye closure myotonia predicted sodium channel myotonia over myotonia congenita. Distinctive "tornado-shaped" neuromyotonia-like discharges may be seen in patients with paramyotonia congenita. In the long exercise test, area decrements from pre-exercise baseline were more sensitive than amplitude decrements-from-maximum-compound muscle action potential (CMAP) in patients with Andersen-Tawil syndrome. Possible ethnic differences in the normative data of the long exercise test argue for the use of appropriate ethnically-matched controls.

INTERPRETATION:

Concordant CMAP amplitude-and-area decrements of >20% allow more reliable interpretation of the short exercise tests and aid accurate DNA-based diagnosis. In patients with negative exercise tests, specific clinical features are helpful in differentiating sodium from chloride channel myotonia. A modified algorithm is suggested.

PMID:
21387378
PMCID:
PMC3051421
DOI:
10.1002/ana.22238
[Indexed for MEDLINE]
Free PMC Article

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