Cold extends electromyography distinction between ion channel mutations causing myotonia.
OBJECTIVE
Myotonias are inherited disorders of the skeletal muscle excitability. Nondystrophic forms are caused by mutations in genes coding for the muscle chloride or sodium channel. Myotonia is either relieved or worsened by repeated exercise and can merge into flaccid weakness during exposure to cold, according to causal mutations. We designed an easy electromyography (EMG) protocol combining repeated short exercise and cold as provocative tests to discriminate groups of mutations.


METHODS
Surface-recorded compound muscle action potential was used to monitor muscle electrical activity. The protocol was applied on 31 unaffected control subjects and on a large population of 54 patients with chloride or sodium channel mutations known to cause the different forms of myotonia.


RESULTS
In patients, repeated short exercise test at room temperature disclosed three distinct abnormal patterns of compound muscle action potential changes (I-III), which matched the clinical symptoms. Combining repeated exercise with cold exposure clarified the EMG patterns in a way that enabled a clear correlation between the electrophysiological and genetic defects.


CONCLUSIONS
We hypothesize that segregation of mutations into the different EMG patterns depended on the underlying pathophysiological mechanisms. Results allow us to suggest EMG guidelines for the molecular diagnosis, which can be used in clinical practice.
Fournier
Emmanuel
E
Fédération de Neurophysiologie Clinique, Groupe Hospitalier Pitié-Salpêtrière, Assistance Publique-Hôpitaux de Paris, Université Pierre et Marie Curie, Paris. emmanuel.fournier@upmc.fr
Viala
Karine
K
Gervais
Hélène
H
Sternberg
Damien
D
Arzel-Hézode
Marianne
M
Laforêt
Pascal
P
Eymard
Bruno
B
Tabti
Nacira
N
Willer
Jean-Claude
JC
Vial
Christophe
C
Fontaine
Bertrand
B
D000200:Action Potentials / Q000502:physiology; D000293:Adolescent; D000328:Adult; D000368:Aged; D015220:Calcium Channels / Q000235:genetics / Q000502:physiology; D003080:Cold Temperature; D004576:Electromyography / Q000379:methods*; D005080:Exercise Test / Q000379:methods; D005260:Female; D006801:Humans; D007473:Ion Channels / Q000145:classification / Q000235:genetics*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology; D009154:Mutation; D009222:Myotonia / Q000175:diagnosis / Q000235:genetics* / Q000503:physiopathology; D015221:Potassium Channels / Q000235:genetics / Q000502:physiology; D015222:Sodium Channels / Q000235:genetics / Q000502:physiology; D013697:Thermosensing / Q000502:physiology; D013997:Time Factors
D015220:Calcium Channels; D007473:Ion Channels; D015221:Potassium Channels; D015222:Sodium Channels