A. Myotonia typically develops after a short period of exercise e.g. 10 min or on resting after a period of exercise.
B. This same pattern may be observed in dominant myotonia congenita. CMAP = compound muscle action potential.
Myopathy
Myopathy may develop in some patients with non-dystrophic myotonia (Becker, 1977; Plassart et al., 1996; Nagamitsu et al., 2000). In a series of 49 genetically confirmed paramyotonia congenita cases, ‘myopathic biopsy findings’ were reported in 33% of those biopsied although full clinical details of the degree of weakness were not available (Miller et al., 2004). Permanent severe myopathy seems to be more common in patients with periodic paralysis than in the non-dystrophic myotonias (Miller et al., 2004). In periodic paralysis it has been postulated that the severity of myopathy may not relate to paralytic attack frequency (Buruma et al., 1978; Links et al., 1990) but the exact relationship remains unclear. There is some evidence that the severity of myopathy associated with periodic paralysis does correlate with increasing age (Links et al., 1990; Plassart et al., 1994). It is not known if a similar relationship between age and severity of myopathy exists in the non-dystrophic myotonias or if symptom frequency or severity has a direct influence on the development of myopathy. In periodic paralysis there is some evidence that the frequency of paralytic attacks may decline with age (Miller et al., 2004). However, it is not established if myotonia severity alters over time in patients with non-dystrophic myotonia. Importantly, there are no published studies to provide accurate detailed data on the natural history of the non-dystrophic myotonias in order to address the above questions. Such a large natural history study is currently in progress as part of the Consortium for Clinical Investigation of Neurological Channelopathies.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2801326/
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