Myotonia is a stiffness of muscles with inability to relax after voluntary contraction (action myotonia), or induced by electrical or mechanical (e.g., percussion myotonia) excitation. The phenomenon is often described by patients as "cramp" or stiffness. This is a reflection of primary muscle disease (i.e., myogenic; cf. neuromyotonia, neurogenic muscle stiffness), which persists after peripheral nerve or neuromuscular junction blockade.
Electrophysiology reveals myotonic discharges, with prolonged twitch relaxation phase, which may be provoked by movement, percussion, and electrical stimulation of muscle; discharges typically wax and wane.
A similar clinical phenomenon of slow muscle relaxation may be observed in other circumstances, for example hypothyroidism, but without the characteristic EMG findings of myotonia, hence this is labeled as pseudomyotonia. Paramyotonia is myotonia exacerbated by cold and exertion (paradoxical myotonia).
Recognized causes of myotonia include:
- myotonic dystrophy (myotonia dystrophica; myotonic dystrophy type 1)
- hyperkalaemic periodic paralysis
- myotonia congenita (autosomal dominant Thomsen’s disease, autosomal recessive Becker’s myotonia)
- K+-aggravated myotonia
- Schwartz-Jampel syndrome (chondrodystrophic myotonia)
- proximal myotonic myopathy (PROMM; myotonic dystrophy type 2)
Mutations in genes encoding voltage-gated ion channels have been identified in some of the inherited myotonias, hence these are channelopathies: skeletal muscle voltage-gated Na+ channel mutations have been found in K+-aggravated myotonia, and also paramyotonia congenita and hyperkalaemic periodic paralysis. Chloride (Cl−) channel mutations have been identified in myotonia congenita. These latter conditions respond best to mexiletine.
Harper PS, van Engelen B, Eymard B, Wilcox DE (eds.). Myotonicdystrophy: present management, future therapy. Oxford: OUP, 2004 Mankodi A, Thornton CA. Myotonic syndromes. Current Opinion in Neurology 2002; 15: 545-552