Dystrophic and non-dystrophic myotonias

Abstract

Myotonic syndromes are rare neuromuscular diseases characterized by the clinical or neurophysiological detection of myotonia. The genetic defects involve primarily or secondarily the muscular isoforms of the ion channels. The channel dysfunction consecutively leads to a hyper-excitability of the muscle membrane and the clinical symptom myotonia. Two forms of dystrophic myotonic diseases are currently known: the myotonic dystrophy type 1 (DM1) and the myotonic dystrophy type 2 (DM2). They are multisystemic diseases clinically characterized by a combination of myotonia and other muscular symptoms (muscle weakness, wasting and myalgia) together with the involvement of other organs and systems (cataract, diabetes, heart diseases, hormone dysfunctions). The non-dystrophic myotonic diseases are caused by mutations affecting either the chloride ion channels or the sodium ion channels. The clinical picture is dominated by the presence of myotonia and other minor muscular complaints as mild episodic weakness and muscle hypertrophy. The differential diagnosis among the myotonic syndromes is extremely challenging leading to a significant diagnostic delay. This review will update on the main clinical, diagnostic and therapeutic aspects of myotonic syndromes to guide general neurologists through an earlier diagnosis and better management.