Myotonic dystrophy type 2 (DM2) is characterized by myotonia and muscle dysfunction (proximal and axial weakness, myalgia, and stiffness), and less commonly by posterior subcapsular cataracts, cardiac conduction defects, insulin-insensitive type 2 diabetes mellitus, and other endocrine abnormalities. While myotonia (involuntary muscle contraction with delayed relaxation) has been reported during the first decade, onset is typically in the third to fourth decade, most commonly with fluctuating or episodic muscle pain that can be debilitating and proximal and axial weakness of the neck flexors and the hip flexors. Subsequently, weakness occurs in the elbow extensors and finger flexors. Facial weakness and weakness of the ankle dorsiflexors are less common. Myotonia rarely causes severe symptoms. In a subset of individuals, calf hypertrophy in combination with brisk reflexes is notable.

Acid-labile subunit deficiency is characterized by severely reduced serum insulin-like growth factor I (IGF1; 147440) and IGF-binding protein-3 (IGFBP3; 146732) concentrations that are incongruent with an associated mild growth retardation (height, -2 to -3 SD before and during puberty). Pubertal delay in boys and insulin insensitivity are common findings (summary by Domene et al., 2011).