Use the difference in age of onset between myotonic dystrophy types., weakness distribution differ between myotonic dystrophy types 1 and 2, and facial appearance is characteristic of myotonic dystrophy type 1 to frame the working diagnosis and next step; use it to sharpen the differential and avoid a false label. Make weakness distribution differ between myotonic dystrophy types 1 and 2 the checkpoint that determines whether you escalate testing, narrow the differential, or change treatment.

DM1 typically starts earlier and has more prominent distal weakness and facial involvement; DM2 usually starts later and skews toward proximal weakness, pain, and stiffness; Prominent myotonia and conduction disease are major DM1 clues; Facial phenotype helps separate DM1 from DM2; Cardiac surveillance is important, especially in DM1