Myotonic dystrophy
Myotonic dystrophy types 1 and 2 (DM1, DM2) are autosomal dominantly inherited multisystem disorders and genetically characterised by pathogenic repeat mutations. DM1 is—with an estimated prevalence of 1 in 8,000—the second most common muscular dystrophy worldwide [52]. The clinical phenotype and pattern of muscle affliction differ between DM1 and DM2 (or proximal myotonic myopathy = PROMM) but also show some overlap. Almost all DM1 patients show signs of fatty degeneration and/or oedematous changes in the striated muscles on muscular MRI. Most of the patients show severe fatty degeneration of the proximal and distal lower limb muscles. There is a predominant affliction of the anterior compartment of the thighs compared with the posterior compartment with a relative sparing of the rectus femoris muscles [27, 53, 54]. Particularly, the vastus muscles frequently show a semi-lunar peri-femoral area of fatty degeneration [53, 55]. In the lower legs of DM1 patients, the gastrocnemius muscles show early and frequent involvement, whereas the posterior tibial muscles are relatively unaffected. In addition, whole-body MRI protocols can frequently detect the involvement of other organs in DM1 patients. Dypshagia is a common clinical finding in DM1 patients, which is reflected by a substantial dilatation of the oesophagus that can be easily detected and rated on MRI (Fig. 2).
Contrary to DM1, patients with DM2 are less affected based on MRI findings. A recently published whole-body MRI study including DM1 and DM2 patients showed that most of the DM2 patients showed no fatty degeneration, and none of the patients showed any inflammatory changes. The affected patients were exclusively men and showed damage predominantly of the trunk muscles (such as the erector spinae and gluteus) and in some cases involvement of the proximal and lower leg muscles with sparing of the rectus femoris and gracilis muscles in all DM2 patients [27].