MMD is an autosomal dominant multicystic disorder.
2 subtypes exists: Type 1 caused by an unstable trinucleotide (CTG) repeat expansion in the 3′ untranslated region of the dystrophia myotonica-protein kinase (DMPK) gene.
Type 2 MMD is a tetra-nucleotide (CTTG) repeat expansion in intron 1 of the zinc finger 9 (ZNF9) gene.
It is the most common adult muscle dystrophy.
Estimated prevalence 1 in 8000 (Suominen T et al).
Type 1 associated with a more severe phenotype and can present at age.
Type I also know as Steinert disease, is the most common inherited neuromuscular disease in adults, with an incidence of 1 to 8000.
Type I MMD is in autosomal dominant disorder caused by expansion of CTG interest with repeat in the untranslated 3′ region of the gene encoding dystrophia myotonic a protein kinase (DMPK).
At presentation 20-30 years and result in premature death at 50-65 years.
Associated with skeletal muscle weakness which is progressive and wasting.
Type I disease manifests by muscle weakness, myotonia, multiple endocrine disorders, respiratory insufficiency, and cardiac abnormalities.
Suddenly death occurs in up to one third agents with progression of conduction system disease to complete AV block is the presumed cause.
Permanent pacing is recommended when complete AV block or advanced high degree AV block is present, or prophylactically for patients presenting with first-degree AV block or fascicular block on EKG.
Both subtypes are a result of the interactions between CUG or CCUG repeat are in a count2242egulatory binding proteins, mainly the muscle blind-like (MBNL) protein family leading to abnormal regulation of free messenger RNA alternative splicing (Wheeler TM et al).
Depletion of MBNL1 in MMD has been implicated with the development of myotonia, cataracts and skin cancer.
Manifestations other than muscle weakness include cardiac conduction defects, insulin resistance, testicular atrophy, respiratory insufficiency, premature cataracts and cognitive impairment (Harper PS).
Patients with MMD had increased risk of benign and malignant tumors, particularly pilomatricoma.