Résumé :
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Myotonic dystrophy type 1 (MD1) is an autosomal-dominant multisystemic disorder linked to a monoallelic expansion of the CTG n repeat in the 3 untranslated region of the DM protein kinase (DMPK) gene. Healthy individuals have repeats of n= 5-37, whereas affectid individuals have more then 50 repeats. The prevalence of this most frequent muscular dystrophy is in Europe 4/100 000. Myasthenia gravis is an autoimmune neuromuscular disorder with estimated prevalence 8- 15/100 000. Cases of myasthenia gravis in association with MD1 are extremely rare and only 4 cases are described in the literature up to date. It is presented 55 years old man with schizophrenic disorder, symmetric semiptosis, but with the normal muscle performance (cycling), who quickly developed the muscle weakness, diplopia, the bulbar involvement and the respiratory insufficiency. Acetylcholine receptor antibodies were highly indicative for myasthenia gravis (12.4 nmol/l, normal range 0.0- 0.4 nmol/l). Thymoma was not found. Patient was successfully treated with intravenous immunoglobuline and pyridostigmine. After the myasthenic weakness resolved, we observed an impaired muscle relaxation with the grip myotonia. DNA analysis focused to DMPK was performed and 39 CTG repeats were found. We suppose that the myotonia was in the acute stage covered by the myasthenic weakness. Bilateral semiptosis in combination with diplopia was considered as typical myasthenic sign. Manifestation of the mild premutation DM1 was in this patient's history underestimated probably due to long-lasting schizophrenic disorder and related medication. It is extremely rare combination of two orphan neuromuscular diseases; the chance has been estimated at 1 per 40 million.
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