Introduction: Myotonia is a common clinical feature in myotonic disorder, characterized by a delayed relaxation or sustained contraction of skeletal muscle. Myotonia involving the stomatognatic muscle system may lead to difficulties of mandible movements, including fundamental physiologic functions as mastication and conversation. Myotonic Dystrophy (MD), Steinert's Disease, is one of the most frequent dystrophic muscular disorder in adulthood, with incidence of 1:7000 individuals in the population.

Objective: to analyze the prevalence of myotonic phenomena in masticatory muscles on patients with MD.

Subjects and methods: 31 patients with molecular diagnosis of MD were studied by clinical exam of the stomatognatic system, and by timing the mouth opening and closing.

Results: 16 (52%) patients presented myotonic phenomena on masticatory muscles, during mouth opening in 3, and closing in one. Others referred the symptom as if mouth locked when completely opened. Mouth opening took 9.25 seconds and 1.77 secs in closing in the first condition, and 1.03 secs in opening and 18.59 secs in closing in the second condition. Normal individual opens and closes the mouth in less than one second. Sign of paradoxical myotonia, myotonia becoming worse with successive movement, was mentioned only during conversation. Unrecognition of this phenomena lead to incorrect orthodontic treatment in 8 patients.

Conclusion: The stomatognatic system is involved in more than 50% of patients with MD. The present finding emphasizes the importance of diagnosing this phenomena before the clinical decision about dental treatment.

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