Painful muscle stiffness with markedly elevated serum creatine kinase (CK) levels after twenty weeks of gestation in four patients with myotonic dystrophy type 1 (DM1) and a patient with paramyotonia congenita (PMC).

Four patients with myotonic dystrophy type 1 (DM1) and a patient with paramyotonia congenita (PMC) developed devastating painful muscle stiffness with markedly elevated serum creatine kinase (CK) levels after 20 weeks of gestation. Immediately after delivery, painful muscle stiffness completely disappeared and the CK levels returned to the baseline. In a patient with DM1, muscle biopsy at delivery and skeletal muscle MRI in six days after delivery showed inflammatory changes, which disappeared in MRI on postpartum day 41. Pregnancy-associated aggravation of myotonia has been reported in myotonic disorders, but painful muscle stiffness has been scarcely reported. Inactivation of muscle Cl channel by progesterone that is prominently increased in the middle phase of gestation is likely to be the underlying mechanism of the pregnancy-associated painful muscle stiffness in myotonic disorders.