Study of Ranolazine in Myotonia Congenita, Paramyotonia Congenita and Myotonic Dystrophy Type 1

Inclusion Criteria: * Diagnosis of myotonia congenital, paramyotonia congenital or Myotonic Dystrophy Type 1 established by genetic testing in the subject or in a first-degree relative. * Clinically evident myotonia Exclusion Criteria: * Contraindications to ranolazine use: * for fungus infection: ketoconazole (Nizoral), itraconazole (Sporanox, Onmel) * for infection: clarithromycin (Biaxin) * for depression: nefazodone * for HIV: nelfinavir (Viracept), ritonavir (Norvir), lopinavir and ritonavir (Kaletra), indinavir (Crixivan), saquinavir (Invirase). * for tuberculosis (TB): rifampin (Rifadin), rifabutin (Mycobutin), rifapentine (Priftin) * for seizures: phenobarbital, phenytoin (Phenytek, Dilantin, Dilantin-125), carbamazepine (Tegretol) * the herbal supplement St. John's wort * you have scarring (cirrhosis) of your liver * Concurrent use of mexiletine, lacosamide, acetazolamide, phenytoin, quinine, procainamide, Saint John wort or tocainide. Patients who were previously treated with these medications may participate. They need to be off of the medication for at least a week prior to enrollment. * QTc \>470 ms for men and \>480 ms for women. * Women who are pregnant or breastfeeding * Direct family history of sudden cardiac death