The purpose of this study is to gather preliminary data to determine if ranolazine is a safe and effective treatment for the symptoms of myotonia congenital, paramyotonia congenita, and myotonic dystrophy type 1. The duration of the study is 5 weeks.

Recent advances in the understanding of myotonia congenita have identified potential areas that could possibly respond to treatment in a drug study. The drug ranolazine (trade name Ranexa) is a FDA-approved medication to treat chest pain in patients with heart disease. Ranolazine has been studied in mice with myotonia congenita. The data from this animal model suggest that ranolazine may improve the symptoms and signs of myotonia. All individuals that participate will be placed on active drug. The investigators want to see if this drug is safe to take without causing too many side effects for people with myotonia congenita, paramyotonia congenital and myotonic dystrophy type 1. Participants will go to The Ohio State University for study visits. Participants will take ranolazine for four weeks. Participants can expect a total of 4 study visits and 2 phone calls over the 5 week period.

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