Long Term Safety Study of Amifampridine Phosphate in MuSK-MG (Muscle Specific Tyrosine Kinase Myasthenia Gravis)

Inclusion Criteria 1. Participated in the MSK-002 study 2. Willing and able to provide written informed consent after the nature of the study has been explained and before the start of any research-related procedures. 3. Female patients of childbearing potential must have a negative pregnancy test (urine human chorionic gonadotropin \[HCG\] at the end of MSK-002 study); and must practice an effective, reliable contraceptive regimen during the study and for up to 30 days following discontinuation of treatment. 4. Ability to participate in the study based on overall health of the patient and disease prognosis, as applicable, in the opinion of the Investigator; and able to comply with all requirements of the protocol, including completion of study questionnaires. Individuals who met any of the exclusion criteria in the original protocol or those listed below are not eligible to participate in the study: 1. Epilepsy and currently on medication. 2. Clinically significant abnormalities in 12 lead ECG, in the opinion of the Investigator. 3. Breastfeeding or pregnant at Screening or planning to become pregnant at any time during the study. 4. Intolerable amifampridine-related side effects 5. Treatment with an investigational drug (other than amifampridine) or device while participating in this study. 6. Any medical condition that, in the opinion of the Investigator, might interfere with the patient's participation in the study, poses an added risk for the patient, or confound the assessment of the patient.