Effectiveness of 3,4-Diaminopyridine in Lambert-Eaton Myasthenic Syndrome

Inclusion Criteria: 1. Age 18 or over 2. Ambulatory while taking 3,4-DAP, i.e. the patient was able to perform the timed up and go (TUG), either with or without an assistive device 3. Established diagnosis of LEMS, with documentation provided 4. Continuous use of Jacobus 3,4-DAP for at least 3 months 5. Minimum of 3 doses per day with no single dose less than 10 mg of 3,4-DAP 6. The patient needed to wait about 15 to 30 minutes to experience an unequivocal improvement in a LEMS-induced dysfunction after they take their first dose of 3,4-DAP in the morning \[a patient who remains in bed past this point by choice may still be eligible\] 7. Stable regimen of all LEMS-related treatments for at least 3 months 8. Stable daily regimen of other medications (prescription and over-the-counter) for a minimum of 1 month 9. Willing to chance being tapered off of 3,4-DAP 10. Fluency in English 11. If applicable, agreed to use birth control during heterosexual intercourse until at least 2 weeks after completion of study 12. A signed informed consent by the study subject Exclusion Criteria: 1. Last monoclonal antibody treatment (e.g. rituximab) was less than 6 months ago (i.e., recent treatment is an exclusion) 2. Clinically significant or poorly controlled condition that in the opinion of the study personnel might pose an unacceptable risk to the patient if entered into the study 3. Respiratory failure requiring intubation while on 3,4-DAP with no precipitating event or medication 4. Use of any investigational drug other than 3,4-DAP within the last 30 days 5. Pregnant or lactating 6. Current use of other aminopyridines (e.g.4-AP) or guanidine 7. Did not display a sufficiently large response to 3,4-DAP during the baseline observation period in the CRU to detect a decline during withdrawal of 3,4-DAP