Study Rationale and Objectives The objective of the study is to determine the efficacy and safety of lacosamide, a sodium channel blocker, in patients with pain due to SCN9A-associated SFN. The proposed study plans to recruit patients with clinically diagnosed SFN, where a mutation in SCN9A has been confirmed genetically, and where possible, has been demonstrated on functional testing, to cause hyperexcitability of DRG neurons. This small, precision medicine population provides an opportunity to evaluate the efficacy and safety of lacosamide in treatment of pain due to SCN9A-associated SFN.