Advances in electrophysiology and pharmacogenomics have led to the characterization of sodium polymorphisms associated with increased risk of chronic pain in conditions such as adult osteoarthritis, fibromyalgia, and post-surgical pains, as well as pathogenic mutations in SCN9A (encoding Nav1.7) that underlie cases of inherited erythromelalgia and extreme pain disorder [95,96,97]. This evidence concerns the gene SCN9A and erythromelalgia.