SCN9A and sodium channelopathy-related small fiber neuropathy: Primary erythromelalgia (PEM) is an autosomal dominant disorder caused by SCN9A gene mutations, and these variants (Q10R, I136V, P187, S211T, F216S, I228M, I234T, S241T, N395K, V400M, P610T, G616R, DII, L823R, F826Y, I848T, G856R, L858H, L858F, A863P, Q875E, DelL955, R1150W, DIII, N1245S, DIV, W1538R, A1632E and A1632G) are strongly linked to PEM.