SCN9A and erythromelalgia: Pathogenic variants in this locus underlie SCN9A-associated neuropathic spectrum disorders (SCN9A-NSD), a rare monogenic condition characterized by neuropathic pain phenotypes comprising three clinical entities: inherited erythromelalgia (IEM, an autosomal-dominant genodermatosis), paroxysmal severe pain disorder (PSPD), and sodium channel-associated small fiber neuropathy (SCN9A-SFN) (Drenth and Waxman, 2007; Caldito et al., 2024).