SCN9A and paroxysmal extreme pain disorder: Recent human studies have directly linked Nav1.7 to four pain disorders: Primary erythromelalgia (PE), paroxysmal extreme pain disorder (PEPD), Nav1.7-associated congenital insensitivity to pain (CIP) and small fibre neuropathy (Dib-Hajj et al., 2007; Faber et al., 2012).