Pain That Challenges Survival: A Novel SCN9A Variant (p.Leu1623Gln) Causing Carbamazepine-Refractory Paroxysmal Extreme Pain Disorder in a Chinese Family — Case Report

<b>Background and Clinical Significance:</b> Paroxysmal extreme pain disorder (PEPD) is an extremely rare autosomal dominant sodium channelopathy caused by <i>SCN9A</i> gain-of-function variants. It is characterized by infantile-onset excruciating paroxysmal pain, typically in rectal, ocular, or mandibular regions, triggered by innocuous stimuli and accompanied by autonomic flares. Carbamazepine is dramatically effective in most reported cases. To date, only two genetically confirmed cases have been documented in Chinese patients, and fewer than 20 disease-causing variants are reported worldwide. We report the third Chinese case harboring a novel likely pathogenic <i>SCN9A</i> variant (p.Leu1623Gln), notable for its unusually severe, progressive, and carbamazepine-refractory phenotype, as well as life-threatening psychiatric sequelae, highlighting phenotypic heterogeneity and the devastating impact when standard therapy fails. <b>Case Presentation:</b> A Chinese male proband with positive family history presented with lifelong trigger-induced catastrophic burning and tearing pain in the perineum and lower limbs, associated with erythema, swelling, and occasional non-epileptic seizures. Attacks worsened with age despite escalating polypharmacy, including high-dose opioids, benzodiazepines, topical lidocaine and carbamazepine. Both the proband and his father developed profound psychosocial sequelae including severe depression and suicidal attempts. Next-generation sequencing in the proband revealed a novel heterozygous likely pathogenic variant NM_001365536.1 (<i>SCN9A</i>): c.4868T>A p.(Leu1623Gln). <b>Conclusions:</b> This third reported ethnic Chinese PEPD case expands the genotypic and phenotypic spectrum of <i>SCN9A</i>-related channelopathies, demonstrating that some variants can produce carbamazepine-refractory, progressive, and profoundly disabling disease with high suicidality risk. Early genetic diagnosis is critical in family planning and cascade testing, and has the potential in guiding targeted therapy that is under active research.