Primary erythromelalgia associated with an SCN9A variant can mimic systemic lupus erythematosus in skin manifestations: a case report.

Primary erythromelalgia (PEM) is a rare autosomal dominant disorder characterized by episodic erythema and burning pain of the extremities, typically triggered by exercise or fatigue. PEM usually begins in childhood and is caused by gain-of-function variants in SCN9A, which encodes the voltage-gated sodium channel Nav1.7. The disease concept of PEM, however, is largely unrecognized among rheumatologists. Here, we report a 20-year-old woman who had experienced recurrent burning pain and redness of the extremities since around age 10. She also exhibited livedo-like rash, leukopenia, and hypocomplementemia and tested positive for antinuclear antibodies. Nerve conduction studies showed abnormalities interpreted as compatible with multifocal mononeuropathy. Based on these findings, she was diagnosed with systemic lupus erythematosus (SLE) and treated with 20 mg of prednisolone; however, the painful erythema of the extremities did not improve. Given a family history of similar episodic symptoms in her father, PEM was suspected. Genetic testing identified a heterozygous SCN9A p.G856D variant, supporting the diagnosis of autosomal dominant PEM. Direct immunofluorescence/lupus band testing of the biopsied lower-leg lesion was interpreted as negative. This case highlights that SCN9A-associated PEM may present with lupus-like cutaneous erythema in young patients, potentially leading to misdiagnosis and ineffective immunosuppression. PEM should be considered in patients with childhood-onset, trigger-induced painful erythema of the extremities, particularly when there is family history or poor response to steroids.