When the mimic meets reality: AQP4-IgG seroconversion in a patient with profound biotinidase deficiency presenting with an NMOSD- like phenotype

Abstract

Neuromyelitis optica spectrum disorder (NMOSD) is a chronic autoimmune inflammatory disease of the central nervous system. While aquaporin-4 immunoglobulin G (AQP4-IgG) antibodies are detected in most adult patients, seronegativity is common in pediatric and early disease stages, complicating diagnosis. Several metabolic disorders may clinically and radiologically mimic NMOSD in these settings. Biotinidase deficiency is a rare, treatable metabolic disorder reported as an NMOSD mimic. We describe a 16-year-old girl with an NMOSD-like phenotype and longitudinally extensive transverse myelitis who was initially AQP4-IgG–negative and diagnosed with profound biotinidase deficiency. Following clinical improvement with biotin supplementation, she experienced a relapse, and repeat testing revealed AQP4-IgG seroconversion, confirming coexistent seropositive NMOSD. This case illustrates that a metabolic disorder may coexist with, rather than solely mimic, NMOSD.