Biotinidase deficiency (BD) is an autosomal recessive metabolic disorder that can present atypically in adulthood and mimic demyelinating disorders. We present a case of adult-onset BD with a relapsing course initially diagnosed as neuromyelitis optica spectrum disorder (NMOSD) and treated with immunosuppressive therapy for over a decade.

Initial brain and spine MRI showed minimal findings with punctate T2 hyperintensities. CSF analysis was consistently unremarkable with negative oligoclonal bands and negative NMO/MOG antibodies. The patient symptomatically responded to corticosteroids, plasmapheresis, and rituximab therapy, but remained wheelchair dependent. After 13 years of presumed stable inflammatory demyelinating disease, serum biotinidase activity was found to be markedly deficient (<0.5 U/L; normal 4.4-12.0 U/L) following progression of symptoms. Genetic testing confirmed two pathogenic variants in the BTD gene: c.1612C>T (p.Arg538Cys) and c245C> (p.Ala82Val). Oral biotin supplementation was initiated with improvement in multiple symptoms.

Adult-onset BD can mimic demyelinating disorders, particularly when presenting with optic neuropathy and myelopathy. This case highlights the importance of considering metabolic disorders in atypical presentations of presumed demyelinating conditions, as early diagnosis and inexpensive biotin supplementation can prevent unnecessary immunosuppressive therapy and irreversible neurological damage.