Unclear symptoms, early diagnosis and perfect outcome: a case diagnosed as sepiapterin reductase deficiency hidden behind vitamin B12 deficiency

Objectives: Sepiapterin reductase deficiency (SRD) is a rare dopa-sensitive neurotransmitter disorder caused by autosomal recessive mutations in the sepiapterin reductase gene. The triad of paroxysmal stiffening, oculogyric crises, and hypotonia are highly suggestive in some patients. However, in other patients, the clinical picture may be nonspecific and remain under-recognized and misdiagnosed as cerebral palsy. Case presentation: We present a nine-month-old boy who initially presented with hypotonia and developmental delay, diagnosed as vitamin B12 deficiency. Upon he did not respond to vitamin replacement treatment, he was diagnosed with SRD by whole-exome sequencing (WES). The boy improved dramatically under treatment with L-dopa, 5-hydroxy-tryptophan and BH4. Conclusions: We aim to emphasize that SRD can present with nonspecific symptoms, leading to a diagnostic delay for this rare but treatable disease. Moreover, our case is the first to demonstrate the clinical benefit of BH4 add-on treatment. Early intervention is crucial for good outcome and neurodevelopment.