Biotinidase is an enzyme that recycles biotin, a water-soluble vitamin essential as a coenzyme for four carboxylases that are involved in gluconeogenesis, fatty acid synthesis, and in the catabolism of several branch-chain amino acids. Biotinidase deficiency (BD) is an autosomal recessively inherited disorder. Patients with profound BD (<10% of mean normal serum biotinidase activity) presents, usually during early childhood, with neurological (seizures, hypotonia, ataxia, developmental delay, vision problems, and/or hearing loss) and non-neurological findings (metabolic acidosis, respiratory difficulties, alopecia and/or skin rash) that may progress to coma or death if untreated.

Three cases of adult-onset biotinidase deficiency with reversible optic neuropathy have recently been described in France, where there is no neonatal screening of BP. Once treated with Biotin, patients' vision was fully restored.

This study aims to assess the prevalence of BP among a population of patients with idiopathic optic neuropathy, and to assess the efficacy of Biotin supplementation on visual impairment in these patients.

Inclusion Criteria:

• patient with bilateral optic neuropathy
• symptoms beginning before 50 years old
• diagnosed for more than 1 months
• etiology unknown

Exclusion Criteria:

• none

Patients with idiopatic bilateral optic neuropathy