A Rare Case of a Movement Disorder due to Late-onset Isolated Sulfite Oxidase Deficiency – A Case Report from India

Isolated sulfite oxidase deficiency is an inborn error of the metabolism of sulfated amino acids caused by mutations in the SUOX gene (OMIM #272300). This report details the case of late-onset sulfite oxidase deficiency diagnosed at the age of 18 years in an intellectually normal girl. She presented with gradually increasing involuntary movements in the form of dystonia in the upper and lower limbs. Sulfite oxidase is a mitochondrial enzyme that plays a crucial role in detoxifying sulfites to sulfates in the metabolism of sulfur-containing amino acids. The whole-exome sequencing result of the patient revealed a homozygous pathogenic variant c. 1096C>T (p.Arg366Cys) in the SUOX gene. The enzyme deficiency typically presents as neonatal encephalopathy and intractable seizures resembling stage 3 hypoxic–ischemic encephalopathy and later manifests as multicystic leukoencephalopathy and profound global developmental delay. Late-onset ISOD presents usually between 6 and 18 months of age, often precipitated by febrile illness with episodic encephalopathy, ataxia, choreoathetosis, dystonia, etc., and an acute hemiparesis as a result of metabolic stroke. In summary, we described the first case of a milder, adult-inherited metabolic disease with preserved intelligence, which has biochemical markers available for the diagnosis.