Classical homocystinuria presenting with transient basal ganglia pathology and dystonia

Abstract Classical homocystinuria is caused by pathogenic variants in the CBS gene leading to a deficiency of the vitamin B6‐dependent enzyme cystathionine beta synthase. The disease is typically associated with high blood homocysteine concentrations. Clinical features include developmental delay/intellectual disability, psychiatric problems, thromboembolism, lens dislocation, and marfanoid habitus. We report on a child with classical homocystinuria presenting with acute episodes of dystonia and symmetrical basal ganglia abnormalities mimicking a mitochondrial disease. After starting treatment with vitamin B6, homocysteine levels rapidly normalized and dystonic episodes did not re‐occur. Moreover, brain‐imaging findings almost completely disappeared. The case illustrates that homocystinuria should be considered as a treatable differential diagnosis of dystonia.