Isolated sulfite oxidase deficiency in the newborn: Lactic acidaemia and leukoencephalopathy

We report a newborn with progressive leukoencephalopathy and lactic acidaemia, diagnosed with isolated sulfite oxidase deficiency. We show that low plasma total homocysteine (PTHcy) is a valuable early indicator of sulfite oxidase dysfunction, providing a crucial first-line screen, whereas plasma cystine is not always informative in the first few days of life.