Biallelic mutations in the SLC25A19 gene impair the function of the thiamine mitochondrial carrier, leading to two distinct clinical phenotypes. Homozygosity for the c.530G > C mutation is invariably associated to Amish lethal microcephaly. The second phenotype, reported only in 8 patients homozygous for different non-Amish mutations (c.373G > A, c.580T > C, c.910G > A, c.869T > A, c.576G > C), is characterized by bilateral striatal necrosis and peripheral polyneuropathy. We report a new patient with the non-Amish SLC25A19 phenotype showing compound heterozygosity for the new variant c.673G > A and the known mutation c.373G > A.

The natural history of non-Amish SLC25A19 deficiency is characterized by acute episodes of fever-induced encephalopathy accompanied by isolated lactic acidosis and Leigh-like features at magnetic resonance imaging (MRI). Acute episodes are prevented by high-dose thiamine treatment (600 mg/day). As shown in the new case, both mild clinical signs and basal ganglia involvement can precede the acute encephalopathic onset of the disease, potentially allowing treatment anticipation and prevention of acute brain damage. Peripheral axonal neuropathy, observed in 7 out of 9 patients, is not improved by thiamine therapy. In two early treated patients, however, peripheral neuropathy did not occur even on long-term follow-up, suggesting a potential preventive role of treatment anticipation also at the peripheral level.

Non-Amish SLC25A19 deficiency is an extra-rare cause of Leigh syndrome responsive to thiamine treatment. Ex adiuvantibus thiamine treatment is mandatory in any patient with Leigh-like features.