An atypical presentation in a child with propionic acidemia? Better think twice!

This report details the case of an infant with confirmed propionic acidemia who presented with progressive neurological deterioration and recurrent episodes of metabolic decompensation with elevated lactate levels, but without hyperammonemia. The child's clinical course and neuroradiological findings increasingly deviated from the known clinical and neuroradiological spectrum of propionic acidemia. A rapid trio exome sequencing identified <i>SLC19A3</i>-related thiamine metabolism dysfunction syndrome 2 as a second genetic disease. The pathomechanisms of both diseases synergize in the impairment of brain energy metabolism, and the associated clinical phenotypes partially overlap, which explains the severe and atypical course of propionic acidemia in the reported case.