seizures and stupor during intravenous mannose therapy in a patient with cdg syndrome type 1b mpicdg mpicdg formally called cdg 1b caused by phosphomannose isomerase mpi deficiency leads to hypoglycaemia protein losing enteropathy hepatopathy and thrombotic events whereas neurologic development remains unaffected dietary supplementation of mannose can reverse clinical symptoms by entering the nglycosylation pathway downstream of mpi when oral intake of mannose in patients with mpicdg is not possible eg due to surgery mannose has to be given intravenously we report a patient with mpicdg on intravenous mannose therapy that showed severe depression of consciousness and seizures without apparent cause eeg and cranial mri findings were compatible with metabolic coma whereas extended laboratory examinations including repeated blood glucose measurements were normal importantly an intravenous bolus of glucose immediately led to clinical recovery and eeg improvement mannose did not interfere with glucose measurement in our assay we suggest that in patients with mpicdg intravenous mannose infusion can lead to intracellular atp deprivation due to several mechanisms 1 in mpi deficiency mannose 6p cannot be isomerised to fructose 6p and therefore is unavailable for glycolysis 2 animal data has shown that accumulating intracellular mannose 6p inhibits glycolysis and 3 elevated intracellular mannose 6p may induce an atp wasting cycle of dephosphorylation and rephosphorylation honey bee effect the mannoseinduced metabolic inhibition may be overcome by highdose glucose treatment we caution that in patients with mpicdg lifethreatening central nervous system disturbances may occur with intravenous mannose treatment these may be due to intracellular energy failure clinical symptoms of energy deficiency should be treated early and aggressively with intravenous glucose regardless of blood glucose levels