Aim The aim of this study was to characterize patients diagnosed with glucose transporter protein‐1 deficiency syndrome (GLUT‐1 DS) clinically and genetically, and to evaluate the effect of treatment with the classic ketogenic or modified Atkins diet. Method We retrospectively studied medical records of 10 patients diagnosed with GLUT‐1 DS. Four females and six males with a median age of 15 years were included. Results The study illustrates the genetic and clinical heterogeneity of GLUT‐1 DS. Analysis of the SLC2A1 gene disclosed a variety of mutation types. The time between onset of symptoms and diagnosis was more than 11 years on average. The outcome in those with early diagnosis and intervention was surprisingly good. All but one patient with the classic phenotype became seizure free after treatment with the classic ketogenic or modified Atkins diet. Acetazolamide was effective in one patient with paroxysmal exercise‐induced dyskinesia. A point prevalence of GLUT‐1 DS in Norway was estimated as 2.6 per 1 000 000 inhabitants. Interpretation Although the long‐term prognosis in patients with GLUT‐1 DS partly depends on the underlying genetics, our study supports the assumption that early initiation of treatment with a ketogenic diet may positively affect the outcome.

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