A multicenter international genetic and clinical study of GLUT1 deficiency received 132 requests for mutational analysis of SLC2A1 gene from 2004-8. Mutations identified in 57 patients (43%) were novel in 37, known in 6, and multiple exon deletions in 6. Clinical data retrospectively collected by questionnaire revealed three different phenotypes: 1) classical (84%), early onset (<2 years) (65%) and late-onset (18%); 2) non-classical with mental retardation and movement disorder, without epilepsy (15%); and 3) one adult case with minimal symptoms. The ketogenic diet controlled epilepsy in 86% and reduced movement disorders in 48% patients with classical phenotype and in 71% of non-classical. Delay in diagnosis of classical cases was 6.6 years (range, 1 month-16 years). CSF glucose was <2.5 mmol/1 (range 0.9-2.4 mmol/1), and CSF:blood glucose ratio was <0.5 in all but one (range 0.19-0.52). CSF lactate was low to normal. Type of mutation was related to severity of phenotype. CSF:blood glucose ratio was related to type of mutation and phenotype. 
COMMENT. Lumbar puncture with CSF glucose determination was the key to diagnosis of GLUT1 deficiency and prompt treatment with the ketogenic diet. Seizures were not a symptom in 15% of patients.