Mothers of infants with this disorder usually have uneventful pregnancies and deliveries, with the child appearing normal and within typical birth weight and length ranges. Infants with GLUT1 deficiency syndrome have a normal head size at birth, but the growth of the brain and skull is slow, in severe cases resulting in an abnormally small head size (microcephaly). Typically, seizures start between one and four months in 90% of cases with abnormal eye movements and apneic episodes preceding the onset of seizures in some cases. Seizures usually are complex to begin with and later become more generalized. Seizure frequency is variable and a history of decreasing frequency during times of ketosis may prompt a diagnosis. It is estimated that 10% of individuals with Glut 1 Deficiency do not have seizures and symptoms are typically less severe in these cases. Most of these non-epileptic cases will still have developmental delay, intellectual delays, and movement disorders such as ataxia, alternating hemiplegia, or dystonia.