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Background  The  SYNGAP1  gene encodes for a small GTPase-regulating protein critical to dendritic spine maturation and synaptic plasticity. Mutations have recently been identified to cause a breadth of neurodevelopmental disorders including autism, intellectual disability, and epilepsy. The purpose of this work is to define the phenotypic spectrum of  SYNGAP1  gene mutations and identify potential biomarkers of clinical severity and developmental progression.    Methods  A retrospective clinical data analysis of individuals with  SYNGAP1  mutations was conducted. Data included genetic diagnosis, clinical history and examinations, neurophysiologic data, neuroimaging, and serial neurodevelopmental/behavioral assessments. All patients were seen longitudinally within a 6-year period; data analysis was completed on June 30, 2018. Records for all individuals diagnosed with deleterious  SYNGAP1  variants (by clinical sequencing or exome sequencing panels) were reviewed.    Results  Fifteen individuals (53% male) with seventeen unique  SYNGAP1  mutations are reported. Mean age at genetic diagnosis was 65.9 months (28–174 months). All individuals had epilepsy, with atypical absence seizures being the most common semiology (60%). EEG abnormalities included intermittent rhythmic delta activity (60%), slow or absent posterior dominant rhythm (87%), and epileptiform activity (93%), with generalized discharges being more common than focal. Neuroimaging revealed nonspecific abnormalities (53%). Neurodevelopmental evaluation revealed impairment in all individuals, with gross motor function being the least affected. Autism spectrum disorder was diagnosed in 73% and aggression in 60% of cases. Analysis of biomarkers revealed a trend toward a moderate positive correlation between visual-perceptual/fine motor/adaptive skills and language development, with posterior dominant rhythm on electroencephalogram (EEG), independent of age. No other neurophysiology-development associations or correlations were identified.    Conclusions  A broad spectrum of neurologic and neurodevelopmental features are found with pathogenic variants of  SYNGAP1 . An abnormal posterior dominant rhythm on EEG correlated with abnormal developmental progression, providing a possible prognostic biomarker.    Electronic supplementary material  The online version of this article (10.1186/s11689-019-9276-y) contains supplementary material, which is available to authorized users.

journal of neurodevelopmental disorders

Phenotypic characterization of individuals with SYNGAP1 pathogenic variants reveals a potential correlation between posterior dominant rhythm and developmental progression