Hereditary hyperekplexia is a rare neuronal disorder, caused by genetic defects leading to dysfunction of glycinergic neurotransmission. The clinical presentation is characterized by stiffness and exaggerated startle responses to unexpected stimuli, that appear shortly after birth. The generalised stiffness can lead to apnea and sudden infant death syndrome. Several genes are known to be associated with hereditary hyperekplexia. The most frequent are Glycine Receptor Alpha 1 (GLRA1), Glycine Receptor Beta (GLRB) and Solute Carrier Family 6 Member 5 (SLC6A5). They encode for the postsynaptic glycine receptor (GLRA1, GLRB) and the presynaptic glycine transport (SLC6A5). Genetic mutations in these genes lead to dysfunction in the glycinergic inhibitory neurotransmission. The neurodevelopment was initially described as normal, or as delayed due to the motor difficulties. Global development delay and intellectual disability are reported as well, in the most recent studies. Nevertheless, the degree of severity of the learning difficulties and the adaptive faculties of the patients is not specified. Similarly, the efficacy of clonazepam in hyperekplexia is well known, but the evolution of dosage over time and the frequency of complete withdrawal have never been studied. The primary endpoint of this study is to describe adaptive skills using a standardized questionnaire, Vineland Adaptive Behavior Scale (VABS2). Secondary endpoints are: * Neurodevelopmental course study * Description of the evolution of the clinical manifestations over the years * Evaluation of the efficacity of the treatment CLONAZEPAM, initially and over time, and evolution of the dosage * Comparison of clinical and therapeutical characteristics according to the genotype