Hyperekplexia

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

A few years ago, a new genetic disorder (OMIM # 615075) has been associated with loss-of-function variations in the CTNNB1 gene. The clinical features include a delayed psychomotor development usually leading to severe intellectual disability with or without autistic spectrum disorders, progressive spastic diplegia, and various visual defects. Among over 30 cases described worldwide, 2 were reported with an exaggerated startle response to sudden stimulus corresponding to a very rare neurological phenomenon called hyperekplexia. The investigators also have a 3rd patient carrying a CTNNB1 syndrome associated with hyperekplexia.

Mutations in genes affecting pain transmission start to be known, the investigators are investigating a mutation in a glycine channel, which has an influence on pain modulation. Pain modulation is the ability of the central nervous system to enhance or diminish the sensation of pain. The investigators therefore will test patients and healthy volunteers with quantitative sensory tests, basically determining the point at which a stimulation just starts to induce pain. These tests are reliable and permit a direct comparison between healthy volunteers and patients with the affected glycine gene.