Neurodevelopmental disorders (NDD) encompass conditions that impair cognitive and/or emotional development in children, significantly impacting school, social, and family life. They are often linked to genetic causes and, in most cases, lack curative treatment. Among these disorders, monoallelic variations in the CTNNB1 gene cause a rare syndrome known as NEDSDV (Neurodevelopmental disorder with spastic diplegia and visual defects, OMIM: 615075). About twenty patients are reported in France. This syndrome is characterized by global developmental delay, intellectual disability, axial hypotonia, autistic traits, microcephaly, and sometimes ocular anomalies. The clinical profile resembles that of cerebral palsy, and CTNNB1 syndrome is considered a genetic form of this condition, accounting for roughly 4% of cases where a gene has been identified.

Motor impairment is a core feature, with a wide range of movement disorders. Research remains limited, except for a recent publication. Dystonic hypertonia of the lower limbs is frequently described, more pronounced distally than proximally, without pyramidal signs. Spasticity is less common. Gait has been poorly studied: it may be absent or, when acquired, unstable, often tiptoe, and sometimes broad-based, resembling ataxia despite the absence of cerebellar signs. These motor features are difficult to detect before one year of age. To date, no longitudinal studies exist on motor or cognitive progression in CTNNB1 patients; available data are cross-sectional and do not suggest cognitive decline.

From a pathophysiological perspective, the CTNNB1 gene encodes β-catenin, a key protein in cell adhesion and Wnt signaling, involved in cell differentiation and tissue homeostasis. It plays an essential role in embryonic brain development, particularly neuritogenesis and synaptic organization, with a specific impact on dopaminergic structures in the midbrain. Knock-out animal models show severe reduction in dopaminergic neurogenesis. These findings suggest that CTNNB1 anomalies lead to secondary dopaminergic deficits, contributing to clinical signs. The hypothesis is that this deficit could be partially corrected by dopamine supplementation.

Regarding treatment, L-dopa (levodopa), used in dopaminergic disorders, has shown beneficial effects in a CTNNB1 patient. In our neuropediatrics department, two patients treated with L-dopa exhibited notable improvements in alertness, language, and motor skills within two months. These observations support the hypothesis that L-dopa may improve certain motor and non-motor symptoms in these patients.

In summary, CTNNB1 syndrome is a rare form of NDD, clinically similar to cerebral palsy, with complex motor disorders and a probable dopaminergic deficit. Current evidence calls for further research, including longitudinal studies and therapeutic trials targeting the dopaminergic pathway.

Inclusion Criteria:

• Aged between 1 and 15 years inclusive,
• Carrier of a pathogenic variant of CTNNB1,
• Patient with dystonia,
• Patient willing to comply with the contraception requirements detailed in the protocol.

Exclusion Criteria:

• Contraindication to treatment with L-dopa and carbidopa or any of its excipients,
• Current treatment with L-dopa, dopamine agonist, or dopamine blocker,
• Patients with peptic ulcer disease,
• Patients with open-angle glaucoma,
• Patients with orthostatic hypotension,
• Failure to obtain informed consent signed by both parents or legal guardians and the child's assent, if possible,
• Patients not affiliated with or not covered by a social security scheme,
• Individuals participating in another study with an exclusion period still in progress,
• Individuals who are pregnant or wish to become pregnant within 12 months of inclusion.