Severe intellectual disability-progressive spastic diplegia syndrome is a rare condition that has been described in a few people with severe intellectual disability. Other signs and symptoms include progressive microcephaly (very small head); ataxia (lack of coordination); spasticity ; and/or skin, hair and mild facial anomalies. It is caused by changes (mutations) in the CTNNB1 gene and it is inherited in an autosomal dominant fashion. Treatment is based on the signs and symptoms present in each person. The disease is Monarch Disease Ontology term MONDO_0014035 (severe intellectual disability-progressive spastic diplegia syndrome). Also known as: CTNNB1 syndrome, MRD19, autosomal dominant intellectual disability 19, autosomal dominant mental retardation 19, intellectual disability, autosomal dominant type 19, mental retardation, autosomal dominant type 19, neurodevelopmental disorder with spastic diplegia and visual defects.