RESUMO
INTRODUCTION: Intellectual disability (ID) is a neurodevelopmental disorder characterized by limitations in intellec tual and adaptive functioning, of various etiologies, including genetic causes. OBJECTIVE: to describe genetic studies carried out in a series of children and adolescents with ID of previously undetermined etiology, considering their phenotypic characteristics. PATIENTS AND METHOD: Descriptive study of a series of patients with ID aged 6 to 18 years. Clinical records, cognitive assessment results (Wechsler -TADI), and genetic study performed were reviewed. They were classified according to phenotypic characteristics into Group 1 patients without a specific phenotype, Group 2: patients with Angel- man- and Rett-like neurodevelopmental disorders phenotype, and Group 3: patients with difficult- to-control seizures. Group 1 was studied with CMA and Groups 2 and 3 with specific genetic panels. RESULTS: 18 patients were described, average age 11 years, male predominance, non-consanguineous parents, and with history of psychomotor retardation. Common comorbidities were epilepsy, autism spectrum disorder (ASD), and behavioral difficulties. Most had a neurological examination without focus and had TADI with very poor developmental ages. In Group 1, there was one patient with a 16p11.2 microdeletion and in Group 3 a duplication of the IQSEC2 gene was found in a patient with difficult-to-control seizures. CONCLUSIONS: The phenotypic characteristics allow to guide the choice of specific genetic studies in children and adolescents with ID of previously undetermined etiology to approach the etiological diagnosis.