RESUMEN
INTRODUCTION: Rett syndrome (RS) is a neurodevelopmental disorder that affects girls almost exclusively. The identification of mutations in the MECP2 and CDKL5 genes offers genetic confirmation of the clinical diagnosis. The FOXG1 gene appears to be a novel cause of the congenital variant of RS. CASE REPORT: We describe the first Spanish patient with the atypical (congenital) variant of RS with mutation of the FOXG1 gene and the case is compared with 12 patients previously reported in the literature; clinical criteria that suggest alterations in FOXG1 are proposed. The patient was referred at the age of 6 months due to overall retardation, axial hypotonia, microcephaly and a peculiar phenotype. Magnetic resonance imaging of the brain revealed hypoplasia of the corpus callosum, frontal atrophy and ventriculomegaly. The appearance of hand-to-mouth stereotypic movements at 12 months pointed the clinical diagnosis towards an atypical variant of RS, the congenital form; there was progressive improvement of visual contact and interest in her surroundings. Frequent respiratory infections and obstructive sleep apnoea syndrome. At the age of 5 years there was partial control over the axial tone, grasping with the hands, good contact and babbling, without epilepsy or behavioural disorders. The MECP2 and subtelomeric deletion study did not reveal any alterations; two polymorphisms were identified in the CDKL5 gene and a pathogenic mutation was found in FOXG1 (c.624C>G p.Tyr203X). CONCLUSIONS: It has been shown that 92% of patients with mutations in the FOXG1 gene present the congenital form of RS with severe generalised hypotonia, early acquired microcephaly (-3 to -6 standard deviations) and peculiar phenotype. When faced with a diagnosis of RS with no alterations in the MECP2 and CDKL5 genes, especially in the case of the congenital variant, the FOXG1 gene must be investigated. The molecular diagnosis confirms the clinical diagnosis and provides the family with genetic counselling.