is a birth defect of the brain characterized by the premature closure of one or more of the joints between the bones of the skull (called the cranial sutures) before brain growth is complete. The first sign is an abnormally shaped skull; other features can include signs of increased intracranial pressure, developmental delays, intellectual disability, seizures, and blindness, caused by constriction of the growing brain. Closure of a single suture is most common and does not prevent the brain from expanding to a normal volume. When many sutures close prematurely, the skull cannot expand to accommodate the growing brain, leading to increased pressure and impaired development of the brain. Craniosynostosis can be caused by genetic mutations, certain metabolic diseases (such as rickets or an overactive thyroid) or can be associated with other disorders such as microcephaly and hydrocephalus. Treatment generally consists of surgery to improve the symmetry and appearance of the head and to relieve pressure on the brain and the cranial nerves. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.