Craniosynostosis is a birth defect of the brain characterized by the premature closure of one or more of the fibrous joints between the bones of the skull (called the cranial sutures) before brain growth is complete. Closure of a single suture is most common. The abnormally shaped skull that results is due to the brain not being able to grow in its natural shape because of the closure. Instead it compensates with growth in areas of the skull where the cranial sutures have not yet closed. The condition can be gene-linked, or caused by metabolic diseases, such as rickets or an overactive thyroid. Some cases are associated with other disorders such as microcephaly (abnormally small head) and hydrocephalus (excessive accumulation of cerebrospinal fluid in the brain). The first sign of craniosynostosis is an abnormally shaped skull. Other features can include signs of increased intracranial pressure, developmental delays, or mental retardation, which are caused by constriction of the growing brain. Seizures and blindness may also occ