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Craniosynostosis, or premature fusion of the skull plates, occurs in about one in 2,000 infants. It can be an isolated abnormality or part of a broader malformation syndrome.
When a baby is born, the skull, which must be compressible to get through the narrow birth canal, consists of six plates that are designed to slide partially over each other, temporarily reducing the diameter of the head. For the first month or two, parents can feel the ridges on their newborn's head where these plates overlap. These plates then slide back out and lay flat as the brain grows and expands.
The joints between the plates are called the sutures. Craniosynostosis occurs when for whatever reason, sporadic or as part of a larger problem, one or more of these sutures fuse prematurely. Constrained abnormally in one part of the skull, brain growth forces the remainder of the skull to expand out of proportion, leading to abnormal skull shape.
Isolated sagittal synostosis (the suture that runs from the anterior fontanelle to the posterior fontanelle) occurs in about 55% of cases and isolated coronal suture synostosis (the suture that runs down laterally from the anterior fontanelle) occurs in 20% of cases.
Diagnosis of craniosynostosis is by xrays, and increasingly by CT scan. Early evaluation and possible neurosurgical intervention is imperative, since the synostosis may be a marker for a larger pattern of abnormalities, and if synostosis is left uncorrected, head and facial shape can be severely affected. There is also concern that some forms of synostosis can cause damage to the infant brain by constraint of normal brain growth.
A good set of diagrams is available here.