As the brain grows during fetal life (in the womb) and childhood, the overlying skull also enlarges by adding new bone at these sutures. Craniosynostosis causes distortion of the shape of the skull due to both to failure of bone growth at the prematurely closed suture, and to compensatory overgrowth at the sutures that remain open.
The different types of craniosynostosis are classified by which sutures have closed prematurely:
- sagittal synostosis - which gives a long, narrow head
- coronal synostosis - may affect one side (unilateral) or both sides (bilateral) and gives a broad, flat head that is asymmetric in unilateral cases
- metopic synostosis - causes a triangular-shaped forehead.
- lambdoid synostosis - causes flattening at the back of the head.
The remaining 20 per cent of craniosynostosis is more complex and either involves the fusion of multiple sutures, and/or is combined with additional changes in the face, limbs or other parts of the body, indicating a syndrome.
Over 100 craniosynostosis syndromes have been described - the most common are Crouzon, Pfeiffer, Apert, Muenke, Saethre-Chotzen and craniofrontonasal syndromes, together with TCF12- and ERF-related craniosynostosis.