1. Thickening of the skull.
2. Premature fusion of the cranial sutures, usually occurring before birth, and resulting in deformity of the skull.

The fusion prevents the normal shape given to the cranium by the growth of the brain; therefore, fusion of the sagittal suture (line of union between the two parietal bones or side bones of the skull), the most common form, prevents lateral growth and allows compensatory expansion forward and backward, giving a long, narrow shape of the head, sometimes with a ridge at the metopic suture (persistent frontal suture or the seam or line at which two edges have been joined between the two halves of the frontal bone line (scaphocephaly).

Asymmetrical forms also occur.