In Down's syndrome the sphenoid sinus is missing and the sphenoid body is hypoplastic. The absence of the frontal sinus results in a straight, infantile forehead. The cranial base is still nearly in a straight line, as in the foetal skull, but the angle to the horizontal is steeper. The anterior cavity of the cranium is shorter, steeper and on a higher plane than normal. There is absence of diploe in the vault bones, resulting in a thin, light skull.
Nasion is retracted and the maxilla is hypoplastic.