Crouzon syndrome and Apert syndrome are both craniosynostosis syndromes caused by a mutated FGFR2 gene. Apert syndrome is similar but more severe. With Apert syndrome, your baby has the craniofacial characteristics of Crouzon syndrome. In addition, they may have fused or webbed fingers and toes. They may have short fingers and broad thumbs and big toes. Intellectual disability is also more common in Apert syndrome.