Pfeiffer syndrome is a genetic disorder characterized by the premature fusion of certain bones of the skull (craniosynostosis), which prevents further growth of the skull and affects the shape of the head and face. In addition, the thumbs and big toes are broader and often shorter than normal. Many of the characteristic facial features of Pfeiffer syndrome result from the premature fusion of the skull bones. The head is unable to grow normally, which leads to bulging and wide-set eyes, an underdeveloped upper jaw, and a beaked nose. About 50 percent of children with Pfeiffer syndrome have hearing loss, and dental problems are also common. Additionally, the thumbs and big toes are broader than normal and bend away from the other digits. Unusually short fingers and toes (brachydactyly) are also common, and there may be some webbing or fusion between the digits (syndactyly). Pfeiffer syndrome is divided into three subtypes. Type 1 or "classic" Pfeiffer syndrome has symptoms as described above. Most individuals with type 1 have normal intelligence and a normal life span. Types 2 and 3 are more severe forms of Pfeiffer syndrome, often involving problems with the nervous system. Type 2 is distinguished from type 3 by more extensive fusion of bones in the skull, leading to a "cloverleaf" shaped head.