Development of the face and cranium during embryogenesis is a complex and orchestrated process that involves cellular proliferation, differentiation, migration, and selective apoptosis. Facial clefts and holoprosencephaly (HPE) are conditions caused by the disruption of these normal embryonic processes.
Facial clefts are deformations of the face and/or cranium caused by relative excesses or deficits of tissue along linear anatomic planes [1,2]. Although the exact incidence of facial clefts is unknown, they are estimated to affect 1.4 to 4.9 newborns per 100,000 live births [1,3,4].
Embryology — More than one theory exists regarding the embryologic pathogenesis of facial clefts. Fetal craniofacial development is a complex series of events that occurs between the third and eighth weeks of gestation. During the initial stages, five facial processes (one frontal, two maxillary, two mandibular) form and subsequently fuse (by the sixth week of gestation) to form the human face. The classic theory states that facial clefting occurs when the fusion process is disrupted. However, other theories propose that the pathogenesis is related to the infarction of primordial blood vessels, amniotic bands, failure of certain developmental zones of the face to develop completely, or errors in cellular migration, penetration, and differentiation .
Etiology — Although the precise etiology is unknown, multiple genetic and environmental factors may be involved. The environmental risk factors implicated include antenatal exposure to radiation, viral infections, metabolic abnormalities, and teratogenic compounds .
Classification of clefts — Paul Tessier, the father of modern craniofacial surgery, introduced a classification system to describe the anatomic planes along which facial clefts fall . The classification system numbers clefts based upon their location surrounding the orbit. Numbers 0 through 7 run inferior to the orbit (facial clefts) from medial to lateral, number 8 runs on the lateral portion of the orbit, and numbers 9 through 14 run superior to the orbit from lateral to medial (cranial clefts) (figure 1).