Cleft lip is caused by failure of the the paired maxillary prominences to fuse with the medial nasal process.
Cleft lip is one of the most common congenital craniofacial deformities, and occurs both sporadically and in the context of a syndrome. Cleft lip develops in the first 6 weeks of embryogenesis, while cleft palate develops in weeks 7-12. Prenatal ultrasonography can usually detect cleft lip but will often miss cleft palate. Cleft lip contributes substantially to difficulty with breastfeeding, therefore it is surgically corrected within the first 3 months of life.
Shkoukani et al. provide a comprehensive review of the epidemiology of cleft lip. They suggest that cleft lip occurs most commonly in Asians and Native Americans, and most infrequently in Africans, with a male:female ratio of 2:1. The overall incidence of cleft lip ranges from 0.2 to 2.3 per 1000 births.
Dixon et al. review the genetic and environmental influences contributing to cleft lip and palate. They state that the exact etiology of non-syndromic/isolated forms of the disease is unknown, but twin studies suggest a genetic component. A few genes have been implicated, including transforming growth factor alpha and fibroblast growth factor receptor 2. It is likely that the etiology of cleft lip and palate is multifactorial with genetically susceptible individuals being exposed to environmental factors causing development of the anomaly.
Figure A shows a child with unilateral complete cleft lip. Illustration A reviews the embryologic development of the face, and the fusion of the maxillary and nasal prominences. Illustration B displays the different forms of cleft lip and palate.
Answers 1,4: Both of these result in cleft palate.
Answers 3,5: Both are not associated with known congenital abnormalities.
Shkoukani MA, Chen M, Vong A. Cleft Lip - A Comprehensive Review. Front Pediatr. 2013 Dec 27;1:53.
PMID:24400297 (Link to Abstract)
Dixon MJ, Marazita ML, Beaty TH, Murray JC. Cleft lip and palate: understanding genetic and environmental influences. Nat Rev Genet. 2011 Mar;12(3):167-78.
PMID:21331089 (Link to Abstract)