In large series, the distribution of clefts is about 50% cleft lip and palate, 30% cleft palate only, and 20% cleft lip only. Cleft lip occurs most often on the left side; the distribution of left to right to bilateral cleft lip is approximately 6:3:1. Right-sided clefts are more commonly associated with syndromes. There is a slightly higher incidence in males.
Modern ultrasound can identify cleft lip by the absence of muscle fibers crossing the lip. Specific efforts must be made to obtain a frontal view to make a prenatal diagnosis. Newer ultrasounds have increasing accuracy. Although fetal surgery for clefts is not yet feasible in humans, prenatal diagnosis makes it possible to counsel parents earlier and prepare them for the care that their new child will require (Figure 20–1).
(A) Ultrasound of a child with bilateral cleft, incomplete on the left. (B) Photo of the same child postnatally before lip repair.
It is important to remember the embryology of clefting; the primary palate includes the lip and premaxilla, whereas the secondary palate extends from the incisive foramen back. The lip and alveolus are formed by the fusion of the frontonasal process and the lateral maxillary processes; this fusion is reinforced by the migration of mesenchymal tissue derived from neuroectoderm (Figure 20–2). The stabilization of neuroectoderm by folate during the first trimester of pregnancy has been shown to reduce the incidence of clefting as well as that of other neural crest defects such as myelomeningocele.
Diagram of a 6-week-old embryo. The frontonasal process will give rise to the central lip and premaxilla, the lateral nasal process will develop into the alae of the nose, and the maxillary processes will produce the lateral lip and maxillary segments.
An understanding of the anatomic derangements is critical to proper repair. In the cleft lip, the orbicularis oris muscle is interrupted, and the remnants of the muscle adjacent to the cleft flow toward the upper portion of the cleft, at the base of the columella medially and at the alar base laterally. Incomplete clefts have variable amounts of muscle intact across the upper portion of the lip. In bilateral complete clefts, there is no muscle in the central portion (the prolabium).
Normally, the levator palatini muscle forms a sling that elevates the soft palate and excludes the nasopharynx from the oropharynx during speech and swallowing. In the cleft palate, the levator muscle is oriented longitudinally, parallel with the cleft margin. This abnormal orientation of the muscle is even seen in submucous cleft palate, when the mucosa is intact (Figure 20–3). The most ...