Clinical Correlates

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Birth Defects Involving the Pharyngeal Region Ectopic Thymic and Parathyroid Tissue

Since glandular tissue derived from the pouches undergoes migration, it is not unusual for accessory glands or remnants of tissue to persist along the pathway. This is true particularly for thymic tissue, which may remain in the neck, and for the parathyroid glands. The inferior parathyroids are more variable in position than the superior ones and are sometimes found at the bifurcation of the common carotid artery.

Branchial Fistulas

Branchial fistulas occur when the second pharyngeal arch fails to grow cau-dally over the third and fourth arches, leaving remnants of the second, third, and fourth clefts in contact with the surface by a narrow canal (Fig. 15.14A). Such a fistula, found on the lateral aspect of the neck directly anterior to the sternocleidomastoid muscle, usually provides drainage for a lateral cervical cyst (Fig. 15.14B). These cysts, remnants of the cervical sinus, are most often just below the angle of the jaw (Fig. 15.15), although they may be found anywhere along the anterior border of the sternocleidomastoid muscle. Frequently a lateral cervical cyst is not visible at birth but becomes evident as it enlarges during childhood.

Internal branchial fistulas are rare; they occur when the cervical sinus is connected to the lumen of the pharynx by a small canal, which usually opens in the tonsillar region (Fig. 15.14C). Such a fistula results from a rupture of the membrane between the second pharyngeal cleft and pouch at some time during development.

Lateral Cervical Cyst

Figure 15.14 A. Lateral cervical cyst opening at the side of the neck byway of a fistula. B. Lateral cervical cysts and fistulas in front of the sternocleidomastoid muscle. Note also the region of preauricular fistulas. C. A lateral cervical cyst opening into the pharynx at the level of the palatine tonsil.

Figure 15.14 A. Lateral cervical cyst opening at the side of the neck byway of a fistula. B. Lateral cervical cysts and fistulas in front of the sternocleidomastoid muscle. Note also the region of preauricular fistulas. C. A lateral cervical cyst opening into the pharynx at the level of the palatine tonsil.

Preauricular Fistula
Figure 15.15 Patient with a lateral cervical cyst. These cysts are always on the lateral side of the neck in front of the sternocleidomastoid muscle. They commonly lie under the angle of the mandible and do not enlarge until later in life.

Neural Crest Cells and Craniofacial Defects

Neural crest cells (Fig. 15.2) are essential for formation of much of the cran-iofacial region. Consequently, disruption of crest cell development results in severe craniofacial malformations. Since crest cells also contribute to the conotruncal endocardial cushions, which septate the outflow tract of the heart into pulmonary and aortic channels, many infants with craniofacial defects also have cardiac abnormalities, including persistent truncus arteriosus, tetralogy of Fallot, and transposition of the great vessels. Unfortunately, crest cells appear to be a particularly vulnerable cell population and are easily killed by compounds such as alcohol and retinoic acid. One reason for this vulnerability may be that they are deficient in superoxide dismutase (SOD) and catalase enzymes that are responsible for scavenging free radicals that damage cells. Examples of craniofacial defects involving crest cells include the following:

Treacher Collins syndrome (mandibulofacial dysostosis) is characterized by malar hypoplasia due to underdevelopment of the zygomatic bones, mandibular hypoplasia, down-slanting palpebral fissures, lower eyelid colobo-mas, and malformed external ears (Fig. 15.16A). Treacher Collins is inherited as an autosomal dominant trait, with 60% arising as new mutations. However, phenocopies can be produced in laboratory animals following exposure to teratogenic doses of retinoic acid, suggesting that some cases in humans may be caused by teratogens.

Robin sequence may occur independently or in association with other syndromes and malformations. Like Treacher Collins syndrome, Robin sequence alters first-arch structures, with development of the mandible most severely affected. Infants usually have a triad of micrognathia, cleft palate, and glossoptosis (posteriorly placed tongue) (Fig. 15.16B). Robin sequence may be due to genetic and/or environmental factors. It may also occur as a deformation, as for example when the chin is compressed against the chest in cases of oligohydramnios. The primary defect includes poor growth of the mandible and, as a result, a posteriorly placed tongue that fails to drop from between the palatal shelves, preventing their fusion. Robin sequence occurs in approximately 1/8500 births.

DiGeorge anomaly occurs in approximately 1 in 2000 to 3000 births and represents the most severe example of a collection of disorders that also includes velocardiofacial syndrome (VCFS) and conotruncal anomalies face syndrome (Fig. 15.16C). All of these disorders are part of a spectrum called CATCH22 because they include cardiac defects, abnormal facies, thymic hy-poplasia, cleft palate, and hypocalcemia and are a result of a deletion on the long arm of chromosome 22 (22q11). Patients with complete DiGeorge anomaly have immunological deficiencies, hypocalcemia, and a poor prognosis. Origin of the defects is caused by abnormal development of neural crest cells that contribute to formation of all of the affected structures. In addition

Craniofacial Abnormalities Pictures

Figure 15.16 Patients with craniofacial defects thought to arise from insults to neural crest cells. A. Treacher Collins syndrome (mandibulofacial dysostosis). Note underdevelopment of the zygomatic bones, small mandible, and malformed ears. B. Robin sequence. Note the very small mandible (micrognathia). C. DiGeorge anomaly. In addition to craniofacial defects, such as hypertelorism and microstomia, these individuals have partial or complete absence of the thymus. D. Hemifacial microsomia (oculoauriculover-tebral spectrum, or Goldenhar syndrome).

Figure 15.16 Patients with craniofacial defects thought to arise from insults to neural crest cells. A. Treacher Collins syndrome (mandibulofacial dysostosis). Note underdevelopment of the zygomatic bones, small mandible, and malformed ears. B. Robin sequence. Note the very small mandible (micrognathia). C. DiGeorge anomaly. In addition to craniofacial defects, such as hypertelorism and microstomia, these individuals have partial or complete absence of the thymus. D. Hemifacial microsomia (oculoauriculover-tebral spectrum, or Goldenhar syndrome).

to genetic causes, exposure to retinoids (vitamin A), alcohol, and maternal diabetes can produce the defects.

Hemifacial microsomia (oculoauriculovertebral spectrum, Goldenhar syndrome) includes a number of craniofacial abnormalities that usually involve the maxillary, temporal, and zygomatic bones, which are small and flat. Ear (anotia, microtia), eye (tumors and dermoids in the eyeball), and vertebral (fused and hemivertebrae, spina bifida) defects are commonly observed in these patients (Fig. 15.16D). Asymmetry is present in 65% of the cases, which occur in 1/5600 births. Other malformations, which occur in 50% of cases, include cardiac abnormalities, such as tetralogy of Fallot and ventricular septal defects. Causes of hemifacial microsomia are unknown.

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Responses

  • lily
    Who discovered treacher collins syndrome?
    8 years ago
  • Alan
    Why are inferior parathyroids are more variable in position?
    8 years ago
  • Tracie
    Can branchial fistulas kill a person?
    8 years ago

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