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The Practical Guide to the Genetic Family History - Bennett L.R.

Bennett L.R. The Practical Guide to the Genetic Family History - Wiley-liss, 1999. - 265 p.
ISBN 0-471-22391-3
Download (direct link): thepracticalguidetot1999.pdf
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Deformation refers to an abnormal shape or position of a part of the body caused by mechanical forces in utero. Examples of deformations include limb positioning defects, such as clubfoot or congenital hip dislocation, and minor facial deformities, such as a small chin (micrognathia) or facial asymmetry. Deformations usually occur during the third trimester, and often represent intrauterine molding from mechanical constraint (e.g., breech presentation, decreased or lack of amniotic fluid, multiple births). Most deformations due to these factors spontaneously correct themselves because the infant is no longer subjected to intrauterine constraints. However, deformations secondary to an intrinsic cause
(such as abnormal formation of the central nervous system, renal dysfunction, or neuromuscular dysfunction) are associated with neonatal morbidity. These intrinsic factors often have a genetic etiology such as hereditary neuropathies and myopathies or renal malformations.
Disruptions are the result of interference with an originally normal development process. There is extensive clinical variability in disruptions. Examples of a disruption include digit amputation or facial clefting from amniotic bands. Structural abnormalities due to disruptions often have a vascular etiology such as the rare occurrence of limb reduction anomalies following chorionic villus sampling. Maternal factors such as infections and teratogens can be at the root of birth defects from disruptions. The clinician should obtain a detailed pregnancy history regarding the mother of the affected child. For congenital anomalies due to disruptions, the recurrence risk for the parents to have another affected child is usually small.
Although the separation of birth anomalies into the singular categories of malformations, deformations, and disruptions is a valuable clinical tool for determining the etiology of birth defects, the three categories are interrelated. A single extraneous variable can have different physical effects. Decreased amniotic fluid (oligohydramnios) in the third trimester can result in minor deformations (e.g., clubfoot, micrognathia), whereas oligohydramnios in early embryologic development can be related to the disruptive limb-body wall complex (thoracoabdominal wall deficiency with craniofacial anomalies). Micrognathia caused by intrauterine constraint in early fetal development can lead to failure of the tongue to descend resulting in a developmental malformation—cleft palate. The malformation spina bifida may produce leg paralysis leading to the deformations of congential hip dislocation and clubfoot (Cohen, 1997).
Table 4.4 summarizes the general medical-family history questions to pose when there is a family history of one or more birth defects. The medical-family history inquiry is similar for any history of birth anomalies. Brief discussions on three categories of common malformations (cleft lip with and without cleft palate, neural tube defects and congenital heart defects) follow.
Cleft Lip with and without Cleft Palate
Cleft palate (CP) is a different condition from cleft lip with or without cleft palate (CL/P). The formation of the palate and lips do not occur at the same time in em-bryologic development; consequently, CP and CL/P are associated with different genetic risks (Robinson and Linden, 1993). Associated birth anomalies occur in a significant number of individuals with isolated cleft palate (40%) (Khoury et al.,
1997). Between 7 and 13% of individuals with cleft lip are born with associated birth anomalies, as are 11-14% of individuals with both cleft lip and palate (Robinson and Linden, 1993). More than 300 syndromes are associated with cleft lip and palate (Cohen, 1997). Chromosome anomalies, particularly trisomy 13 and trisomy 18, are common causes of CL/P and cleft palate. A medical geneticist should evaluate newborns with a clefting condition to see if a syndrome can be identified. Likewise, individuals with a clefting condition who are interested in genetic risk assess-
TABLE 4.4 Medical-Family History Questions for Congenital Anomalies
Inquiries Related to the Child/Adult With a Birth Variant or Anomaly
• Does the child/adult have:
Other birth anomalies (particularly of the hands and feet)? Explain.
Anything unusual about his or her facial appearance, such as unusual placement or appearance of the eyes, nose, mouth, or ears?
Any birthmarks? If yes, describe their color, number, shape, size, and location.
Any hearing problems? (If yes, see Section 4.3)
Any visual problems? (If yes, see Section 4.4)
Any learning disabilities or problems with schooling? (If yes, see Section 4.5)
Any delays in achieving developmental milestones?
Any medical problems, particularly neurological or muscle weakness? Explain.
• Does he or she resemble other family members in appearance?
• Is this person of normal stature? Are the limbs in proportion? (If not, see Section 4.14)
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