Biology in Medicine

Figure 14-1 Polydactyly and syndactyly malformations. A, Insertional polydactyly. This patient has heptadactyly with insertion of a digit in the central ray of the hand and a supernumerary postaxial digit. This malformation is typically associated with metacarpal fusion of the third and fourth digits. Insertional polydactyly is common in patients with Pallister-Hall syndrome. B, Postaxial polydactyly with severe cutaneous syndactyly of digits two through five. This type of malformation is seen in patients with Greig cephalopolysyndactyly syndrome. See Sources & Acknowledgments.

In contrast to malformations, deformations are caused by extrinsic factors impinging physically on the fetus during development. They are especially common during the second trimester of development when the fetus is constrained within the amniotic sac and uterus. For example, contractions of the joints of the extremities, known as arthrogryposes, in combination with deformation of the developing skull, occasionally accompany constraint of the fetus due to twin or triplet gestations or prolonged leakage of amniotic fluid (Fig. 14-2). Most deformations apparent at birth either resolve spontaneously or can be treated by external fixation devices to reverse the effects of the instigating cause.


Figure 14-2 Deformation known as congenital arthrogryposis seen with a condition referred to as amyoplasia. There are multiple, symmetrical joint contractures due to abnormal muscle development caused by severe fetal constraint in a pregnancy complicated by oligohydramnios. Intelligence is generally normal, and orthopedic rehabilitation is often successful. See Sources & Acknowledgments.

Disruptions, the third category of birth defect, result from destruction of irreplaceable normal fetal tissue. Disruptions are more difficult to treat than deformations because they involve actual loss of normal tissue. Disruptions may be the result of vascular insufficiency, trauma, or teratogens. One example is amnion disruption, the partial amputation of a fetal limb associated with strands of amniotic tissue. Amnion disruption is often recognized clinically by the presence of partial and irregular digit amputations in conjunction with constriction rings (Fig. 14-3).


Figure 14-3 Disruption of limb development associated with amniotic bands. This 26-week fetus shows nearly complete disruption of the thumb with only a nubbin remaining. The third and fifth fingers have constriction rings of the middle and distal phalanges, respectively. The fourth digit is amputated distally with a small fragment of amnion attached to the tip. See Sources & Acknowledgments.

The pathophysiological concepts of malformations, deformations, and disruptions are useful clinical guides to the recognition, diagnosis, and treatment of birth defects, but they sometimes overlap. For example, vascular malformations may lead to disruption of distal structures, and urogenital malformations that cause oligohydramnios can cause fetal deformations. Thus a given constellation of birth defects in an individual may represent combinations of malformations, deformations, and disruptions.

Genetic, Genomic, and Environmental Causes of Malformations


Figure 14-4 The relative contribution of single-gene defects, chromosome abnormalities, copy number variants, multifactorial traits, and teratogens to birth defects.

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Nov 27, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Biology in Medicine

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