How to determine the causes and recurrence risks of congenital anomalies - a clinical approach

Professor P A Farndon

There are three main types of abnormal morphogenesis - malformations, deformations and disruptions (table 3).

A malformation is caused by an abnormality of morphogenesis due to an intrinsic problem within the developing structure.

Underlying mechanisms include altered tissue formation, growth or differentiation due to genetic, environmental or a combination of factors.

Examples include spina bifida, cleft lip/palate, congenital heart defect, and neural tube defects.

A deformation is an abnormality of morphogenesis caused by extrinsic force on a normally developing or developed structure.

Deformations usually occur in late fetal life and are caused by lack of fetal movement through mechanical, malformational or functional factors.

Examples include craniofacial asymmetry, arthrogryposis and talipes. Mechanical causes include uterine anomalies and abnormal fetal positions. Malformations such as spina bifida can cause intrinsic lack of movement (leading to a deformation - talipes, for instance); renal agenesis results in a lack of amniotic fluid which constrains movement giving a "squashed appearance". Functional disorders include fetal neuromuscular disorders (neonatal myotonic dystrophy, spinal muscular atrophy).

A disruption is due to a destructive force acting upon an otherwise normal developing structure. Anomalies caused by disruptive forces can present a particularly distinctive appearance because of the loss of tissue and aberrant differentiation of adjacent tissues with which adhesions may have developed.

The mechanisms include cell death or tissue destruction due to vascular anomalies, anoxia, teratogens, infections, or mechanical forces. Examples include some cases of facial clefts and missing digits or limbs.

"Amniotic bands" encircling a limb are thought to be one possible mechanism (as shown in the diagram)