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Micrognathia (very small jaw) and Pierre Robin Sequence

Clinical types

Some cases of micrognathia, usually when it is an isolated finding, are transmitted by autosomal dominant inheritance. In Pierre Robin sequence, because there is a very small jaw the tongue is retracted and this interferes with the closure of the palate, resulting in a cleft palate in the newborn. Most cases are sporadic, but hereditary cases have been described (see under inheritance pattern). There are many syndromes featuring micrognathia and/or cleft palate: Wanger syndrome, Stickler syndrome, chromosomal syndromes of various types, etc.

Clinical signs

In most cases of autosomal dominant micrognathia when this is an isolated finding the problem is mainly esthetic. In addition, however, there is a problem with the occlusion of the teeth in that the upper teeth protrude compared to the lower ones. This requires orthodontic treatment and occasionally extension of the mandible (lower jaw). In exceptional cases in the family, there may be more severe micrognathia causing the Pierre Robin sequence.

In the Pierre Robin sequence, because there is a very small jaw the tongue is retracted and this interferes with the closure of the palate, resulting in a cleft palate in the newborn. In most of these cases, the micrognathia is severe. Sometimes polyhydramnios resulting from blockage of the digestive tract may occur. After birth, it is common to find problems with swallowing and also choking. It is necessary to feed the infant with a special bottle, which can be filled with the mother's milk to allow breastfeeding. Ear infections that may affect hearing are common in these children unless they are properly treated. In some cases, breathing problems are severe enough to require intubation to bypass the obstructing tongue.

In some studies, mental retardation has been reported in about 15% of cases, most of which appear to result from problems of oxygenation during breathing immediately after birth, because at that point in time the presence of the defect in the fetus was not known. Heart defects are present in about 15% of cases. Some studies have reported a higher frequency in males.

There are many syndromes of which micrognathia is a part. These include trisomy 13 and trisomy 18.

Inheritance pattern

Isolated (non-syndromic) micrognathia is transmitted by autosomal dominant inheritance.

Regarding the Pierre Robin sequence, most cases are sporadic, but individual families with autosomal recessive inheritance and others with X-linked inheritance have been described. In many hereditary cases there may be other defects, especially in the digits and heart, and also clubfoot.

In the various syndromes, the pattern of inheritance depends on the specific syndrome involved.

Penetrance

Not known.

Associated features that can be demonstrated by imagery

Heart defects. Other defects that can indicate a syndrome featuring micrognathia should be looked for. Cleft palate can be demonstrated reliably in utero by ultrasound examination.

What is the risk of recurrence in a subsequent pregnancy?

If one of the parents has micrognathia, the risk for recurrence is 50% in every pregnancy. Regarding the Pierre Robin sequence, parents who have had a child with this defect usually have a small risk of having another affected child. However, in families with a number of cases, the risk is 50%.

In the various syndromes - depends on the specific syndrome involved.

Molecular genetic information

The gene for the disease

Genes associated with micrognathia and the Pierre Robin sequence have not yet been identified. In all cases, a blood chromosome test (karyotype) should be performed to look for a chromosomal problem, which is present in about 25% of cases.

Location

Not known

Genetic testing

Diagnostic testing

There are to date no clinical genetic tests for diagnosing this condition. Only in families with multiple cases is it possible to carry out investigations that may lead to the possibility of performing practical tests in some families in the future, initially by indirect tests (linkage analysis), and in the more distant future, once the gene itself has been found, by identifying the mutation or mutations. This will be done in cooperation with a genetic institute. See information sheet: Indirect testing for genetic markers in a family that has one or several patients- when there are a number of different genes that can each cause the disease- the gene, or most of the genes not having been located / identified / mapped- multifactorial diseases.

Carrier testing

This cannot usually be performed. See above under diagnostic test.

Fetal testing

This cannot usually be performed. See above under diagnostic test. Ultrasound detection as stated above is possible.
 
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