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Syndactyly (Fusion of Digits)


Types

Syndactyly can occur with or without polydactyly (supernumerary digits). The fusion can involve the skin between the digits or the entire digit, including fusion of the bones, which is demonstrable on X-ray, when the fused digit appears wider than normal. The fusion can occur only in the proximal parts of the digits involved or along their entire length. The fusion can include only two digits or all the digits of that limb, and can involve more than one limb.

The existence of other defects or findings and additional cases in the family, as specified below, is also important.

Clinical signs and inheritance pattern

The most common type of fusion is between the second and third toes. Usually only some of the skin between the two toes is fused, and this does not constitute a medical problem. This is hereditary and is very common.

Medically important genetic syndromes involving syndactyly

  • In some cases there is only digital involvement without any associated problems- the most frequent of these is multi-digit syndactyly. Usually this involves the third and fourth fingers and the second and third toes. However, there may be other types featuring involvement of other digits, with or without a supernumerary fourth digit. In the more severe types, there is involvement of more digits and/or greater fusion. Altogether, there are at least five types that can be differentiated by the distribution of fusion of the digits. Essentially, the milder types are transmitted by autosomal dominant inheritance. Fusion of the fourth and fifth metacarpal bones in the hand is transmitted by X-linked inheritance. The more severe type involving fusion of all the digits is transmitted by autosomal recessive inheritance.

  • Syndactyly can occur together with polydactyly (supernumerary digits). Apart from the syndactyly and the polydactyly, there are no other defects, although there may be variations in the structure of the earlobe. This condition is known as poly-syndactyly (see information sheet on polydactyly - supernumerary digits). In these cases, the pattern of inheritance is usually autosomal dominant. If the same condition is present in one of the parents, or in other relatives, this supports the autosomal dominant pattern of inheritance. Sometimes it only becomes apparent that an individual is affected after he or she has undergone a thorough examination in order to look for the condition. Without asking questions and making an in-depth investigation of the family, it is not always possible to know this, as some cases are expressed very mildly and other relatives may not be aware of them. If the same condition is found in other family members, including in one of the parents, it can be assumed that this is isolated (non-syndromic) poly-syndactyly as long as the affected individuals in the family have no other additional problems.

  • Syndromic type - there are more than 150 syndromes involving other body systems. This is a heterogeneous group of disorders, including chromosome disorders and other syndromes resulting from a mutation in a single gene with varying patterns of inheritance. These syndromes may involve defects in the eye structure, small head circumference with mental retardation, cleft palate and lip, etc.

Penetrance

Penetrance is not full. In other words, there may be instances where members of a family with many cases of syndactyly are born with normal digits, but will give birth to children with abnormal digits - these family members carry the abnormal gene for the condition but do not express it. In some families there may be some children with syndactyly, with or without polydactyly, restricted to a single limb, and others where all four limbs are involved.

Associated features that can be identified by imagery

In a certain percentage of cases there may be additional problems, and defects in the eye structure, small head circumference (usually associated with a relatively high risk for mental retardation), cleft palate and lip, etc., must be looked for on ultrasound examination.

If other problems are identified, this is not isolated (non-syndromic) syndactyly, but a genetic syndrome of which the digit anomaly is only a part.

Some syndromes include structural defects detectable by ultrasound examination. Only a relatively small number of these syndromes feature only functional problems such as mental retardation, etc. without defects or changes in the body structure that are detectable by ultrasound examination.
Therefore, when syndactyly is detected, further testing is very important in order to confirm or rule out the presence of a syndrome, and performing amniocentesis to confirm or rule out a chromosomal problem should also be considered. It is also necessary to check for other defects in the digits such as polydactyly, shortening of other digits, etc., and also in other body organs. An ultrasound examination that gives a normal result apart from the syndactyly indicates a significantly reduced risk of a genetic syndrome, but there is still a risk that is slightly higher than the regular risk for defects and mental retardation (2- 4%). Thus there may be syndromes in which the only additional feature apart from the syndactyly is mental retardation, although the likelihood of this is not high.

What is the risk of recurrence in a subsequent pregnancy?

In isolated (non-syndromic) cases
If this is the first case in the family and is mild, with involvement of only some digits, with or without polydactyly and without other defects, there is a high probability that the condition in the child/fetus is a result of a new mutation in the gene for syndactyly- i.e. the hereditary transmission starts with the child/fetus, and there is a 50% chance in every pregnancy that he or she will transmit the abnormal gene to his or her offspring. On the other hand, in these cases, the parents and other family members who are not offspring of the patient apparently have no increased risk for having another affected child. It must always be taken into account that some cases are transmitted by autosomal recessive or X-linked inheritance, in which case the risk of recurrence in the parents and in other relatives is relatively high. Therefore it is important to establish the exact diagnosis and the risks for recurrence within genetic counseling.

If there are many affected individuals in the family
The situation is different if there are already other cases in the family, because here one of the parents of each affected individual also carries the abnormal gene for syndactyly or poly-syndactyly, and the risk of transmitting this to the offspring is 50% in every pregnancy, although it may not always be expressed. This risk also applies to more distant relatives who carry the same abnormal gene. Because the gene is not always expressed, one of the parents may carry it without having a defect in his or her digits.

In severe cases of the syndromic type
In these cases, the exact risk for recurrence must be evaluated. This varies depending on which syndrome the patient has. In some cases the condition is sporadic and occurs as a result of a new mutation, and in others it is hereditary, with a recurrence risk of 25% in every pregnancy for syndromes transmitted by autosomal recessive inheritance.

Molecular genetic information

The gene for the disease
Because there is genetic heterogeneity, it is necessary first to establish the exact diagnosis:

For the type involving a fusion of digits 4-5, the gene has been mapped to chromosome 6q22-q24. The gene responsible is the gene causing oculodentodigital syndrome, which also involves the eyes and teeth.

The gene for the type involving fingers 3-4 and the fifth toe has been mapped to chromosome 2q31-q32 and is called HOXD13.

The gene for the type involving fusion of the base of fingers 4-5 (not the fingers themselves), which is transmitted by X-linked inheritance, has been located, and is called MF4.

There are probably other genes that have not yet been found and that are responsible for other types.

Locations
As stated above by type.

Genetic testing

Diagnostic testing
At the present time there is no simple genetic test for diagnosing the mutations associated with the non-syndromic types of syndactyly. However, when the type is clinically obvious and the gene has been identified, an attempt can be made to identify the mutation.

In very large families it is possible to perform linkage analysis (see: indirect testing for genetic markers in a family with one or more patients in the family - when there are a number of different genes that can each cause the disease - the gene or most of the genes have not yet been located / identified / mapped - autosomal dominant disease).

Carrier testing
The test for detecting a carrier of an abnormal gene in an autosomal dominant disease where the condition is not clinically expressed is in fact the same as a diagnostic test. In the recessive types, there are a very few syndromes involving syndactyly where the gene has been found, and carrier status can be tested for in these families. However, this is not usually necessary.

Fetal testing
When syndactyly is detected in pregnancy, where there are no other known cases of syndactyly in the family, it cannot be determined definitively that this is the non-syndromic type. An attempt can be made to rule out some of the differential diagnoses, mainly by ultrasound examination and by ruling out the presence of other defects.
 
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