Moebius syndrome

Definition

Polydactyly and syndactyly are congenital irregularities of the hands and feet. Polydactyly is the occurrence of extra fingers or toes, and syndactyly is the webbing or fusing together of two or more fingers or toes.

Description

Polydactyly can vary from an unnoticeable rudimentary finger or toe to fully developed extra digits.

Syndactyly also exhibits a large degree of variation. Digits can be partially fused or fused along their entire length. The fusion can be simple with the digits connected only by skin, or it can be complicated with shared bones, nerves, vessels, or nails.

Polydactyly and syndactyly can occur simultaneously when extra digits are fused. This condition is known as polysyndactyly.

Causes & symptoms

Polydactyly and syndactyly are due to errors in the process of fetal development. For example, syndactyly results from the failure of the programmed cell death that normally occurs between digits. Most often these errors are due to genetic defects.

Polydactyly and syndactyly can both occur by themselves as isolated conditions or in conjunction with other symptoms as one aspect of a multi-symptom disease. There are several forms of isolated syndactyly and several forms of isolated polydactyly; each of these, where the genetics is understood, is caused by an autosomal dominant gene. This means that since the gene is autosomal (not sex-linked), males and females are equally likely to inherit the trait. This also means that since the gene is dominant, children who have only one parent with the trait have a 50% chance of inheriting it. However, people in the same family carrying the same gene can have different degrees of polydactyly or syndactyly.

Polydactyly and syndactyly are also possible outcomes of a large number of rare inherited and developmental disorders. One or both of them can be present in over 100 different disorders where they are minor features compared to other characteristics of these diseases.

For example, polydactyly is a characteristic of Meckel syndrome and Laurence-Moon-Biedl syndrome. Polydactyly may also be present in Patau's syndrome, asphyxiating thoracic dystrophy, hereditary spherocytic hemolytic anemia, Moebius syndrome, VACTERL association, and Klippel-Trenaunay syndrome.

Syndactyly is a characteristic of Apert syndrome, Poland syndrome, Jarcho-Levin syndrome, oral-facial-digital syndrome, Pfeiffer syndrome, and Edwards syndrome. Syndactyly may also occur with Gordon syndrome, Fraser syndrome, Greig cephalopolysyndactyly, phenylketonuria, Saethre-Chotzen syndrome, Russell-Silver syndrome, and triploidy.

In some isolated cases of polydactyly or syndactyly, it is not possible to determine the cause. Some of these cases might nevertheless be due to genetic defects; sometimes there is too little information to demonstrate a genetic cause. Some cases might be due external factors like exposure to toxins or womb anomalies.

Diagnosis

Polydactyly and syndactyly can be diagnosed by external observation, x ray, and fetal sonogram.

Treatment

Polydactyly can be corrected by surgical removal of the extra digit or partial digit. Syndactyly can also be corrected surgically, usually with the addition of a skin graft from the groin.

Prognosis

The prognosis for isolated polydactyly and syndactyly is excellent. When polydactyly or syndactyly are part of a larger condition, the prognosis depends on the condition. Many of these conditions are quite serious, and early death may be the probable outcome.

Prevention

There is no known prevention for these conditions.

Key Terms

Autosomal chromosome

One of the non-X or non-Y chromosomes.

Congenital

A condition present at birth.

Digit

A finger or a toe.

Dominant trait

A genetic trait that will always express itself when present as one of a pair of genes (as opposed to a recessive trait where two copies of the gene are necessary to give the individual the trait).

Gene

A portion of a DNA molecule that either codes for a protein or RNA molecule or has a regulatory function.

Triploidy

The condition where an individual has three entire sets of chromosomes instead of the usual two.

Trisomy

An abnormal condition where three copies of one chromosome are present in the cells of an individual's body instead of two, the normal number.

Further Reading

For Your Information

Books

• Jones, Kenneth Lyons. Smith's Recognizable Patterns of Human Malformation. 5th ed. Philadelphia: W.B. Saunders, 1997.
• Rimoin, David L, J. Michael Connor, and Reed E. Pyeritz, eds. Emery and Rimoin's Principles and Practice of Medical Genetics. 3rd ed. New York: Churchill Livingstone, 1997.

  Organizations

• March of Dimes Birth Defects Foundation. 1275 Mamaroneck Avenue, White Plains, NY 10605. (888) 663-4637. http://www.modimes.org/
• NIH/National Institute of Child Health and Human Development. 9000 Rockville Pike, Building 31, Rm 2A32, MSC 2425, Bethesda MD 20892. (301) 496-5133. Fax: (301) 496-7101. http://www.nih.gov/nichd/

  Other

• OMIM Homepage, Online Mendelian Inheritance in Man. Searchable Database. http://www3.ncbi.nlm.nih.gov/Omim/ (19 June 1998).
• Mih, Alex D. and Gary Schnitz. Congenital Deformities of the Hand. 1997. http://www.indianahandcenter.com/htcong.html#polydactyly (19 June 1998).

Gale Encyclopedia of Medicine. Gale Research, 1999.