Supernumerary Digit 

Updated: Feb 13, 2019
Author: Luke Lennox, MD; Chief Editor: Dirk M Elston, MD 

Overview

Background

Polydactyly is the most common congenital digital anomaly of the hand and foot. It may appear in isolation or in association with other birth defects. Isolated polydactyly is often autosomal dominant, while syndromic polydactyly is often autosomal recessive.[1]  See the image below.

Supernumerary digit, no magnification. Supernumerary digit, no magnification.

Pathophysiology

Early theories for polydactyly concerned disorders in the programmed cell death cycle of fetal limb development. Current theories focus on mutations in specific genetic locations that cause limb development to go awry.[2]

Many specific mutations have been linked to polydactyly; however, a molecular etiology has not been found in a third of human disorders associated with polydactyly.[3]

Mammals have been shown to have genetic clusters identified as homeobox or Hox genes corresponding to five domains across the limb bud. According to Muragaki et al,[4] mutations in the HOXD13 gene are associated with synpolydactyly.

The steps in limb development and outgrowth are controlled by at least two described signal centers, as follows[3] :

  1. Zone of polarizing activity (ZPA) - Sonic Hedgehog is a molecule found to mediate ZPA activity

  2. Apical ectodermal ridge (AER) - Expresses fibroblast growth factors

As limb growth in utero progresses along a preset timeline, elongation of the limb, development of soft tissue, and differentiation of digits progresses.

The Medscape Genomic Medicine Research Center may be of interest.

Epidemiology

Frequency

United States

Despite being a common malformation, the true incidence of polydactyly is not fully known. The epidemiologic data on polydactyly is limited because most birth defect registries do not include the entity. One study by Finley et al[5] combined data from Jefferson County, Alabama and Uppsala County, Sweden. This study showed incidence of all types of polydactyly to be 2.3 per 1000 in white males, 0.6 per 1000 in white females, 13.5 per 1000 in black males, and 11.1 per 1000 in black females. The Swedish data alone showed polydactyly of all types to have an incidence of 1.0 per 1000, equally distributed between males and females.

International

Genetic and ethnic factors seem to influence the nature of polydactyly in the world population. Unfortunately, the knowledge of polydactyly is limited to select studies of certain subpopulations. Preaxial polydactyly is very common in various Asian populations, accounting for 90% of cases in South China, Hong Kong, and Japan. Duplicated thumbs at the metacarpophalangeal level are the usual anomaly.[6, 7]

Race

Postaxial hand polydactyly is a common isolated disorder in black children, and autosomal dominant transmission is suspected. In contrast, postaxial polydactyly seen in white children is usually syndromic and associated with an autosomal recessive transmission. Postaxial polydactyly is approximately 10 times more frequent in blacks than in whites and is more frequent in male children. 

Other factors associated with postaxial hand polydactyly include twinning, low maternal education, parental consanguinity, occurrence in first-degree relatives, and maternal bleeding in the first trimester.

 

Presentation

History

Polydactyly is the most common congenital digital anomaly of the hand and foot. It may appear in isolation or in association with other birth defects.[8]

Physical Examination

Temtamy and McKusick[9] classified polydactyly into preaxial, central, and postaxial types. Preaxial polydactyly refers to the duplication of the first digit. Central polydactyly involves duplication of the second, third, or fourth digit. Postaxial polydactyly involves the fifth digit. The terms radial, central, and ulnar can be used to describe polydactyly in the upper extremity. Similarly, the terms tibial, central, and fibular can be used to describe polydactyly in the lower extremity.

The association of hand and foot polydactyly is uncommon. If hand and foot polydactyly does occur, it may be a case of "crossed polydactyly"—when preaxial involement of the hand and postaxial involvement of the foot occurs or vice versa.

Synpolydactyly is the combination of syndactyly and polydactyly.[10, 11]

 

DDx

Diagnostic Considerations

Polydactyly is not often associated with congenital anomalies, except in recognizable syndromes. In one study by Castilla et al, trisomy 13, Meckel syndrome, and Down syndrome accounted for most cases of polydactyly.[12] Down syndrome was associated strongly with duplication of the first digit.

Many other syndromes have been identified to include polydactyly as an associated clinical manifestations, including the following:

  • Acrocallosal syndrome
  • Basal cell nevus syndrome
  • Bardet-Biedl syndrome
  • Biemond syndrome
  • Ectrodactyly-ectodermal dysplasias-cleft lip/palate syndrome
  • Ellis van Creveld syndrome
  • McKusick-Kaufman syndrome
  • Mirror hand deformity (ulnar dimelia) [13]
  • Mohr syndrome
  • Oral-facial-digital syndrome
  • Pallister-Hall syndrome
  • Rubinstein-Taybi syndrome
  • Short rib polydactyly
  • VATER association (vertebral [defects], [imperforate] anus, tracheoesophageal [fistula], radial and renal [dysplasia])

Differential Diagnoses

 

Workup

Imaging Studies

Prenatal ultrasound

Fetal finger buds can be seen using transvaginal ultrasound as early as 9 weeks and reliably by 13 weeks of pregnancy. Once polydactyly is established, a thorough ultrasound evaluation, especially of the heart, nervous system, limbs, and kidneys, to identify an associated syndrome (eg, Meckel-Gruber syndrome, trisomy 13) should be performed. Follow-up ultrasound between 17 and 34 weeks with biometric profile is recommended to establish the diagnosis of isolated polydactyly.[14, 15]

Radiographs

Radiographs of the affected limb are recommended to show whether the rudimentary digit contains skeletal elements. The degree of deviation of the digit and the size of the articulating metacarpal or metatarsal also may be helpful in surgical planning.

Histologic Findings

Rudimentary polydactyly, which may be a type of congenital traumatic neuroma, shows hyperkeratosis and acanthosis overlying many nerve bundles in the dermis. It may appear as a papule on the base of the ulnar side of the little finger. Abundant Merkel cells may appear at first and then disappear after the development of these nerve bundles, which form Meissner corpuscles in the dermal papillae and stain positively with S-100 protein.[16]

See the histology slides below.

Supernumerary digit, no magnification. Supernumerary digit, no magnification.
Twenty-eight-day-old supernumerary digit, connecti Twenty-eight-day-old supernumerary digit, connective tissue and epidermis (40x).
Twenty-eight-day-old supernumerary digit, nerve bu Twenty-eight-day-old supernumerary digit, nerve bundle base (40x).

 

Twenty-eight-day-old supernumerary digit, rudiment Twenty-eight-day-old supernumerary digit, rudimentary nail unit cartilage (2x).
 

Treatment

Surgical Care

A child with distal extremity anomalies experiences emotional stress.[17] By age 3 years, the child has become aware of the anomaly. By age 7 years, the child has begun to experience the close scrutiny of his peers at school. For these reasons, along with others, surgical correction should be initiated early in life.

Surgical management depends greatly on the complexity and location of the deformity. Traditionally, postaxial polydactyly was managed by pediatricians with suture ligation and only syndactyly was treated in the operating room. However, the increased risk of painful neuromas when using suture ligation has led to the use of sharp excision for postaxial polydactyly. The accessory digital nerve in postaxial polydactyly needs to be identified and transected as far proximally as is safe in order to decrease the risk of neuroma. Soft tissue then covers the end of the divided nerve. In cases of preaxial and central polydactyly, treatment is variable and the surgeon looks to find a balance between aesthetics and functionality. Waiting until age 9-12 months is advisable to decrease anesthesia risk.[18, 19, 20, 21]

Conversely, if an infant has postaxial type B polydactyly (rudimentary extra digit attached to the ulnar side by a soft tissue stalk), excision in the neonatal nursery is a safe and simple procedure with consistently positive outcomes. Excision is done with a single swipe of a scalpel after the use of topical anesthesia.[22] .

When preaxial polydactyly involves the radial hand and thumb, reconstructive techniques are usually needed to ensure a functional and stable thumb.[23]  Consultation with a surgeon with special expertise in hand problems is typically warranted.

A related Medscape article is Polydactyly of the Foot.

Complications

Complications in the perioperative timeframe include those secondary to bleeding and anesthesia.

Later, decreased function due to ligamentous laxity or contracture can occur. Painful neuromas can also occur at the treatment site, especially when using suture ligation as therapy.[21]