Polydactyly
Polydactyly is a birth defect that results in extra fingers or toes. The hands are more commonly involved than the feet. Extra fingers may be painful, affect self-esteem, or result in clumsiness.
It is associated with at least 39 genetic mutations. It may either present alone or with other defects. Cases may run in families. The underlying mechanism involves an error in limb bud formation during early development. Diagnosis may occur before birth via prenatal ultrasound as early as nine weeks. X-rays may be useful after a child is a year old. The opposite is oligodactyly.
Treatment varies from removal by cautery to more involved surgery. While putting a tight band around the base has been carried out, this is not typically recommended. If surgery is required, this is often done around two years of age. Occasionally multiple surgeries are required.
Polydactyly is present in about 4 to 12 per 10,000 newborns. It is the most common defect of the hands and feet. In the United States, Black people are more commonly affected than White people. The term is.
Signs and symptoms
In humans/animals this condition can present itself on one or both hands or feet. The extra digit is usually a small piece of soft tissue that can be removed. Occasionally it contains bone without joints; rarely it may be a complete functioning digit. The extra digit is most common on the ulnar side of the hand, less common on the radial side, and very rarely within the middle three digits. These are respectively known as postaxial, preaxial, and central polydactyly. The extra digit is most commonly an abnormal fork in an existing digit, or it may rarely originate at the wrist as a normal digit does.Polydactyly can be divided into three major types, which are discussed below, which depend on the location of the additional digit.
In 2019 it was found that in cases of polydactyly with a fully functional additional digit, muscles to control the extra digit may be duplicated, resulting in increased motor control that allows the patient to carry out certain tasks with one hand that would normally require two.
Ulnar or postaxial
This is the most common situation, in which the extra digit is on the ulnar side of the hand, thus the side of the little finger. This can also be called postaxial polydactyly. It can manifest itself very subtly, for instance only as a nubbin on the ulnar side of the little finger, or very distinctly, as a fully developed finger. Most commonly, the extra finger is rudimentary, consisting of an end phalanx with a nail, and connected to the hand with a small skin pedicle. Mostly one neurovascular bundle can be identified, with no tendons present in the extra digit. In case of a fully developed extra finger, the duplication usually presents itself at the level of the metacarpophalangeal joint. A triplication of the little finger is very rare. Ulnar polydactyly occurs ten times more often in African populations. The incidence in Caucasians is reported as 1 in 1,339 live births, compared with 1 in 143 live births in Africans. Ulnar polydactyly is also often part of a syndrome. In patients with African ancestry ulnar polydactyly mostly occurs isolated, whereas the presentation in Caucasians is often associated with a syndrome, though in a retrospective review, only 4 of 37 cases of ulnar polydactyly in Caucasians were syndromic.Radial or preaxial
This is a less common situation, in which the affectation is on the side of the hand towards the thumb. Radial polydactyly refers to the presence of an extra digit on the radial side of the hand. It is most frequent in Indian populations and it is the second most common congenital hand disorder. The incidence of radial polydactyly is reported as 1 in every 3,000 live births. The clinical features of radial polydactyly will depend upon the extent of duplication.Radial polydactyly varies from a barely visible radial skin tag to complete duplication. Thumb polydactyly varies from barely visible broadening of the distal phalanx to full duplication of the thumb including the first metacarpal.
Radial polydactyly is frequently associated with several syndromes.
Central
This is a rare condition, in which the extra digit is on the ring, middle or index finger. Of these fingers, the index finger is most often affected, whereas the ring finger is rarely affected.This type of polydactyly can be associated with syndactyly, cleft hand, and several other syndromes.
Polysyndactyly presents various degrees of syndactyly affecting fingers three and four.
Causes
Polydactyly is associated with different mutations, either mutations in a gene itself or in a cis-regulatory element responsible for the expression of a specific gene. Mutations in Hoxa- or Hoxd clusters are reported leading to polydactyly. Interactions of Hoxd13 and GLI3 induce synpolydactyly, a combination of extra and consolidated digits. Other signal transduction pathways in this context are the Wnt signaling pathway or Notch.In the specific case of preaxial polydactyly, a cis-acting mutation approximately 1Mb upstream of SHH gene has been implicated. Normally SHH is expressed in an organiser region, called the zone of polarizing activity on the posterior limb side. From there it diffuses anteriorly, laterally to the growth direction of the limb. In the mutant, smaller ectopic expression in a new organiser region is seen on the anterior side of the limb. This ectopic expression causes cell proliferation delivering the raw material for one or more new digits.
Polydactyly can occur by itself, or more commonly, as one feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes, i.e. it is not a multifactorial trait. But mutation in a variety of genes can give rise to polydactyly. Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two.
Polydactyly has been linked to the prenatal environment in a 2020 study showing a relationship to maternal PM10 pollution exposure in China.
Types include:
| OMIM | Type | Locus |
| Postaxial A1 | GLI3 at 7p13 | |
| Postaxial A2 | 13q21-q32 | |
| Postaxial A3 | 19p13.2-p13.1 | |
| Postaxial A4 | 7q22 | |
| Preaxial I | ? | |
| Preaxial II | SHH at 7q36 | |
| Preaxial III | ? | |
| Preaxial IV | GLI3 at 7p13 |
Syndromes
Because polydactyly can be part of a syndrome or association, children with a congenital upper extremity deformity should be examined by a geneticist for other congenital anomalies. This should also be done if a syndrome is suspected, or if more than two or three generations of the family are affected.As of 2009, 97 genetic syndromes have been associated with different kinds of polydactyly.
Examples of syndromes include Diamond–Blackfan anemia, and the VACTERL association, acrocallosal syndrome, basal cell nevus syndrome, Biemond syndrome, ectrodactyly-ectodermal dysplasias-cleft lip/palate syndrome, mirror hand deformity, Mohr syndrome, oral-facial-digital syndrome, Rubinstein–Taybi syndrome, short rib polydactyly.
Ulnar
Ulnar polydactyly is often bilateral and associated with syndactyly and polydactyly of the feet. This can be a simple or complex polydactyly. Ulnar polydactyly occurs as an isolated congenital condition, but can also be part of a syndrome, such as: Trisomy 13, Greig cephalopolysyndactyly syndrome, Meckel syndrome, Ellis–van Creveld syndrome, McKusick–Kaufman syndrome, Down syndrome, Bardet–Biedl syndrome, Smith–Lemli–Opitz syndrome.Radial
Type VII of radial polydactyly is associated with several syndromes:Holt–Oram syndrome, Fanconi anemia, Townes–Brocks syndrome, and Greig cephalopolysyndactyly.
Central
The syndromes associated with central polydactyly are:Bardet–Biedl syndrome, Meckel syndrome, Pallister–Hall syndrome, Legius syndrome, Holt–Oram syndrome. Central polydactyly can also be associated with syndactyly and cleft hand.