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Polysyndactyly is a congenital anomaly, combining Polysyndactyly and Syndactyly, in which affected individuals have an extra digit that is connected, via fusing or webbing, to an adjacent digit^[1].

The most common presentation of polysyndactyly is postaxial^[2], in which the extra digit is on the same side as the pinky or little toe.^[3] Preaxial polysyndactyly, in which the duplicated digit is on the side of the thumb or big toe, is less common.^[3] Crossed polysyndactyly, in which both the upper and lower extremities are affected, and the polysyndactyly is preaxial on one and postaxial on the other, is extremely rare, and often occurs with other genetic disorders.^[4]

The way polysyndactyly presents itself varies; when the 'webbing' is mild, it gives the appearance of the extra digit being partially connected to the (otherwise normal) digit next to it, when the webbing is extreme, the extra digit and the digit next to it will have the appearance of one rather enlarged digit (macrodactyly) and will have two or more finger/toe nails growing in the same digit (polyonychia). How the nails look will also vary, from separate supernumerary finger/toe nails to one very wide finger/toe nail.

Polysyndactyly is typically inherited, in an autosomal dominant pattern.^[4][5] The specific mutations leading to polysyndactyly are varied between different types of the disease and between different families carrying the disease. However, many cases of polysyndactyly are caused by changes to genetic elements affecting the signaling molecule Sonic Hedgehog (SHH). Primarily, mutations are found in a specific regulatory sequence, referred to as ZRS, that is known to control the expression of SHH in developing limbs. Many cases of polysyndactyly have been found to be the result of duplications of the ZRS^[6][7][8] or regions close to the ZRS known as the pre-ZRS region.^[9][10]

Polysyndactyly can also be associated with the presence of other genetic disorders. It is a hallmark of Carpenter's syndrome, an autosomal recessive disorder that is also associated with craniosynostosis, obesity, short stature, and other malformations.^[11] Patients with other syndromes, including those with Pallister-Hall syndrome^[12] and Greig cephalopolysyndactyly syndrome^[13] may also display polysyndactyly of varying severity.

Polysyndactyly has full penetrance but variable expressivity. This means that individuals possess an allele for polysyndactyly, but may have a different severity of polysyndactyly. This has been seen in case studies where a parent has hexadactyly in their 4th and 5th fingers but their child has hexadactyly in their 1st, 2nd, 3rd and 4th fingers.^[14]

Polysyndactyly can be diagnosed in-utero through sonographic and genetic testing. Ultrasounds done at the 19th week of pregnancy look for the presence of six metacarpals to detect this condition. Genetic testing done on the human fetus looks at disruptions in the HOXD13 gene at 2q31-q32 and in the GLI3 gene at 7p13. These regions are known to regulate proliferation and differentiation in the limb bud, which is why mutations in the genes impair that process. Due to the cost and risk associated with genetic testing, sonographic methods are usually preferred to diagnose polysyndactyly during fetal development. Postnatally, polysyndactyly is diagnosed simply by observing the presence of an extra digit and using X-rays to confirm the presence of an extra metacarpal.^[15][16]

Polysyndactyly is treated through surgical excision of the extra digits. The choice of which digit to remove needs to be made carefully and affects the post-operative outcomes for the patients. Factors that must be considered when determining which digit to excise include looking at the neurovascular bundles, angle differences, risks for impaired circulation, post-operative appearance, and residual deformities. ^[17]

The underlying cause of polysyndactyly determines the quality of life for individuals diagnosed with this condition. Treatment is generally aimed at normalizing both function and appearance of the affected extremity^[3] and, in the case of polysyndactyly of the foot, shoe fit and comfort.^[18] If there is no other genetic condition causing the formation of the extra digit, then the removal of the digit through surgery results in a high quality of life. Parents of children with polysyndactyly have reported high physical, social, emotional, and school functioning and good psychosocial health after removal of the extra digit. ^[19]