Polydactyly

POLYDACTYLY, multi-fingeredness, the complete or partial development of supernumerary digits, which is a not very rare malformation in humans (Broman), although precise statistical data on its prevalence are lacking. Polydactyly is encountered approximately equally often on both the hands and the feet, very frequently in combination on both hands or feet and even sometimes on all four limbs; less often on one hand and one foot. Usually, supernumerary digits develop near the fifth and even more often near the first digit, and very frequently a fibrous membrane remains between the supernumerary digit and the main one. The total number of digits on a limb in cases of polydactyly is most often six, but can reach a greater number. Supernumerary digits do not always have a developed independent neurovascular system, which of course affects their function. The number of phalanges in a supernumerary digit may be less than normal. The main mechanisms for the formation of polydactyly appear to be budding, splitting, and doubling. Doubling, which implies the presence of two germinal primordia, is disputed by many (Rauber). There is no complete unity of views on the origin of polydactyly, but the majority believe that polydactyly is a hereditary malformation, often not manifesting for a number of generations; in many known cases, it was possible to establish the manifestation of this malformation to one degree or another in a whole series of members of a given family. Some researchers (Ahlfeld) consider polydactyly to be a congenital malformation arising as a consequence of a pathological state of the mother's organism. Infections, trauma, starvation, psychological influences, and so on cause, among other changes, an alteration of the amnion. Adhesions, sclerosis, and so on can form in the amnion, sometimes leading to the splitting of the digit primordia. The usual site of formation of supernumerary digits at the outer edge of the limb, i.e., closer to the amnion, the presence of parts of the amnion in the webs between the digits, and the lack of definite indications of heredity are the main arguments of proponents of the congenital nature of polydactyly. Such a crudely mechanistic view of the development of polydactyly is held, however, by very few researchers. Heredity in each individual case can be difficult to establish due to the fact that the malformation did not manifest in a number of generations at all or manifested in the form of so-called hidden polydactyly, i.e., with an increase in the number of metatarsal and metacarpal bones without complete splitting of the phalanges (Burckhard). Finally, according to some authors (Schwalbe), what is inherited is not polydactyly as such, but amniotic anomalies, which in turn cause polydactyly. The view of polydactyly as an atavism has not gained widespread acceptance in human pathology.