Hexadactyly is a congenital malformation which results in the presence of an extra digit on a hand or foot. This extra digit can vary in size and functionality; sometimes, it is little more than a skin tag, while in other instances the extraneous digit is fully functional. Most people remove these extra digits surgically, although they are generally quite harmless, because extra fingers and toes are viewed as aesthetically displeasing, and in some cultures they are subject to superstitious beliefs.
When the extra digit appears next to the pinky finger or toe, it is known as ulnar hexadactyly. This form is by far the most common. Hexadactyly which presents on the other side of the hand or foot, near the thumb or big toe, is known as radial hexadactyly. Very rarely, the extra finger or toe may grow between normal fingers and toes, in which case the patient is said to have intercalary hexadactly.
When people grow extra digits in general, it is known as “polydactyly,” a word which literally means “many fingers.” Polydactyly is especially common in some feline communities, and some cats have been documented with as many as eight toes on each foot!
When two thumbs grow on one hand, this is known as preaxial polydactyly, and it is extremely unusual. In a few rare cases, people have been born with two working thumbs on one hand, which can have interesting implications for manual dexterity.
In many cases, hexadactyly is simply the result of a random mutation. Other than the extra digit, the patient should have no other problems. In other instances, the condition may be linked with other, more serious medical conditions which require more extensive medical attention. As a general rule, parents are forewarned about these conditions, thanks to prenatal testing. Ultrasounds can also reveal hexadactyly, in which case the parents may choose to pursue additional prenatal testing to determine the cause of the abnormal number of fingers and toes.
In most cases, hexadactyly is treated shortly after birth, especially if the extra digit contains no bone. In other instances, parents may choose to leave the hexadactyly intact to allow the child to make a decision about it later, especially if the extra digit appears to be fully functional. While most people view hexadactyly as a “birth defect,” implying that it needs to be corrected, some people rather enjoy their six-fingered existence.