These days, popular media has made polydactylism a well-known genetic anomaly. From Gravity Falls to The Princess Bride, tons of fictional characters have more than five digits on their hands or feet. However, not everyone knows what Ulnar Dimelia is, in part because less than 100 cases have ever been diagnosed. People affected by Ulnar Dimelia can have a second hand attached to their wrist that mirrors the movements of the first. For this reason, Ulnar Dimelia is colloquially known as mirror hand syndrome.
As of 2018, researchers are still discovering new mirror hand syndrome facts on a regular basis. The disease doesn't seem to be hereditary, and it may be caused by a genetic mutation. Unfortunately, this is just a hypothesis, as no one knows exactly what causes the rare hand condition.
Ulnar Dimelia (AKA mirror hand syndrome) is a rare condition that affects carriers' arms and hands. It was first reportedly documented in 1587, but since that discovery, only 72 documented cases have appeared in medical literature, though some may have gone unnoticed in undeveloped parts of the world.
As cases pop up, medical experts aim to gather additional information about the syndrome, and new studies are published whenever possible.
A person who has Ulnar Dimelia can appear to have two hands on one wrist. In these cases, the hands are often joined at the palm and generally symmetrical. The affected person's hand looks as though it has been duplicated, and there may be a cleft in the palm where the hands join. The extra fingers connect to additional bones that run down the carrier's wrist, sometimes extending all the way to their elbow.
However, this isn't the only way mirror hand syndrome manifests. Sometimes, a person has additional fingers, as though multiple hands were molded together. There may also be multiple fingertips branching off a single finger, depending on what classification the specific case falls into.
Though the hand of a person with Ulnar Dimelia may appear to be duplicated down the middle, this usually does not include the thumb. Instead, hands affected by Ulnar Dimelia have either two index fingers in the center or a single, shared index finger. In such cases, no thumb bones are present in the affected individual's hand.
When this condition is repaired via surgery, the procedure usually involves creating a thumb using one of the extra fingers. This is not just for cosmetic purposes, as it's a lot easier to navigate the world with working thumbs.
People with mirror hand syndrome often cannot move their conjoined appendages independently of one another. Instead, many affected individuals find that when they move one half of their hand, the other half mimics the motion.
When grabbing at things, these people's hands open and shut similar to Venus flytraps, with fingers curling inward towards each other. If some fingers are extended, the others may not be able to flex. Not only are a person's digits mirrored symmetrically, but their nerves also seem to react in unison.