Girl Born with ‘Mirror Hand’ Syndrome: A Rare Condition Documented in <100 People

A rare case of mirror hand syndrome.Photo byAmericanRoentgenRaySociety

Imagine having a hand that’s perfectly symmetrical, like a mirror image. While this might seem like something out of a science fiction story, it's a reality for a small number of people who have a rare congenital condition known as ulnar dimelia, or mirror hand, with fewer than 100 known cases worldwide.

This intriguing and rare disorder is characterized by the duplication of digits, often without a thumb, and a mirror-like appearance on either side of the central finger.

Understanding Mirror Hand Syndrome

Mirror hand is an incredibly rare condition where individuals are born with a symmetrical hand, usually featuring a central digit and three mirrored fingers on either side.

What makes this condition even more unique is the absence of the thumb, as well as the duplication of the ulna (the bone that runs along the forearm) and the absence of the radius (the other main forearm bone). This results in a hand structure that defies the typical anatomy we’re used to.

A 2017 case study detailed the experience of a 15-year-old female patient who presented with a mirror hand. Her right hand had seven digits symmetrically aligned but no thumb. X-rays revealed the presence of two ulnae and a complete absence of the radius, which further compounded her restricted mobility.

Another case study involving mirror hand syndrome was documented as recently as 2024, involving a one-year-old girl who underwent reconstructive surgery to improve both function and appearance.

(A) Mirror hand of a 1-year-old girl; (B) Surgical wound; (C) Grasping movement 2 months after surgery.Photo byLife (Basel), 2024.

What Causes Ulnar Dimelia?

The exact cause of mirror hand is still unclear, though it’s believed to stem from genetic factors during fetal development.

The condition is linked to the improper signaling of the Sonic Hedgehog protein, a molecule crucial in the development of the limbs, particularly in the differentiation of the radius and ulna. This miscommunication during early development leads to the duplication of the ulna and the absence of the radius.

Despite its rarity, the condition can often be identified through prenatal ultrasounds or immediately after birth when the physical malformations are clear. There is no known environmental or lifestyle factor linked to the development of ulnar dimelia, making its occurrence difficult to predict.

Challenges and Surgical Treatment Options

Living with mirror hand can be challenging, particularly due to the functional limitations it imposes. Patients often experience restricted movement, especially in the wrist and elbow, and many have difficulty with fine motor skills due to the unusual anatomy. Surgery is the primary form of treatment for those with ulnar dimelia, aimed at improving both the function and appearance of the hand.

In the case studies, the treatment plan involved a procedure known as pollicization, in which one of the more functional digits is reconstructed to serve as a thumb. The two lateral digits were amputated, and the skin from these amputated digits was used to create a wider web space between the thumb and index finger, allowing for improved grip.

During the operation, the muscles and tendons were carefully shortened and repositioned to maximize the hand’s functionality. By fixing the third ray (the middle radial digit) into the desired position for a thumb using surgical pins, the patient gained greater dexterity post-surgery.

Conclusion

As fascinating as it is rare, mirror hand syndrome challenges our understanding of normal human anatomy. Although you may never meet someone with mirror hand, this anomaly reminds us of the incredible complexity of human development and the importance of ongoing medical research.