Mirror hand, or ulnar dimelia, is one of the least common but most notable congenital differences of the upper extremity. This condition affects one extremity in a variable way: all patients have extra fingers and a missing thumb with the forearm and elbow affected differently. There may be 7 or 8 fingers with two ulnae bones (and no radius) and an abnormal elbow joint where the two proximal ulnae meet the humerus.
There is one reported syndrome with mirror hands and feet, Laurin- Sandrow Syndrome. The OMIM description in the link provides interesting information with ulna dimelia and fibular hemimelia of both hands and feet. It is caused by a duplication of the ZRS which regulates sonic hedgehog and the LMBR1 gene on chromosome 7q36. One of the reasons I became so interested in limb development and congenital hand surgery is this diagnosis and one of the best summaries I have read (still), is a 1999 description in Scientific American on “How Limbs Develop.”
I have learned so much in caring for children with mirror hand and believe strongly that certain principles guide care. The condition varies a great deal from child to child. This can make it difficult to compare outcomes as the state of the hand, wrist, and elbow all can be different. Nonetheless, the principles hold as I outline below.
- The goal with surgery is to create the best possible hand and wrist. Typically there are either 4 or 5 good fingers. The other 2 or 3 fingers are not very functional in most cases. If there are 5 good fingers, then we can create a hand with four fingers and a new thumb (pollicized digit). But if only 4 good fingers, then I recommend creating a hand with 3 fingers and a new thumb. This is a very difficult conversation as going from 8 fingers to 3+ a thumb is difficult for any parent. And, of course, ultimately, we want the parents to agree with the course of care – there have been kids where we create the 4 finger hand with a thumb even when the new thumb is not as strong/ functional as it might have been.
- The wrist is always in flexion because the muscles on top of the forearm (the extensors) are weak and underdeveloped. At the time of the hand surgery/ pollicization surgery, we use available tendons from excised fingers and potentially from the wrist as a transfer to power wrist extension. Most kids end up with the wrist in neutral (ie, straight) which is highly functional.
- The elbow is very interesting as the condition does not overlap with other birth conditions. I prefer to operate at an earlier age to allow cartilage (which is a big part of the young elbow joint) a chance to remodel. The fundamental problem is that the ulna bones are both rotated and prevent elbow flexion AND the muscles are not powerful to flex the elbow. When we operate early, we can adjust the bones and cartilage to remove a bony block to elbow flexion.
- The future for all kids with mirror hand is bright. Kids are so adaptable and will learn how to best function. I feel that if we can provide a very good hand, a straight wrist, and an elbow that can bend (ideally, 90 degrees), the child will have a great extremity. Here are a few preoperative and teenage pictures for two amazing kids (with amazing families). Both of these kids (Owyn and Evan) and families have consented to my sharing pictures.
