What Is Mirror Hand Syndrome?

Medically Reviewed by Brunilda Nazario, MD on May 08, 2023
3 min read

Mirror hand syndrome, also known as ulnar dimelia, is an extremely rare congenital (meaning it's there from the time you're born) birth defect that affects your hand. In some cases, your forearm and elbow might be impacted, too. The first case was seen in 1852. Since then, there have been fewer than 100 cases reported. 

Here’s a look at what causes it, its symptoms, and how to treat it. 

If your child has this condition, they're typically born with 7-10 fingers on one or both hands.

This means from the midpoint of their hand, their fingers look like mirror images. Usually, there’s no thumb. 

Instead, in most cases, they’ll have a central finger. And on either side, they’ll have a mirrored symmetry of three fingers – the middle finger, the ring finger, and the little finger. 

Studies show it affects men and women equally. 

Usually, you have two bones in your forearm – the ulna and the radius. Both bones lend to different hand and finger growth and function. 

But when you have mirror hand syndrome, typically there are two ulna bones, and the radius is missing. This tends to happen when your limb is still in the developing stages as an embryo. Your wrist bones might duplicate, too. This then causes your forearm and hand to look like mirror images. 

There are experts who believe there are different types of mirror hand syndrome. In some cases, it’s possible for you to develop two ulnae along with a radius bone. If the bones develop differently, it can affect how your hand appears physically. For example, it could cause you to have more than five fingers (polydactyly). It’s also possible to develop multiple hands at the end of your forearm. 

Doctors don’t know what causes this birth defect, but they believe genetics play a role. 

Besides the physical appearance of multiple fingers and no thumb, the condition may cause the wrist to be bent slightly. Your child might be able to bend their elbow regularly. But in some cases, the elbow might be straight and difficult to bend or rotate.

Mirror hand syndrome can cause you to have a limited range of motion in your wrist and hand. This can lead to dexterity issues, which means it may be hard to use your hand and fingers to perform certain day-to-day tasks such as grasping or picking things up.

As this condition can happen when the limbs are forming during the embryo’s developing stages, mirror hand syndrome is often diagnosed at the prenatal ultrasound screening during pregnancy. It can also be diagnosed at birth. 

Your child will need reconstruction surgery to fix the physical appearance as well as the function of the affected hand. This may include multiple surgeries. 

Usually, the surgeon will wait until your child is between 18-24 months to perform this surgery. That’s because this allows the hand to grow almost twice the size it was at birth. 

During the surgery, they’ll remove additional fingers and most likely pick one of the digits to use as a thumb. The doctor will reposition it where the thumb would have naturally developed. This is known as pollicization. The aim of the surgery is to rebuild their hand with five digits including a thumb. 

They also try to keep as much muscle, tendon, and growth plates on their hand as possible and reconstruct their wrist. This will help your child to develop hand movement, growth, and function as they grow up.  

If mirror hand syndrome affects their elbow joint or forearm, they’ll need surgery to fix their ability to bend and rotate it.

They might need motion exercises and physical therapy for the hand to improve function as it grows. Speak to your doctor, physical therapist, or an occupational therapist about it.