Hitchhiker’s Thumb

What is Hitchhiker’s Thumb?

Hitchhiker’s thumb is a genetic condition that causes a person to bend his or her thumbs backwards while stretching. It is also known as distal hyperextensibility¹.

The thumb consists of a distal joint which keep the thumb in a straight line. As the distal joint extends, it enables you to bend the thumb backwards leading to hitchhiker’s thumb. Hitchhiker’s thumb does not cause any pain in those who have it.

Thumb Genes

Normally, our bodies contain genes that are responsible for the size, color and shape of an individual. Bendy thumb gene is one of those genes in your body that regulate the way the thumbs extend. This type of gene has numerous alleles in its genetic material¹.

One allele can form a bend thumb while another one a straight thumb. This depends on the type of alleles an individual inherits from both parents.

There are two genetic traits that determine whether a person has a straight or bend thumb². They are:

Recessive trait

The recessive traits manifest when dominant traits are suppressed. An individual with a recessive trait has a thumb that bends backwards. You can have hitchhiker’s thumb if both of your parents donates recessive alleles to you.

Dominant trait

Two or more alleles can come together and become more powerful. The stronger alleles form dominant trait. An individual with a dominant trait is born with a straight thumb that can be folded towards the palm.

A person born with dominant and recessive genes is born with straight thumbs. This person is only a carrier of hitchhiker’s thumb. A person with only dominant genes is born with a straight thumb only and there is no possibility of having a hitchhiker’s thumb. If you are born with recessive genes only, you will have hitchhiker’s thumb.

Medical conditions Associated with Hitchhiker’s thumb

There are several conditions that cause hitchhiker’s thumb. They include^{3, 4}:

Joint Hypermobility

This is a disorder where an individual can move his/her limbs and joints. Joint hypermobility is genetic condition where genes are passed from the parent to the child. Changes in genes that make collagen, a protein that is found in your bones, muscles and skin are responsible for joint hypermobility.

• Joint hypermobility causes pain in the persons’ knees, elbows and finger joints. This condition is common and affects women and children mostly.

Diastrophic dysplasia

This disorder is also associated with hitchhiker’s thumb. It is present at birth but it rare. It is also known as diastrophic dwarfism. Diastrophic dysplasia affects development of cartilage and bones.

• Diastrophic dysplasia is a genetic condition. The gene that causes it is called diastrophic dysplasia sulfate transporter. This condition causes your thumbs to bend, foot to turn upwards, short arms and legs and unusual spine curvature.

Diagnosis

Hitchhiker’s thumb is a genetic condition and can be diagnosed either after birth or before. Before birth, your doctor can order the following to confirm diagnosis⁵:

Ultrasonography

This is a medical procedure that uses sound waves to create images of your internal body structures on the screen. This procedure is safe for pregnant women since it does not use ionizing radiation like X-ray.

In this procedure, the doctor puts a device known as transducer on your body which releases sound waves into your body. The waves hit your internal body tissues and bones and bounces back where they are recorded by the transducer.

For pregnant woman, images of her womb are produced as the sound waves are released. The images of the womb allow the doctor to check for abnormalities in the bone and tissues as the fetus develop.

Since there are many thumb disorders that can affect your thumb, your doctor may order other tests to rule them out.

Treatment

There is no treatment for hitchhiker’s thumb because it is a genetic condition.

Reference List

1. Hitchhiker’s thumb. https://www.news-medical.net/health/Genetics-of-Hitchhikers-Thumb.aspx
2. https://www.karger.com/Article/Pdf/150741
3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3279100/
4. Hitchhiker’s thumb. http://www.fetalultrasound.com/online/text/30-019.HTM
5. Ultrasonography. http://study.com/academy/lesson/what-is-ultrasonography-definition-history-uses.html