Hitchhiker's thumb

Hitchhiker's thumb, also known as distal hyperextensibility of the thumb, is the condition of having a thumb that has a distal phalange that bends backwards in an angle of 90°. This condition is benign (when isolated) and does not cause pain or affect the thumb with the trait negatively.

Hitchhiker's thumb
Other names	Distal hyperextensibility of the thumb, thumb hypermobility, Z-shaped deformity,[1] duck-bill thumb [2]
Example of bilateral hitchhiker's thumb
Specialty	Medical genetics
Symptoms	A thumb that can bend backwards at more than a 90° degree angle
Complications	If it presents as an isolated trait, none
Duration	Life-long
Causes	May either present as an isolated trait, or be a symptom of a condition such as a hypermobility spectrum disorder[medical citation needed]
Risk factors	Family history
Frequency	24–36% of the U.S. population [3]

If a person has the genes for this condition, it might present bilaterally, unilaterally, or on none of the thumbs.

Symptoms vary among people with hitchhiker's thumbs, those who have it as an isolated trait are not affected by any symptoms, however, this trait is sometimes a symptom of other conditions, such as a hypermobility spectrum disorder.

The signs of this trait are having a thumb with a distal phalange that is able to bend backwards beyond the normal range of motion. Some people with hitchhiker's thumb also have a Z-shaped deformity, named after the appearance of this deformity, which is that of the letter Z.

The exact cause of isolated hitchhiker's thumb is not known, it is thought to be inherited in an autosomal recessive manner.^[4]

However, this trait is often a symptom of an underlying condition, such as

• Diastrophic dysplasia
• Hypermobility spectrum disorder
• Rheumatoid arthritis

It does not need treatment.

Hitchhiker's thumbs affect 24–36% of the U.S. population.^[3]

• Hypermobility (joints)
• Ligamentous laxity
• Hypermobility spectrum disorder
• Ehlers–Danlos syndromes
• Diastrophic dysplasia