Achondroplasia

Definition

Achondroplasia is a disorder of bone growth that causes the most common type of dwarfism.

Causes

Achondroplasia is an inherited disorder of bone growth. It is one of the group of disorders that are collectively called chondrodystrophies or osteochondrodysplasias.

The disorder causes a type of dwarfism that is recognized by a characteristic normal- to large-sized head, shortened arms and legs (especially the upper arm and thigh), a normal-sized trunk, and waddling gait. Achondroplasia is the most common type of dwarfism.

Achondroplasia may be inherited as an autosomal dominant trait, which means that if a child gets the defective gene from one parent, the child will have the disorder.

However, approximately 80% of cases appear as spontaneous mutations. This means that two parents without achondroplasia may give birth to a baby with achondroplasia. If one parent has achondroplasia, the infant has a 50% chance of inheriting the disorder. If both parents have the condition, the infant's chances of being affected increase to 75%.

Symptoms

Typical appearance of achondroplastic dwarfism is apparent at birth:

• Short stature
• Short limbs (upper arm and thigh)
• Skeletal (limb) abnormalities
• Abnormal hand appearance (trident hand) with persistent space between the long and ring fingers
• Marked kyphosis and lordosis (spine curvatures)
• Waddling gait
• Prominent (conspicuous) forehead (frontal bossing)
• Disproportionately large head-to-body size difference
• Hypotonia
• Polyhydramnios
• Bowed legs
• Spinal stenosis

A family history of achondroplasia should alert parents to the possibility of having an affected child.

Exams and Tests

Examination of the infant shows increased front-to-back head size (occipital-frontal circumference). There may be signs of hydrocephalus

X-rays of the long bones can demonstrate achondroplasia in the newborn.

Treatment

There is no specific treatment for achondroplasia. Related abnormalities, including spinal stenosis and spinal cord compression, should be treated when they cause problems.

Outlook (Prognosis)

People with achondroplasia seldom reach 5 feet in height. Intelligence is in the normal range. Infants who are homozygous for achondroplasia (both parents are achondroplastic and each has contributed an affected gene) seldom live beyond a few months.

Possible Complications

• Hydrocephalus
• Clubbed feet

When to Contact a Medical Professional

If there is a family history of achondroplasia and you plan to have children, you may find it helpful to speak to your health care provider.

Prevention

Genetic counseling may be helpful for prospective parents when one or both have achondroplasia. However, because achondroplasia usually arises as a spontaneous mutation, prevention is not always possible.