Achondroplasia

Definition

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

Alternative Names

Causes, incidence, and risk factors

Achondroplasia is one of a group of disorders called chondrodystrophies or osteochondrodysplasias.

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. 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%.

However, most cases appear as spontaneous mutations. This means that two parents without achondroplasia may give birth to a baby with the condition.

Symptoms

The typical appearance of achondroplastic dwarfism can be seen at birth. Symptoms may include:

• Abnormal hand appearance with persistent space between the long and ring fingers
• Bowed legs
• Decreased muscle tone
• Disproportionately large head-to-body size difference
• Prominent forehead (frontal bossing)
• Shortened arms and legs (especially the upper arm and thigh)
• Short stature (significantly below the average height for a person of the same age and sex)
• Spinal stenosis
• Spine curvatures called kyphosis and lordosis

Signs and tests

During pregnancy, a prenatal ultrasound may show excessive amniotic fluid surrounding the unborn infant.

Examination of the infant after birth shows increased front-to-back head size. There may be signs of hydrocephalus ("water on the brain").

X-rays of the long bones can reveal 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.

Support Groups

Expectations (prognosis)

People with achondroplasia seldom reach 5 feet in height. Intelligence is in the normal range. Infants who receives the abnormal gene from both parents do not often live beyond a few months.

Complications

• Clubbed feet
• Fluid buildup in the brain (hydrocephalus)

Calling your health care provider

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 most often develops spontaneously, prevention is not always possible.

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References

Horton WA, Hecht JT. Disorders involving transmembrane receptors. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 18th ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 694.