Achondroplasia

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

Causes, incidence, and risk factors  

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 its 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 can be inherited as an autosomal dominant trait, which means that if a child gets the defective gene from one parent, he or she will have the disorder.

However, the majority of cases, approximately 80%, appear as spontaneous mutations. This means that two patients 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, proximal extremity (upper arm and thigh)  
  • Large appearing head  
  • 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)  
  • Head appears disproportionately large for body  
  • Hypotonia  
  • Polyhydramnios  
  • Bowed legs

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

Signs and tests  

Examination of the infant shows increased front-to-back head size (occipital-frontal circumference). There may be signs of hydrocephalus associated with enlargement of the chambers within the center of the brain (ventricular dilatation).

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

Treatment  
There is no specific treatment for achondroplasia. Associated orthopedic abnormalities such as club feet should be corrected.

Expectations (prognosis) 

People with achondroplasia seldom reach five 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.

Complications  

     
  • Hydrocephalus  
  • Clubbed feet

Calling your health care provider  

Call for an appointment with your health care provider if you are planning to have children and there is a family history of achondroplasia.

Prevention  

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

Johns Hopkins patient information

Last revised: December 3, 2012
by Martin A. Harms, M.D.

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