Defective bone growth due to an abnormal gene, causing short stature
Achondroplasia causes abnormal body proportions and short stature. The disorder affects the bones of the limbs and base of the skull and prevents normal growth. As a result, although the child’s trunk and head grow at a near normal rate, the arms and legs are disproportionately short and the forehead often protrudes. However, the child’s intelligence is not usually affected. In the UK, about 2,000 people are affected.
Achondroplasia is caused by an abnormal gene inherited in an autosomal dominant manner (see Gene disorders). Usually, the gene abnormality occurs spontaneously, and there is no family history of the condition. In a few cases, there is a family history of the disorder. People with achondroplasia have a 1 in 2 chance of passing the disorder on to a child, and they may wish to have genetic counselling if they are planning a pregnancy.
Symptoms such as short, bowed legs and a prominent forehead are usually apparent at birth. Other symptoms develop as the baby grows and include:
Short, broad hands and feet.
Forward curve to the lower spine (see Kyphosis and lordosis).
Psychological difficulties may develop when a child starts to realize that other children are different. Rarely, a baby also has hydrocephalus, in which there is excess fluid around the brain.
Achondroplasia is sometimes diagnosed during routine ultrasound scanning in pregnancy at about 18–20 weeks, but more often the condition is diagnosed at birth. The doctor may arrange for X-rays to be taken to confirm the diagnosis, and CT scanning or MRI scanning may be carried out to look at the baby’s head.
There is no cure for achondroplasia, and treatment is rarely needed. The legs can be lengthened slightly by surgery, but many children prefer their natural height. A baby with hydrocephalus may need an operation to drain excess fluid from the brain (see Shunt for hydrocephalus). Affected children usually have a normal life expectancy, but may have spinal problems as adults.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.