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Quick Reference and Fact Sheets |
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Achondroplasia
What is achondroplasia?
Achondroplasia is a genetic disorder of bone growth that is evident at birth. It affects about one in every 25,000 births and it occurs in all races and in both sexes. Its depiction in ancient Egyptian art makes it one of the oldest recorded birth defects.
It is the most common of a group of growth defects characterized by abnormal body proportions — affected individuals have arms and legs that are very short, while the torso is more nearly normal size.
The word achondroplasia is derived from Greek and means "without cartilage formation," although individuals with achondroplasia do have cartilage. During fetal development and childhood, cartilage normally develops into bone, except in a few places, such as the nose and ears. In individuals with achondroplasia, something goes wrong during this process, especially in the long bones (such as those of the upper arms and thighs). The rate at which cartilage cells in the growth plates of the long bones turn into bone is slow, leading to short bones and reduced height.
What does a person with achondroplasia look like?
A child with achondroplasia has a relatively normal torso and short arms and legs. The upper arms and thighs are more shortened than the forearms and lower legs. Generally, the head is large, the forehead is prominent and the nose is flat at the bridge. Sometimes, the large head size reflects hydrocephalus (excess fluid in the brain), and require surgery.
Hands are short with stubby fingers. There is a separation between the middle and ring fingers (trident hand). Most individuals with achondroplasia eventually reach an adult height of about 4 feet.
How does achondroplasia affect development?
Before beginning to walk, a baby with achondroplasia often develops a small hump (kyphosis) on his upper back. This is due to poor muscle tone, and usually goes away after the child starts walking.
Babies with achondroplasia should not be placed in umbrella-type strollers or other carriers that do not provide good back support, because lack of support can contribute to development of a hump in the back. Once walking, the child usually develops a markedly curved lower spine (lordosis or sway- back), and the lower legs often become bowed. The feet are generally short, broad and flat.
Because of the large head, short arms and legs, poor muscle tone and loose joints, a baby with achondroplasia is slow to sit, stand and walk alone. Though babies with achondroplasia may be delayed in these motor skills, they generally have normal intelligence. Children with achondroplasia also have narrow passages in the nose that can contribute to ear infections and, without treatment, to hearing loss. Due to a small jaw, teeth may be crowded and upper and lower teeth may be poorly aligned.
Low back and leg pains are common in adolescents and adults, partly because there is pressure on the spinal cord from a small spinal canal. This pressure on the spinal cord also can cause paralysis of the legs, requiring surgery to relieve the pressure.
Occasionally, a baby or young child with achondroplasia may die suddenly, often during sleep.These deaths are thought to result from compression of the upper end of the spinal cord, which can interfere with breathing. The compression is caused by abnormalities in the size and structure of the opening in the base of the skull (foramen magnum) and vertebrae in the neck through which the spinal cord descends.
All babies and young children with achondroplasia should be monitored for foramen magnum compression (using tests such as computed tomography or magnetic resonance imaging). When necessary, surgery can widen the opening and relieve pressure on the spinal cord. Breathing problems also may develop as a result of small chest size, large tonsils and small facial structure.
What causes achondroplasia?
Achondroplasia is caused by an abnormal gene located on one of the chromosome 4 pair (humans have 23 pairs of chromosomes). In some cases, a child inherits achondroplasia from a parent who also has the condition. If one parent has the condition and the other does not, there is a 50 percent chance that their child will be affected. If both parents have achondroplasia, there is a 50 percent chance that the child will inherit the condition, a 25 percent chance that the child will not have it, and a 25 percent chance that the child will inherit one abnormal gene from each parent and have severe skeletal abnormalities that lead to early death. A child who does not inherit the gene will be completely free of the condition, and cannot pass it on to his or her own children.
In more than 80 percent of cases, however, achondroplasia is not inherited but results from a new mutation (change) that occurred in the egg or sperm cell that formed the embryo. The parents of children with achondroplasia resulting from new mutations are usually normal-sized.
Typically, these parents have no other children with achondroplasia, and the chances of their having a second affected child are extremely small. Geneticists have observed that older-than-average fathers (age 40 and older) are more likely to have children with achondroplasia and certain other autosomal dominant conditions caused by new mutations. Individuals with achondroplasia resulting from new mutations transmit the disorder to their children as described above.
Can achondroplasia be diagnosed before birth?
In 1994, researchers identified the gene that causes achondroplasia. This discovery allowed the development of highly accurate prenatal tests that can diagnose or rule out achondroplasia. These tests can be offered when both expectant parents have achondroplasia. In such cases, the baby has a one- in-four chance of inheriting an abnormal gene from each parent and developing a fatal form of achondroplasia. After birth, it generally is diagnosed by physical examination and X-rays.
How is achondroplasia treated?
There currently is no way to normalize skeletal development of children with the disorder. Several medical centers are evaluating the use of human growth hormone for children with the disorder. To date, some children have achieved modest increases in growth; however, it is not yet known whether treatment will significantly increase their eventual adult height. Leg-lengthening surgeries can increase the height of someone with achondroplasia by up to 12 inches. These procedures require a long duration of treatment, however, and are associated with many complications. They should only be done in a center that is well- experienced in the procedure.
Infants and children with achondroplasia should be thoroughly evaluated for skeletal abnormalities by a doctor experienced with the disorder. The doctor will follow the child's growth using special charts of head and body growth developed for children with achondroplasia. If the head is becoming too large, the doctor will test the child for hydrocephalus and, if necessary, a neurosurgeon will insert a shunt to drain the excess fluid and relieve pressure on the brain. The child also will be monitored for bone abnormalities that may cause spinal cord compression with breathing difficulty or leg pain and paralysis. If kyphosis does not go away after a child begins walking, it may have to be corrected by surgery. Bowing of the legs, especially if it becomes severe or causes pain, also can be corrected with surgery.
Children with achondroplasia often require placement of middle- ear drainage tubes. This helps to prevent the hearing loss that can occur with frequent ear infections. Dental problems caused by overcrowding of teeth may require extra routine care and braces and, sometimes, removal of one or more teeth. Children with achondroplasia also tend to put on extra weight, starting in early childhood. Because excessive weight can further aggravate skeletal problems, they should receive nutritional guidance to help prevent obesity.
Can achondroplasia be prevented?
There is no way to prevent the majority of cases of achondroplasia, since these births result from totally unexpected gene mutations in unaffected parents. Genetic counseling can help affected adults make informed decisions about family planning.
What research is being done on achondroplasia?
As noted above, scientists have identified the gene, and the exact mutation (change) in the gene, that causes achondroplasia. The gene is one of a family of genes that makes proteins called fibroblast growth factor receptors. Scientists have recently linked these genes with several skeletal disorders.
The fibroblast growth factor receptor gene dictates the production of a protein that is located on the surface of cells of different tissue types, including cartilage. The protein normally responds to signals from chemicals called growth factors which stimulate cell growth and maturation.
Now scientists are investigating how the faulty protein actually causes the features of achondroplasia. This eventually may lead to improved treatment for this disorder, as well as improved understanding and treatment of other skeletal disorders caused by this family of genes. March of Dimes scientists and other researchers also are working to identify the genes that cause some of the other more than 100 forms of disproportionate short structure.
Additional Sources of Information
For further information on support groups for individuals with achondroplasia or other forms of growth deficiency, contact:
Little People of America (LPA)
P.O. Box 745
Lubbock, TX 79408
888-LPA-2001
Human Growth Foundation
997 Glen Cove Ave.
Glen Head, NY 11545
800-451-6434
The Magic Foundation for Children's Growth and Related Adult Disorders
1327 N. Harlem Ave.
Oak Park, IL 60302
708-383-0808
09-321-00 8/00
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