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Achondroplasia
Monday, 18 August 2003
Thursday, 18 November 2004
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What

Achondroplasia is an inherited disorder of bone growth. It is one of the most common causes of growth defects characterized by abnormal body proportions (arms and legs not proportional to head and torso). Individuals with this disorder are short, have short arms and legs, a large head, and usually a normal-sized torso. Although the word achondroplasia means "without cartilage formation," these individuals do indeed have cartilage. What differs in a child with achondroplasia, is that the cartilage is changed to bone much more slowly than normal. This is especially so in the growth plates of long bones such as the upper arm (humerus) and thigh (femur). As a result, the child has short bones and reduced height.

Who

Achondroplasia, a genetic condition that is present at birth, affects about one in every 25,000 births. It occurs in all races, in both sexes and was depicted in ancient Egyptian art making it one of the earliest known birth defects.

Signs and Symptoms

An individual with achondroplasia may have the following symptoms:

  1. Physical characteristics include short stature (height), short limbs especially in the upper arm and thigh, short hands and fingers, abnormal separations between the middle and ring fingers called a trident hand, a prominent forehead, and a large head. The large head size may be the result of hydrocephalus (excess fluid in the brain) which may require surgical treatment.
  2. Other orthopedic problems include abnormal spine curvatures such as "kyphosis" (increased outward curve of upper back making back appear slightly hunched) and "lordosis" (increased inward curve of lower back). The kyphosis is due to poor back support from low muscle tone (hypotonia), and usually goes away after the child starts walking. However, once walking, the child usually develops a lordosis and the lower legs often become bowed (bowlegged) producing a waddling gait.
  3. Because of the large head, short arms and legs, poor muscle tone, and loose joints, children may develop motor skills slower than normal. They may be slow to learn how to sit, stand, and walk. However, they generally have normal intelligence.
  4. Complications include frequent ear infections (due to narrow nasal passages), which if left untreated, may lead to hearing loss. In adolescents and adults, a small spinal canal can create pressure on the spinal cord, causing lower back and leg pains, and even paralysis of the legs that requires surgery to relieve the pressure.
  5. The greatest danger in a baby or young child with achondroplasia is foramen magnum compression. The foramen magnum is the opening in the underside of the skull. The spinal cord normally comes down through this opening into the spinal canal in the back. In children with achondroplasia, the opening may be abnormal in size and shape which can lead to pressure on the spinal cord. This can interfere with breathing and result in death often during sleep.

Possible Causes

All of us have genetic material that makes us who we are. The genetic material, referred to as “chromosomes” are numbered. The gene for achondroplasia has been found on chromosome 4 called the FGFR3 gene. Mistakes in this gene cause achondroplasia. Achondroplasia is an autosomal dominant disorder. This means that an affected parent has one abnormal gene on chromosome 4 and one normal gene on chromosome 4. If the child receives the abnormal gene from the affected parent, he/she will also have achondroplasia. However, in the majority of cases, approximately 80% of occurrences appear as spontaneous mutations meaning that there is no affected parent and a mistake just spontaneously occurred in the child's gene. There have been observations that the risk of achondroplasia may be higher with advanced paternal age, or fathers aged 40 and older.

Diagnosis

Diagnosis of achondroplasia after birth includes physical examination, X-rays of the long bones, and genetic testing. Highly accurate prenatal tests can also be offered when both expectant parents have achondroplasia. This is important because the baby has a 25% chance of inheriting an abnormal gene from each parent and developing a fatal form of achondroplasia.

Treatment

Currently, there is no cure for achondroplasia, but treatment includes corrective and preventative measures. Monitoring for foramen magnum compression can be done with CT (computed tomography) or MRI (magnetic resonance imaging). When necessary, surgery can widen the opening and relieve pressure on the spinal cord. Surgery can also correct some of the symptoms such as club feet, severe spinal defects, and severe bowing of the legs. In children with frequent ear infections, placement of middle-ear drainage tubes helps to prevent hearing loss. Other preventative measures include providing babies with good back support, such as when sitting in a stroller, to avoid kyphosis. Also, nutritional guidance may help to prevent obesity, because extra weight can make the skeletal problems worse. Growth hormone does not seem to improve height very well. But orthopedic surgeons may be able to perform surgeries to lengthen the child's legs to make them taller.

Prognosis

Most individuals with achondroplasia eventually reach an adult height of about 4 feet and have normal intelligence. However, babies with the fatal form of achondroplasia who inherit two abnormal genes (one abnormal gene from each parent) seldom live beyond a few months.

Connect with other parents

In the spirit of community and support, Madisons Foundation offers the unique service of connecting parents of children with rare diseases. If you would like to be connected to other parents of children with this disease, please fill out this brief form.

Weblinks

March of Dimes article
Helpful factsheet on Achondroplasia in English and Spanish, with links to other informational and support group sites.

Medicine Net
Easy to understand information sheets written by doctors.

People of America (LPA)
Great place for articles, stories, current news & events, local chapters, & information. Also has numerous web links and information about many different support groups around the world.

Human Growth Foundation
A foundation that helps children with disorders related to growth through research, education, support and advocacy.

Google Search for Achondroplasia

References and Sources

www.geneclinics.org/query?dz=achondroplasia www.hopkinsmedicine.org/greenberg.center/achon.htm www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd. section.256