Achondroplasia Causes Symptoms Treatment

 

Achondroplasia

Definition

Achondroplasia

Achondroplasia, or short-limb dwarfism, is the most common form of
abnormally short stature in adults. It is caused by a mutation in a single gene
on chromosome 4 that regulates the conversion of cartilage to bone. This
gene is the only gene that is known to be associated with achondroplasia.

Description

Achondroplasia is basically a disorder of bone development. The skeleton
of a human fetus is composed primarily of cartilage, a dense and somewhat
elastic form of connective tissue that gradually turns to bone during
normal development. In a person with achondroplasia, a gene that is
involved in the process of bone formation produces too much of a protein
that limits bone growth. As a result, the person with achondroplasia has
unusually short bones in the arms and legs and other skeletal abnormalities.
They also usually have difficulties with posture, joint disorders, and
breathing problems in later life.

Demographics

Researchers estimate that achondroplasia occurs in one in every 15,000 to
40,000 live births. About 20 percent of cases are children who have one
parent with achondroplasia; however, 75–80 percent of cases involve new
mutations of the gene responsible for the disorder.
These new mutations are more likely to occur in
the sperm of fathers over 35; the mother’s age does
not matter, as far as is presently known.
The disorder affects both sexes and all races
equally.
The average adult height of people with achondroplasia
is 4 feet 4 inches (1.3 meters) for men and
4 feet one-half inch (1.24 m) for women. The
shortest living person with achondroplasia as of
2008 was Jyoti Amge, a teenager from Nagpur,
India, who stands 23 inches (58 centimeters) tall
and weighs 11 pounds (5 kilograms).

Achondroplasia

Causes and Symptoms

Achondroplasia is caused by a mutation in the
FGFR3 gene on chromosome 4. A normal gene
helps the body convert cartilage to bone. The
mutation in the gene related to achondroplasia
results in severely shortened bones and weak
muscle tone in the body.
The symptoms of achondroplasia are usually obvious by the first year
of life. They include:
• Poor muscle tone.
• Slowness in learning to walk; a child with achondroplasia may not
walk until some point between 24 and 35 months.
• Abnormal skull structure leading to frequent ear infections, apnea
(temporary slowing or stopping of breathing), and overcrowding of
the teeth.
• Distinctive facial features, particularly a prominent forehead and
underdevelopment of the nose and midface.
• Greater than normal separation of the little finger from the ring
finger, giving the hand a distinctive trident (three-pronged) shape.
• A tendency toward obesity.
• Postural problems, including a pronounced curvature of the spine
in the lower back and bowed legs.
• Back and leg pain in adult life.
 

Diagnosis

Babies with achondroplasia can be diagnosed before birth by ultrasound
measurements of the growth of their long bones and head size. After
birth, the diagnosis is usually based on x-ray studies of the child’s bones
and head size. The x-ray images usually reveal a small skull base, shortened
growth plates in the long bones, square-shaped long bones, and
normal-sized bones in the trunk area.
A genetic test can also be performed to confirm the diagnosis in children
whose symptoms may be less clear.
Treatment

The management of achondroplasia includes careful monitoring of the
child’s growth, head size, and weight pattern. There are special growth
charts that doctors use to evaluate the rate of growth in children with
achondroplasia. It is particularly important to prevent obesity if possible.
In addition, children with achondroplasia need social support because of
ongoing prejudice against people with dwarfism. Organizations such as
Little People of America can offer helpful advice on the education and
other future plans of children with achondroplasia.
Surgery may be performed if necessary to relieve pressure on the spinal
cord or the brain. In some cases, there may be a buildup of fluid in the
brain (hydrocephalus) that needs to be drained surgically. The child’s tonsils
and adenoids may be removed in order to lower the risk of apnea. The
shape of bowed legs can be corrected surgically, but doctors disagree about
the value of surgery intended to lengthen the legs.
Children with achondroplasia may need extra dental work because of
the mismatch between the size of their teeth and the size of the jaw. In
addition, they should be watched carefully for recurrent ear infections in
order to minimize the risk of deafness and later learning difficulties.
There is disagreement among doctors as of the early 2000s regarding
treatment with human growth hormone. It is considered an experimental
treatment. Many doctors are concerned that using growth hormone in
children with achondroplasia will lead to abnormal bone deposits and
worsening of the spinal curvature.

Prognosis

The prognosis of a baby with achondroplasia depends on whether the
defective gene is inherited from only one parent or from both. A child
who inherits the gene from both parents will die before birth or shortly
after birth from respiratory failure. About 3 percent of children with
achondroplasia die suddenly and unexpectedly during the first year of life,
usually from compression of the spinal cord.
A child who has one normal copy of the gene will usually have
normal intelligence and a normal life expectancy in spite of bone and
joint problems.

Prevention

There is no known way to prevent achondroplasia because the gene
responsible for the condition can undergo mutation in families with no
history of the disorder, and 75–80 percent of cases result from new
mutations.

The Future

Although new mutations of the gene associated with achondroplasia
cannot always be foreseen, people who have the condition should seek
genetic counseling before marriage. If the partner does not have the
mutation, there is a 50 percent chance with each pregnancy that the child
will have achondroplasia. If both spouses have the mutation, they have a
25 percent chance of a normal child with each pregnancy, a 50 percent
chance that the child will have achondroplasia, and a 25 percent chance
that the child will die before or shortly after birth.
SEE ALSO Childhood obesity; Ear infection; Gigantism; Hydrocephalus

For more information
BOOKS
Adelson, Betty M. Dwarfism: Medical and Psychosocial Aspects of Profound Short
Stature. Baltimore, MD: Johns Hopkins University Press, 2005.
Barty, Billy. Little People of America, Inc., Souvenir Book. Studio City, CA: Billy
Barty Foundation, 1982.
Copeland, Michael, and Debra Copeland. Within Reach: An Inspirational
Journey into the Life, Legacy, and Influence of Billy Barty. Fairfax, VA: Xulon
Press, 2002.
PERIODICALS
Rabin, Roni. “It Seems the Fertility Clock Clicks for Men, Too.” New York
Times, February 27, 2007. Available online at http://query.nytimes.com/gst/