Achondroplasia
What is achondroplasia?
Achondroplasia is a genetic (inherited) bone disorder that occurs in one
in 20,000 live births. Achondroplasia is the most common type of dwarfism,
in which the child's arms and legs are short in proportion to body length.
Further, the head is often large and the trunk is normal size. The average
height of adult males with achondroplasia is 52 inches (or 4 feet, 4
inches). The average height of adult females with achondroplasia is 49
inches (or 4 feet, 1 inch).
What causes achondroplasia?
Achondroplasia is inherited by an autosomal dominant gene that causes
abnormal cartilage formation. Autosomal dominant inheritance means that
the gene is located on one of the autosomes (chromosome pairs 1 through
22). This means that males and females are equally affected. Dominant
means that only one gene is necessary to have the trait. When a parent has
a dominant trait, there is a 50 percent chance that any child they have
will also inherit the trait. So, in some cases, the child inherits the
achondroplasia from a parent with achondroplasia. The majority of
achondroplasia cases (80 percent), however, are the result of a new
mutation in the family - the parents are of average height and do not have
the abnormal gene.
As mentioned, persons with achondroplasia have a 50 percent chance to
pass the gene to a child, resulting in the condition. If both parents have
achondroplasia, with each pregnancy, there is a 50 percent chance to have
a child with achondroplasia, a 25 percent chance that the child will not
inherit the gene and be of average height, and a 25 percent chance that
the child will inherit one abnormal gene from each parent, which can lead
to severe skeletal problems that often result in early death.
Geneticists have found that fathers who are older than 45 have a higher
chance of having children with certain autosomal dominant conditions such
as achondroplasia, but no cause for the new mutations in sperm has been
discovered at this time.
The gene responsible for achondroplasia was discovered in 1994 making
accurate prenatal diagnosis available, in most cases.
What are the symptoms of achondroplasia?
The following are the most common symptoms of achondroplasia. However,
each child may experience symptoms differently. Symptoms may include:
- shortened arms and legs, with the upper arms and thighs more
shortened than the forearms and lower legs
- large head size with prominent forehead and a flattened nasal bridge
- crowded or misaligned teeth
- curved lower spine - a condition also called lordosis (or
"sway-back") which may lead to kyphosis, or the development of a small
hump near the shoulders that usually goes away after the child begins
walking.
- small vertebral canals (back bones) - may lead to spinal cord
compression in adolescence. Occasionally children with achondroplasia
may die suddenly in infancy or early childhood in their sleep due to
compression of the upper end of the spinal cord, which interferes with
breathing.
- bowed lower legs
- flat feet that are short and broad
- extra space between the middle and ring fingers (Also called a
trident hand.)
- poor muscle tone and loose joints
- frequent middle ear infections which may lead to hearing loss
- normal intelligence
- delayed developmental milestones such as walking (which may occur
between 18 to 24 months instead of around one year of age)
The symptoms of achondroplasia may resemble other problems or medical
conditions. Always consult your child's physician for a diagnosis.
How is achondroplasia diagnosed?
Achondroplasia can be diagnosed before birth by fetal ultrasound or after
birth by complete medical history and physical examination. DNA testing is
now available before birth to confirm fetal ultrasound findings for
parents who are at increased risk for having a child with achondroplasia.
Treatment of achondroplasia:Currently, there is no way to prevent or treat achondroplasia,
since the majority of cases result from unexpected new mutations.
Treatment with growth hormone does not substantially affect the height of
an individual with achondroplasia. Leg-lengthening surgeries may be
considered, in some very specialized cases.
Detection of bone abnormalities, particularly in the back, are
important to prevent breathing difficulties and leg pain or loss of
function. Kyphosis (or hunch-back) may need to be surgically corrected if
it does not disappear when the child begins walking. Surgery may also help
bowing of the legs. Ear infections need to be treated immediately to avoid
the risk of hearing loss. Dental problems may need to be addressed by an
orthodontist (dentist with special training in the alignment of teeth.)
There is research into the family of genes called fibroblast growth factors,
in which the gene that causes achondroplasia is included. The goal
is to understand how the faulty gene causes the features seen in
achondroplasia, in order to lead to improved treatment. These genes
have been linked to many heritable skeletal disorders.
More Information
Schneider Children's Hospital Division
of Endocrinology
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