Achondroplasia

Definition:
Achondroplasia is a genetic (inherited) bone disorder that occurs in one in 25,000 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).

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When a baby is developing in the womb, the skeleton first forms out of cartilage, which then this develops into bone (except in certain areas such as the nose or ears where cartilage remains).

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In achondroplasia, this process doesn’t happen as it should, especially in the long bones of the arms and legs. Instead, the cartilage cells in the growth plates turn only very slowly into bone.


Symptoms:

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)

These bone abnormalities can lead to a range of health problems, as well as psychological trauma caused by the stigma of looking different from the crowd. Most children with achondroplasia have normal intelligence.

Babies with achondroplasia may have poor muscle tone and be slow to stand and walk. Spinal problems can develop, and the lower part of the legs may become bowed. There may be dental problems because teeth are often crowded and poorly aligned. Middle ear infections occur frequently and can cause mild to moderate hearing loss.

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Causes:
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.

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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.

There are two other syndromes with a genetic basis similar to achondroplasia: hypochondroplasia and thanatophoric dysplasia.

In approximately 75 per cent of cases the problem results from a new mutation of a gene (that is, neither parent carries the faulty gene), but in some cases a child inherits achondroplasia from a parent who also has the condition. There is also a link with older fathers, over the age of 40.

Diagnosis:

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.A DNA test can be performed before birth to detect homozygosity, wherein two copies of the mutant gene are inherited, a lethal condition leading to stillbirths.
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Radiologic findings:
A skeletal survey is useful to confirm the diagnosis of achondroplasia. The skull is large, with a narrow foramen magnum, and relatively small skull base. The vertebral bodies are short and flattened with relatively large intervertebral disk height, and there is congenitally narrowed spinal canal. The iliac wings are small and squared,[4] with a narrow sciatic notch and horizontal acetabular roof. The tubular bones are short and thick with metaphyseal cupping and flaring and irregular growth plates. Fibular overgrowth is present. The hand is broad with short metacarpals and phalanges, and a trident configuration. The ribs are short with cupped anterior ends. If the radiographic features are not classic, a search for a different diagnosis should be entertained. Because of the extremely deformed bone structure, people with achondroplasia are often double jointed.

The diagnosis can be made by fetal ultrasound by progressive discordance between the femur length and biparietal diameter by age. The trident hand configuration can be seen if the fingers are fully extended.

Another distinct characteristic of the syndrome is thoracolumbar gibbus in infancy.


Treatment :

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.

However, if desired, the controversial surgery of limb-lengthening will lengthen the legs and arms of someone with achondroplasia.

Usually, the best results appear within the first and second year of therapy.  After the second year of GH therapy, beneficial bone growth decreases. Therefore, GH therapy is not a satisfactory long term treatment.

Prognosis:
People with achondroplasia seldom reach 5 feet in height. Intelligence is in the normal range. Infants who receives the abnormal gene from both parents do not often live beyond a few months.

Complications:
* Clubbed feet
* Fluid build up in the brain (hydrocephalus)

Epidemiology:

Achondroplasia and is one of several congenital conditions with similar presentations, such as osteogenesis imperfecta, multiple epiphyseal dysplasia tarda, achondrogenesis, osteopetrosis, and thanatophoric dysplasia. This makes estimates of prevalence difficult, with changing and subjective diagnostic criteria over time. One detailed and long-running study in the Netherlands found that the prevalence determined at birth was only 1.3 per 100,000 live births.[8] However, another study at the same time found a rate of 1 per 10,000.


Prevention:

Genetic counseling may be helpful for prospective parents when one or both have achondroplasia. However, because achondroplasia most often develops spontaneously, prevention is not always possible.

* Reviewed last on: 11/2/2009
* Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

Disclaimer: This information is not meant to be a substitute for professional medical advise or help. It is always best to consult with a Physician about serious health concerns. This information is in no way intended to diagnose or prescribe remedies.This is purely for educational purpose.

Resources:
http://www.yalemedicalgroup.org/stw/Page.asp?PageID=STW026342
http://en.wikipedia.org/wiki/Achondroplasia
http://www.bbc.co.uk/health/physical_health/conditions/achondroplasia1.shtml
http://www.umm.edu/ency/article/001577all.htm

http://wikis.lib.ncsu.edu/index.php/Group_6_HYALOS_(hyaline_cartilage)

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