Achondroplasia is a rare genetic (inherited) bone disorder that occurs in one in 15,000 to 40,000 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 about 52 inches (or 4 feet, 4 inches). The average height of adult females with achondroplasia is about 49 inches (or 4 feet, 1 inch).
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, result from a new mutation in the families in which the parents are average height and do not have the abnormal gene.
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.
The following are the most common symptoms of achondroplasia. However, each child may experience symptoms differently. Symptoms may include:
- Shortened arms, legs, and fingers 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) which 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
- Apnea or stopping or slowing down of breathing for short periods
- 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)
- Normal lifespan
The symptoms of achondroplasia may resemble other problems or medical conditions. Always consult your child's doctor for a diagnosis.
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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.
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).
Research into the family of genes called fibroblast growth factors discovered the gene that causes achondroplasia . 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 other heritable skeletal disorders.
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