Achondroplasia

What is Achondroplasia?

Achondroplasia is a rare genetic bone disorder. The word achondroplasia means “without cartilage formation”. Cartilage is a smooth, elastic connective tissue that makes up the skeleton during early development. As a baby develops, cartilage turns into a bone in most parts of the body. But in achondroplasia, cartilage is not converted to bone, especially in the long bones of the arms and legs.

So a child’s arms and legs become disproportionate to a body’s length. The average height of adult males with achondroplasia is 4 feet, 4 inches and in adult females with this disorder it is about 4 feet, 1 inch.

Occurrence

Achondroplasia is the most common type of short limbed dwarfism. It is seen in 1 in 15,000-40,000 newborns. Males and females are equally affected.

Causes

Mutations (changes) in Fibroblast growth factor receptor (FGFR3) gene causes achondroplasia. This gene makes a protein that is responsible for converting cartilage to bone. This protein is also responsible for development and maintenance of bone and brain tissue. Two specific mutations in the FGFR3 gene make the protein to be overly active, which in turn interferes with skeletal development and leads to bone growth disturbances.  

Inheritance pattern

The mode of inheritance is autosomal dominant, which means a single copy of the altered gene in each cell is sufficient to cause the disorder. The altered copy of the gene could be from either of the parents or because of mutations in the FGFR gene. In 80% of the cases, this disorder is due to spontaneous genetic mutation, whereas in the remaining 20% it is inherited from the parent. 

If either of the parents has the condition, the child has a 50 percent chance of getting it.

If both parents have the condition, the child has:

·      A 25% chance of normal stature

·      A 50% chance of having one defective gene that causes achondroplasia

·      A 25% chance of inheriting two defective genes, which could result in a severe form of achondroplasia that causes extreme shortening of the bones and an underdeveloped rib cage.

Signs and Symptoms

Some of the common physical characteristics of the disease are listed below.

Short upper arms and thighs, in comparison to body height

A disproportionately large forehead compared to the body

Flattened nasal bridge

Crowded or crooked teeth

Curved lower spine

Flat feet that are short and broad

Bowed lower legs

Poor muscle tone and loose joints

Delayed developmental milestones, such as walking (which may occur between 18 to 24 months instead of around one year of age)

Diagnosis

Before birth: During an ultrasound examination, some features of achondroplasia could be seen. If an abnormally large head is seen, a doctor may suspect for this disorder and prenatal genetic tests to identify mutations in FGFR gene could be done to confirm the diagnosis. Genetic tests will be done through amniocentesis.

After birth: Complete physical examination, X- rays to measure infant’s bones and blood tests to detect gene mutations.

Treatment

There is no specific treatment for achondroplasia. Treatment will be based on the existing symptoms.

1. Apnea: This happens when a baby stops breathing for 15 to 20 seconds. These breathing problems are corrected by removing the tonsils and adenoids.

2. Children with middle ear dysfunction may require ear tubes.

3. Use of growth hormones to increase the height has shown only little long term effects.

4. To prevent spinal deformities, unsupported sitting in the first 12-18 months of life is strictly prohibited.

5. Surgeries may be required to correct hunch-back and bowed legs.

Children with achondroplasia have normal lifespan and intelligence. They require adequate attention from the parents and the society to combat their physical illness.