Achondroplasia

Definition

Achondroplasia is a genetic disorder that causes short stature called dwarfism and a large head, also known as macrocephaly. It is a disorder in which bone and cartilage do not grow normally. It is the most common cause of dwarfism.

This condition leads to patients reaching a full-grown height of less than four feet. The greatest shortening occurs in the bone between the shoulder and the elbow and the bone between the hip and the knee. There may also be underdevelopment of the face.

Causes

Achondroplasia is a genetic disorder. It is caused by mutations in the FGFR3 gene. This gene prevents the growth of cartilage at the growth plate. FGFR3 encodes a protein called Fibroblast Growth Factor Receptor 3. This protein is the site of action of a major growth factor responsible for lengthening bones. When this growth factor cannot act properly due to the absence of its receptor, the growth of bones, at the growth plate's cartilage, is slowed. This leads to shorter bones, abnormally shaped bones, and shorter stature.

The gene for achondroplasia can be passed from one generation to the next. If a parent has the disorder, there is a 50% chance of passing the gene for achondroplasia to children. In most cases of achondroplasia, it more commonly is the result of a sudden genetic defect that occurs in the developing embryo.

Risk Factors

Those at risk of inheriting achondroplasia are:

  • Children of a parent with achondroplasia
  • Children of normal-sized parents who carry a mutated FGFR3 gene
  • Advanced paternal age causing spontaneous mutations

Symptoms

Symptoms of achondroplasia include:

  • Short stature, a long trunk, and shortened limbs, which are noticeable at birth
  • An adult height of between 42-56 inches
  • A head that is large and a forehead that is prominent
  • Portions of the face can be underdeveloped
  • At birth, the legs appear straight, but as a child begins to walk, he or she develops a knock-knee or bowed-leg deformity
  • The hands and the feet appear large, but the fingers and toes are short and stubby
  • Straightening of the arm at the elbow may be limited, but usually does not keep a patient from doing any specific activities
  • Children may develop an excessive curve of the lower back and a waddling walking pattern
  • Dental problems

Other common symptoms include:

  • Weight control problems
  • Dental problems from overcrowding of teeth
  • Neurologic and respiratory problems
  • Fatigue, pain, and numbness in the:
    • Lower back
    • Spine

Diagnosis

The doctor will ask about your symptoms and medical history. A physical exam will be done. The strength of your extremities and your bladder control will be evaluated. Weakness and loss of bladder are both signs of spinal stenosis, which is a narrowing of the spinal canal. It is important to follow the doctor's advice to make sure that spinal stenosis does not develop.

Your bodily fluids may be tested. This can be done with blood tests.

Images may be taken of your bodily structures. This can be done with:

  • Radiographs
  • X-rays
  • MRI scan
  • CT scan
Spinal Stenosis
Stenosis of spine with punched nerve
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Treatment

Unfortunately, there is currently no treatment that can cure this condition. Scientists are exploring ways to create alternate growth factors which can bypass the missing receptor and lead to normal bone growth. They may offer the possibility of enhanced stature to future families who have children with achondroplasia.

Medication

Treatment with human growth hormone has been used for over a decade. It effectively increases bone growth rate, at least in the first year of life. There have been few studies looking at whether children treated with growth hormone achieve greater adult height.

Surgery

Surgery is sometimes needed to correct specific skeletal deformities.

  • Spinal fusion —a surgery to permanently connect otherwise separate vertebrae
  • Laminectomy —a surgery to open the spinal canal to relieve pressure on the compressed spinal cord from spinal stenosis
  • Osteotomy—the bones of the leg are cut and allowed to heal in the correct anatomical position. This procedure is for patients with severe knock-knee or bowed legs.

Osteotomy has primarily been used to correct deformities. In recent years, bone lengthening procedures have been used for many short children, including those with achondroplasia. The procedures are lengthy, traumatic, and demanding for both children and their families. Complications, sometimes serious, are common. One center has reported an average leg length gain of about seven inches and an average increase in arm length of about four inches for achondroplastic individuals who undergo surgery. The combination of growth hormone therapy followed by lengthening surgery may provide benefit in achieving near-normal stature and proportions.

Prevention

There are no steps to prevent a genetic disorder. Genetic counseling can be used to discuss the chances that your child will have achondroplasia. You may consider this counseling if you are planning on having a child and have a family history of genetic conditions.

Revisions

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