Achondroplasia

What is achondroplasia?

Achondroplasia, a bone growth disorder, is the most common cause of dwarfism. Achondroplasia means ‘absence of cartilage formation’. However, the defect in achondroplasia is not in the formation of the cartilage but is in the conversion of cartilage to bone (particularly the long bones). Achondroplasia may run in families or may result from a spontaneous mutation.

The average height of a man with achondroplasia is 52 inches (131 cm, or 4 foot 4 inches). A woman with achondroplasia reaches an average height of 49 inches (124 cm, or 4 foot 1 inch). The appearance of a person with achondroplasia is distinctive as they have a short height with disproportionately small limbs. The features of achondroplasia are apparent at birth. Achondroplasia can also be detected in the fetal stage.

How common is achondroplasia?

Achondroplasia affects nearly 1 in every 25,000 to 40,000 children. It is estimated that around 10,000 individuals in the United States and 150,000 persons worldwide suffer from achrondroplasia. It can affect all races and any gender. It is thought to account for almost 80% of the dwarfism cases in the world.

Effects of Achondroplasia

Achondroplasia is a short-limb dwarfing condition, which means that the limbs are affected more than the trunk. This is evident in patients’ sitting height, which is within the normal range. In achondroplasia, the proximal parts of limbs – the thighs and arms – are more severely affected than the legs, feet, hands, and forearms.

Cartilage is the flexible material that makes up the skeleton in infants. Over time it becomes ossified meaning that it is converted to bone. This happens over years and all bones are ossified by about 25 years of age. In patients with achondroplasia, the formation of cartilages occurs normally but the subsequent conversion of cartilage to bone is defective. This results in short limbs.

Signs and Symptoms

In people with achondroplasia, following symptoms are seen:

  • Low standing height
  • Normal sitting height
  • Delay in the development of motor skills (like head control, sitting, standing, and walking without support)
  • Tongue thrust or reverse swallow, which causes difficulty in speaking
  • Normal intelligence
  • Enlarged head
  • Recurrent middle ear infections
  • Enlarged tonsils
  • Misaligned teeth
  • Breathing problems
  • Narrow spinal canal
  • Pain in lower back, hip, knee, ankle or leg
  • Tingling, numbness, and weakness in leg
  • Curved spine
  • Bowed bones
  • Flat nasal bridge
  • Prominent lower jaw

Picture from Wolf Wikis North Carolina State University

Complications of Achondroplasia

The following complications are associated with achondroplasia:

  • In children younger than 4 years, the brain stem might get pressed due to short base of the skull. This may result in sudden death.
  • In teens, respiratory problems are frequent and may become severe enough to cause death.
  • In adults aged 25 to 50 years, heart-related problems are the most frequent causes of death.
  • Recurrent ear infections may cause hearing loss.
  • Due to pressure on the brain, various neurological complications may arise (loss of muscle tone, apnea, feeding problems, weakness of limbs, and sudden death).

Achondroplasia Causes

Achondroplasia can be caused by a random new mutation (in 80% cases) or through inherited defect in FGFR3 gene (in 20% cases). FGFR3 or fibroblast growth factor receptors are important for cell growth and wound healing. A mutation in FGFR3 can cause achondroplasia.

The gene responsible for achondroplasia can be passed on from parents to children as an autosomal dominant trait, which means just one copy of the defective gene, from any of the parents, is sufficient to produce the disease in the child. The disease can be transmitted in two ways:

  • When both parents have achondroplasia, 50% of their children have one copy of the defective gene and are affected. 25% offspring get two copies, which is fatal. Such children rarely survive beyond the first few months of life. 25% of the offspring, however, are unaffected.
  • When just one parent suffers from achondroplasia, there is 50% chance of transmitting the defective gene to each child.

In majority of the cases, a random new mutation in the gene causes achondroplasia. Becoming a father after 35 years of age may increase the risk of having a baby with achondroplasia.

Diagnosis and Tests

There are various tests to diagnose achondroplasia both in the unborn fetus and in early life.

Genetic testing

Blood samples of pregnant women can be used for genetic testing to reveal the presence of the FGFR3 mutation associated with achondroplasia. If both parents have achondroplasia, DNA testing should be performed to detect the presence of this condition in the unborn fetus.

Imaging studies

X-rays of the skull, spine, legs and arms can reveal the characteristic features of achondroplasia (like an undeveloped middle part of face, enlarged skull cap, shape of pelvis, prominent forehead, short neck of femur bone of leg, short fingers and toes, and short base of the skull).

A computed tomography (CT) scan or a magnetic resonance imaging (MRI) scan can show a narrow spinal canal. An ultrasound scan can detect limb defects only in the latter half of pregnancy.

Treatment of Achondroplasia

Medication

A growth hormone called somatotropin is given to the patients with achondroplasia. The therapy works best when started as early as possible.

Surgery

Most of the patients with achondroplasia report problems with the spine, which can be addressed in the following manner:

  • For a narrow spinal canal, a surgical procedure called laminectomy can be performed to remove the back arch of a vertebra.
  • Curved spine usually does not need treatment and the problem goes away on its own. However, sitting without a support is discouraged. In rare cases, a surgery is performed for the curved spine.
  • Surgery may be required for bowed legs. Usually a procedure called osteotomy (cutting of the bone) is performed, in which small cuts are made in the bones.
  • Limb lengthening (of arms and legs) is also a recommended procedure.

Other therapies

  • Dental problems may require orthodontic treatment.
  • Children with persistent speech problems may benefit from speech therapy.
  • In children who get recurrent middle ear infections, it is important to recognize and treat the infections as soon as possible to prevent hearing loss.
  • Enlarged tonsils may require surgery.
  • Care should be taken to avoid respiratory complications such as pauses in breathing (apnea) and pneumonia.
  • Obesity is frequently seen in patients of achondroplasia. Therefore, diet should be regulated since from the early years of life.

References :

http://www.genome.gov/19517823

http://emedicine.medscape.com/article/1258401-overview

http://www.nlm.nih.gov/medlineplus/ency/article/001577.htm

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