Keypoints

• Achondroplasia is a genetic disorder of bone growth.
• Achondroplasia is the most common cause of short stature with disproportionately short limbs.
• The appearance of the person with achondroplasia is characteristic.
• Intelligence is entirely normal in people with achondroplasia.
• Complications of achondroplasia can affect the brain and the spinal cord.

• The parents of children with achondroplasia are usually normal.
• Achondroplasia is inherited as a dominant trait.
• Achondroplasia can be diagnosed before birth.

What is Achondroplasia?

Achondroplasia is a genetic (inherited) condition that results in abnormally short stature. All persons with achondroplasia are little people. The average height of an adult with achondroplasia is 131 cm (52 inches, or 4 foot 4) in males and 124 cm (49 inches, or 4 foot 1) in females.

Achondroplasia is the most common cause of short stature with disproportionately short limbs.

Achondroplasia is a disorder of bone growth. Although achondroplasia literally means "without cartilage formation," the problem in achondroplasia is not in forming cartilage but in converting it to bone, particularly in the long bones.

Achondroplasia is one of the oldest known birth defects. The frequency of achondroplasia is estimated to range from about 1 in 10,000 births in Latin America to about 12 in 77,000 in Denmark. An average figure worldwide is approximately 1 in 25,000 births.

Achondroplasia is the most frequent form of short-limb dwarfism. Affected individuals exhibit short stature caused by rhizomelic shortening of the limbs, characteristic facies with frontal bossing and midface hypoplasia, exaggerated lumbar lordosis, limitation of elbow extension, genu varum, and trident hand.

Achondroplasia is a genetic disorder of bone growth that is evident at birth. It affects about one in every 25,000 births and it occurs in all races and in both sexes. Its depiction in ancient Egyptian art makes it one of the oldest recorded birth defects.

It is the most common of a group of growth defects characterized by abnormal body proportions — affected individuals have arms and legs that are very short, while the torso is more nearly normal size.

The word achondroplasia is derived from Greek and means "without cartilage formation," although individuals with achondroplasia do have cartilage. During fetal development and childhood, cartilage normally develops into bone, except in a few places, such as the nose and ears. In individuals with achondroplasia, something goes wrong during this process, especially in the long bones (such as those of the upper arms and thighs). The rate at which cartilage cells in the growth plates of the long bones turn into bone is slow, leading to short bones and reduced height.

Achondroplasia is the most common form of short-limb dwarfism. It occurs in approximately 1 in 26,000 to 1 in 40,000 births. The characteristic features of achondroplasia are apparent at birth. These include typical facial features, disproportionate short stature, and rhizomelic (the proximal ends of the limbs) shortening. Diagnosis of achondroplasia is made by physical exam and skeletal x-rays. Most individuals have normal intelligence. Infants and children often have motor delays but cognitive delays are not present. A special infant developmental chart has been made for children with achondroplasia. Final adult height is in the range of 4 feet. Special achondroplasia growth curves have also been made. These should be used by the child's pediatrician to monitor growth and development.

The facial features include a large head with a prominent forehead. The midface is often small with a flat nasal bridge and narrow nasal passages. In infancy and childhood, middle ear infections often occur because of the small nasal passages and Eustachian tube dysfunction. If the ear infections are left untreated, hearing loss can occur. "Ear tubes" are probably indicated for a child with multiple ear infections. The jaw appears to be prominent. Occasionally dental crowding can occur since the jaw is small.

Respiratory problems can occur in infants and children. Airway obstruction can be "central" in origin (due to foramen magnum compression) or "obstructive" in origin (due to narrowed nasal passages). Symptoms of airway obstruction include snoring, sleeping with the neck in a hyperextended position, or apnea. A sleep study is done if these symptoms exist to determine the cause of the airway obstruction. Treatment depends on the type of obstruction. I f central obstruction is present, a foramen magnum decompression is performed (see below). If obstructive airway obstruction is present, therapy can include tonsillectomy, adenoidectomy, or tracheostomy placement.

The limbs have rhizomelic shortening. The legs are straight in infancy but when a child starts walking, they can develop a valgus (knock-knee) position. As the child continues to walk, the legs assume a varus (bowleg) appearance. Occasionally, children have these leg curvatures corrected. The fingers and toes are short. Infants have a thoracolumbar kyphosis in the sitting position. Since infants with achondroplasia often have reduced tone, it is recommended that they not be placed in umbrella-type strollers, jumpers, or swings that do not provide good back support since these devices may lead to the development of a gibbus or hump in the back. As the child begins to walk, the kyphosis disappears and the back assumes a lordotic posture. If a child is delayed in walking, the spine should be monitored closely for signs of gibbus formation.

Neurologic complications can occur in achondroplasia. In infancy, hydrocephalus can develop. Infants should be monitored monthly with measurements of their head circumference to detest a rapidly enlarging head size that can indicate hydrocephalus. The child's pediatrician should have a copy of the head circumference curves for children with achondroplasia. Radiologic studies are indicated if the head circumference increases disproportionately or if symptoms of hydrocephalus develop. Common radiologic procedures include head ultrasound, CT scan, or MRI of the head. If intervention is necessary, a ventriculoperitoneal (VP)-shunt is placed to relieve the excess pressure. Infants should also be monitored for foramen magnum compression. The foramen magnum is the opening at the base of the skull through which the brainstem and cervical spinal cord exit. Individuals with achondroplasia have narrowed foramen magnums that can then compress the brain stem and spinal cord. Symptoms of narrowing include apnea (cessation of breathing) and cervical myelopathy. CT-scans and MRI scans are done to examine the size of the foramen magnum. If a child is having symptoms, a neurosurgical procedure called foramen magnum decompression is done to enlarge the foramen magnum and alleviate further symptoms. Adolescents and adults are at risk of developing lumbosacral spinal stenosis. The lumbar spinal cord or nerve roots become compressed producing neurologic symptoms. Initial symptoms include weakness, tingling, and pain of the legs. Often the pain is alleviated by assuming a squatting position. As the condition worsens, pain in the low back or buttocks occurs. Diagnosis is made by a neurologic exam, SERs (somatic evoked responses), and CT or MRI scans. Treatment is a neurosurgical procedure called a lumbar laminectomy.

Achondroplasia is an autosomal dominant condition (see the genetics section for further details). This means that a person with achondroplasia has a 1 in 2 or 50% chance of having children with achondroplasia. However, approximately 75% of individuals with achondroplasia are born to parents of average size. In these cases, achondroplasia is due to a new mutation or genetic change. The gene for achondroplasia has been found. It is called fibroblast growth factor receptor 3 (FGFR3). Often, two individuals with achondroplasia have children. These couples are at risk of having a child with 2 copies of the changed gene or double homozygosity. Infants with homozygous achondroplasia are either stillborn or die shortly after birth. Couples at risk often have prenatal diagnosis via serial ultrasounds. A DNA test is now also available to detect double homozygosity.

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