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.
The baby exhibits some decrease in muscle tone (hypotonia). Because of the large head, especially compared to rest of the body, and the decreased muscle tone, the child with achondroplasia will run behind "schedule" in reaching the usual motor milestones. The schedule to which an achondroplastic child's development should be compared is not that for all children in the general population, but rather the timetable followed by achondroplastic children.
Intelligence is entirely normal in patients with achondroplasia. Enlargement of the brain (megalencephaly) is common and normal with achondroplasia.
The molecular diagnosis of achondroplasia before birth is feasible. The limited number of DNA changes responsible for achondroplasia and the ease with which they can be detected provide the basis for a simple method for prenatal diagnosis. In families in which both parents have achondroplasia, prenatal diagnosis may be particularly useful, the aim being to distinguish fatal homozygous achondroplasia from heterozygous achondroplasia (with one copy of the achondroplasia gene) from normal. Diagnosis before birth is accomplished by examining cells obtained by chorionic villus sampling (CVS) or amniocentesis.
When special problems complicate achondroplasia, prompt and expert intervention is important. For example, the foramen magnum (the large opening under the skull) should be surgically enlarged in cases of severe narrowing (stenosis) and compression of the spinal cord. The back of patients with achondroplasia can develop a marked sway (lordosis) to the lower back while abnormalities in the mid-back may cause a small hump (kyphosis) in infancy and compression of the spinal cord in adolescence. The spinal cord compression can require surgery to decompress it. Orthopedic procedures may be required for lengthening of the limb bones and correction of bowed legs (usually after full growth has been achieved). Surgery (lumbar laminectomy) is also indicated when spinal stenosis (narrowing) causes symptoms, which tends to be evident in young adults.
Disproportion between the brain and the base of the skull can sometimes result in hydrocephalus ("water on the brain") which needs to be promptly detected and treated.
The large head with achondroplasia increases the chance of bleeding within the baby's head during vaginal delivery. This should be taken into account in planning the birth and postnatal care. The brainstem (which contains a center for controlling respiration) may be compressed in achondroplasia and contribute to abnormal breathing.
Pregnant women with achondroplasia should have their babies delivered by cesarean birth.
Middle ear infections are frequent and can lead to mild to moderate hearing loss. Therefore, ear infections should be readily suspected and promptly and fully treated with antibiotics or ear tubes. Dental crowding is also common. Teeth should be straightened and, if necessary, removed to alleviate this problem.
Control of obesity is essential. The child with achondroplasia must not be allowed to become overweight. Adults with achondroplasia should also monitor and control their weight because excess weight aggravates back and joint problems.
Treatment with human growth hormone, which is still considered experimental, has been preliminarily reported to increase the growth rate.
In only about an eighth of cases is the gene inherited from a parent who has achondroplasia. Conversely, about seven-eighths of cases are due to a new mutation (a new change in the gene). This means that most cases of achondroplasia occur sporadically (out of the blue) and are the result of a new mutation in a sperm or ovum of one of the normal- appearing parents. The chance of a new mutation rises with the age of the father. As early as 1912 it was noted that sporadic (new) cases were more often last-born than first-born children. This fits with the fact that the chance of an achondroplastic birth has been shown to increase with paternal age (age of the father).
What if someone with achondroplasia has children?
What if two people with achondroplasia have children?
Homozygous achondroplasia, although fatal, has led to insights into other medical conditions. For example, similarities were noticed between homozygous achondroplasia and a condition called thanatophoric dwarfism. It was proposed that the two disorders might be due to different mutations in the same gene and this has proven to be the case. Another skeletal disorder called hypochondroplasia has also turned out to be due to yet another mutation (change) in the same gene. Achondroplasia, thanatophoric dwarfism, and hypochondroplasia are thus due to mutational changes at the same gene locus.
What gene causes achondroplasia?
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.