Pseudoachondroplasia
Overview
Pseudoachondroplasia is moderately severe skeletal dysplasia characterized by disproportionate short stature, hypermobile joints, normal head size, and normal length and appearance at birth. Individuals with this condition are usually not diagnosed until early childhood. Complications of this disorder include early-onset arthritis of the weight-bearing joints and other orthopedic complications. This genetic disorder has autosomal dominant inheritance.
Symptoms
Individuals with pseudoachondroplasia have disproportionate short stature, normal-sized heads, limb differences, and rhizomelic shortening of their limbs; rhizomelic means "root limb." Rhizomelic shortening of the limbs means that those segments of a limb closest to the body (the root of the limb) are more severely affected. In individuals with pseudoachondroplasia, the upper arms are shorter than the forearms, and the upper leg (thigh) is shorter than the lower leg. In addition to shortened limbs, individuals with pseudoachondroplasia have other characteristic limb differences. They have a limited ability to rotate and extend their elbows. They also have joint laxity, particularly of the hands, ankles, and knees. Because of this joint laxity, they can be bowed legged or knock kneed and may have in-turned toes. Their hands and feet are short and broad, as are their fingers and toes. Their fingers and other joints are hyperextensible, or very flexible. In addition to limb differences, individuals with pseudoachondroplasia have other characteristic skeletal differences. Because of malformed vertebra and lax ligaments, individuals with pseudoachondroplasia can have spinal problems, including kyphosis (hunchback), lordosis (swayback), and scoliosis.
Diagnosis
Pseudoachondroplasia is diagnosed by a combination of physical exam, x rays, and molecular testing. The characteristic findings of short stature and rhizomelic shortening of the limbs become apparent around two years of age and become more pronounced over time. In addition to being diagnosed by physical examination, individuals with pseudoachondroplasia have some specific bone changes that can be seen on an x ray. A DNA blood test to look for mutations in the COMP gene may also help clarify the diagnosis. Unlike other skeletal dysplasias, pseudoachondroplasia cannot be diagnosed by a prenatal ultrasound or sonogram because the characteristic changes in the bones and the growth delays do not appear until the child is two years of age. The diagnosis of pseudoachondroplasia can be made prenatally by DNA testing if the mutation for that family has been characterized. A sample of tissue from a fetus is obtained by either chorionic villi sampling (CVS) or by amniocentesis. Chorionic villi sampling is generally done between 10 and 12 weeks of pregnancy, and amniocentesis is done between 14 and 18 weeks of pregnancy. Chorionic villi sampling involves removing a small amount of tissue from the developing placenta. The tissue in the placenta contains the same DNA as the fetus. Amniocentesis involves removing a small amount of fluid from around the fetus. This fluid contains some fetal skin cells from which DNA can be isolated. The fetal DNA is then tested to determine if it contains the mutation that is responsible for pseudoachondroplasia in that family.
Prognosis
The prognosis for most people with pseudoachondroplasia is very good. In general, they have minimal medical problems, normal IQ, and most achieve success and have a long life, regardless of their stature. The most serious medical barriers to an excellent prognosis are the orthopedic complications and early-onset arthritis that can limit the activities of an individual and cause significant pain. Most individuals with pseudoachondroplasia will have multiple orthopedic surgeries over their lifetime.
Treatment
There is no cure for pseudoachondroplasia. All children with pseudoachondroplasia should have their height, weight, and head circumference measured and plotted on growth curves specifically developed for children with pseudoachondroplasia. The most common medical complication is early-onset osteoarthritis and other orthopedic problems. Some patients experience significant pain that can be controlled by analgesics, although the effectiveness of various forms of analgesics has not been thoroughly studied in pseudoachondroplasia. Early-onset osteoarthritis is caused by malformations of the weight-bearing joints and deficient cartilage production. Approximately 50% of patients with pseudoachondroplasia will require hip replacements. By being aware of the potential medical complications and by taking some preventative measures, it may be possible to avoid or delay the onset of some of the long-term consequences of these complications.