“Pseudo” means “false.” Thereby, this disorder is one that resembles, but is clinically distinct from, achondroplasia.The incidence of pseudoachondroplasia is estimated at 1 in 30,000, however the birth prevalence is not yet known (2).
Pseudoachondroplasia results from a mutation in the gene coding for cartilage oligomeric matrix protein (COMP) (1). COMP is a normal constituent of the extra-cellular matrix in cartilage, ligaments, and tendons. Defective COMP results in the accumulation of proteoglycans within cartilage cells.
Both the epiphyses and metaphyses are affected in pseudoachondroplasia. Clinically, it is recognized as a form of short-limbed dwarfism, with body proportions similar to those of achondroplasia, yet with normal-sized heads and facial features.
The postnatal onset of short-limbed growth deficiency will not become apparent until between 18 and 24 months of age. Pseudoachondroplasia manifests itself over time. Ultimately, adult stature is between
82 and 130 cm.
Face and Skull
- normal head size and facial features
Trunk, Chest and Spine:
- disproportionately long trunk
- prominent abdomen
- exaggerated lumbar lordosis
- possible thoracolumbar kyphosis
- mild to moderate scoliosis
Arms and Legs:
What Are the X-Ray Characteristics?
The radiographic features of pseudoachondroplastic patients include short and broad long bones with flaring of the metaphyses. Epiphyseal ossification is delayed. The epiphyses appear irregular and fragmented. The hips and knees are primarily affected. Due to their dysplastic nature, the carpals ossify late.
In the pelvis, the acetabulum (hip socket) is shallow and accentuates hip dysplasia. The triradiate cartilage is also late to mature and ossify. Arthrograms are helpful in identifying joint surfaces and planning surgery for angular deformities. The capital femoral epiphyses are small and irregular in children; in adults, there is marked dysplasia of the femoral head. The femoral head is flattened and fragmented. This leads to hip joint incongruity and exacerbates the effects of hip subluxation.
X-rays of the spine show platyspondyly and flame-shaped anterior projections. The interpedicular distance does not progressively decrease in the lumbar spine. In the neck, lateral X-rays of the cervical spine may reveal odontoid hypoplasia. The vertebrae will at first seem deformed, but the irregularities generally disappear by adolescence. Flexion-extension radiographs should be obtained to rule out atlantoaxial instability. MRI scans of the cervical spine (static, flexion/extension views and CSF flow studies) are helpful in identifying any compression of the spinal cord.
The average length at birth is 49 cm, which is within the normal range. Pseudoachondroplasia is therefore not readily recognized at birth. But, lack of longitudinal growth manifests itself in the first 2 years of life (below 5th percentile on standard growth charts). By this point the abnormal gait is present and measurements suggest pseudoachondroplasia. Diagnosis is typically made between 1 and 4 years of age and is based on clinical examination and characteristic X-ray appearances. Prenatal testing is now available by direct DNA analysis. The test detects the abnormal COMP gene by mutation scanning. Prenatal diagnosis may be appropriate during pregnancy in women with pseudoachondroplasia. It must be stressed that the majority of cases are spontaneous mutations.
The cervical spine should be monitored for the presence of atlantoaxial instability. Lateral flexion-extension x-rays of the cervical spine is recommended, if a pre-existing abnormality such as hypoplastic odontoid is present. Posterior cervical decompression and fusion should be performed if the instability exceeds 8 mm or neurological symptoms (cervical myelopathy) occur. Scoliosis should be looked for and is managed similar to idiopathic curves. Lateral c-spine x-rays should be routinely obtained in all children with pseudoachondroplasia undergoing surgery for any reason.
Angular deformities around the knee are corrected using osteotomies. Careful pre-operative planning is essential to restore normal mechanical axes in sagittal and coronal planes (down the middle of the body). Since the epiphyses are distorted, intraoperative arthrography may be necessary to properly visualize the joint surfaces. The effect of ligamentous laxity on alignment should be ascertained as part of the pre-operative planning. Recurrence of deformity is common and several procedures may be necessary to achieve lower extremity skeletal alignment at maturity. Up to 50 percent of adults will require joint replacement surgery for early onset degenerative arthritis. Hip/ knee replacement surgery in patients with skeletal dysplasia is a technically demanding exercise due to abnormal skeletal size and shape. Subluxation of the hips is a combination of femoral deformity, failure of epiphyseal ossification, acetabular dysplasia (failure of hip socket development), and joint contractures (flexion and adduction). A combination of femoral and pelvic osteotomies may be necessary. Since the femoral head is flattened, a valgus proximal femoral osteotomy is preferred to a varus procedure. If the hip joint is not congruous, acetabular augmentation procedures (Chiari osteotomy or Shelf procedure) are used to salvage the hip.
Few problems, if any, occur and good general health can be expected.
Pseudoachondroplastic patients should look out for neurological symptoms such as weakness of the lower limbs, incontinence, pain in the legs, reduced endurance, and tingling/ numbness of the legs. These symptoms may indicate compression of the spinal cord in the neck.
Lower extremity pain of gradual onset or changes in walking (waddling/ limping) may also result from altered alignment of the legs. In later life, pain in the hips and knees is usually the result of degenerative arthritis.
Generally all skeletal dysplasias warrant multidisciplinary attention. Regular assessment by an orthopedist, geneticist, pediatrician, dentist, neurologist, and physical therapist will provide the most comprehensive treatment.
- Jones, Kenneth L. Recognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006.
- Posey, Karen L. Hayes, Elizabeth. Haynes, Richard. Hecht, Jacqueline T. 2004. Role of TSP-5/COMP in Pseudoachondroplasia. The International Journal of Biochemistry and Cell Biology. 36: 1005-1012.
- Scott, Charles I. Dwarfism. Clinical Symposium, 1988; 40(1);11-14.
- Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.
From Nemours' KidsHealth
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Genetic tests are done by analyzing small samples of blood or body tissues. They determine whether you, your partner, or your baby carry genes for certain inherited disorders.
Genetic testing has developed enough so that doctors can often pinpoint missing or defective genes. The type of genetic test needed to make a specific diagnosis depends on the particular illness that a doctor suspects.
Many different types of body fluids and tissues can be used in genetic testing. For deoxyribonucleic acid (DNA) screening, only a very tiny bit of blood, skin, bone, or other tissue is needed.
Genetic Testing During Pregnancy
For genetic testing before birth, pregnant women may decide to undergo amniocentesis or chorionic villus sampling. There is also a blood test available to women to screen for some disorders. If this screening test finds a possible problem, amniocentesis or chorionic villus sampling may be recommended.
Amniocentesis is a test usually performed between weeks 15 and 20 of a woman's pregnancy. The doctor inserts a hollow needle into the woman's abdomen to remove a small amount of amniotic fluid from around the developing fetus. This fluid can be tested to check for genetic problems and to determine the sex of the child. When there's risk of premature birth, amniocentesis may be done to see how far the baby's lungs have matured. Amniocentesis carries a slight risk of inducing a miscarriage.
Chorionic villus sampling (CVS) is usually performed between the 10th and 12th weeks of pregnancy. The doctor removes a small piece of the placenta to check for genetic problems in the fetus. Because chorionic villus sampling is an invasive test, there's a small risk that it can induce a miscarriage.
Why Doctors Recommend Genetic Testing
A doctor may recommend genetic counseling or testing for any of the following reasons:
- A couple plans to start a family and one of them or a close relative has an inherited illness. Some people are carriers of genes for genetic illnesses, even though they don't show, or manifest, the illness themselves. This happens because some genetic illnesses are recessive — meaning that they're only expressed if a person inherits two copies of the problem gene, one from each parent. Offspring who inherit one problem gene from one parent but a normal gene from the other parent won't have symptoms of a recessive illness but will have a 50% chance of passing the problem gene on to their children.
- A parent already has one child with a severe birth defect. Not all children who have birth defects have genetic problems. Sometimes, birth defects are caused by exposure to a toxin (poison), infection, or physical trauma before birth. Often, the cause of a birth defect isn't known. Even if a child does have a genetic problem, there's always a chance that it wasn't inherited and that it happened because of some spontaneous error in the child's cells, not the parents' cells.
- A woman has had two or more miscarriages. Severe chromosome problems in the fetus can sometimes lead to a spontaneous miscarriage. Several miscarriages may point to a genetic problem.
- A woman has delivered a stillborn child with physical signs of a genetic illness. Many serious genetic illnesses cause specific physical abnormalities that give an affected child a very distinctive appearance.
- The pregnant woman is over age 34. Chances of having a child with a chromosomal problem (such as trisomy) increase when a pregnant woman is older. Older fathers are at risk to have children with new dominant genetic mutations (those caused by a single genetic defect that hasn't run in the family before).
- A standard prenatal screening test had an abnormal result. If a screening test indicates a possible genetic problem, genetic testing may be recommended.
- A child has medical problems that might be genetic. When a child has medical problems involving more than one body system, genetic testing may be recommended to identify the cause and make a diagnosis.
- A child has medical problems that are recognized as a specific genetic syndrome. Genetic testing is performed to confirm the diagnosis. In some cases, it also might aid in identifying the specific type or severity of a genetic illness, which can help identify the most appropriate treatment.
A Word of Caution
Although advances in genetic testing have improved doctors' ability to diagnose and treat certain illnesses, there are still some limits. Genetic tests can identify a particular problem gene, but can't always predict how severely that gene will affect the person who carries it. In cystic fibrosis, for example, finding a problem gene on chromosome number 7 can't necessarily predict whether a child will have serious lung problems or milder respiratory symptoms.
Also, simply having problem genes is only half the story because many illnesses develop from a mix of high-risk genes and environmental factors. Knowing that you carry high-risk genes may actually be an advantage if it gives you the chance to modify your lifestyle to avoid becoming sick.
As research continues, genes are being identified that put people at risk for illnesses like cancer, heart disease, psychiatric disorders, and many other medical problems. The hope is that someday it will be possible to develop specific types of gene therapy to totally prevent some diseases and illnesses.
Gene therapy is already being studied as a possible way to treat conditions like cystic fibrosis, cancer, and ADA deficiency (an immune deficiency), sickle cell disease, hemophilia, and thalassemia. However, severe complications have occurred in some patients receiving gene therapy, so current research with gene therapy is very carefully controlled.
Although genetic treatments for some conditions may be a long way off, there is still great hope that many more genetic cures will be found. The Human Genome Project, which was completed in 2003, identified and mapped out all of the genes (about 25,000) carried in our human chromosomes. The map is just the start, but it's a very hopeful beginning.
Reviewed by: Larissa Hirsch, MD
Date reviewed: September 26, 2016