About Hypochondroplasia

A two and half year old child with hypochondroplasia

“Hypo” is a prefix meaning “below” or “less.” It follows that this dysplasia is considered a more mild or atypical form of achondroplasia. The incidence of hypochondroplasia is approximately 180,000 to 312,000 live births (2).

Hypochondroplasia is genetically heterogeneous. Due to its mild nature, it is often times difficult to differentiate between “familial” shortness of stature and hypochondroplasia.

How Hypochondroplasia Is Inherited

The mode of inheritance for hypochondroplasia is autosomal dominant. Most cases result from spontaneous mutations (4).

Causes of Hypochondroplasia

Hypochondroplasia is genetically heterogeneous. Approximately one-half of hypochondroplastic patients have a mutation within the fibroblast growth factor receptor 3 (FGFR3) gene (2).

Physical Characteristics

Due to its mild nature, it is often times difficult to differentiate between “familial” shortness of stature and hypochondroplasia. Hypochondroplasia seems to be the grey area between achondroplasia and being constitutionally shorter than average. The average adult height of hypochondroplastic patient varies between 52 and 58 inches.

Drawing of children with hypochondroplasia's faces
Face and Skull
  • average head circumference or slight macrocephaly
  • slightly prominent forehead
Trunk, Chest and Spine:
Arms and Legs:
  • short and stocky limbs
  • mild genu varus in infancy
  • ligamentous laxity with limited elbow extension and supination
  • broad and short hands without trident formation
  • with age, prominent aching knees, elbows and ankles
What Are the X-Ray Characteristics?
A hypochondroplasia patient

The major radiographic features of hypochondroplasia include narrowing of interpediculate distances with anterioposterior shortening of pedicles. Vertebral bodies in lumbar region of spine have increased dorsal concavity. The height of the vertebral bodies is normal. The deformities of the spine, however, are not as pronounced as in the case of achondroplasia.

The pelvis is square with short ilia, although the flare of the iliac crests is normal. The sacrum is hypoplastic and low set on the iliac bones, effectively narrowing the transverse diameter of the pelvis. The tubular bones are short and with mild metaphyseal flare (most evident at the knees). The styloid processes of the ulnae are frequently long. Femoral necks are short and broad.

Distal fibulae are long in comparison to tibia. In children, growth plates of distal femurs exhibit a shallow, V-shaped indentation. This is due to slower enchondral bone growth at the center of the growth plate as compared to growth at the periphery. Again, this change is more mild in hypochondroplasia than in achondroplasia. Generalized brachydactyly is mild to moderate. Occasionally, the neurocranium is slightly larger.

Making the Diagnosis

Considering that the skeletal deformities of hypochondroplasia are moderately similar to those of achondroplasia, radiographic findings must be well-scrutinized to give a correct diagnosis. The best features to examine are the skull and pelvis; each are more severely affected in the case of achondroplasia.

In order to differentiate between hypochondroplasia and familial short stature, vertebral and pelvic changes should be considered. Vertebral abnormalities are characteristic only of hypochondroplasia. The appearance of the long bones may be similar to metaphyseal chondrodysplasia, Schmid type. Again, the differential feature is the vertebral abnormalities, which are only present in hypochondroplastic patients. It is difficult to diagnosis hypochondroplasia in infancy, although birth length may be slightly below average. By 3 years of age, slow growth and bowlegs are early indicators of this skeletal dysplasia.

Musculoskeletal Problems
Genu Varum

Genu varum and outward bowing become pronounced as children age and weight bearing increases. Surgical straightening may be necessary.

Inversion of the Feet

Inversion of the feet may result because of the relatively longer fibulae.

Considerable Discomfort

Considerable discomfort in the knees, ankles, or elbows may occur, especially during childhood. Into adulthood, the pain is most prominent in the lower back.

Spinal Stenosis

Spinal stenosis may result in cord compression. Symptoms include activity-related leg pain that is relieved on squatting down, tingling, pins and needles, numbness in the feet (paraesthesias), weakness of the legs, or disturbances in control of bladder or bowel function (incontinence). X-rays, CT and MRI scans of the lower spine, confirm the diagnosis. Obesity greatly increases the risk of this problem developing.

Problems Elsewhere in the Body
Developmental Delay

Approximately 10 percent of hypochondroplastic persons have learning problems.

Neurological Complications

Compressive myelopathy and radiculopathy occur, albeit less frequent
than achondroplasia.


Woman who become pregnant often times require a cesarean section, albeit vaginal delivery is still possible.

What to Look for

Considering that the course and complications of hypochondroplasia are slightly different from achondroplasia, it is important for a radiologist to correctly diagnosis this specific skeletal dysplasia early on.

For children and adults of any size or stature, obesity should be avoided. However, in the case of hypochondroplasia, patients must put forth greater effort to stay active and physically fit. Increased weight bearing of the joints can lead to extreme discomfort and possible neurological complications. Diminishing motor milestones, decreased endurance, apnea or any neurological symptoms should be quickly evaluated by an experienced physician.

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.

  1. Desch, Larry W. Horton, Willaim A. An Autosomal Recessive Bone Dysplasia Syndrome Resembling Hypochondroplasia. Pediatrics. 75, No 4: 786-789. 1985.
  2. Jones, Kenneth L. Recognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006
  3. Newman, Donald E. Dunbar, Scott. Hypochondroplasia. Journal of the Canadian Association of Radiologists. 26: 95-103. 1975.
  4. Scott, Charles I. Dwarfism. Clinical Symposium, 1988; 40(1):9-10.
  5. Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.
  6. Walker, Bryan A. Murdoch, Lamont J. McKusick, Victor A. Langer, Leonard O. Beals, Rodney K. Hypochondroplasia. American Journal of Disease of Children. 122: 95-104. 1971.

Genetic Testing

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