“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.
Hypochondroplasia is genetically heterogeneous. Approximately one-half of hypochondroplastic patients have a mutation within the fibroblast growth factor receptor 3 (FGFR3) gene (2).
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.
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.
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.
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 in the knees, ankles, or elbows may occur, especially during childhood. Into adulthood, the pain is most prominent in the lower back.
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.
Approximately 10 percent of hypochondroplastic persons have learning problems.
Compressive myelopathy and radiculopathy occur, albeit less frequent
Woman who become pregnant often times require a cesarean section, albeit vaginal delivery is still possible.
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.
- Desch, Larry W. Horton, Willaim A. An Autosomal Recessive Bone Dysplasia Syndrome Resembling Hypochondroplasia. Pediatrics. 75, No 4: 786-789. 1985.
- Jones, Kenneth L. Recognizable Patterns of Human Malformation. Philadelphia, PA: Elsevier Saunders. 2006
- Newman, Donald E. Dunbar, Scott. Hypochondroplasia. Journal of the Canadian Association of Radiologists. 26: 95-103. 1975.
- Scott, Charles I. Dwarfism. Clinical Symposium, 1988; 40(1):9-10.
- Spranger, Jurgen W. Brill, Paula W. Poznanski, Andrew. Bone Dysplasias: An Atlas of Genetic Disorder of Skeletal Development. Oxford: Oxford University Press. 2002.
- 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.
From Nemours' KidsHealth
Trusted External Resources
Prenatal Genetic Counseling
If you and your partner are newly pregnant, you may be amazed at the number and variety of prenatal tests available to you. Blood tests, urine tests, monthly medical exams, screening tests, and family history tracking — each helps to assess the health of you and your baby, and to predict any potential health risks.
You may also have the option of genetic testing. These tests identify the likelihood of passing certain genetic diseases or disorders (those caused by a defect in the genes — the tiny, DNA-containing units of heredity that determine the characteristics and functioning of the entire body) to your children.
Some of the more familiar genetic disorders are:
- Down syndrome
- cystic fibrosis
- sickle cell disease
- Tay-Sachs disease (a fatal disease affecting the central nervous system)
If your history suggests that genetic testing would be helpful, you may be referred to a genetic counselor. Or, you might decide to seek out genetic counseling yourself.
But what do genetic counselors do, and how can they help your family?
What Is Genetic Counseling?
Genetic counseling is the process of:
- evaluating family history and medical records
- ordering genetic tests
- evaluating the results of this investigation
- helping parents understand and reach decisions about what to do next
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.
Genes are made up of DNA molecules, which are the building blocks of heredity. They're grouped together in specific patterns within a person's chromosomes, forming the unique "blueprint" for every physical and biological characteristic of that person.
Humans have 46 chromosomes, arranged in pairs in every living cell of our bodies. When the egg and sperm join at conception, half of each chromosomal pair is inherited from each parent. This newly formed combination of chromosomes then copies itself again and again during fetal growth and development, passing identical genetic information to each new cell in the growing fetus.
Current science suggests that every human has about 25,000 genes per cell. An error in just one gene (and in some instances, even the alteration of a single piece of DNA) can sometimes be the cause for a serious medical condition.
Some diseases, such as Huntington's disease (a degenerative nerve disease) and Marfan syndrome (a connective tissue disorder), can be inherited from just one parent. But most disorders, including cystic fibrosis, sickle cell anemia, and Tay-Sachs disease, cannot occur unless both the mother and father pass along the gene.
Other genetic conditions, such as Down syndrome, are usually not inherited. In general, they result from an error (mutation) in the cell division process during conception or fetal development. Still others, such as achondroplasia (the most common form of dwarfism), may either be inherited or the result of a genetic mutation.
Genetic tests don't yield easy-to-understand results. They can reveal the presence, absence, or malformation of genes or chromosomes. Deciphering what these complex tests mean is where a genetic counselor comes in.
About Genetic Counselors
Genetic counselors are professionals who have completed a master's program in medical genetics and counseling skills. They then pass a certification exam administered by the American Board of Genetic Counseling.
Genetic counselors can help identify and interpret the risks of an inherited disorder, explain inheritance patterns, suggest testing, and lay out possible scenarios. (They refer you to a doctor or a laboratory for the actual tests.) They will explain the meaning of the medical science involved, provide support, and address any emotional issues raised by the results of the genetic testing.
Who Should See One?
Most couples planning a pregnancy or who are expecting don't need genetic counseling. About 3% of babies are born with birth defects each year, according to the Centers for Disease Control and Prevention (CDC) — and of the malformations that do occur, the most common are also among the most treatable. Cleft palate and clubfoot, two of the more common birth defects, can be surgically repaired, as can many heart malformations.
The best time to seek genetic counseling is before becoming pregnant, when a counselor can help assess your risk factors. But even after you become pregnant, a meeting with a genetic counselor can still be helpful. A genetic counselor can help determine what testing is appropriate for your pregnancy.
Experts recommend that all pregnant women, regardless of age or circumstance, be offered genetic counseling and testing to screen for Down syndrome.
It's especially important to consider genetic counseling if any of the following risk factors apply to you:
- a standard prenatal screening test (such as the alpha fetoprotein test) yields an abnormal result
- an amniocentesis yields an unexpected result (such as a chromosomal defect in the unborn baby)
- either parent or a close relative has an inherited disease or birth defect
- either parent already has children with birth defects, intellectual disabilities, or genetic disorders
- the mother-to-be has had two or more miscarriages or babies that died in infancy
- the mother-to-be will be 35 or older when the baby is born. Chances of having a child with Down syndrome increase with the mother's age: a woman has about a 1 in 350 chance of conceiving a child with Down syndrome at age 35, a 1 in 110 chance at age 40, and a 1 in 30 chance at age 45.
- you are concerned about genetic defects that occur frequently in certain ethnic or racial groups. For example, couples of African descent are most at risk for having a child with sickle cell anemia; couples of central or eastern European Jewish (Ashekenazi), Cajun, or Irish descent may be carriers of Tay-Sachs disease; and couples of Italian, Greek, Middle Eastern, southern Asian, or African descent may carry the gene for thalassemia, a red blood cell disorder.
- either parent is concerned about the effects of exposures they have had to radiation, medications, illegal drugs, infections, or chemicals
Meeting With a Genetic Counselor
Before you meet with a genetic counselor in person, you may be asked to gather information about your family history. The counselor will want to know of any relatives with genetic disorders, multiple miscarriages, and early or unexplained deaths. The counselor will also want to look over your medical records, including any ultrasounds, prenatal test results, past pregnancies, and medications you may have taken before or during pregnancy.
When you meet with the counselor, you'll go over any gaps or potential problem areas in your family or medical history. The counselor can help you understand the inheritance patterns of any potential disorders and help assess your chances of having a child with those disorders.
The counselor will distinguish between risks that every pregnancy faces and risks that you personally face. Even if you discover you have a particular problem gene, science can't always predict the severity of the related disease. For instance, a child with cystic fibrosis can have debilitating lung problems or, less commonly, milder respiratory symptoms.
If more tests are necessary, the counselor will help you set up those appointments and track the paperwork. When the results come in, the counselor will call you with the news and may ask you to come in for another discussion.
Genetic counselors can help you understand your options and adjust to any uncertainties you face, but you and your family will have to decide what to do next.
If you've learned before conception that you and/or your partner are at high risk for having a child with a severe or fatal defect, your options might include:
- pre-implantation diagnosis — when eggs that have been fertilized in vitro (in a laboratory, outside of the womb) are tested for defects at the 8-cell (blastocyst) stage, and only nonaffected blastocysts are implanted in the uterus to establish a pregnancy
- using donor sperm or donor eggs
- taking the risk and having a child
- establishing pregnancy and have specific prenatal testing
If you've received a diagnosis of a severe or fatal defect after conception, your options might include:
- preparing yourself for the challenges you'll face when you have your baby
- fetal surgery to repair the defect before birth (surgery can only be used to treat some defects, such as spina bifida or congenital diaphragmatic hernia, a hole in the diaphragm that can cause severely underdeveloped lungs. Most defects cannot be surgically repaired.)
- ending the pregnancy
For some families, knowing that they'll have an infant with a severe or fatal genetic condition seems too much to bear. Other families are able to adapt to the news — and to the birth — remarkably well.
Genetic counselors can share the experiences they've had with other families in your situation. But they will not suggest a particular course of action. A genetic counselor understands that what is right for one family may not be right for another.
Genetic counselors can, however, refer you to specialists for further help. For instance, many babies with Down syndrome are born with heart defects. Your counselor might encourage you to meet with a cardiologist to discuss heart surgery, and a neonatologist to discuss the care of a post-operative newborn. Genetic counselors can also refer you to social workers, support groups, or mental health professionals to help you adjust to and prepare for your complex new reality.
Finding a Genetic Counselor
Working with a genetic counselor can be reassuring and informative, especially if you or your partner have known risk factors. Talk to your doctor if you feel you would benefit from genetic counseling. Many doctors have a list of local genetic counselors they work with. You also can contact the National Society of Genetic Counselors for more information.
Reviewed by: Larissa Hirsch, MD
Date reviewed: August 11, 2016