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).

 
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.

Face & Skull
  • Average head circumference or slight macrocephaly
  • Slightly prominent forehead
Trunk, Chest, & Spine:
Arms & 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?

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 becuase 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% of hypochondroplastic persons have learning problems.

Neurological Complications

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

Pregnancy

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.

 
References
  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.

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