HETEROZYGOUS ACHONDROPLASIA

• About 80% of the cases are due to a spontaneous mutation (1).
• Physical features include:
• Moderate rhizomelic shortening of the limbs.
• Large head with frontal bossing.
• Depressed nasal bridge.
• Short trident hands.
• Lumbar lordosis.
• Normal mental and sexual development.
• Life span may be normal.

REFERENCES

1. Patel MD, Filly RA. Homozygous Achondroplasia: US distinction between homozygous, heterozygous, and unaffected fetuses in the second trimester. Radiology 1995;196:541-545.

HYPOCHONDROPLASIA

Hypochondroplasia is another form of short-limb dwarfism with rhizomelic. Many cases are overlooked or misdiagnosed as being familial short stature because the short stature and limb shortening can be mild. The diagnosis is also often made later in childhood (2-4 years of age), when the child is noted to "fall off the growth curve" and a careful physical exam notes the rhizomelic shortening.

Hypochondroplasia is inherited in an autosomal dominant manner. This means that a person with hypochondroplasia who partner is average-sized, has a 50% or 1 in 2 chance of having children with hypochondroplasia. Like achondroplasia, many children with hypochondroplasia are born to average-sized parents. In these cases, the hypochondroplasia is due to a new mutation or genetic change. The gene for hypochondroplasia has been found. It is the same gene responsible for achondroplasia called fibroblast growth factor receptor 3 (FGFR3). Although the same gene causes these two conditions, different changes occur in the gene.

Facial features:

· Similar to achondroplasia but are less striking.

· The head circumference is often in the normal range but the forehead is often prominent as in achondroplasia.

· The nasal bridge and midface are not depressed as in achondroplasia.

Extremities:

· The extremities are short with milder shortening of the proximal portions as compared to achondroplasia.

· Mild bow-legs can occur but often disappear as the child gets older.

· Range of motion at the elbows is often limited with limitation of extension and supination.

· The fingers are short but do not show the characteristic trident formation seen in achondroplasia.

Spine:

· The back shows increased lumbar lordosis.

· Neurologic complications such as lumbar stenosis can occur in individuals with hypochondroplasia but much less frequently than seen in patients with achondroplasia.

· It has been reported that a small percentage of individuals with hypochondroplasia have mental retardation.