Achondroplasia
Medical Author: Melissa Conrad Stöppler, MD
Medical Editor: David Perlstein, MD, FAAP
What is achondroplasia?
Achondroplasia is a genetic (inherited) condition that results in abnormally
short stature and is the most common cause of short stature with
disproportionately short limbs. The average height of an adult with
achondroplasia is 131 cm (52 inches, or 4 foot 4 inches) in males and 124 cm (49
inches, or 4 foot 1 inch) in females.
Although achondroplasia literally means "without cartilage formation," the
defect in achondroplasia is not in forming cartilage but in converting it to
bone, particularly in the long bones.
Achondroplasia is one of the oldest known birth defects. The frequency of
achondroplasia is estimated to range from about 1 in 10,000 births in Latin
America to about 12 in 77,000 in Denmark. An average figure worldwide is
approximately 1 in 25,000 births.
What are the characteristics of achondroplasia?
Achondroplasia is a distinctive condition that usually
can be noted at birth.
- The baby with achondroplasia has a relatively long,
narrow torso (trunk) with short extremities (arms and legs) and a disproportionate shortening of the
proximal (near the
torso) segments of the limbs (the upper arms and thighs).
- There is a typically
large head with prominence of the forehead (frontal
bossing), underdevelopment (hypoplasia) of the
midface with cheekbones that lack prominence, and a low nasal bridge with narrow
nasal passages.
- The baby's fingers appear short and the ringer and middle
fingers diverge giving the hand a trident (three-pronged) appearance. Most
joints can extend more than normal. For example, the knees can hyperextend
beyond the normal stopping point. Not all joints are lax in this way. To the
contrary, extension and rotation of the elbow
are abnormally limited. Hip extension also tends to be limited.
- At birth there is often prominence of the mid-to-lower back with a small
gibbus (a hump). With walking, the hump goes away and a pronounced sway
(lordosis) of the lumbar
region (the lower back) becomes apparent. The lumbar lordosis is persistent. The
legs are bowed (genu varum).
- The baby exhibits some decrease in muscle tone (hypotonia). Because of
the large head, especially compared to rest of the body, and the decreased
muscle tone, the child with achondroplasia will run behind "schedule" in
reaching the usual motor developmental milestones. The schedule to which an achondroplastic
child's development should be compared is not that for all children in the
general population, but rather the timetable followed by achondroplastic
children.
- Intelligence is generally normal in patients with
achondroplasia. Enlargement of the brain (megalencephaly) is common and normal with achondroplasia.
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