Achondroplasia
What is achondroplasia?
Achondroplasia is a genetic (inherited) condition that results
in
abnormally short stature. All persons with achondroplasia are
little
people. The average height of an adult with achondroplasia is
131 cm (52
inches, or 4 foot 4) in males and 124 cm (49 inches, or 4 foot
1) in
females.
Achondroplasia is the most common cause of short stature with
disproportionately short limbs.
Achondroplasia is a disorder of bone growth. Although
achondroplasia
literally means "without cartilage formation," the
problem 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 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 entirely normal in patients with
achondroplasia.
Enlargement of the brain (megalencephaly) is common and normal
with
achondroplasia.
Next: How is achondroplasia diagnosed? »
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