Definition

Achondroplasia is the most common cause of
dwarfism, or significantly abnormal short stature.

Description

Achondroplasia is one of a number of chondodystrophies,
in which the development of cartilage, and therefore,
bone is disturbed. The disorder appears in approximately
one in every 10,000 births. Achondroplasia is
usually diagnosed at birth, owing to the characteristic
appearance of the newborn.
Normal bone growth depends on the production of
cartilage (a fibrous connective tissue). Over time, calcium
is deposited within the cartilage, causing it to harden
and become bone. In achondroplasia, abnormalities
of this process prevent the bones (especially those in
the limbs) from growing as long as they normally
should, at the same time allowing the bones to become
abnormally thickened. The bones in the trunk of the
body and the skull are mostly not affected, although the
opening from the skull through which the spinal cord
passes (foramen magnum) is often narrower than normal,
and the opening (spinal canal) through which the
spinal cord runs in the back bones (vertebrae) becomes
increasingly and abnormally small down the length of
the spine.

Causes and symptoms

Achondroplasia is caused by a genetic defect. It is a
dominant trait, meaning that anybody with the genetic
defect will display all the symptoms of the disorder. A
parent with the disorder has a 50% chance of passing it
on to the offspring. Although achondroplasia can be
passed on to subsequent offspring, the majority of cases
occur due to a new mutation (change) in a gene. Interestingly
enough, the defect seen in achondroplasia is one of
only a few defects known to increase in frequency with
increasing age of the father (many genetic defects are
linked to increased age of the mother).

People with achondroplasia have abnormally short
arms and legs. Their trunk is usually of normal size, as is
their head. The appearance of short limbs and normal
head size actually makes the head appear to be oversized.
The bridge of the nose often has a scooped out appearance
termed