ACHONDROPLASIA
Achondroplasia is a genetic condition which primarily involves the skeleton
and so restricts growth. It is rare, affecting only about 1 in 20,000 people.
Cause:
Each human cell contains 46 chromosomes in 23 pairs, one of each pair
inherited from the mother and one from the father. Each chromosome carries
many genes like a row of pearls. Achrondroplasia is caused by a fault in
a single dominant gene, this implies that the disease is expressed although
only one of a pair of genes is affected. Males and females are equally
likely to be affected and have a 50% chance of passing the abnormal gene,
and so the condition, on to children of either sex. However, in achondroplasia
about three-quarters of affected people represent new mutations, a situation
in which the abnormality of the gene arises spontaneously before birth
and is not inherited from either parent. Unfortunately, the gene can then
be passed to later generations. In 1994 it was found that the gene for
achondroplasia lies on the short arm (each gene has a short and a long
arm) of chromosome 4 at the site of the gene for a growth factor receptor
(FGFR3), a protein in the outer wall of cells in growing bone which receive
the message to divide. Nearly all affected people studied so far have had
identical mutations in this gene; this is very unusual and indicates that
this is the gene most highly susceptible to mutation in the human. The
mutation is easily detected so for couples at risk (because one parent
is affected) prenatal diagnosis from chorionic villus sampling (removal
of a sample from the placenta in early pregnancy) is now possible.
Physical features:
The physical and X-ray features of achondroplasia are remarkably constant.
Affected babies are short at birth and grow slowly throughout childhood,
the average final height for women is 126cm (4ft 2in) and for men is 131cm
(4ft 4in). The limbs are relatively shorter than the trunk with the the
upper arm and the thigh especially shortened The head is usually large
with prominence of the forehead and a flat nasal bridge. The fingers and
toes are short and the hands and feet small and wide. The spine seldom
has a sideways curve but may have an exaggerated lumbar curve. Children
often show some early hypotonia (floppiness) of the limbs with delayed
motor development but this usually improves and disappears in childhood.
There may be dental problems from malocclusion of the teeth. In childhood
recurrent episodes of ear infection and glue ear are common which may lead
to temporary loss of hearing and interference with speech development.
Achondroplasia may be complicated by hydrocephalus, an excess of fluid
within the head resulting from obstruction of the flow of cerebrospinal
fluid or by compression of the nerves leaving the spinal cord.
Treatment:
There is no specific treatment but it is desirable to monitor the progress
of affected children to offer support and to detect early the occurrence
of any complications. Most children cope remarkably well with the problems
arising from their short stature and most schools are helpful. Attempts
to increase final height with growth hormone treatment have not been successful
but in recent years surgical bone-lengthening procedures have been developed
in several centres. These require major surgery, which is generally done
in at least two operations, and the wearing of heavy and uncomfortable
expandable bone braces for many months, typically about 18 months. If all
goes well a really significant gain in height can be achieved, about 10-12in
(25-30cm). Clearly such surgery is a major undertaking for both the child
and the family and the decision to go ahead has to be most carefully considered.
Outlook:
Most people with achondroplasia cope extraordinarily well with the physical
difficulties and lead successful and fulfilled lives. Many have distinguished
themselves in various fields. The Restricted Growth Association (103 St
Thomas Avenue, West Town, Hayling Island, Hants PO11 OEU) is an important
source of help on this and other related conditions. NDB |
