|
Myasthenia Gravis (MG) is a neuromuscular
autoimmune disease that affects the use of muscles - normal communication
between the nerve and the muscle is interrupted, leaving the muscle
weak and fatigued.
An autoimmune disease is one where
the body's immune system appears to attack healthy tissue and produces
so many antibodies (immune blood proteins that recognise and fight
infections and other foreign things in the body) that the healthy
tissue becomes damaged.
With MG, it is the voluntary (striated)
muscles that are weakened. Voluntary muscles are the muscles that
we can control, where the message is transmitted via our nervous
system to contract the muscle. They are the muscles in our legs,
our arms and our neck; those that move the eyeball and keep the
eyelids open; some of those involved with facial expression, and
those involving chewing, swallowing and breathing. MG does not affect
bowel and bladder function or the myasthenic's mental capacity.
Nor is it restricted to humans!
For a muscle to contract, the chemical
acetylcholine normally transmits nerve impulses to muscle fibres
at the place where the nerve and muscle connect (the neuromuscular
junction). The physical weakness of the myasthenic's muscle is caused
by a defect at this neuromuscular junction. However, whilst the
he myasthenic has a problem with the transmission of nerve impulses
to the muscle, the nerves and muscles themselves may remain normal.
Research has shown that most myasthenics
form abnormal antibodies against the acetylcholine receptor (sites
at which the chemical can be received on the surface of the muscle).
The number of acetylcholine receptors in a myasthenic is reduced
due to an attack on the receptors by the body's immune system -
if receptors are missing, the response of the muscle to the nerve
impulses is poor, and so weakness occurs. Scientists are investigating
what triggers the body to develop an autoimmune response. In many
myathenics, the thymus gland appears to be involved.
The onset of MG may be sudden, with
severe and generalised muscle weakness, but more often the symptoms
in the early stages are subtle and variable, making it difficult
to diagnose correctly. Some have only ocular myasthenia involving
the eye muscles and eyelids; others have mainly swallowing difficulties
or slurred speech; others have generalised MG affecting many muscle
groups.
Muscle weakness may develop over a
few days or weeks, or remain at the same level for long periods
of time. The severity varies at different times of the day, and
from person to person. The maximum extent of involvement in an individual
patient usually manifests itself within the first 5 to 7 years,
and thereafter it tends not to be progressive, even though muscle
involvement and severity of weakness may still fluctuate from hour-to-hour
and day-to-day.
MG is normally not a degenerative
disease - that is, with continued use, the muscle itself should
not deteriorate. A distinctive feature of MG, however, is fluctuating
weakness of muscles, made worse by use of those muscles and improved
by rest of the same muscles. Consequently, a person with MG experiences
phases of muscular weakness that alternate with periods of normal
health.
MG is not considered a hereditary
disease as there is no well documented case of MG occurring in a
child of a mother with MG. However, around 12% of babies born to
myasthenic mothers suffer from neonatal myasthenia where the baby
develops a feeble cry, poor sucking, and respiratory distress. These
symptoms disappear over the first few months of life. It is rare
that infants born to non-myasthenic mothers have myasthenic symptoms
- these infants are said to have congenital
myasthenia.
These days, MG is not considered fatal.
It can be life threatening if muscle weakness interferes with breathing.
It is treatable, but at this stage, not curable. Treatments provide
patients with a marked relief of symptoms and often allow them to
lead full, normal lives.
Next...
|
