Your doctor thinks that you have Myasthenia Gravis
(MG). This is an autoimmune condition where the body's immune system
has damaged receptors on your muscles. This results in muscle weakness
as receptors are necessary for the muscles to know when to contract.
If this involves the muscles of the eyelid, it can result in lid
droop (ptosis). If it involves the muscles of eye movement, it can
result in double vision. The double vision and lid droop may vary,
being worse when tired or later in the day.
Anatomy
When you want to move a particular muscle, the brain sends a signal
via the nerves that go to the muscle. When this signal reaches the
end of the nerve, it causes release of a chemical neuro-transmitter
that diffuses across a narrow gap (the synapse) between the nerve
and muscle. On the muscle side of the synapse, there are chemical
receptors that are waiting to detect this neuro-transmitter. When
they sense the presence of the neuro-transmitter, they begin the
process that results in the muscle contracting. In the case of a
patient with MG, when the neuro-transmitter is released and diffuses
across the synapse, it does not produce normal muscular muscle contraction.
This is because the normal receptors have been blocked or eliminated
by antibodies produced by the body's immune system. Repeat attempts
at moving the muscle will result in a gradual increase in weakness.
Hence, muscles that are used most frequently are most likely to
be weak. This is particularly true of the muscles that move the
eye, resulting in ocular misalignment (double vision), or the muscles
that lift up the lid, resulting in lid droop. Muscles anywhere within
the body may be affected.
Physiology
The reason for the body's immune system's attack on the muscles
is unclear. Patients with MG may have other autoimmune problems
(such as thyroid orbitopathy). Like other autoimmune diseases, MG
often spontaneously gets better but may come back. Over time, there
may be some lessening effect, but MG produces variable disease that
may last indefinitely or at least for years.
Symptoms
The most common symptoms of MG relate to weakness of the muscles
that lift up the lid (ptosis) or move the eyes (double vision).
MG can affect muscles anywhere in the body including those of swallowing
or even breathing. Shortness of breath or difficulty swallowing
may be very serious symptoms of MG and must be brought to your doctor's
attention immediately. MG does not produce pain or numbness. If
pain is present there must be something else going on and you need
to tell your doctor.
Signs The
most common sign seen by ophthalmologists are lid droop
and eyes that don't work together. The eye movement
problems may be difficult to detect and often patients with double
vision related to MG have had it years before a diagnosis is made.
It is not unusual for previous evaluations to have been normal.
Diagnosis
MG can be confirmed by a biopsy of muscles and studies that demonstrate
decreased receptors. In most cases, the diagnosis can be made without
this procedure. A blood test may make the diagnosis, but about half
of patients with MG restricted to the eye muscles may not have a
positive test. Thus the finding of a negative blood test does not
exclude MG. If Tensilon (a substance that decreases the turnover
of the neuro-transmitter) is injected into a vein a patient with
MG may demonstrate rapid improvement in the apparently weak muscle.
Immediate elevation of the eyelid or improvement in eye movement
can diagnose MG. A Tensilon test may be difficult to interpret if
there are only mild findings. The sleep test or ice test depends
on improved strength in rested muscles. If the patient is examined
immediately upon awakening from rest, the findings of improved motility
or resolution of lid droop may strongly suggest a diagnosis of MG.
It is also possible to do studies of conduction between the nerve
and muscle. This test, an EMG (electromyogram), has a relatively
high sensitivity for the diagnosis of myasthenia. This requires
sophisticated equipment and may be done only infrequently.
Prognosis
As with other auto-immune conditions, MG tends to come and go. Symptoms
may suddenly become worse. This can be precipitated by the use of
certain antibiotics and other drugs. Once the diagnosis of MG is
made, it is important that patients inform all their physicians,
particularly if they are ever seen in an Emergency Room.
Treatment
MG can be treated by medications that decrease the turnover of normal
neuro-muscular transmitters. These may be taken in pill form and
can be effective in improving weak muscles. Unfortunately, often
this will not relieve lid droop or problems with the eye movements
that result in double vision. Occasionally surgery to remove the
thymus gland may be effective in treatment of MG and it is common
to check for thymus enlargement with a chest CT scan. Steroid therapy
(prednisone) may also be effective in improving muscle function.
As steroids need to be used for long term, it is important to consider
whether or not the benefits are outweighed by the risks of side
effects associated with steroids. Occasionally other medications
may help decrease the need for prednisone. This needs to be discussed
with your physician.
Most frequently asked questions
How did I get Myasthenia?
As with other autoimmune diseases, the reason for
development of MG remains unclear. It is possible that some external
challenge (viruses, etc.) may have caused the immune system to start
acting in this fashion. At this time, however, we do not have full
understanding of why certain patients develop MG.
Is there a cure for Myasthenia?
At this time there is no known cure for the autoimmune
condition MG. Medical therapy may be effective in improving muscular
function but needs to be tailored to the particular activity of
the disease process. Drugs that influence the immune system response
may also be helpful.
What are my chances of developing weakness in muscles
other than my eye muscles?
MG may seem to involve only certain muscles,
especially the muscles that move the eye and lift the lid. It has
been felt that when MG fails to cause general muscle weakness over
2 years or more that the chances of developing weakness in other
non-ocular muscles in the future is low. This is probably true but
not a guarantee that systemic weakness will not occur. It remains
very important, that if problems with breathing or swallowing ever
develop, it needs to be brought to the attention of your physician
immediately.
