2. Myasthenia Gravis Causes Muscle
Weakness
occurs in about 1 in every 20,000 Persons
causes muscle weakness
because of the inability of the neuromuscular junctions to transmit
enough signals from the nerve fibers to the muscle fibers.
Pathologically, antibodies that attack the acetylcholine receptors have
been demonstrated in the blood of most patients with myasthenia
gravis.
Therefore, myasthenia gravis is believed to be an autoimmune disease
in which the patients have developed antibodies that block or destroy
their own acetylcholine receptors at the postsynaptic neuromuscular
junction.
3. Regardless of the cause, the end plate potentials that occur in the
muscle fibers are mostly too weak to initiate opening of the voltage-
gated sodium channels, and thus muscle fiber depolarization does not
occur.
If the disease is intense enough, the patient may die of respiratory
failure as a result of severe weakness of the respiratory muscles.
The disease can usually be ameliorated for several hours by
administering neostigmine or some other anticholinesterase drug,
which allows larger than normal amounts of acetylcholine to
accumulate in the synaptic space.
Within minutes, some of these people can begin to function almost
normally, until a new dose of neostigmine is required a few hours
later.
17. Summary for anticholinesterases
Anticholinesterase drugs enhance neuromuscular transmission in voluntary and
involuntary muscle in myasthenia gravis. They prolong the action of acetylcholine by
inhibiting the action of the enzyme acetylcholinesterase. Excessive dosage of these drugs
can impair neuromuscular transmission and precipitate cholinergic crises by causing a
depolarising block. This may be difficult to distinguish from a worsening myasthenic state.
Muscarinic side-effects of anticholinesterases include increased sweating, increased
salivary and gastric secretions, increased gastro-intestinal and uterine motility, and
bradycardia. These parasympathomimetic effects are antagonised by atropine.
Neostigmine produces a therapeutic effect for up to 4 hours. Its pronounced muscarinic
action is a disadvantage, and simultaneous administration of an antimuscarinic drug such
as atropine or propantheline may be required to prevent colic, excessive salivation, or
diarrhoea. In severe disease neostigmine can be given every 2 hours. The maximum that
most patients can tolerate is 180 mg daily.
18. Summary for anticholinesterases
Pyridostigmine is less powerful and slower in action than neostigmine but it has a
longer duration of action. It is preferable to neostigmine because of its smoother
action and the need for less frequent dosage. It is particularly preferred in patients
whose muscles are weak on waking. It has a comparatively mild gastro-intestinal effect
but an antimuscarinic drug may still be required. It is inadvisable to exceed a total
daily dose of 450 mg in order to avoid acetylcholine receptor downregulation; patients
requiring doses exceeding 450 mg daily will usually require input from a specialised
neuromuscular service. Immunosuppressant therapy is usually considered if the dose
of pyridostigmine exceeds 360 mg daily.
Neostigmine is also used to reverse the actions of the non-depolarising
neuromuscular blocking drugs
19.
20.
21.
22. Anticholinesterases are used as first-line treatment in ocular
myasthenia gravis and as an adjunct to immunosuppressant therapy
for generalised myasthenia gravis.
Corticosteroids are used when anticholinesterases do not control
symptoms completely. A second-line immunosuppressant such as
azathioprine is frequently used to reduce the dose of corticosteroid.
Plasmapheresis or infusion of intravenous immunoglobulin
[unlicensed indication] may induce temporary remission in severe
relapses, particularly where bulbar or respiratory function is
compromised or before thymectomy.