After an individual is admitted to the hospital, it determined that he displays a variable weakness of cranial nerve and limb muscles but shows no clinical signs of denervation from tests, which include dectromyogram (EMG) recordings. This disorder was partially reversed by the administration of drugs that inhibit acetyicholinesterase. Which of the following is the most likely diagnosis?
A. Multiple sclerosis (MS)B. Amyotrophic lateral sclerosis (ALS)
C. Myasthenia gravis
D. Combined system disease
Answer. C. Myasthenia gravis
Myasthenia gravis is an
autoimmune disease that causes cranial nerve and limb muscle weakness by
producing antibodies that act against the nicotinic receptor at the
neuromuscular junction. The result is that the action of nerve fibers that
innervate skeletal muscle are affected producing loss of the effects of ACI at
the. neuromuscular junction. The net result is a reduction of the size of the
action potential in the muscle, producing a weakness in the affected muscle.
This disorder is reversed by administration of drugs that inhibit the efizyme,
acetyicholinesterase, that degrades ACh. Multiple sclerosis, ALS, and combined
systerndisease (see the chapter entitled “The Spinal Cord11) involve damage to
axons and/or nerve cells within the CNS, producing much more profound damage to
motor functions and, in the case of combined system disease, damage to both
motor and sensory systems. Muscular dystrophy is typically characterized, in
part, by progressive weakness of muscles and degeneration of the muscle fibers.
The other disorders listed all involve disorders affecting the CNS, and thus,
the symptoms associated with these disorders differ significantly from those
described in this case. Excessive release of ACh is not a realistic event that
is likely to occur (except from the bite of a black widow spider). In theory,
if it were to occur, therc is no reason to believe that muscular weakness would
be a symptom. Instead, there would be some rigidity and muscle spasms.
