A 34-year-old female complains of weakness and double vision
for the last 3 weeks. She has also noted a change in her speech, and her
friends tell her that she is “more nasal.” She has noticed decreased exercise
tolerance and difficulty lifting objects and getting out of a chair. The
patient denies pain. The symptoms are worse at the end of the day and with
repeated muscle use. You suspect myasthenia gravis. All the following are
useful in the diagnosis of myasthenia gravis EXCEPT:
A. Acetylcholine receptor (AChR) antibodies
B. Edrophonium
C. Electrodiagnostic testing
D. Muscle-specific kinase (MuSK) antibodies
E. Voltage-gated calcium channel antibodies
The answer
is E.
(Harrison 18th edn Chap. 386) Myasthenia gravis
(MG) is a neuromuscular disorder characterized by weakness and fatigability of
skeletal muscles. The primary defect is a decrease in the number of
acetylcholine receptors at the neuromuscular junction secondary to autoimmune
antibodies. MG is not rare, affecting at least 1 in 7500 individuals. Women are
affected more frequently than men. Women typically present in the second and
third decades of life, and men present in the fifth and sixth decades. The key
features of MG are weakness and fatigability. Clinical features include
weakness of the cranial muscles, particularly the eyelids and extraocular
muscles. Diplopia and ptosis are common initial complaints. Weakness in chewing
is noticeable after prolonged effort. Speech may be affected secondary to
weakness of the palate or tongue. Swallowing may result from weakness of the
palate, tongue, or pharynx. In the majority of patients the weakness becomes
generalized. The diagnosis is suspected after the appearance of the
characteristic symptoms and signs. Edrophonium is an acetylcholinesterase
inhibitor that allows ACh to interact repeatedly with the limited number of
AChRs, producing improvement in the strength of myasthenic muscles.
False-positive tests may occur in patients with other neurologic diseases.
Electrodiagnostic testing may show evidence of reduction in the amplitude of
the evoked muscle action potentials with repeated stimulation. Testing for the
specific antibodies to AChR are diagnostic. In addition to anti-AChR
antibodies, antibodies to MuSK have been found in some patients with clinical
MG. Antibodies to voltage-gated calcium channels are found in patients with the
Lambert-Eaton syndrome.
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