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20 Cards in this Set

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Define: myasthenia gravis

An autoimmune disease characterized by muscle weakness.

How does MG affect cranial muscles? (4)

1. Oculomotor - diplopia (double vision)

2. Elevator - ptosis (drooping lids)

3. Oropharyngeal -difficulty swallowing

4. Facial - difficulty smiling, speaking

How does MG affect trunk and limb muscles?

Diaphragm weakness is most common.

May result in respiratory failure and death.

Etiology of seropositive MG? (2)

1. Circulating serum antibodies to nicotinic receptors.

2. Can be passed from mother to newborn.

What are other complications seen in patients with seropositive MG?

15% of patients have thymoma (tumor in thymus); 80-90% have an abnormality in thymus gland.

What percentage of patients have seropositive and seronegative MG?

80-90% of patients have seropositive.

10-20% seronegative.

Etiology of seronegative MG? (3)
1. Antibodies against muscle-specific kinases (MuSK), used in membrane signalling.

2. Congenital deficiency in nicotinic receptor.

3. Mutation in membrane signalling protein rapsyn

What are methods of diagnosing MG?

1. Muscle power tests

2. Edrophonium test

3. Tubocurarine

4. Detection of antibodies to AChR, MuSK and Rapsyn

5. Radiolabled a-bungarotoxin to measure AChR density

How is edrophonium used to test for MG?

Edrophonium is a short acting acetylcholinesterase inhibitor (prevents breakdown of ACh).

Temporary increase in muscle strength.

Indicates NM defect but not MG.

How is tubocurarine used to test for MG?

Tubocurarine is a non-depolarizing NM blocker.

Competes with ACh for nicotinic receptor.

Give at 1/20th normal dose; results in decrease of muscle strength/ paralysis.

No longer used.

Which acetylcholinesterase inhibitor is prefered for treatment of MG?

a. pyridostigmine

b. neostigmine

a. pyridostigmine

Has a longer action than neostigmine.

What are side effects of using acetylcholinesterase inhibitors?
Excessive doses lead to:

1. AChR down-regulation
2. Cholinergic toxicity or crisis
3. GI spasms and diarhea

How is corticosteroid therapy used to treat MG?

Used when MG is generalized or related to eye muscles and does not respond to other medication.

Example: prednisone

Dose titration needed to avoid myasthenic crisis.

Slowly reduce when remission is reached.

What are side effects to using corticosteroid therapy?

Weight gain, depression, diabetes, insomnia, cataracts and osteoporosis.

Depression may cause muscle weakness, leading to increased dosage by prescriber.

How are immunosuppressive drugs used to treat MG?

Only used in most serious cases.

Causes profound decrease in immune responses.

Examples: azathioprine, cyclosporine, cyclophophamide

How is plasma exchanged used to treat MG?

All circulating antibodies are removed (separated from blood cells).

Only temporary improvement.

How is thymectomy used in treating MG?

Thymus has potential to be tumorous.

Used as an option for moderate to severe seropositive MG in addition to drugs.

Used in patients under 45 (over 45, thymus plays little role in immune response)

What is the difference between cholinergic crisis and myasthenic crisis?

Cholinergic crisis: caused by excessive acetylcholine.

Myasthenic crisis: a severe and sudden exacerbation of MG. Breathing may be affected.

What are the symptoms of cholinergic crisis?




Miosis (contraction of pupil)

Muscle weakness



Lacrimation (tears)




How can you diagnose between cholinergic and myasthenic crisis?

Edrophonium makes cholinergic crisis worse, while improves in myasthenic crisis.