32. Myasthenia gravis

Question 1

Definitions of MG

Answer 1

Chronic autoimmune disorder in which antibodies destroy the communication between nerves and muscle, resulting in weakness of the skeletal muscles. Myasthenia gravis affects the voluntary muscles of the body, especially those that control the eyes, mouth, throat and limbs

Question 2

Pathophysiology MG

Answer 2

• Autoimmune response against AChR of postsynapctic membrane in neuromuscular junction
• Antibodies produced in thyroid
• 10-20% have no anti-AChR, they have ab against muscle-specific kinase

Question 3

Etiology of MG

Answer 3

• Persistent thymus (70%)
• Thymoma (10%)
• Other autoimmune disorders: thyroiditis, RA, SLE etc.

Question 4

Symptoms of MG

Answer 4

• Painless fluctating weakness
• Distinctive patterns of weakness
• No muscle atrophy, reflex abnormalities, sensory loss of autonomic dysfunction

Question 5

Clinical forms of MG

Answer 5

1. Ocular form
2. Bulbar form
3. Generalized form

Question 6

Characteristics of painless fluctating weakness in MG

Answer 6

Worse with exertion, worse in evening
Better with rest and cold environm.

Question 7

Muscles with weakness in MG

Answer 7

Most commonly muscles innervated by CNS:
- extraocular, bulbar, psotural and prox. limb muscles

Question 8

Ocular form of MG

Answer 8

Fluctating ptosis, diplopia
50% of MG pt. have ocular symptoms initially
90% of MG pt. have ocular symptoms at one point

Question 10

Symptoms of bulbar form of MG

Answer 10

Dysarthria, dysphagia
Nasal regurgitation
Difficulty chewing, coughing
Hypophonic voice, nasal voice
Facial weakness, head drop, myasthenic snarl

Question 11

What is bulbar muscles?

Answer 11

Group of muscles in the head and neck. These muscles are involved in speaking, swallowing, chewing, and holding the jaw in place. The nerves that control these muscles are located in a part of the brainstem called the bulbar region

Question 12

Symptoms of generalized form of MG

Answer 12

Proximal limb weakness
Hyperreactive reflexes

Question 13

What is myasthenic crisis?

Answer 13

• Severe generalized and/or bulbar weakness and respiratory failure
• 10-30% pt. experience it
• Precipitating factors: resp. infection, aspiration, surgeries, drugs

Question 14

Diagnosis of MG

Answer 14

• Clinical
• IV edrophonium test (tensilon)
• Electromyography
• Serology: anti-AChR antibodies
• CXR/CCT: check for thymoma or presistent thymus
• Spirometry

Question 15

IV edrophonium test of MG

Answer 15

Distinguish a myasthenic crisis from a cholinergic crisis in individuals undergoing treatment for myasthenia gravis. Give tensilon (acetylcholinesterase inh.) and assess improvement over 2 min, + if clear improvement of weakness.

Question 16

Treatment of MG

Answer 16

• Cholinesterase inhibitor: pyridostigmine
• Corticosteroids
• Immunomodulator: Azathioprine

Question 18

Treatment of myasthenic crisis

Answer 18

Plasma exhange or IVIG