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Neuromuscular Junction Flashcards

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1
Q

Which structures may be involved in the pathology of a neuromuscular disease?

A
  1. Muscle
  2. Neuromuscular Junction
  3. Peripheral Nerve + Cranial Nerves
  4. Spinal Nerve Root
  5. Nerve Plexus
  6. Nuclei in the brainstem or spinal cord
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2
Q

What is the most common NMJ disease?

A

Myasthenia Gravis

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3
Q

What are some examples of neuromuscular junction diseases (4)?

A
  1. Myasthenia Gravis
  2. Lambert-Eaton
  3. Botulism
  4. Tetanus
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4
Q

What is the age distribution for Myasthenia Gravis?

A

Bimodal- teens-30 (females) and 50-70 (males)

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5
Q

What is seen histologically at the nerve terminal in Myasthenia Gravis?

A

No invaginations of the nerve terminal

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6
Q

What are the three pathological causes of MG?

A
  1. Blocking
  2. Accelerated Internalization of ACh Receptors
  3. Complement lysis of the muscle end plate
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7
Q

Describe the blocking pathology of MG

A

Direct blocking of the skeletal muscle nicotinic ACh Receptor site by antibodies

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8
Q

Describe the accelerated internalization pathology of MG

A

ACh Receptors are more rapidly internalized and degraded due to the cross linking of IgG Antibodies

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9
Q

Describe the role of complement in MG

A

Complement mediated lysis results in distortion of the muscle end plate (fewer invaginations/less surface area)

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10
Q

Which pathophysiological cause of MG describes the decreased effectiveness of medications with time?

A

Complement destruction of the muscle end plate

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11
Q

50% of AChR Antibody + MG patients have what finding?

A

Thymic hyperplasia

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12
Q

What percent of MG patients have thymic tumors?

A

10-15%

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13
Q

How does a hyper plastic thymus correlate to MG?

A

Thymocytes can produce anti-ACh Receptor Antibodies

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14
Q

Typical presentation of MG

A

Fatiguable weakness often affecting the extra ocular, oropharyngeal, axial or limb muscles

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15
Q

How will reflexes be on a patient with MG?

A

Normal

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16
Q

How will the sensory exam be on a patient with MG?

A

Normal

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17
Q

What are the three types of MG and which muscle groups are involved with each?

A

Bulbar- Craniofascial weakness
Ocular- EOM
General- Entire body

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18
Q

Which EOM is usually most severely involved?

A

Medial Rectus

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19
Q

What is tested for by having the patient count aloud?

A

MG- enhances dysarthria

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20
Q

What are the clinical fatiguing maneuvers used to test for MG?

A
  1. Sustained Upgaze
  2. Sustained aBduction of the arms
  3. Sustained elevation of the leg while laying supine
  4. Repeated arising from chair w/o arms
  5. Counting aloud
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21
Q

Bulbar MG will be detected by

A

Changing pitch while counting aloud

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22
Q

What is the most common immunologic finding in MG?

A

ACh Receptor Binding Antibody

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23
Q

What percent of MG patients have the AChR Binding Antibody?

A

80-85%

as low as 50% in some ocular cases

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24
Q

Why is the AChR binding antibody the gold standard?

A

Low false positive rate (high specificity)

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25
What is the anti-muscle specific tyrosine kinase?
A Neuromuscular Junction protein that clusters ACh Receptors
26
Which patients may test positive for anti-muscle specific tyrosine kinase?
Generalized MG patients
27
What percent of patients who test (-) for ACh Receptor Binding Antibody test + for the Anti-muscle specific tyrosine kinase?
Up to 40%
28
Which antibody is highly associated with thymomas in MG?
Anti-striated muscle
29
When do you test for the anti-striated muscle antibody?
1. Young patients as an adjunct way to detect thymomas | 2. Older patients with mild disease (because this may be the only abnormality)
30
Repetitive Nerve Stimulation is more sensitive in diagnosing which type of MG?
Generalized MG
31
What is the next step if the RNS is normal but the patient has symptoms of MG?
Do a single fiber electromyography (SFEMG)
32
Describe the EMG findings in MG
The end plate potential (EPP) is decreased in MG and there is a lower safety factor; repeated stimulation --> the EPP to fall below threshold needed for muscle fiber activation
33
How does the edrophonium test work?
IV administration of edrophonium (ACh Esterase Inhibitor) results in rapid improvement of ptosis/weakness and only lasts 5-10mins
34
What is the sensitivity of the edrophonium test?
70-95%
35
What are potential drawbacks to the edrophonium test?
1. Must blind physicians 2. Bradycardia and Hypotension (keep Atropine close) 3. Not specific/reliable
36
What is the symptomatic treatment of MG?
Pyridostigmine (Mestinon)
37
What drug class is pyridostigmine?
ACh Esterase Inhibitor
38
What is the effect of pyridostigmine?
It increases the size and length of the end plate potential within 30mins and lasts 3-6hrs
39
What are common side effects of pyridostigmine?
1. Stomach Cramps/Diarrhea/Nausea/Vomiting 2. Sweating 3. Bronchial and nasal secretions 4. Bradycardia
40
What are the general types of treatments for MG?
1. Symptomatic | 2. Immunosuppressant (short and long term)
41
What are short term immunotherapies used for MG?
Plasma Exchange and IV Ig
42
What are the long term immune-directed therapies for MG?
1. Thymectomy 2. Corticosteroids 3. Azathioprine and Mycophenolate
43
How is plasma exchange used to treat MG?
It temporarily reduces the levels of circulating antibodies
44
When do patients experience relief from plasma exchange for MG and how often are treatments?
Relief in days, usually receive 3-6 exchanges every other day
45
Why is plasma exchange not done daily?
It depletes clotting factors
46
When is plasma exchange done for MG?
1. Acute Exacerbations | 2. Drug resistant patients
47
What is the MOA of IV Ig?
Either neutralizing or down regulation
48
When will an MG patient experience relief on IV Ig and how long does it last?
Relief in days, effects last 4-8 weeks
49
What is the dosing of IV Ig?
2g/kg given over 2-5 days
50
When is IV Ig used?
Similar to PLEX- 1. Acute Exacerbations 2. Drug Resistant patients
51
What is the indication for a thymectomy in an MG patient?
1. Thymoma (only absolute indication) | 2. Usually in patients under 50
52
Which percent of SCLC patients will have LEMS?
3%
53
What percent of LEMS have an identifiable malignancy?
50%
54
Which patients develop LEMS who do not have SCLC?
Young females with other autoimmune diseases
55
Describe the pathophysiology of LEMS
IgG antibody binds to voltage gated Ca++ Channels in the presynaptic neuron
56
How does the antibody to VGCC cause LEMS?
Thought that the antibodies cross link multiple VGCCs which renders them ineffective by disrupting their proper positioning and function
57
How does SCLC cause LEMS?
An antigen that is expressed in SCLC results in the formation of VGCC antibodies
58
What is the consequence of decreased Ca++ influx into the presynaptic cell?
Decreased ACh released into the neuromuscular junction --> decreased end plate potential --> failed transmission
59
What are the clinical symptoms of LEMS?
1. Slow progressive proximal leg and arm weakness 2. Tender/Aching 3. Mild bulbar (cranial nerve) and respiratory symptoms 4. Dry Mouth and Metallic Taste 5. Weakness of Ca++ Channel Blocker meds 6. Prolonged paralysis prior to intubation
60
When does LEMS present in SCLC patients?
Up to 9 months before detection of the cancer
61
What will be seen on physical exam for LEMS?
1. Proximal weakness that improves with exercise 2. Minimal Bulbar Findings (ptosis, blurred vision, difficulty chewing, dysphagia, dysarthria) 3. Dry eyes and Dry mouth
62
How will the sensory exam of a LEMS patient be?
Normal
63
How will the reflexes be of a LEMS patient?
Reduced/absent
64
How is LEMS diagnosed?
1. Serum testing of voltage gates calcium channel antibody | 2. Electrodiagnostic testing (repetitive stimulatio; 3Hz- decrement; 30-50Hz results in increment)
65
Which disease (MG or LEMS) presents with a weakness that has fluctuation?
MG
66
Which disease (MG or LEMS) presents with autonomic issues?
LEMS
67
What is the treatment for LEMS?
1. Symptomatic and immunosuppressive | 2. PLEX or IV Ig
68
What is the most common cause of myopathy?
Statins (followed by inclusion body, MG, poly/dermatomyositis and DMD)

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