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Neurologic Emergencies > Neuro Emerg 24: Myasthenia Gravis > Flashcards

Neuro Emerg 24: Myasthenia Gravis Flashcards

1
Q

What are the three forms of acquired Myasthenia Gravis?

A
  • focal AMG: localized weakness of esophageal, pharyngeal, laryngeal and/or facial muscles - no evidence of appendicular weakness
  • generalized AMG: obvious appendicular weakness +/- megaesophagus and facial/pharyngeal/laryngeal weakness
  • acute fulminant AMG: profound appendicular weakness with acute onset, +/- facial, pharyngeal, esophageal, laryngeal weakness - rapidly fatal due to tetraparesis and respiratory muscle failure (diaphragm and intercostal!)
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2
Q

Fill the green gaps

A

Motor neuron axon

Motor neuron terminal or Axonal terminal

Motor end plate

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3
Q

Describe the steps of neuromuscular action potential as shown in this picture

A
  1. Action potential of motor neuron increases permeability of Ca++ channels –> increased IC [Ca++]
  2. Increase [Ca++] triggers exocytosis of Ach-containing vesicles
  3. Ach reaches synaptic cleft –> reaches nicotonic Ach receptors
  4. Two Ach molecules bind to each receptor –> 5. increases ligand-gated Na and K channels –> Na influx into cell –> threshold potential
  5. when threshold potential is reached –> voltage-gates Na+ channels open –> propagation of the action potential
  6. Acetylcholineesterase hydrolizes Ach –> hydrolyzed to choline and acetate
  7. Choline is taken up into the cholinergic axon terminal
  8. Acetylcholine synthesized from choline and acetyl-coenzyme A (acetyl-CoA) (enzyme: choline acetyl-transferase)
  9. Acetyl-CoA synthesized in the mitochondria
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4
Q

Which type of congenital myasthenia gravis is more common in dogs?

A
  • post-synatpic - i.e., defect is related to the amount Acetylcholine-Receptors expressed on the postsynaptic region
  • other forms: presynaptic or synaptic (observed in humans)
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5
Q

What 2 breeds have shown an absolute ACh-R deficiency leading to congenital MG?

A

Jack Russel Terrier, Springer Spaniel

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6
Q

What type of antibody is mostly implicated in acquired myasthenia gravis?

A

IgG

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7
Q

What subunit of the ACh-R do autoantibodies bind to in MG?

A

alpha subunit - separate region from the ACh binding site - called MIR (main immunogenic region)

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8
Q

Name the breeds with the highest risk of acquired myasthenia gravis

A
  • Akitas
  • Terrier breeds
  • scottish terriers
  • GSP
  • Chihuahuas
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9
Q

What proportion of dogs with acquired MG present with megaesophagus?

A

84%

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10
Q

Why can dogs with myasthenia gravis develop urinary incontinence?

A

external urethral sphincter is made of a skeletal muscle

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11
Q

How does myasthenia gravis affect spinal reflexes and proprioceptic testing?

A

should remail normal - can have poor knuckling or hopping if profound weakness present

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12
Q

What do you expect to see from a facial nerve exam on a dog with myasthenia gravis?

A

Normal facial nerves, except decreased palpebral reflexes if facial muscle fatigue present

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13
Q

List autoimmune disorders reported to occur with acquired myasthenia gravis

A
  • Hypothyroidism
  • Hypoadrenocorticism
  • Thrombocytopenia
  • IMHA
  • Polymyositis
  • IBD
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14
Q

What types of neoplasia have been shown to be associated with paraneoplastic syndromes with acquired myasthenia gravis?

A
  • Thyomoma
  • Osteosarcoma
  • Cholangiocellular carcinoma
  • AGASACA
  • Cutaneous lymphoma
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15
Q

How is thymoma proposed to contribute to the development of myasthenia gravis?

A

thyomoma can express antigenic epitopes similar to those of nicotinic ACh-R

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16
Q

What is the most common form of acquired myasthenia gravis in cats?

A

generalized AMG

17
Q

What is the most common cardiac disease detected in dogs with acquired myasthenia gravis?

A

Third degree AV block

18
Q

Name 4 examples for neuromuscular disorders mimicking myasthenia gravis

A
  1. botulism
  2. polyradiculoneuritis
  3. tick paralysis
  4. infectious neuropathies/myopathies
19
Q

What 2 medications can you administer for presumptive diagnostic tests for myasthenia gravis?

A

Edrophonium challenge test
alternatively: neostigmine methyl sulphate

20
Q

Explain how the Edrophonium challenge test works

A

ultra-short acting anticholineesterase agent –> decreases breakdown of ACh –> more ACh available for AChR -

Exercise dog until weakness occurs –> then give Edrophonium IV –> assess for transiet improvement

21
Q

What adverse effects should be expected from Edrophonium administration in MG patients?

A
  • Overstimulation of the AChR can produce depolarizing blockade –> worsens muscle weakness = cholinergic crisis –> respiratory paralysis
  • SLUDGE + bradycardia from muscarinic overstimulation
22
Q

Why do some patients with myasthenia gravis not respond to edrophonium?

A

may not have enough function AChR left - more common in acute fulminant MG

23
Q

What is the gold standard diagnostic test for myasthenia gravis?

A

Serum AChR antibody assay

24
Q

List your diagnostic plan for a dog suspected to have myasthenia gravis

A
  • CBC/Chem/UA
  • thyroid panel
  • Thoracic radiographs –> assess for megaesophagus, if aspiration pneumonia suspected –> BAL or tracheal wash
  • edrophonium challenge test (neostigmine alternatively
  • Electrodiagnostic tests
  • Immunocytochemical staining of muscle end plates (needs biopsy)
  • Serum AChR antibody assay
25
Q

What are the three components of treating a patient for myasthenia gravis?

A
  • Anticholinesterase therapy
  • Immunomodulatory therapy
  • Supportive Care
26
Q

List the treatment options for Anticholinesterase therapy in MG

A
  • pyridostigmine bromide PO versus IV as CRI
  • neostigmine bromide IV q6h IM
27
Q

How can you monitor treatment of dogs with MG with pyridostigmine/ differentiate over or under-treatment?

A
  • edrophonium chlordie challenge test –> if treatment with pyridostigmine is indadequate –> challenge will improve signs; if overtreated –> edrophonium makes signs worse
28
Q

List indications for immunomodulatory therapy in dogs with myasthenia gravis

A
  • dogs with persistently elevated AChR antibody titres
  • seropositve dogs with negative edrophonium challenge test
  • dogs with inadequate response to anticholinesterase therapy
  • dogs with unacceptable side effects from anticholinesterase therapy
  • cats
  • only acquired MG - congenital is not immune-mediated
29
Q

What is the main cause of death in dogs with myasthenia gravis?

A

aspiration pneumonia - associated with megaesophagus

30
Q

How can corticosteroids worsen myasthenia gravis in dogs?

A
  • glucocorticoids have negative effects on excitation-coupling of the contractile elements within myofibres
  • cause altered function of the ion channel of the AChR
31
Q

What is the mechanism of action of Mycophenolate?

A

active compound - mycophenolic acid –> selectively inhibits inosine monophosphate dehydrogenase - necessary for GTP synthesis –> impairs cells ability to synthesize DNA, RNA, proteins, glycoproteins –> targets preferentially lymphocytes

32
Q

What are the main side effects of Mycophenolate?

A
  • gastrointestinal signs
  • bone marrow suppression
33
Q

What is the mechanism of action of Azathioprine?

A

cytotoxic antimetabolite, purine antagonist –> incorperates into nucleid acid and terminates proliferation process
active metabolite: 6-mercaptopurine –> decreased lymphocyte proliferation

targets mostly T lymphocytes

34
Q

What are the adverse effects of Azathioprine?

A
  • bone marrow suppression
  • hepatotoxicity
  • GI signs
  • pancreatitis
35
Q

What is the mechanism of action of Cyclosporine?

A
  • calcineurin inhibitor –> inhibits T-cell activation
  • reduces IL-2 synthesis –> further inhibits T cell proliferation
36
Q

What are the adverse effects of cyclosporine?

A
  • gingival hyperplasia
  • GI upset
37
Q

Which antibiotics are contraindicated in myesthenia gravis and why?

A
  • erythromycin
  • imipenem
  • aminoglycosides
  • ampicillin
  • ciprofloxacin

can potentially have adverse effects on the neuromuscular junction

Neurologic Emergencies (3 decks)

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