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Internal Medicine/ Ettinger > Neuromuscular Junction Disorders > Flashcards

Neuromuscular Junction Disorders Flashcards

1
Q

What kind of clinical signs are (UMN vs LMN) or seen with neuromuscular junction disease?

A

LMN

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2
Q

What happens at a neuromuscular junction?

A

Action potential –> via ventral horn motor neuron –> pre-synaptic terminal –> release of Acetylcholine (with help of Ca2+) –> into synaptic clef –> Acetylcholine receptors on the post-synaptic muscle membrane –> muscle contraction
- there is usually excess ACh and AChR to as “safety factor”
- ACh in the synaptic cleft is degraded by acetylcholinesterase

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3
Q

What are some clinical signs of NMJ dissease?

A
  • Symmetrical flaccid paresis
  • in more severe cases can have decreased segmental spinal reflexes, CN deficits, and autonomic signs
  • usually have normal mentation and sensory
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4
Q

What are some pre-synaptic diseases of NMJ?

A
  • Tick paralysis
  • Botulism
  • Elapid snake envenomation
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5
Q

How dose tick paralysis cause NMJ?

A
  • Dermacentor sp or Ixodes sp
  • neurotoxin released from female tick salivary gland
  • inability to release Ach from pre-synaptic terminal due to mechanisms associated with Ca2+
  • tx = remove tick
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6
Q

What are some clinical signs associated with tick paralysis?

A

In North America, Dermacentor more common.
- c/s noted 5-9 days after tick attachment
- acute, rapidly progressive ascending flaccid paresis
tetraplegia can happen in 12-72h
- segmental spinal reflexes severely diminished, decreased muscle tone
- hypoventilation can occur in severe cases
- autonomic, sensory, and sphincter function = normal
- tick removal = dramatic improvement within hours

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7
Q

What are some c/s with Ixodes paralysis in Australia?

A
  • in addition to the rapid flaccid paresis, can also have autonomic, sensory, and sphincter abnormalities
  • congestive heart failure (diastolic dysfunction)
  • pupillary dilation
  • can get pulmonary edema, aspiration pneumonia
  • may continue to decline several days after tick removal
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8
Q

What’s the most common botulism neurotoxin in dogs?

A

neurotoxin type-C (BoNT-C)

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9
Q

How does botulism cause NMJ disease?

A

Botulism toxin irreversibly binds to SNARE protein, which is essential in docking of Ach in the pre-synaptic terminal –> therefore, there is no release of ACh
- it’s at pre-synaptic terminal of skeletal muscle and cholinergic autonomic synapsis

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10
Q

What are some clinical signs of botulism?

A

acute, progressive, tetra paresis
- also has autonomic signs (mydriasis, changes in heart rate)
- CN deficits (palpebral, megaesophagus, PLR) also common
- can be seen within hours or up to 6d

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11
Q

What are the treatment for botulism?

A
  • supportive therapy
  • anti-toxins don’t work (most are not against BoNT-C), watch for anaphylaxis
  • don’t use aminoglycosides or ampicillin –> may potentiate NM blockade
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12
Q

How does snake venom cause NMJ disease?

A

It can block pre-synaptic release of ACh

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13
Q

Where are some clinical signs of envenomation?

A
  • flaccid tetraparesis/ tetrplegia
  • hypotonia
  • reduced/ absenet segmental spinal reflexes
  • hypoventilation
  • CN deficits: drooling, dhysphagia, dysphonia, facial paresis/ paralysis
  • discolored urine (esp in dogs): not a feature in other pre-synaptic NMJ disease
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14
Q

What are some causes of post-synaptic NMJ disease?

A

acquired or congenital myasthenia gravis

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15
Q

How does acquired myasthenia gravis cause NMJ disease?

A
  • autoimmune
  • produces antibodies against muscarinic AChR in skeletal muscles
  • therefore, post-synaptic terminal less responsive to ACh
  • skeletal muscle weakness, often exacerbated by activity
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16
Q

What’s the typical signalment for acquired myasthenia gravis?

A
  • <4y or >9y
  • Golden, German Short-hair pointers, Akita, Chihuahua, GSH, Newfoundlanders
  • less common in cats (Abyssinians, Somali, other purebreds)
17
Q

What are the 3 forms of myasthenia gravis? Which one is the most common one?

A
  1. Generalized (most common)
  2. focal
  3. fulminant
18
Q

What are the c/s related to generalized myasthenia gravis?

A
  • neurological normal at rest
  • has a choppy, tilted gait, first seen in hind limbs
  • with increased activity, will see it in the front limbs
  • segmental spinal reflexes typically normal
  • can have fatigable palpebral reflex
  • megaesophagus/ regurgitation common
  • ptyalism, dysphonia possible
  • ventroflexion of neck is more common in cats
19
Q

What are some signs associated with fulminant myasthenia gravis?

A
  • rapidly progressive severe diffuse weakness –> nonambulatory tetraparesis, lateral recumbency
  • doesn’t get better with rest
  • lots of regurgitation and aspiration pneumonia = common
  • respiration muscle weakness and urine retention
  • thymoma
  • seen in 16% of dogs, and 15% of cats with MG
20
Q

What are some signs of focal myasthenia gravis?

A

disorder isolated to a single muscle group: ocular, facial, esophageal, pharyngeal, or laryngeal muscles

21
Q

What are some conditions associated with acquired myasthenia graivs?

A
  • thymoma
  • hypothyroidism
  • hypoadrenocorticism
  • polymyositis
  • masticatory myositis
  • hyperthyroid cats on methimazole
22
Q

How is acquired myasthenia gravis diagnosed?

A
  • no change with CBC or chemistry panel
  • Gold standard = demonstration of anti-AChR antibodies
  • a decrease in CAMP can be noted from repeated nerve stimulation
  • edrophonium test (short acting anti-acetylcholinesterase) – not 100% specific to MG
  • watch for cholinergic crisis with edrophonium (weakness, hypersalivation, tremors, vomiting, bradycardia, bronchoconstriction, respiratory distress)
23
Q

What are some treatment options for acquired myasthenia gravis?

A
  • acetylcholinesterase inhibitors: neostigmine, pyridostigmine bromide (oral). Both have narrow therapeutic window
  • immunosuppressive drugs: cyclosporin, mycophenoalte, azathioprine (targets adaptive and spares the innate immunity)
  • corticosteroids
  • spontaneous recovery possible
24
Q

Which breeds have congenital myasthenia gravis been reported in?

A
  • Jack Russel Terriers
  • Min Dachshund
  • Smooth Fox Terriers
  • Gammel Dansk Honsehund
  • others
25
Q

What are the clinical signs of congenital myasthenia gravis?

A
  • usually evident by 6-9 weeks, multiple puppies affected in one litter
  • generalized limb and cervical muscle weakness, worse with activity
  • +/- megaesophagus
  • in Gammel Dansk Honsehund, it’s seen around 4m
26
Q

What’s the treatment for congenital myasthenia gravis?

A
  • neostigmine or pyridostigmine
  • euthanasia
  • in Min Dachshund, spontaneous resolve by 6m
  • Gammel Dansk Honsehund, c/s mild, so no tx necessary
27
Q

What’s exercise induced collapse of Labrador Retrievers?

A
  • fatigue and flaccid para or tetra paresis during or after strenuous exercise
  • ataxia, pelvic limb weakness, collapse
  • normal at rest
  • recover within 30min of activity
  • severe episode can lead to death
28
Q

How is congenital myasthenia gravis diagnosed?

A

Muscle biopsy = definitive diagnosis

29
Q

What’s the etiology of exercise induced collapse in Labrador Retrievers?

A
  • heritable mutation
  • autosomal recessive Arg256Leu, for protein dyamin 1
  • this protein encodes a family of proteins that maintain synaptic vesicle function during sustained neurotransmission
  • important in recycling or synaptic vesicles
  • usually seen before 2yo

Internal Medicine/ Ettinger (93 decks)

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