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Neurology > Disorders of Neuromuscular Junction > Flashcards

Disorders of Neuromuscular Junction Flashcards

1
Q

3 general features of lower motor neurone disorders?

A

weakness
low tone
fasciculations

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2
Q

3 general features of UPN disorders?

A

stiffness
spasticity
increased tone

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3
Q

cell bodies of motor neurones arise where?

A

ventral horn of spinal cord

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4
Q

what is the motor end plate?

A

the terminal portion of motor neurones give rise to very fine projections which run along the muscle cell
synapses formed between these projections and the muscle = end plate

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5
Q

each muscle can only respond to one motor neurone>

A

true

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6
Q

describe what happens at the NMJ?

A

AP moves along nerve
voltage gated calcium channels open allowing influx of calcium
vesicles of acetyl choline released into synaptic cleft
ACh diffuses across synaptic cleft
ACh receptor opens and renders membrane permeable to Na/K ions
the depolarisation starts an AP at the motor end plate
acetylcholinesterase breaks down ACh into acetate and choline
- choline is sequestered into presynaptic vesicles

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7
Q

what is curare?

A

chemical compound which ooccupies same position on ACh receptor but doesn’t open ion channel, so no muscle contraction can take place
- causes resp arrest when given IV or IM

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8
Q

where can problems occur at NMJ?

A

presynaptic (e.g botulism, lambert eaton)
synaptic cleft
ACh receptor (e.g myasthenia gravis)

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9
Q

what is botulism?

A

aka clostridium botulinum

  • organism in soil which can infect food and wounds
  • common in black tar heroin
  • toxin cleaves presynaptic proteins involved in vesicle formation and blocks vesicle docking with the presynaptic membrane
  • rapid onset weakness (6 hrs) without sensory loss
  • causes paralysis
  • not permanent?
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10
Q

what is LEMS?

A

lambert eaton myasthenic syndrome

  • antibodies to presynaptic calcium channels leads to less vesicle release
  • strong association with underlying small cell carcinoma
  • treatment = 3-4 diaminomyridine
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11
Q

what is myasthenia gravis?

A

most common NMJ disorder
autoimmune - antibodies to ACh receptors
causes reduced number of functioning receptors leads to weakness and fatiguability
- often in unusual muscle groups - eyes
- can also cause resp arrest
- bilateral facial weakness (can be missed as bilateral)

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12
Q

pathophysiology of MG?

A

reduced number of ACh receptors and flattening of endplate folds causes inefficient ACh transmission, even wit normal amounts of ACh
ACh antibodies found in most patients which block binding of ACh and trigger inflammatory acscades which damage folds of posysynaptic membrane

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13
Q

when do MG symptoms start?

A

symptoms start when ACh receptors reduced to 30% normal

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14
Q

which organ is often involved in MG?

A

thymus

75% have hyperplasia or thymoma

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15
Q

epidemiology of MG?

A

peaks of incidence - females in 3rd decade and males in 60s-70s
more common in females overall
more common in thyroid disease/addisons disease

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16
Q

clinical features of MG?

A

fluctuating weakness (worse through the day)
extraocular weakness, facial and bulbar weakness
limb weakness when it occurs is usually proximal (not usually first symptom)

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17
Q

how is MG manages acutely?

A

acetylcholinesterase inhibitor (pyridostigmine)
IV Ig
thymectomy (even in the absence of thymus abnormality it helps)

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18
Q

long term treatment of MG?

A

immunomodulating
steroids (prednisolone)
steroid sparing agents (azathioprine/mycophenolate)
plasma exchange or Ig in emergency

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19
Q

what must be avoided in MG?

A

gentamicin!
- causes myasthenic crisis > resp failure
depolarizing anaesthetic agents

20
Q

mortality of MG?

A

3-4%

21
Q

what causes death in MG?

A

usually due to resp failure in elderly

side effects of immunosuppression in elderly can also cause problems

22
Q

types of muscle?

A

skeletal muscle
cardiac muscle
smooth muscle

23
Q

describe features of skeletal muscle

A

striated
highly organised
muscle fibres each surrounded by thin layer of endomysium
20-80 fibres grouped to form a fascicle encapsulated by perimysium
large number of fascicles encapsulated in epimysium = muscle

24
Q

smooth muscle?

A 
not striated
single central nucleus
gap junctions between cells
significant connective tissue around them
no sarcomeres
25
Q

muscle fibre types?

A

type 1 = slow oxidative, dense capillaries, myoglobin, resist fatigue
type 2a = fast oxidative, aerobic metabolism
type 2b = fast glycolytic, easily fatigued

26
Q

what is a faciculation?

A

visible fast, fine, spontaneous twitch

27
Q

what causes fasciculations?

A

may occur in healthy muscle - precipitated by stress, caffeine and fatigue
occur in denervated muscle which become hyperexcitebale
usually sign of disease in MN, not the muscle

28
Q

what is myotonia?

A

failure of muscle relaxation after use

due to problems in Cl channel

29
Q

signs and symptoms of muscle disease?

A

myalgia
muscle weakness (often specific patterns of weakness - neck, shoulder and pelvic girdle)
wasting
hyporeflexia

30
Q

3 groups of muscle disorders?

A

immune mediated
inherited
congenital

31
Q

examples of immune mediated muscle disorders?

A

polymyositis
- symmetrical progressive proximal weakness over weeks-months
- CK in thousands
- responds well to steroids
dermatomyositis
- similar but with skin lesions (heliotrope rash etc)
- can have underlying malignancy

32
Q

what is the most common inherited muscle disorder?

A

myotonic dystrophy (type of muscular dystrophy)

  • AD
  • trinucleotide repeat disorder with anticipation
33
Q

other less common muscle disorder types?

A

toxic

infective

34
Q

describe degenerative muscle disorders and what category they fit into?

A

used to think they were inflammatory but don’t respond to steroids
inclusion body myositis
typically presents as slowly progressing weakness in 6th decade of life with thumb sparing

35
Q

features of myotonic dystrophy?

A 
myotonia
weakness
cataracts
ptosis
frontal balding
cardiac defects
36
Q

what are muscular dystrophies?

A

inherited, non-inflammatory progressive disorders of muscle
no central or peripheral nerve abnormality
Duchenne and becker muscular dystrophies most common

37
Q

examples of infective muscle disorders?

A

viral - coxsaccie
trypanosomiasis
cistercercosis - uncooked pork
borrelia

38
Q

examples of toxic muscle disorders?

A

caused by drugs or venoms

39
Q

examples of congenital muscle disorders?

A

congenital myasthenic syndromes

congenital myopathies

40
Q

examples of drugs which can cause a myopathy?

A 
statins
some steroids (dexamethasone)
amiodarone
colchicine
diuretics
oral contraceptives
D-penicillamine 
interferon alpha
41
Q

what is rhabdomyolysis?

A

dissolution of muscle

damage to skeletal muscle causes leakage of large quantities of toxic intracellular contents into plasma

42
Q

what can cause rhabdomyolysis?

A 
crush injury
toxins
post convulsions
extreme exercise
elderly lying on floor for long time after a fall
43
Q

features of rhabdomyolysis?

A

triad of myalgia, muscle weakness and myoglobinuria

can cause acute renal failure and DIC

44
Q

how is muscle examined?

A

inspection - thin? wasting? fasciculation?
palpating
strength testing across joints
neck strength (should never be able to overcome neck muscles)
core strength
fatiguability

45
Q

how is muscle power graded?

A

MRC grading
0 = no movement
1 = flicker of movement
2 = some muscle movement if gravity removed but none against gravity
3 = movement against gravity but not against resistance
4 = movement against resistance but not full strength
5 = normal strength

Neurology (53 decks)

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