Myasthenia Gravis Flashcards
What signs are seen in LMN conditions?
weakness
hypotonia
fasciculations
where do motor cell bodies arise?
ventral (anterior) horns of the spinal cord
What is the name of the synapse formed between neurones and muscle?
motor end plate
How many motor neurones do each muscle cell respond to?
1
what is the process behind muscle contraction?
action potential moves along the nerve
depolarisation
calcium channels open allowing Ca2+ influx
ACh released into the synaptic cleft
which enzyme terminates the action of ACh?
acetylcholinesterase splits Ach into acetate and choline
What is curare?
plant extract used in arrow poisons
What effect does curare have?
occupies Ach receptor but doesn’t open ion channels - no contraction - no breathing
Name some presynaptic disorders
botulism - clostridium botulinum
botulinum toxin
Lambert Eaton Myasthenic syndrome
What is Lambert Eaton Myasthenic Syndrome?
antibodies to presynaptic calcium channels leads to less vesicle release
Which malignancy has a strong association with LEMS?
small cell lung carcinoma
name a postsynaptic disorder?
myasthenia gravis
What happens in Myasthenia Gravis?
antibodies to acetylcholine receptors
reduced number of receptors = muscle weakness and fatiguability
How do postsynaptic membranes become damaged?
antibodies trigger an inflammatory cascade that damages them
What is the relation between the thymus and myasthenia gravis?
75% of patients have thymus hyperplasia or thymoma
What are the common clinical features of MG?
worsening weakness throughout the day
extra ocular weakness
facial and bulbar weakness
limb weakness typically proximal
How is MG treated?
Ach inhibitors
thymectomy
Name an Ach inhibitor used to treat MG?
pyridostigmine
Which medication should be avoided in the acute illness setting?
steroids - cause paradoxical worsening
What causes death in MG patients?
respiratory failure and aspiration pneumonia
immunosuppressant side-effects
Describe some features of smooth muscle
not striated
single central nucleus
gap junctions
actin myosin ratio 10:1
What are Type 1 muscles?
slow oxidative - fatigue resistant
What are type 2a muscles?
fast oxidative - aerobic metabolism
What are Type 2b muscles?
easy glycolytic - easily fatigues
What are fasciculations?
visible, fast twitches of muscle
What are fasciculations a sign of in an unhealthy individual?
disease of the motor neurone, not the muscle
What is myotonia?
failure of muscle relaxation after use
Name some inflammatory muscle disorders?
dermatomyositis
polymyositis
Name some inherited muscle disorders
muscular dystrophies
What is polymyositis?
symmetrical, progressive proximal weakness developing over weeks to months
What investigation findings are found in polymyositis?
raised CK
how is polymyositis treated?
steroids
What is the difference between dermatomyositis and polymyositis?
skin involvement - heliotrope rash
underlying malignancy common
What is characteristic in inclusion body myositis?
thumb sparing
What is rhabdomyolysis?
dissolution of muscle, damage to muscle causes leakage of large quantities of toxic intracellular contents into plasma
What is the common triad of rhabdo symptoms?
myalgia
muscle weakness
myoglobinuria
Can drugs cause problems in muscle?
Yes - HMG co-enzyme A reductase inhibitors
hydroxychloroquine
amiodarone
What is the most common muscular dystrophy?
myotonic dystrophy
autosomal dominant
weakness, cataracts, ptosis, balding, cardiac defects
