Neuromuscular Disorders Flashcards
how do lower motor neurone disorders present?
weak, flaccid and wasted muscles
low tone
fasciculations
reduced/absent reflexes
what is the role of the neuromuscular junction?
makes the muscle contract by providing the connection between the motor neurone and the muscle
where do the motor neurones that innervate skeletal muscle fibres arise?
in the ventral horn of the spinal cord
what are motor end plates?
the synapses formed between motor neurons and muscles
how many motor neurons does each muscle cell respond to?
only one
however, a single motor neurone can control many muscle cells
what enzyme breaks down acetylcholine and what are the products?
acetylcholinesterase
acetate and choline
choline is sequestered into presynaptic vesicles
name two presynaptic disorders of the neuromuscular junction
botulism
lambert eaton myasthenic syndrome (LEMS)
what causes botulism?
clostridium botulinum
what is the pathophysiology of botulism?
botulinum toxin cleaves presynaptic proteins involved in vesicle formation, blockiung the vesicle from docking with the presynaptic membrane
this prevents muscle contraction
how does botulism present?
rapid onset weakness without sensory loss
ascending paralysis
what is the treatment for botulism?
supportive care
how long does botulism last?
around 12 weeks
what is the pathophysiology behind LEMS?
antibodies to presynaptic calcium channels lead to less vesicle release = reduced muscle contraction
what condition does LEMS have a strong association with?
underlying small cell lung carcinoma
how is LEMS managed?
3-4 diaminopyridine
name a postsynaptic disorder of the neuromuscular junction
myasthenia gravis
what is the pathophysiology behind myasthenia gravis?
autoimmune condition where antibodies to acetylcholine receptors (AChR) are produced
this reduces the number of functioning receptors causing muscle weakness and fatigue
what condition is myasthenia gravis associated with?
thymic hyperplasia/thymoma
when do symptoms start in myasthenia gravis?
when AChR levels reduced to 30% of normal
when are the two peaks of incidence of myasthenia gravis?
females in 20’s
males in 50’s-60’s
what is the most common initial presentation of myasthenia gravis?
extraocular weakness
facial weakness
bulba weakness
then becomes generalised
how does myasthenia gravis present?
fatigable weakness of skeletal muscle
what is ocular myasthenia gravis?
when weakness remains only in the extraocular muscles
what parts of the limbs are usually involved in myasthenia gravis?
proximal parts become weak
how is myasthenia gravis diagnsoed?
serum AChR antibodies
single fibre EMG
CT chest to look for a thymoma
what three things are included in the acute treatment of myasthenia gravis?
acetylcholinesterase inhibitor
IV Ig or plasma exchange
thymectomy if needed
name an acetylcholinesterase inhibitor that can be given for myasthenia gravis
pyridostigmine
this improves muscle strength
what emergency treatments can be given for myasthenia gravis?
IV immunoglobulin
plasma exchange
what drugs can be given for long term myasthenia gravis management?
immunomodulating drugs
steroids
what should be given alongside steroids in myasthenia gravis?
steroid sparing agents such as azathioprine or mycophenolate
what antibiotic should be avoided in patients with myasthenia gravis?
gentamicin
what is the smallest contractile unit of skeletal muscle?
the muscle fibre
what is each skeletal muscle fibre surrounded by?
endomysium
what is a fascicle and what encapsulates this?
20-80 smooth muscle fibres
encapsulated by perimysium
what forms an individual skeletal muscle?
many smooth muscle fascicles surrounded by epimysium
name six possible signs and symptoms of muscle disease
myalgia weakness wasting hyporeflexia fasciculations myotonia
what are fasciculations?
visible, fast, fine and spontaneous twitches
usually a sign of disease in the motor neuron, not the muscle
what is myotonia?
failure of muscle relaxation after use
when can fasciculations occur in normal muscles?
precipitated by stress, caffeine and fatigue
what blood test is often done if muscle disease is suspected?
CK
elevated in many muscle disorders
what is the characteristic feature of dermatomyositis?
heliotrope rash
what underlying condition do many patients with dermatomyositis have?
malignancy
how does polymyositis present?
symmetrical, progressive proximal weakness developing over weeks - months
raised CK
what drug does polymyositis respond to?
steroids
what is the most common type of muscular dystrophy?
myotonic dystrophy
what is the inheritance pattern of myotonic dystrophy?
autosomal dominant
how does myotonic dystrophy present?
myotonia weakness cataracts ptosis frontal balding cardiac defects
how does inclusion body myositis usually present?
slowly progressive weakness in the sixth decade of life with characteristic thumb sparing
