Disorders of the neuromuscular junction Flashcards
specific characteristic of the symptoms that patients with myasthenia gravis experience?
FATIGUABILITY
e.g. can be demonstrated with ptosis: ask pt to maintain upward gaze, eyelid will gradually droop over 60-90s, can then ask pt to close their eyes to relax the muscles and then reopen and the ptosis will have resolved.
And will see frontalis contracting to try and maintain upward gaze.
Abs assoc. with MG?
acetylcholine receptor Ab
muscle-specific kinase Ab
note patients may not have any Abs detectable
who is affected by MG?
bimodal presentation: young people 20-30s, females more than males
and 60-80s, mainly males
note increasing diagnoses being made in the elderly.
effect of drugs on presentation of MG?
MG can be aggravated or induced by drug treatment, most well known=penicillamine.
define bulbar palsy
result of disease affecting the lower cranial nerves (7-12)
presenting features of myasthenia gravis?
limb weakness, more marked in proximal muscles
extraocular muscle weakness-ptosis, and also results in diplopia
weakness is painless and fluctuating
bulbar symptoms due to bulbar muscle weakness-dysarthria, dysphonia, dysphagia
difficulty chewing food
*resp muscle involvement-life threatening
note no sensory or reflex loss, normal muscle tone, no fasciculations and no muscle wasting (atrophy may rarely occur due to disuse because of weakness).
other than drugs, what can exacerbate weakness in MG and precipitate a myasthenic crisis?
intercurrent illness
pregnancy
emotions
hypokalaemia
complications of myasthenia gravis?
acute respiratory failure
aspiration pneumonia
best technique of monitoring MG patients for resp compromise?
regular measuring of FVC using spirometry
note vital capacity can decreased markedly before any change in SpO2 or PaO2.
differentiate between the symptoms of myasthenia gravis and those of lambert-eaton-myasthenic syndrome
LEMS-disorder of NMJ which involves impaired ACh release PRE-SYNAPTICALLY
pre-synapse-lots of ANS receptors, hence disease assoc. with ANS symptoms-dry mouth, impotence in males, postural hypotension.
initially slight benefit from exercise
no auto-Ab or EMG findings
no ocular weakness
investigations for myasthenia gravis?
ice test
Cold improves neuromuscular transmission so will be able to do upward gaze test.
autoantibodies against AChR and muscle specific tyrosine kinase
CK usually normal (may be raised in other myopathies)
tensilon test-IV SA acetylcholine esterase inhibitor given to the pt to see if symptoms resolve, BUT can cause life threat bradycardia, must do test trial with pt connected to cardiac monitor
neurophysiology-specifically need repetitive nerve stimulation EMG to show fatiguability
CT thorax to exclude presence of underlying thymoma
management of myasthenia gravis?
AChesterase inhibitors e.g. neostigmine, pyridostigmine, provide temporary symptomatic relief of muscle weakness, must give 30mins before eating to allow effect to take placw
AI disease so need immunosuppressants-steroids-start low, and steroid sparing drugs-azathioprine, mycophenolate
IV Ig
plasmapharesis
may need diet modifications to aid with swallowing difficulties
must reassure patients that disease can take several mnths to get under control, and can need treatment for years, even lifelong.
thymectomy for thymoma
why are steroids started at LOW dose in MG treatment?
starting high risks myasthenic crisis
features of a myasthenic crisis?
rapid worsening of muscular weakness
whole face drooping, dribbling, SNS response
similar px to cholinergic crisis-result of overtreatment of MG often due to pt taking more med than should as works so well, but this has PNS features**
can do edrophonium test if in doubt-readily reversible AChE inhibitor, pt with myasthenic crisis will improve
tx of lambert-eaton myasthenic syndrome?
3,4-diaminopyridine (amifampridine)-increases pre synaptic ACh release.
