L7 Myasthenia Gravis Flashcards
where is the term myasthenia gravis derived from
greek
- my → muscle
- asthenia → weakness
gravis → severe
what is myasthenia gravis
a chronic autoimmune neuromusclular disease characterised by weakness in voluntary skeletal muscle, fluctuating symptoms and a marked fatiguability effect
key characteristics of MG
- skeletal muscle weakness after periods of activity
- rapid improvement after rest
- skeletal muscle weakness isolated to specific muscle groups
- chronic and gradual onset
how does the neuromuscular junction work
- axons release the neurotransmitter acetylcholine
- acetylcholine travels across the synapse and binds to receptors on post-synaptic membranes
- receptors are stimulated by acetylcholine causing muscular contraction
pathophysiology of MG
- antibodies are produced by the immune system
- they block, alter or destroy acetylcholine receptors at the neuromuscular junction
- this prevents proper muscular contraction
global prevalence
100-200 per million
(700,000+ cases worldwide
incidence
10-20 per 100,000 (likely underdiagnosed)
when does prevalence peak in women
2nd/3rd decade
when does prevalence peak for men
7th/8th decade
prevalence sex ratio
3:1 female to male
occular muscle signs
- fatigable ptosis (drooping eyelids) - first symptom in 2/3 of MG patients
- extraoccular muscle weakness leading to diplopia (double vision) with limited eye movements
facial muscle signs of MG
- facial weakness
- weak eye closure
how many cases have bulbar signs
15%
bulbar muscle signs of MG
- nasal regurgitation
- difficulty chewing
- breathy voice
respiratory muscle involvement in MG
can cause acute respiratory symptoms which is a medical emergency
limb muscle signs of MG
worse after exercise or at the end of hte day
weakness involves arms more than legs
contributing factors in MG
- emotional upset
- systemic illness (eg. viral infections)
- thyroid disorders (hypo/hyperthyroidism)
- pregnancy
- menstrual cycle
- increased body temperature
- medications affecting neuromuscular transmission
what classification system is used for MG
osserman classification system
class 1 of osserman classification system
- Any eye muscle weakness
- Possible ptosis
- All other muscle strength is normal
class 2 of osserman classification system
- Mild weakness of other muscles; may have eye muscle weakness of any severity
- 2a → Predominantly limb or axial muscles weakness or both
- 2b → Predominantly oropharyngeal or respiratory muscle weakness or both
class 3 of osserman classification system
- Moderate weakness of other muscles; may have eye muscle weakness of any severity:
- 3a → Predominantly limb or axial muscles weakness or both
- 3b → Predominantly oropharyngeal or respiratory muscle weakness or both
class 4 of osserman classification system
- Severe weakness of other muscles; may have eye muscle weakness of any severity:
- 4a → Predominantly limb or axial muscles weakness or both
- 4b → Predominantly oropharyngeal or respiratory muscle weakness or both
class 5 of osserman classification system
intubation needed to maintain airway
what is the link between MG and the thymus gland
there is a link between MG and thymomas (tumours of the thymus gland)
what percentage of people with MG have a thymoma
10-20%
what percentage of people with thymoma develop MG
20-40%
what does evalution for MG involve
- Aimed at eliciting fatigue:
- Repeated blinking for ptosis
- Counting to 100 to assess dysarthria
- Abducting arm for unilateral weakness
- Forced vital capacity test
- Upward gaze for double vision
- History of thymoma
what tests can be used to diagnose MG
- Bedside Tests: Tensilon (edrophonium) and ice-pack test
- Electrophysiological Tests
- Auto-Antibody Tests
how does the tensilon (enrophonium) test work
- a short-acting acetylcholinesterase iniibitor prolnong acetylcholine action in the NMJ
- this increases the amplitude and duration of end-plate potentials
- it slows down the overt sypmtoms which confirms MG
what is the sensitivity of the tensilon (endrophonium) test in diagnosing MG
71.5-95%
what MG symptoms is the tensilon (endrophonium) test most reliable for
otosis, nasal speech and strabismus (eye misalignment)
6 treatment types for MG
- cholinesterase inhibitors
- corticosteroids
- plasma exchange (plasmapheresis)
- immunossuppression
- IVIG
- thymectomy
two examples of cholinesterase inhibitors
- neostigmine
- mestinon
cholinesterase inhibitors
- Provide symptomatic relief
- Allow acetylcholine to accumulate at the NMJ
- Prolongs effects of acetylcholine
- Has adverse effects.
corticosteriods
- Much improvement occurs in the first 6-8 weeks
- Strength may increase to total remission in the months that follow.
- Best responses occur in patients with recent onset of symptoms.
- The major disadvantages are the side effects.
plasma exchange (plasmaperesis)
- In plasmapheresis, blood is routed through a machine that removes plasma containing harmful antibodies and replaces it with antibody-free plasma.
- Often 4-5 treatments over a two week period in hospital.
example of a corticosteroid
prednisone
immunosuppression
- eg. prednisone
- Alter the body’s immune system and reduce production of the antibodies which cause myasthenia gravis.
what does IVIG stand for
intravenous immunoglobulin therapy
IVIG
Person is injected with normal antibodies that change the way your immune system acts.
thymectomy
- Recommended for most patients
- Most favourable response is 2-5 years after the surgery.
two potential medical events in MG
- myasthenic crisis
- cholinergic crisis
what is myasthenic crisis
Severe weakness requiring intubation for ventilatory support or airway protection.
indicators for intubation during myasthenic crisis
- Respiratory muscle fatigue, with increasing tachypnea, declining tidal volumes.
- Hypoxaemia, hypercapnea
- Difficulty handling secretions
preferred treatment for myasthenic crisis
plasma exchange
what is cholinergic crisis
Respiratory crisis due to cholinesterase inhibitor overdose.
symptoms of cholinergic crisis
- sweating
- constricted pupils
- excessive salivation
- muscle fasciculations
prognosis of MG
- Some cases may enter remission temporarily.
- Muscle weakness may disappear completely so medications can be discontinued.
- Thymectomy aims for stable, long-lasting remission
- Severe weakness may lead to respiratory crises requiring emergency medical care.
two MG rating scales
- MG-ADL
MG- QOL15
what is the MG-ADL
rating scale for myasthenia gravis which assesses activities of daily livnig
what is the MG-QOL15
rating scale for myasthenia gravis measuring quality of life
what is SLT’s role in myasthenia gravis
- sometimes aid in differential diagnosis
- main role is managing dysarthria and dysphagia
SLT assessment components
- Observe muscle fatigue at rest and during oro-facial examination
- Check for low tone/flaccidity
- Evaluate over time → massively important to identify possible MG.
stress/fatigue test for speech
count from 1-100 and compare speech quality at the start and the end
stress/fatigue test for swallow
include solids (eg. apple, sandwich) in evaulations to assess mastitory fatigue
what type of dysarthria is present in myasthenia gravis
flaccid
flaccid dysarthria characteristics in MG
- Hypernasality, weak breathy voice, low volume
- Misarticulation
- Fatigability → speech deterioration over time
dysarthria management in MG
- Monitor effects of medical interventions.
- Educate patients about speech factors and energy conservation strategies.
examples of conservation strategies for dysarthria in MG
- Rest before conversations, use short sentences, take breaks.
- Calls earlier in the day, non-verbal communication options, quiet environments.
are muscle strengthening exercises recommended for MG
no, these are contraindicated due to the fatigability in MG
two examples of prostheses which may be used in MG
- Ptosis props.
- Palatal lift prostheses.
symptoms of dysphagia in MG
- Difficulty masticating solids.
- Nasal regurgitation of fluids due to weak velopharyngeal seal
- Weak tongue movement (lingual muscles)
- Reduced hyolaryngeal excursion (suprahyoid muscles).
- Poor airway protection (vocal cord adduction).
- Impaired upper eosphageal sphincter tone.
- Weak cough response (expiratory muscles).
dysphagia management in MG
- Observe effects of medical management.
- Educate patients.
- Direct rehabilitation strategies are contra-indicated.
compensation strategies for dysphagia in MG
- Small frequent meals, avoid chewy solids, smaller bolus sizes.
- Multiple swallows.
SLT role in differential diagnosis
- Myasthenia gravis will not present with fasculations unlike MND.
- Additionally, Myasthenia gravis has the fatiguability and rapid rest and recovery as a key characteristic which MND does not.
