Myasthenia Gravis Flashcards
Define myasthenia gravis
An autoimmune neuromuscular disorder characterized by fluctuating motor weakness involving ocular, bulbar, limb and respiratory muscles
What effect can impairment of the bulbar nerves cause?
Dysphagia
Reduced gag reflex
Slurred speech
Aspiration of secretions
Impaired cough
What is the weakness associated with MG caused by?
An immunological attack on the post synaptic membrane where acetylcholine receptors are located
What is the most common disorder of neuromuscular transmission?
Myasthenia gravis
How does MG block neuromuscular impulses?
Antibodies attack nicotonic receptors preventing them from binding with acetylcholine
What is a characteristic of the weakness associated with MG?
Improves after rest
Are the eyes affected equally in MG?
Not necessarily
Describe ocular MG
Muscle weakness limited to eyes and extraocular muscles
Describe generalized MG
Variable muscle weakness in bulbar muscles, limbs, and respiratory muscles
How may a patient with generalized MG present?
Drooped head
Expressionless
Ptosis
Diplopia Double vision
Slurred speech (dysarthria)
How is MG a respiratory problem?
May cause weakness of the respiratory muscles which can lead to respiratory insufficiency or failure
What antibody is responsible for the neuromuscular weakness seen in MG?
IgG
How does IgG disrupt the transmission of acetylcholine?
Not well understood
May block receptor sites
Destroy receptor sites
Accelerate breakdown of acetylcholine
What kind of motion is more susceptible to the weakness associated with MG?
Repetitive
What patients would raise a suspicion of MG?
Patients with fatigable muscle weakness
Patient w isolated ptosis (drooping eyelid) and/or diplopia (double vision)
Describe how the icebag test works on patients with MG
Neuromuscular transmission improves at lower temperatures so cooling the eyelids allows for improved function IF the dysfunction is due to impaired neuromuscular transmission
What is a pharmacological test for MG?
Edrophonium (tensilon) test
What does the edrophonium or tensilon test do?
Short acting drug that inhibits acetylcholinesterase from functioning allowing for higher concentrations of acetylcholine and increases signal transmission
Why does steve hate edrophonium?
Its not used in US
Supposedly unreliable
Associated with adverse events
What is serologic testing?
Testing for antibodies to acetylcholine receptors
If a patient is seropositive, what antibodies are present?
AChR
MuSK
What does it mean if a patient is seronegative?
It means theyh dont have the AChR or MuSK antibodies and will need further testing
How many patients with generalized MG are seronegative?
90%
What are the 3 electrodiagnostic studies used for MG?
Nerve conduction
Electromyography
Single fiber electromyography
Describe repetitive nerve stimulation testing for MG
Stimulate motor nerve of a selected muscle 6-10 times at low rates and observe for changes ins compound muscle action potential (CAMP) amplitude
What constitutes a positive result from a repetitive nerve stimulation test for MG?
Progressive decline in CAMP amplitude after 4-5 stimuli
>10% decrease required for a positive test
What is the most sensitive test available for MG?
Single fiber electromyography (SFEMG)
Describe single fiber electromyography (SFEMG)
A special needle electrode allows simultaneous recording of action potential of two muscle fibers innervated by the same motor neurons
What constitutes a positive test with the single fiber electromyography test?
When the muscle is stimulated, the second fiber will falter behind the first, causing a jitter that is measurable
Why should the thymus be investigated when considering MG?
Thymus creates antibodies
A thymoma or thymic mass may be causing the thymus to create bad antibodies
What kind of lung pathology is associated with MG?
Restrictive lung pathology
Decreased flows
Decreased volumes
Decreased capacities
What is an ABG of someone with MG likely to look like?
Respiratory acidosis
How will the CXR of a patient with MG look?
Normal
How are patients with MG managed in the hospital?
Repeated NIF + VC to assess need for intubation
SpO2
ABGs
When is a patient with MG considered to be in respiratory failure?
RR > 30
SpO2 < 92
FVC < 20 (or 30% reduction since last measurement)
NIF < 30
MEP < 40
PaCO2 > 50
What are the 4 main treatment pathways for MG?
Acetylcholinesterase inhibition
Immunosuppressive or immunomodulatory agents
Short acting immunomodulating agents
Thymectomy
What medication is used to inhibit acetylcholinesterase?
Pyridostigmine (mestinon)
When is pyridostigmine (mestinon) used?
Mild cases
Less effective in severe cases
What drugs can be used on patients who do not tolerate acetylcholinesterase inhibitors?
Glucocorticoids
What treatments are used as a rapid response to a myasthenic crisis?
PLEX
IVIG
What are options for chronic management of MG?
Avoid medications that may exacerbate MG
Seek vaccinations
