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Flashcards in Myasthenia Gravis Deck (21):

what is the neuromuscular junction

the synapse bet the motor neuron and a skeletal muscle fibre


transmission of impulses at the neuromuscular junction is mediated by which neurotransmitter
what is the enzyme that degrades it

acetylcholine (-esterase)


where does myasthenia gravis occur?
what is the basic problem (what fx is lost)

at the neuromuscular junction there is a disorder of transmission bet the motor neuron and innervated muscle cell


when does myastehnia gravis typically occur
how does gender relate to it

bet 20-30
3x more prevalent in women


what is Lambert Eaton myasthenic syndrome?

a special type of myasthenic syndrome that develops in assoc w neoplasms, particularly small cell carcinoma of the lung


patho of myasthenia gravis. how does it start? how does it progress?

-autoimmune, caused by Ab mediated loss of Ach receptors in neuromuscular junction. This may be triggered by sensitized T cells and Ab directed attack on Ach receptor in neuromuscular junction.


what damage results from the Abs

-dec number of Ach receptors
-shedding of ACh receptor rich terminal part of the folds in muscle fiber
-widened space that inhibits transmission


do the Ab directly ihibit binding at neuromuscular

• The antibodies do not directly block binding of acetylcholine to prevent neuromuscular transmission


one way this disease can be transmitted

by placenta (10%)
spontaneous resolution occurs


75% of myasthenia gravis pts have abn with

thymic problems eg thymoma or thymic hyperplasia


if pt w MG has reduced postsynaptic membrane area and fewer Ach receptors what happens when Ach is released?
what does this result in?

each release of Ach from the presynaptic membrane results in a lower amplitude endplate potential

->muscle weakness and fatigability w sustained effort


which areas are most commonly affected?
what else is affected?

Basically, affects eye, face (speech impairment), weakness (can inculde chew/swallow/respiratory) that is more in proximal, gets worse through the day

Most commonly affected are the eye
-periorbital muscles, with ptosis due to eyelid weakness or diplopia d/t weakness of the extraocular muscles as an initial symptom.
• May progress to generalized weakness, including respiratory weakness.Chewing and swallowing may be difficult.
• Weakness in limb movement usually is more pronounced in proximal than in distal parts of the extremity
• Muscles of the lower face are affected, causing speech impairment
• Symptoms grow worse as the day proceeds


what is myasthenic crisis

myasthenic crisis- becomes severe enough to compromise ventilation and need support and airway protection. Occurs during a period of stress, emotional upset, pregnancy, alcohol ingestion, cold exposure, or surgery or inadequate or excessive dose of the anticholinesterase drugs used in treatment of the disorder.


what is a motor endplate

the specialied end of the muscle is where the axon terminls of a motor neuron end


how can muscles have repeated contractions and gradations of contractile force

rapid inactivation of Ach



-anticholinesterase test
-immunoassay for detecting the presence of anti-Ach receptor Abs circulating int he blood


anticholinesterase test as Dx tool

• Anticholinesterase test uses a drug in that inhibits acetylcholinesterase (enzyme theat breaks down acetylcholine). Whenweakness is caused by this disease, a dramatic transitory improvement in muscle function occurs.


electromyography as Dx tool

• Single-fiber electromyography detects delayed or failed neuromuscular transmission in muscle fibers supplied by a single nerve fiber.



• Immunosuppressive therapy (corticosteroid drugs), management of myasthenic crisis, thymectomy, and plasmaphresis or IV immunoglobulin.

• Corticosteroid drugs (eg. Prednisone) suppresses IR and used in cases of poor response to anticholinesterase drugs and thymectomy. May be used in combination with plasmapheresis
• Phasmapheresis removes antibodies from the circulation and provides short-term clinical improvement. It is used to stabilize the condition of persons in myasthenic crisis or for short term treatment if undergoing thymectomy. (circulate the blood and remove the plasma, replace it)


myasthenic crisis

• Symptoms: respiratory distress, dysphagia, dysarthria (difficulty speaking), eyelid ptosis, diplopia, and muscle weakness
• Placed in ICU for monitoring
• Ventilation assistance. Monitor vital capacity, negative inspiratory force, and rate, depth and breath sounds.
• If abdominal, intercostal and pharyngeal muscles are weak, the patient cannot cough, take deep breaths or clear secretions.
• Chest physical therapy and postural drainage (should not be performed 30 min after feeding) to remove secretions may be done.
• ABGs, I&O, electrolytes, and weights are monitored.
• Tube feeds may be done
• Sedatives and tranquilers are avoided because aggregates hypoxia.


events at a neuromuscular junction

AP is propagated along a neuron until the axon terminal where the AP stimulates the release of calcium ions from voltage gated calcium channels.
-vesicles w Ach move to surface of axon terminal and rel Ach into synaptic cleft
-Ach binds to its receptor on motor end plate making memb permeable to sodium and potassium
-this causes depolarization of the motor end plate
-Ap is propagated through muscle fiber involving calcium ions
-Ach is degraded by acetylcholinesterase
-the muscle acts