pathophysiology of myasthenia gravis

8/12/2019 Pathophysiology of Myasthenia Gravis

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PATHOPHYSIOLOGY

OF MYASTHENIAGRAVIS


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Normally, an impulse frombrain is sent down to Spinalcord to its peripheral nervous

system.


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It will go to the Motorneuron where the muscle

and nerve meet at theneuromuscular junction.

The nerve will release aneurotransmitter calledacetylcholine.


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Acetylcholine will bind tothe acetylcholine-receptors.

Triggers an actionpotential which willlead to musclecontraction and body

movement.


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In myasthenia gravis, thebodys immune systemcreates antibodies thatattack the receptor sitesfor acetylcholine.

Acetylcholine is nowunable to bind to the

muscle.


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Because of that, the muscle contraction is nowimpaired.


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ANATOMY


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Neuromuscular Junction


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Neuromuscular junction can bedefined as a synapse the motor neuron andthe muscle fiber. Actually the synapseoccurs between the synaptic endbulbs of the motor neuron and motor end plate of muscle fiber. The motor endplate is in fact the highly excitable region ofmuscle fiber plasma membrane and it isresponsible for initiating action potentialsacross the muscle surface. This effectultimately results in muscle contraction.


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The neuromuscular junction is composed ofthree parts: Synaptic end bulbs:

As the axon of the motor neuron enters theskeletal muscle, it forms many branches calledaxon terminals. At the end of each axon terminal,there is a bulbous swelling called synaptic endbulb. Each synaptic end bulb contains manysynaptic vesicles. These vesicles contain the all-

important neurotransmitter substances such asacetylcholine. These neurotransmitter substancesare responsible for transmission of impulse fromaxon to muscle fiber through the synapse.


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Motor end plate:

It is the part of the sarcolemma of muscle cell, which is in closestproximity to the synaptic end bulb. It shows certain specific features

different than those of other regions of muscle cell sarcolemma,including:

Synaptic Gutter: It is the invaginated membrane, which forms space forthe synaptic end bulbs to reach close to the muscle fiber sarcolemma.

Subneural Clefts: These are small folds of the muscle membrane presentat the bottom of the synaptic gutter. They greatly increase the surfacearea at which the neurotransmitter can act.

Increased number of mitochondria: The area of the muscle fibersurrounding the motor end plate shows a considerable increase in thenumber of mitochondria. The obvious reason for this is the energydemand of the neuromuscular junction.


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The Synaptic cleft: Synaptic cleft is the space between the motor end

plate (muscle fiber part) and synaptic end bulb(motor neuron part) of the neuromuscular junction. It is 20 to 30 nanometers wide. Becauseof this cleft, the connection between the motor

neuron and the muscle fiber is not continuous andthere is a break. This break is traversed by theneurotransmitters. This shows the importance ofthese substances in the activity of muscles (and allother nervous control mechanisms).


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Nursing Diagnosis andInterventions


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Ineffective breathing pattern relatedto intercostal muscle weakness

Assess the respiratory rate ,depth and breath soundsand monitor the results of pulmonary functiontests(tidal volume, vital capacity, inspiratory force)

Encourage deep breathing and coughing exercise ifthe patient can tolerate

Chest physiotherapy, postural drainage is given to

immobilize the secretions Suctioning is done to promote the drainage of

secretions Provide psychological support to the patient


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Impaired physical mobility related tovoluntary muscle weakness

The patients goal is the improvement of thestrength of the weakened muscles

The patient should be educated about the basic

facts about anticholinesterase agents-their action,timing, dosage, adjustment symptoms of overdoseand toxic effects. The importance of taking themedication and the time should be emphasized.

The patient is encouraged to keep a diary to notethe fluctuation in symptoms.

Time the meals to coincide with the peak effectsof anticholinesterase medications


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Plan adequate rest periods throughout the

day. Wear appropriate shoes to minimize injury. Eliminate the factors which will cause

elevation of the symptoms (emotional upset,infections, particularly respiratory infections,vigorous physical activity etc.

Encourage to use cervical collar if there isneck weakness


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Risk for aspiration related toweakness of the bulbar muscles

The patient should be assessed for drooling, regurgitationthrough the nose, choking while attempting to swallow etc.

Rest before the meals is encouraged to reduce musclefatigue

The patient is seated in an upright position with the neckslightly flexed to facilitate swallowing

Soft food is encouraged Since the muscles of swallowing will be stronger in the

morning, caloric intake can be increased for breakfast. Thepatient is encouraged to rest after breakfast

Suction should be available at the home if there is risk ofaspiration


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Impaired verbal communicationrelated to weakness of the larynx,

lips, mouth, pharynx and jaw The weakened muscles will interfere with

verbal communication Techniques for improving the communication

includes listening to the patients, repeatingwhat they are trying to communicate andasking the patient to blink their eyes for yesor no answers


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pathophysiology of myasthenia gravis

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