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Year 3 > Myasthenia gravis (N) > Flashcards

Myasthenia gravis (N) Flashcards

1
Q

Define myasthenia gravis.

A

Chronic autoimmune disorder of the post-synaptic membrane at the neuromuscular junction in skeletal muscle, characterised by generalised muscle weakness

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2
Q

What does the pathophysiology of myasthenia gravis involve?

A

Elevated serum autoantibodies against post-synaptic nicotinic ACh receptors (AChR) at the neuromuscular junction, or muscle-specific tyrosine kinase (MuSK)

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3
Q

What antibodies are found in the most common form of myasthenia gravis?

A

Autoantibodies against post-synaptic nicotinic AChR at the NMJ, which interferes with the neuromuscular transmission via depletion of post-synaptic receptor sites

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4
Q

What other diseases is myasthenia gravis associated with? (2)

A
  • thymomas (tumour of anterior mediastinum) - may cause SVC obstruction
  • thymic hyperplasia
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5
Q

Which demographic is myasthenia gravis most commonly seen in?

A

Women

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6
Q

What are some risk factors for myasthenia gravis?

A
  • Fx/Hx other autoimmune conditions
  • genetic markers
  • cancer-targeted therapy
  • beta-blockers - for myasthenic crisis
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7
Q

What is Lambert-Eaton Syndrome (myasthenia gravis)?

A

Paraneoplastic subtype of myasthenia gravis caused by autoantibodies against pre-synaptic calcium channels, leading to impairment of ACh release

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8
Q

What can myasthenia gravis be? (2)

A
  • paraneoplastic (from lung cancer)
  • autoimmune
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9
Q

Name another autoimmune condition that myasthenia gravis is associated with?

A

Pernicious anaemia

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10
Q

What are the clinical features of myasthenia gravis? (9)

A
  • muscle strength fatiguability/weakness - worsens with activity and improves on rest (patients typically complain of it being worse at end of day)
  • ptosis (droopy eyelid)
  • diplopia (double vision)
  • dysphagia (difficulty swallowing)
  • dysarthria (changes in speech)
  • facial paresis
  • proximal limb weakness
  • respiratory muscle weakness (may lead to dyspnoea)
  • bulbar palsy symptoms (CN IX-XII, medulla oblongata):
    • facial weakness (myasthenic snarl)
    • disturbed hypernasal speech (dysarthria)
    • difficulty smiling, chewing, swallowing
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11
Q

Describe the order of muscle weakness/fatiguability in myasthenia gravis. (6)

A
  • extraocular
  • bulbar
  • face
  • neck
  • limb girdle
  • trunk
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12
Q

How does muscle weakness differ in Lambert-Eaton Syndrome compared to myasthenia gravis?

A

Muscle weakness improves after repeated use (vs worsens in MG)

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13
Q

What are the features of bulbar palsy (CN IX-XII - medulla oblongata) in myasthenia gravis?

A
  • facial weakness (myasthenic snarl)
  • disturbed hypernasal speech (dysarthria)
  • difficulty smiling, chewing, swallowing
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14
Q

What are the clinical features of thymoma (associated with myasthenia gravis)? (3)

Thymoma: tumour of anterior mediastinum

A
  • distended head and neck veins - SVC obstruction
  • SOB
  • chest pain
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15
Q

What might be seen on examination of myasthenia gravis? (5)

A
  • ocular signs: ptosis, diplopia, complex ophthalmoplegia, check for ocular fatigue
  • ice on eyes test - placing ice packs on closed eyelids for 2min can improve NM transmission and reduce ptosis
  • bulbar signs - dysarthria/nasal speech + dysphagia
  • facial paresis
  • limbs - power before and after repeated use
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16
Q

How do we check for ocular fatigue in myasthenia gravis?

A

Ask patient to sustain an upward gaze for 1 min and watch the progressive ptosis that develops

17
Q

What might you see on examination of Lambert-Eaton Syndrome (myasthenia gravis)? (3)

A
  • gait difficulty before eye signs
  • autonomic involvement (dry eyes, constipation)
  • hyporeflexia and weakness that improves after exercise - decreased tendon reflexes as less NT released, but reflexes may increase after maximum voluntary contractions due to build up of NT in synaptic cleft
18
Q

What are the classes of myasthenia gravis?

A
  • class I (mild) - any eye muscle weakness; possible ptosis; all other muscle strength normal
  • class II (mild) - mild weakness of any other muscles; may have eye muscle weakness of any severity
    • IIa - predominantly limb or axial muscles or both
    • IIb - predominantly oropharyngeal or respiratory muscles or both
  • class III (moderate) - moderate weakness of other muscles; may have eye muscle weakness of any severity
    • IIIa - predominantly limb or axial muscles or both
    • IIIb - predominantly oropharyngeal or respiratory muscles or both
  • class IV (severe) - severe weakness of other muscles; may have eye muscle weakness of any severity
    • IVa - predominantly limb or axial muscles or both
    • IVb - predominantly oropharyngeal or respiratory muscles or both
  • class V (severe) - intubation needed to maintain airway
19
Q

What medication can make symptoms of myasthenia gravis worse?

A

Beta-blockers –> can cause FVC to drop

20
Q

What are the first-line investigations for myasthenia gravis? (3)

A
  • serum AChR antibody analysis (+ve)
  • muscle-specific tyrosine kinase (MuSK) antibodies (+ve)
  • serial pulmonary function tests
21
Q

What is the main diagnostic test for myasthenia gravis?

A

Serum AChR antibody analysis - positive in 90% & most specific

(If seronegative, look for MuSK antibodies)

22
Q

What might serial pulmonary function tests show in myasthenia gravis?

A
  • especially if SOB or suspected myasthenic crisis
  • low FVC, low NIF
23
Q

What is the most sensitive test for myasthenia gravis?

A

EMG - shows decremental response to repetitive nerve stimulation

24
Q

What test can we do to rule out thymoma in myasthenia gravis?

A

CT thorax

Thymoma/lung malignancies found in 20% of MG patients

25
Q

What investigation can help reduce muscle weakness in myasthenia gravis?

A

Tensilon test - IV edrophonium given to reduce muscle weakness temporarily (not used much now due to risk of cardiac arrhythmia)

26
Q

What are some differential diagnoses for myasthenia gravis? (4)

A
  • Lambert-Eaton Syndrome: prominent proximal leg and arm weakness accompanied by diminished muscle stretch reflexes which improve with brief exercise, autonomic symptoms, SCLC association, anti-VGCC
  • botulism - hypotension, bradycardia, diarrhoea–>constipation, urinary retention, blurred vision
  • penicillamine-induced MG: anti-AChR antibody test with different properties to idiopathic MG
  • primary myopathies - progressive muscle weakness that is not fatigable
27
Q

What is the management plan for myasthenia gravis? (3)

A
  • 1st line: pyridostigmine (long-acting acetylcholinesterase inhibitor)
  • if Sx not resolved, add corticosteroid (prednisolone)
  • consider azathioprine (immunosuppressant)
28
Q

What drug may be used in muscle-specific tyrosine kinase (MuSK)-myasthenia gravis?

A

Rituximab

29
Q

What can happen if too much pyridostigmine is taken for myasthenia gravis?

A

Cholinergic crisis

30
Q

How would you manage an acute myasthenic crisis? (3)

A
  • intubation and mechanical ventilation
  • plasma exchange/plasmapheresis
  • IV immunoglobulins (IVIG)

Myasthenic crisis is a complication of myasthenia gravis characterized by worsening of muscle weakness, resulting in respiratory failure that requires intubation and mechanical ventilation.

31
Q

What surgery is considered in myasthenia gravis?

A

Thymectomy - may minimise the need for immunosuppressants

May be effective without thymoma too if AChR antibody positive

32
Q

What medication should be avoided in myasthenia gravis?

A

Beta blockers e.g. bisoprolol

33
Q

What is eculizumab and when is it used in myasthenia gravis?

A
  • inhibits complement activation
  • useful in patients with treatment-resistant generalised MG and AChR antibodies
34
Q

What are some complications of myasthenia gravis? (5)

A
  • myasthenic crisis
  • respiratory failure
  • impaired swallowing
  • acute aspiration
  • secondary pneumonia
35
Q

What drug-induced adverse reactions can happen in myasthenia gravis? (3)

A
  • pyridostigmine-induced adverse reactions - muscarinic adverse effects can be decreased by also giving glycopyrronium, alternatively loperamide when persistent diarrhoea
  • plasma exchange-induced adverse reactions - coagulopathy, thrombocytopenia, electrolyte disturbance, arrhythmias, hypotension
  • IVIG-induced adverse reactions - prophylactic administration of paracetamol/NSAID and diphenhydramine may reduce acute side effects e.g. fever, chills, headache and back pain
36
Q

Describe the prognosis of myasthenia gravis.

A

Most, but not all, patients enjoy a good QoL and normal lifespan due to advances in diagnosis and immunosuppressive treatment

Year 3 (203 decks)

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