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Neuro > Myasthenia Gravis > Flashcards

Myasthenia Gravis Flashcards

1
Q

What is the definition of Myasthenia Gravis?

A

Myasthenia gravis is a chronic autoimmune disorder of the postsynaptic membrane at the neuromuscular junction of skeletal muscle.

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2
Q

Epidemiology of MG

A
  • Myasthenia gravis is a rare disease with an estimated worldwide prevalence of 200 people per million
  • Symptoms peak in women during their 20s or 30s, and in men in their 50s or 60s
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3
Q

What are the Risk Factors for MG?

A
  • Female gender: twice as common in women
  • Familyhistory
  • Autoimmunity: either a personal or family history of autoimmune disorders such as rheumatoid arthritis and SLE
  • Thymoma or thymic hyperplasia: 10-15% of patients have a thymoma, whilst up to 70% have thymic hyperplasia
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4
Q

Physiology of MG

A
  • Motor nerves communicate with muscles at neuromuscular junctions.
  • At the neuromuscular junction, axons of motor nerves are situated across a synapse from the post-synaptic membrane on the muscle cell.
  • The axons release a neurotransmitter from the pre-synaptic membrane. The neurotransmitter at these junctions is called acetylcholine.
  • This acetylcholine travels across the synapse and attached to receptors on the post-synaptic membrane. They stimulate the receptors, and this signal leads to muscle contraction.
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5
Q

What does MG occur due to?

A
  • circulating autoantibodies against thenicotinic acetylcholine receptor(AChR)
  • this causes fewer available binding sites for acetylcholine (Ach) at the postsynaptic membrane, resulting in weakness.
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6
Q

What happens as the receptors are used more during muscle activity

A

more of them become blocked up. This leads to less effective stimulation of the muscle with increased activity.

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7
Q

What do the antibodies of MG activate

A

activate the complement system within the neuromuscular junction, leading to damage to cells at the postsynaptic membrane. This further worsens the symptoms.

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8
Q

What do a small proportion of MG patients have?

A

have antibodies against muscle-specific kinase(MuSK) and antibodies against low-density lipoprotein receptor-related protein 4 (LRP4).

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9
Q

What is MuSK and LRP4 important proteins for?

A

the creation and organisation of the acetylcholine receptor. Destruction of these proteins by autoantibodies leads to inadequate acetylcholine receptors.

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10
Q

What can MG present as in very rare cases?

A
  • paraneoplastic syndrome - an underlying cancer like bronchogenic carcinoma or thymoma generates an immune response which results in generation of autoantibodies.
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11
Q

Signs of MG

A
  • Proximal muscle weakness with fatigability: often affecting the face and neck
  • Ptosis (drooping eyelid) exacerbated on upward gaze: may be bilateral or unilateral
  • Complex ophthalmoplegia: cannot be isolated to one cranial nerve
  • Head drop: a rare sign due to weakness of cervical extensor muscles
  • Myasthenic snarl: a ‘snarling’ expression when attempting to smile
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12
Q

Symptoms of MG

A
  • Lethargy
  • Muscle weakness: worsens throughout the day, can improve with rest
  • Diplopia: double vision due to effects on eye muscles
  • Slurred speech
  • Dysphagia: difficulty swallowing
  • Fatigue in jaw when chewing
  • Shortness of breath: may suggest a myasthenic crisis
  • Ptosis
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13
Q

Primary investigations for MG

A
  • Antibodies:AchR antibodies are first-line, whilst anti-MuSK and anti-LRP4 can also be tested in patients who are AchR antibodynegative
  • Electrophysiological studies:repetitive nerve stimulation shows a decremental muscle response
  • CT thorax:all patients should have a CT chest to exclude a thymoma; MRI is an alternative
  • Thyroid function:all patients require thyroid function tests, as there is a higher prevalence of autoimmune thyroiditis
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14
Q

Other investigations to consider for MG

A
  • Tensilon test:edrophonium is administered and improves muscle weakness; this is now rarely performed due to the risk of inducing heart block and asystole
  • Serial pulmonary function tests:carried out if respiratory compromise is suspected, for example in patients with shortness of breath and suspected myasthenic crisis
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15
Q

DDs for MG

A
  • Lambert-eaton myasthenic syndrome
  • Botulism
  • Penicillamine-induced myasthenia gravis
  • Primary myopathies
  • Management
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16
Q

What is the management of a stable patient with MG

A
  • First-line: pyridostigmine; an acetylcholinesterase inhibitor
  • Second-line: prednisolone(to dampen down immune system) on alternate days is offered if - given first acutely THIS IS FIRST LINE symptomatic despite pyridostigmine
  • Third-line: azathioprineis offered if symptoms are not controlled on prednisolone
  • Other: if symptoms remain uncontrolled, consider alternatives such as methotrexate orrituximab
  • Thymectomy:indicated if a thymoma is present on imaging,orin patients aged < 45 years old with positive serology; thymectomy may help to induce remissio
17
Q

Management of myasthenic crisis

A
  • IV immunoglobulin or plasmapheresis
  • Intubation:**if severe respiratory compromise
  • Corticosteroidsmay be used as an adjunct
18
Q

What are the complications of myasthenia gravis

A
  • Myasthenic crisis: weakening of the respiratory muscles and is often provoked by infections or medications. Patients present with increasing shortness of breath, which can deteriorate into respiratory failure.
  • Respiratory failure
  • Aspiration pneumonia: dysphagia increases the risk of aspiration
19
Q

Prognosis of MG

A

The goal of therapy is to induce clinical remission. Myasthenia gravis is associated with a reasonably good prognosis and patients who undergo thymectomy tend to have a better prognosis.

However, it is estimated that as many as 10 to 20% of patients may experience at least one myasthenic crisis, and the annual risk of a myasthenic crisis is approximately 2 to 3%.

20
Q

Medications to avoid in MG

A
  • Antibiotics - gentiamiacin, azithromyocin, ciprofloxacin, doxycycline
  • Neuromuscular blocking agent
  • Penicillamine
  • Cardiac drugs - BBs
  • Neurological - Lithium, Phenytoin
21
Q

What percentage of people are anti Ach-r

A

85%

22
Q

What percent of people are Anti Musk

A

15%

Neuro (19 decks)

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