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Strabismus & Ocular Motility > Myasthenia Gravis > Flashcards

Myasthenia Gravis Flashcards

1
Q

What does Myasthenia Gravis (MG) translate to in Greek?

A

First described by Thomas Willis in 16883. Means ‘Weak Muscle’

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2
Q

What is MG?

A

An acquired autoimmune disorder affecting neuromuscular transmission (similar to MS) causing excessive fatigue to striated muscles and leading to longer term, permanent denervation and muscle weakness

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3
Q

What are the basic symptoms of MG?

A
  • Breathing Difficulty
  • Difficulty chewing to swallow
  • Difficulty climbing stairs or lifting objects
  • Difficulty in talking
  • Drooping position of the head
  • Change in voice
  • Facial paralysis
  • Fatigue
  • Weakness of eye muscles
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4
Q

What marks the onset of MG in approx. 70% of cases?

A

Diplopia and ptosis. With time they’re seen in 90% of cases with some later progressing to generalised muscle involvement

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5
Q

What other autoimmune disease is MG associated with?

A

Diabetes, Graves’ orbitopathy, pernicious anaemia and rheumatoid arthritis

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6
Q

What does it mean for MG to have ‘diurnal variation’?

A

Least symptomatic in the morning and becoming worse as the day progresses

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7
Q

What does the ptosis tend to look like in MG?

A
  • Usually bilateral and often asymmetrical
  • Degree of ptosis increases during the day and can vary from day-to-day with the ptosis affecting different sides at different times
  • Frontalis o/a seen as the patient attempts to increase the lid elevation (rare to see upper eyelid retraction instead). This leads to the upper eyelid fold being reduced or absent from lifting eyebrows in attempt to reduce the ptosis degree.
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8
Q

What is the incidence rate in MG?

A

4.1-30 cases per million per year

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9
Q

Who does MG more commonly affect?

A

Women but men are increasingly affected at >40yo

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10
Q

How common is remission in MG?

A

Very variable
- Complete remission in 18% of patients
- Can be 40 years between first signs and next episode

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11
Q

What is the aetiology of MG (simply)?

A

Antibodies to nicotinic Acytylcholine receptors (AChRs) reduce post synaptic receptors by:

  • degrading receptor protein
  • directly blocking receptor sites
  • destroying receptor sites
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12
Q

What are less common antibodies found in MG that can cause MG onset?

A
  • Muscle specific kinase
    MuSK protein (not found on ocular mG)
  • Lipoprotein receptor related protein 4 (LRP4)
    LRP4 can be found in both MG and ocular MG

both cause issues with normal synaptic processes

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13
Q

How is ACh normally released?

A

Normally Ach is released from the terminal portion of the motor axon at the synaptic gap and binds to the receptor sites at the motor endplate which causes depolarisation waves to spread along the muscle membrane causing a contraction.

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14
Q

How is ACh released in MG?

A

In MG they form antibodies to these receptor sites at the muscle endplate that prevent Ach from binding and so reduce the effectiveness of the neurotransmitter.

As a result the ACh is continually released to attempt muscle contractor but this results in the stores becoming exhausted which causes the muscle fatigue characteristic of MG.

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15
Q

What do the antibodies at ACh receptors result in in MG?

A

Over time structural changes occur with loss of receptor sites at the postsynaptic membrane meaning that even where Ach is available, they cannot bind to receptors and contract. Receptor site antibodies can be found in 80-90% of MG patients but only in 40-50% of those with ocular myasthenia.

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16
Q

How many patients have nicotinic ACh receptor antibodies circulating in their blood?

A

85% of MG patients

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17
Q

What does the post-synaptic membrane of the neuromuscular junction look like in MG patients?

A

Neuromuscular junction is less folded and synaptic space is wider

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18
Q

What in MG is enlarged?

A

The thymus gland (in approx 75% of cases)

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19
Q

What gland is affected in MG compared to Grave’s?

A

Thymus gland = MG
Thyroid gland = Grave’s

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20
Q

What is the function of the thymus gland?

A
  • Major role in babies and children up until puberty and then it shrinks in size
  • Programmes white blood cells into attacking invading foreign cells such as viruses and thus is important in normal immune response development
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21
Q

Is thymus removal useful?

A

Cures 35% of MG patients
Improves further 33% of MG patients

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22
Q

Is a thymectomy useful in non-thymomatous MG?

A

Yes - Cataneo et al. (2018) found that remission rate is higher in thymectomy cases where there was non-thymomatous compared to non-surgical treatment

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23
Q

When should we do a thymectomy in MG patients? Why?

A

removal is advocated in MG patients between 10 & 40 years within 3 years of disease onset (to reduce risk of permanent degradation changes)

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24
Q

What are the classifications for MG?

A

Paediatric = neonatal, congenital & juvenile

Adult = ocular, mild/moderate, acute fulminating and late severe

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25
Q

What is neonatal MG?

A

A baby born to an MG mother, usually self-limiting with recovery expected in 6 weeks

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26
Q

What is congenital MG?

A

Affects infants of non-myasthenic mothers and doesn’t appear to be mediated by antibodies

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27
Q

What is juvenile MG?

A

From birth to puberty and similar to the adult form of the disease but tends to progress slower with a higher proportion of spontaneous remissions

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28
Q

What is ocular MG?

A

Where the MG is limited to the eye muscles susceptible to myasthenic changes. It tends to remain ocular only if not progressing to other muscle groups within 2 years of its onset

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29
Q

What is mild/moderate generalised MG?

A

Disease onset is gradual: starts as EOMs but spreads to both skeletal and bulbar muscles (Bulbar muscles are the muscles in the face, mouth, and throat controlled by the medulla) but with respiratory muscle sparing

30
Q

What is acute fulminating MG?

A

Rapid onset with early involvement of respiratory muscles (significant mortality rate associated with acute fulminating MG)

31
Q

What is late severe MG?

A

Develops in the ocular or mild generalised group at least 2 years after the first signs of muscle weakness

32
Q

What is Osserman’s Classification (1958)?

A

A way of measuring the severity of MG

Grade I -
Restricted only to EOMs

Grade IIa -
Mild general disease

Grade IIb -
Moderate general disease

Grade III -
Severe general disease

Grade IV -
Myasthenic crisis, life threatening involvement of respiration

33
Q

What is drug-related MG?

A

Follows D-penicillamine treatment for rheumatoid arthritis

Some other antibiotics, cardiovascular agents, anti-depressants, sedatives and anaesthetics can induce or aggravate the condition

34
Q

What muscle groups are affected in MG?

A
  • EOMs
  • Bulbar Muscles
  • Limb/Trunk Muscles
35
Q

How are EOM’s affected in MG?

A

Diplopia and ptosis are the presenting features in 50% of cases and present in 90% of patients

36
Q

Which Bulbar Muscles are affected in MG?

A

Innervated by V, VII, IX, X and XI cranial nerves

37
Q

Which limb/trunk muscles are affected in MG?

A
  • 15-20% start with arm or leg weakness
  • Weakness of intercostal muscles causes breathing problems
38
Q

What are some of the natural history points in MG?

A
  • Variable disease, affecting individuals very differently (sometimes mimics other diseases that are pupil sparing)
  • If onset initially is in EOMs then prognosis is better than if in limbs
  • Disease my peak 2-5 years from onset then improve
  • Periods of remission quite common
  • MG 50x more common in Grave’s disease patients (both being autoimmune disorders)
39
Q

Why should MG be considered a differential diagnosis?

A

A lot of exams MG is a differential diagnosis – if anything neurogenic or mechanical MG can be discussed and explained as a differential diagnosis in these cases because it mimics so many other conditions (only PUPIL SPARING diseases).

40
Q

What is Myasthenic Crisis?

A

Classed as a medical emergency, when the intercostal muscles are affected and MG patient is unable to breathe without a ventilator

41
Q

What % of patients go through a myasthenic crisis?

A

15-20% of generalised MG patients will have at least one myasthenic crisis

42
Q

What is the cause of myasthenic crisis?

A

MG patients
50% no known cause, others triggered by stress, infection, surgery or drug sensitivity

43
Q

What ocular diseases does ocular MG mimic?

A

Variety of pupil sparing diseases (any pupil sparing painless eye movement problems)

  • Unilateral or bilateral CNIII, CNIV and CNVI nerve palsies
  • Isolated muscle palsy (IR)
  • Gaze palsy, INO, one and a half syndrome

Extremely variable , may change muscles from one visit to the next!

44
Q

How many MG cases are ocular MG only?

A

10-16%

45
Q

What type of ocular muscle palsy is usually classic of myasthenia?

A

Can cause any type of ocular muscle palsy but usually leads to:
- Limited elevation the most common
- Pseudo-internuclear ophthalmoplegia
- Isolated IR palsy
- Pseudo-gaze palsy
- Pseudo-isolated CNIII, CNIV or CNVI palsies
- Gaze-paretic nystagmus

46
Q

How can we see MGs during OMs?

A

During OM’s we may see MG through:

  • Variability of measurements, both during examination and between assessments
  • Increase in limitation on sustained gaze
  • If a moderate amount of limitation + normal saccadic velocities = MG instead of ophthalmoplegia
47
Q

Why are EOMs affected particiularly?

A
  • EOM highly innervated with densely concentrated receptors.
  • Small reduction in ACh receptors and ACh levels quickly results in fatigue
  • Increased sensitivity of EOM neuromuscular junctions
  • Specific antibodies directed at specific sites on EOM
48
Q

What are the clinical features of ocular myasthenia?

A
  • Ptosis
    Unilateral or bilateral and may increase on least affected side if examiner holds the most affected lid
  • Fine fluttering lid movements
  • Patient may use brow to lift lid or adopt AHP (upper eyelid fold may be reduced/absent)
49
Q

What lid abnormality is common in MG?

A
  • ‘Afternoon Ectropion’
    Ectropion as the muscle is so weak it cannot stay resting against the eyelid
  • Cogan’s lid twitch
  • Lids easily forced open (weak orbicularis)
  • ‘Peek’ sign (lids separate slightly on voluntary closure)
  • Nystagmoid jerks of upper lid (rare)
50
Q

How can we test for variability in ptosis in MG?

A

Variability in the ptosis, which can be assessed by asking the patient to stop blinking whilst maintaining fixation on a target in pp. The ptosis slowly increases after a few seconds in most myasthenic patients. This can also be seen if you get the patient to sustain upgaze for a minute or two.

51
Q

What % of MG patients show ‘peek’ sign?

A

‘Peek’ Sign:
Only 12% of myasthenic patients showed the ‘peek’ sign.

52
Q

How might we see unilateral upper lid retraction in MG patients?

A

May see unilateral upper lid retraction where the patient is attempting to lift the ptotic lid through frontalis overaction leading to the contralateral eye showing lid retraction.

53
Q

How do we perform Cogan’s Lid Twitch Sign (Cogan, 1965)?

A

Ask the patient to look down for 15 seconds and then asking them to make a fast refixation movement to the pp.

The lid movement follows eye movement on downgaze but then returning to pp the lid momentarily moves above its previous level before returning to the position of the ptosis.

54
Q

How often is Cogan’s Lid Twitch Sign seen in MG? What can we not rule out?

A
  • This is seen in 50% of patients with myasthenic ptosis
  • However, it is also a sign of 3rd nerve palsy associated with aberrant regeneration as well as other eye conditions.
55
Q

What orthoptic investigation can we use to explore a differential diagnosis of MG?

A
  • History
  • VA
  • CT & OM
  • *Eyelid Assessment
  • *Orbicularis Assessment
  • Lees Screen, Field of BSV
  • Measurement of Deviation
  • Binocular Vision Assessment
  • Assess for Fresnel Prism suitability
56
Q

What is the Tensilon Test used for?

A

This test is useful for assessing the function of any group of muscles but eyelid muscles and EOMs provide an opportunity to make a quantifiable assessment of the effects of injection. Used in the diagnosis of MG where a positive response is diagnostic of MG but a negative response cannot exclude diagnosis (due to the number of antibodies that can cause MG)

57
Q

How is the the Tensilon test carried out?

A
  • 10Mg injected intravenously (2Mg initially followed by rest)-
  • Intravenous atropine (0.6Mg) if side effects occur
  • Improvement in eye signs within 1 minute and lasts for around 5 minutes
58
Q

If the Tensilon test is negative but we still suspect MG, what other test can we do?

A

If suspected but other results are negative from other tests then a trial of pyridostigmine, an oral anticholinesterase is recommended.

59
Q

How can we use blood tests to diagnose MG?

A

Blood test for ACh receptor site antibodies

  • Presence suggests MG highly probable
  • Absence cannot exclude disease
60
Q

How is the Anti-MuSK antibody test used?

A

In the diagnosis of MG

Anti-MuSK antibody test – present in about 50% of those who don’t show ACh receptor site antibodies. Absence doesn’t exclude MG

61
Q

What tests can we use to diagnose MG?

A
  • Blood Tests
  • Tensilon Test
  • Sleep Test
  • Ice Pack Test
  • Repetitive Nerve Stimulation
  • Saccadic Velocity Measurement
  • EMG
  • Single Fibre EMG
  • CT scans/ Chest X-rays to test for enlarged thymus/thymoma
62
Q

How do we do the sleep test in MG?

A

Sleep test - improvement after 30 minutes in darkened room. Symptoms reappear 1-5 minutes after waking

63
Q

What is the Ice Pack Test in MG?

A

Ice pack improves synaptic uptake and so ptosis may improve on cooling in MG. Increasing temperature worsens the symptoms of MG and lowering the temperature improves symptoms.

80% of patients with myasthenia has a positive response, concluding that a negative response did not exclude myasthenia as a diagnosis.

64
Q

What is EMG?

A

EMG/Electromyography:Technique for evaluating and recording the electrical activity of skeletal muscles.
MG patients have more “jitter” shown in charts compared to normal subjects

65
Q

What treatment options are there for MG in general?

A
  • Long acting anticholinesterase drugs (pyridostigmine - Mestinon)
  • Immunosuppressive agents (azathioprine, cyclosporine)
  • Systemic steroids
  • Plasmapheresis / Intravenous immunoglobulin – exchange of blood plasma
  • Thymectomy (not for ocular MG) appears effective in the treatment of non-thymomatous MG (Cataneo et al. 2018)
  • Fresnel prisms in conjunction to medical treatment
66
Q

Why should we consider thymus surgery in MG patients?

A

Patients with evidence on CT scan of thymus gland enlargement should be considered for thymectomy where 2/3rds of patients enter a complete remission or respond to lower levels of medical treatment.

Even those with non-thymomatous MG have been shown to benefit from thymus surgery

67
Q

How are anticholinesterases used in MG?

A

E.g. pyridostigmine given orally to relieve symptoms in patients in the early stages of the disease with mild muscle involvement.

68
Q

How do systemic steroids work in treating MG?

A

Probably effective by their action in reducing antibody production

69
Q

How do plasmapheresis and immunoglobulin work in treating MG?

A

Used in patients with life-threatening breathing or swallowing difficulty – this treatment can reduce the level of circulating antibodies. Benefits last for a few weeks so they’re not suitable as long-term treatments.

70
Q

When is thymectomy contraindicated in MG?

A

This is MG that remains confined to the ocular muscles for longer than 2 years as unlikely to progress to generalised MG. As it’s not life-threatening consideration has to be made to the potential side-effects of treatment and things like thymectomy is not considered unless for thymoma.

71
Q

What Ocular Management options are there for MG?

A
  • Plotting course of disease / serial record
  • Prisms or occlusion to make patient comfortable but can be ineffective due to the variability of the deviation
  • Ptosis props, if helpful (need to give clear and good instructions due to cornea drying out so will often also prescribe drops and need to be careful of sclera and cornea)
  • Botulinum toxin A injection (fast acting treatment, not long-lasting so often a primary treatment option in MG)
  • Strabismus surgery if situation really stable (thus contraindicated in the active stage of the disease)

Strabismus & Ocular Motility (23 decks)

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