Myasthenia Gravis Flashcards by Sanjana Shaunak

Myasthenia Gravis is an __1___ disease which affects the ___2____. It is characterised by ___3____

Myasthenia Gravis is an ____autoimmune___ disease which affects the ___Neuromuscular junction ____. It is characterised by ___fatiguability____

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Which skeletal muscle groups are affected in Myasthenia Gravis?

Extraocular muscles : common
Bulbar involvement: dysphagia, dysphonia, dysarthria

-Limb weakness – proximal symmetric

-Respiratory muscle involvement

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Who gets Myasthenia Gravis? What kind of distribution is it?

Bimodal distribution.

20-30 yr females

60-80 yr males

More common in women (2:1) than men

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What is the pathophysiology of Myasthenia Gravis?

Antibody mediated mechanism blocking Ach NMJ

IgG blocks Ach binding site on the synaptic membrane.

Ach rarely binds to the Ach receptor and Ach esterase begins to break it down

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Myasthenia Gravis is linked to abnormalities in which lymphoid organ?

The thymus

abnormalities in 80% (LEC BB)

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Which abnormalities of the thymus are linked to Myasthenia Gravis?

Thymic hyperplasia + Thymic tumours (thymoma)

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What are some other asociated autoimmiune disorders with Myasthenia Gravis?

pernicious anaemia, autoimmune thyroid disorders, rheumatoid, SLE

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Explain the link between thymic hyperplasia and Myasthenia Gravis
(LEC BB)

65% have early onset MG

-less common in ocular presentation

CT cannot identify hyperplasia

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Explain the link between Thymic tumours and Myasthenia Gravis
(LEC BB)

30% get late onset MG

-Less common in ocular presentations

-Worse prognosis

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What are the clinical manifestations of Myasthenia Gravis?

Fatiguability of skeletal muscle **

Ptosis

Extra-ocular muscle - complex opthalmoplegia

facial weakness

Muscles of limb and trunk can be affected

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Myasthenia Gravis has a variable course, what can trigger episodes?

Stress
pregnancy / period
secondary illness
thyroid dysfunction
Trauma
temperature extremes
hypokalaemia
drugs ***
surgery

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Name some commonly used drugs that affect the neuromuscular junction ?

AB - gentamicin, macrolides, quinolones (ciprofloxacin), tetracyclines

BP - B’blockers, CCBs, ACEi

Anaesthetics: Succinylocholine

Magnesium e.g. enema

Epilepsy: Phenytoin

Psych: Lithium

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How does a myasthenic crisis present?

Slack facial muscles
drooling
Nasal speech
unsafe swallow
weak neck / jaw/ tongue / generally weak

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How many pts get myasthenic crisis? what treatment will they need?

15% to 20% of patients with MG experience a myasthenic crisis (an exacerbation necessitating mechanical ventilation)

acute treatment:
* Mechanical ventialtion
* IV intravenous immunoglobulin or plasma exchange.
* steroids
* supportive care: DVT prevention, sore prevetions, nutrition and fluid support

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How does a cholinergic crisis present in Myasthenia Gravis?

overstimulation of nicotinic and muscarinic receptors at the neuromuscular junctions. This is usually secondary to the inactivation or inhibition of acetylcholinesterase (AChE). leads to excessive muscle simulation causing&raquo_space; flaccid paralysis

Resp failure (risk of sudden apnoea)
Miosis
SSLUDGE syndrome (salivation, sweating, lacrimation, urinary incontinence, diarrhoea, GI upset, hyper-motility, emesis)

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What do you NOT find in Clinical manifestations of Myasthenia Gravis? (Differenciating from other conditions)

comment on:
Pupils
sensory
reflexes
autonomic

Pupils unaffected
No sensory loss
No reflex loss
No autonomic involvement

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What investigations could you do for Myasthenia Gravis?

bedside
lab
special / imaging

Bedside:
* serial pulmonary function tests (SOB/ suspected Myasthenic crisis) - do FVC / NIF (negative inspiratory force) - both low in Mcrisis
* Ice test (ptosis gets better) (not as good as others below)

Lab:
* Serum Acetylcholine Receptir (AChR) antibodies - (+VE)
* muscle-specific tyrosine kinase (MuSK) antibodies (may be +ve if above not / unclear)

Imaging / special tests:
* repetitive nerve stimulation (>10% decline in compound muscle action potential (CMAP)

Single fibre electromyography EMG ( (increased variability (jitter) or / failure of neuromuscular transmission (block) in some muscle fibres)
CT thorax (thyomas or thymic enlargement)

BMJ BP

What findings in electromyography for MG?

2 classic findings
*decrement in amplitude (progressive ‘fatigue’) of compound muscle action potentioal following repetitie muscle stimuation
* increased jitter using a single electrode

Why is thymus imaging (CT/MRI) important in MG?

look for hyperplasia or tumour
removal of hyperplastic thymus improves the condition in some pts
removal of thyomas can stop malignant transformation

What spirometery needs to be done with a pt with MG and why?

vital capacity (standing and lying) needs to be checked
WHY
vital capacity falling below 1.5 L requires transfer to high dependancy / intensive unit BEFOFRE ventilatory failure occurs

What antibodies can you test for in Myasthenia Gravis?

85-90% - AChR antibodies

40% have anti -muscle specific tyrosine kinase antibodies

often no AB seronegative in disease limited to ocular muscles

What is the Tensilon Test?

To evaluate for Myasthenia Gravis

IV Edrophonium injected which temporarily reduces muscle weakness

Contraindications for Tension test in Myasthenia Gravis?

Asthma
MI
bradycardia
risk of cardiac arrhythmia

What are some important complications of myasthenia Gravis?

Respiratory :

Resp failure
Aspiration
Respiratory infection

What is early / beside investigation for Myasthenia Gravis?

Bedside spirometry - looking at FVC

What FVC readings need intervention in Myasthenia Gravis?

If falling or FVC <20ml/kg - senior review incase need ventilation NOTE: Indications for mechanical ventilation are forced vital capacity (FVC) 15 mL/kg or less (normal ≥60 mL/kg) and/or negative inspiratory force (NIF) 20 cm H₂O or less (normal ≥70 cm H₂O)

What blood gas results may you get in Myasthenia Gravis, what does this indicate?

HIGH CO2 LOW 02 acidosis Type 2 respiratory failure

How is Myasthenia Gravis managed / treated ?

- Acetylcholinesterase inhibitors: Pyridostigmine (1st) Immunosuppression: - Corticosteroids: Prednisolone (start low) - Steroid sparing : Azathioprine, cyclosporine - Thyectomy - regular neurology OP appts - mdt managament - OT, PT, SALT - plasmapheresis of immunoglobulins

What is time of onset and duration of action for pyridostigmine?

Time onset of action - 60 mins duration of action - 3-5 hours patients response determines the dose

what can overdoseage of pyridostigmine cause?

(often at start of usage) can cause a cholinergic crisis with severe weakess (can be hard to differenicate from myasthenic weakness)

What are treatment options for managing an acute Myasthenic crisis?

Plasmapheresis - removes AChR AB - short term improvement Intravenous immunoglobulins -60% respond within 7-10 days

What safety netting information does the patient need to be told about myasthenia gravis?

Risk of certain drugs worsening condition e.g. antibiotics, cardiac, neuro, psychiatry Risk of condition worsening after general anaesthesia Advisable to carry a med-alert bracelet as conditions / inform health care workers e.g. pneumonia can make worse + AB e.g. doxycycline

A pt with suspected Myasthenia gravis has an Xray. What abnormal finding might it show?

Smooth mass overlying aortic knuckle in anterior mediastinum = Thymic mass or Thyoma ( thymic disease associated with MG)

What are the causes of an anterior mediastinal mass (four Ts)

Thymic mass/ thyoma Thyroid mass Teratoma Lymp nodes ('Terrible'lymphoma or carcinoma)

What are some precipitants for a Myasthenic crisis?

Lots e.g. emotion - stress exercise Pain Sleep deprivation Temp extremes hypokalaemia drugs e.g. b blockers, some AB

What is the risk of Acetylcholinesterase inhibitor “over treatment” in a pt with Myasthenia Gravis?

Can cause a cholingergic crisis

What spirometery needs to be done with a pt with MG and why?

* vital capacity (standing and lying) needs to be checked WHY * vital capacity falling below 1.5 L requires transfer to high dependancy / intensive unit BEFOFRE ventilatory failure occurs

Explain how myasthenic crisis leads to resp failure? what wil pt need?

Myasthenic crisis is characterised by worsening of muscle weakness, resulting in respiratory failure requiring mechanical ventilation.

Respiratory failure in MG - what is measured in order to decide if mechanical ventilation is needed?

* Serial measurements of forced vital capacity (FVC) are taken. * Indication for mechanical ventilation includes FVC 15 mL/kg or less (normal ≥60 mL/kg) * DO not wait for ABG as it occurs late in the course after clinical decompensation.

List complicatoins of MG

* Muscarinic adverse effects from medications - SLUDGE (loperamide for persisent diarrhoea/glycopyronium) * Respiratory failure (myasthenic crisis) * impaired swallow * acute aspiration * secondary pnyemonia * cardiac e.g. myocarditis * thyroid disorders

What drugs can exacerbate Myasthenia Gravis:

* Beta blockers * Lithium * Penicillamine * Gentamicin * Quinolones * Phenytoin