Neurology/Neurosurgery - Myasthenia Gravis (Core Clinical Problems - Generalised weakness/squint) Flashcards
What is the pathology behind myasthenia gravis?
Autoimmune disease
- Antibodies to acetylcholine receptor at post-synaptic membrane of the NMJ
Why should you get a CXR in a patient with myasthenia gravis?
in 15% of patients there will be a thymus pathology - either hypertrophy or neoplasm; seen as mediastinal widening
How would you see a thymoma or thymus hypertrophy on CXR?
Mediastinal widening
Anterior mediastinal
Still visible lung markings as not lung pathology
What is the incidence of MG?
0.4/100,000
What is the prevalence?
10/100,000
Looking at the incidence and prevalence is long term survival good or bad?
Good; 25x the number of people living with it than diagnosed with it yearly
What age groups does MG typically affect?
all age groups (trick question sorry)
What are the classical muscular symptoms of MG?
1) Fatigue of muscles that gets worse throughout the day
2) Ptosis - worsening throughout the day
3) Diplopia with limited eye movements
4) may include facial weakness (myasthenia snarl)
5) weakness of eye closure
What bulbar symptoms may occur in MG?
1) Dysphasia with nasal regurgitation of liquids
2) dysarthria with nasal qualities
Acute involvement of bulbar or respiratory symptoms is worrying; why?
Neurological emergency; myasthenia crisis, respiratory failure usually involving mechanical ventilation
Which muscles are most commonly affected in MG? What is the pattern?
Proximal pectoral and pelvic girdle, neck and face, >distal musculature.
Usually worse at the end of the day and better at start
Diagnostic factors include:
1) Muscle fatiguability
2) Ptosis
3) Diplopia
4) Dysphagia
5) Dysarthria (with nasal speech)
6) Facial paresis
7) proximal limb weakness
8) SOB - less common
What investigations do you want to do?
1) ACh receptor autoantibodies - 15% false negative rate
2) Tensilon test - transient, rapid improvement when edrophonium is given
3) EMG - including single fibre studies
4) TFTs - to assess for associated thyrotoxicosis
5) Striated muscle antibody - positive un most patients with thymoma
6) CT of anterior mediastinum for thymus enlargement
What is the first line treatment for MG?
Anti-cholinesterases such as pyridostigmine/rivastigmine
How do anti-cholinesterases help?
Symptom control; increases ACh in the cleft and therefore allows more transmission
What are some of the common side effects of anti-cholinesterase?
if requiring increases doses may cause:
- diarrhoea
- abdo pain
- vomiting
- increased salivation
- bradycardia
- extrapyramidal signs
- insomnia
In extreme cases what is the worst thing an anti-cholinesterase might do?
Might make the weakness worse - cholinergic crisis
Prednisolone may be used for MG - when?
When the disease is moderately severe and poorly/unresponsive to treatments
When should a patient be admitted before starting steroids?
When the symptoms are not purely ocular
Why should a patient be admitted before starting steroids?
Can make the disease worse initially - start low go slow.
When should an immunosuppressant be used for MG?
Azathioprine may be used for moderately to severe disease in conjunction with steroids
Thymectomy is useful in all patients but especially which category and why?
Younger patients, early in course, may reduce requirement for subsequent therapy and in rare cases cause complete remission
Plasma exchange or IVIG should be given to which patients?
- in preparation for thymectomy
- severe disease
List some complications of plasma exchange
Coagulopathy Thrombocytopenia Electrolyte disturbances arrythmia hypotension Bleeding Pneumothorax venous thrombosis line infection Sepsis (Monitor coagulation, FBC and U+E daily)
List complications of IVIG
Headache Aseptic meningitis Migraine headache Back Pain Fever Chills Hives Anaphylactoid reaction (rarely) Anaemia/reduced WBC/thrombocytopenia Renal Failure Stroke MI
Renal failure, stroke and MI are more likely to occur in IVIG with what type of patients?
Older patients with pre-existing risk factors for renal and cardiovascular disease
How might we reduce risk of headache, back pain, renal failure, stroke or MI with IVIG?
Slower infusion
Prophylactic paracetamol or NSAIDS may prevent which side effects of IVIG?
Fever
Chills
Headache
Back pain
Prophylactic dexamethasone IV may be useful in patients on IVIG, especially those suffering worsening migraine - true/false
True
Which class of antibiotics should be avoided in myasthenia gravis? Why?
Aminoglycosides
Blocks ACh receptor at NMJ and thus can worsen symptoms
Define myasthenia crisis
- Respiratory failure
- Caused by MG exacerbation
- often requiring hospitalisation and mechanical ventilations
What percentage of patients will experience a myasthenic crisis? What is the usual timeframe?
15-20% will experience one
Most commonly within the first 2 years. May recur
What might provoke myasthenic crisis?
infection aspiration medicines (especially high dose steroid) malignancy Surgery Failure of medication concordance trauma
what do you do in a myasthenic crisis?
- serial FVC; <15ml/Kg and inspiratory pressure <20cmH2O, consider mechanical ventilation
- do not await ABG result; unlikely abnormal except very late in decompensation
What complications are likely due to dysphagia?
- Aspiration pneumonia
- Myasthenic crisis
- rarely requires PEG tube for feeding
What is a mimic of MG?
Lambert eaton myasthenic syndrome (LEMS)
What is the difference between MG and LEMS?
1) LEMS is paraneoplastic
2) does not affect ACh receptors post synaptically; affects pre-synaptic voltage gated calcium channels
3) does no exhibit repeated fatiguability
What is the commonest cause of LEMS?
Small cell lung cancer
