Myasthenia Gravis Flashcards
What is Myasthenia Gravis (MG)?
chronic autoimmune disorder of post-synaptic membrane at NMJ in skeletal muscle
How common is MG?
uncommon
What type of immune cells are involved in MG?
both T and B cells
What type of reaction is happening in MG?
Type II hypersensitivity
What is found in 80-90% of MG?
detectable antibodies against the nicotinic acetylcholine receptor (AChR)
What are RF for MG?
- FHx of autoimmune disorders
- Genetic markers
- Cancer-targeted therapy
When is MG more common in women?
young 20-30
When is MG more common in men?
60-70
What is MG associated with?
- thymic hyperplasia
- thymic atrophy
- thymic tumour
What are common symptoms in MG?
- Muscle fatigability
- Ptosis
- Diplopia
- Dysphagia
- Dysarthria
- Facial paresis
- Proximal limb weakness
- Reflexes normal
What are the muscle groups affected in order in MG?
- Extraocular
- Bulbar
- Face
- Neck
- Limb girdle
- Trunk
When are symptoms of MG exacerbated by?
- Pregnancy
- Low K+
- Infection
- Over treatment
- Change of climate
- Etc
What are possible DDx of MG?
- Lambert-Eaton myasthenic syndrome (LEMS)
- Botulism
- Penicillamine-induced myasthenia gravis
- Primary myopathies
What are possible investigations for MG?
- Serum acetylcholine receptor (AChR) antibody analysis: positive
- Muscle-specific tyrosine kinase (MuSK) antibodies: maybe positive
- Serial Pulmonary function tests
- EMG
- CT
What would serial pulmonary function tests show in MG?
myasthenic crisis: low FVC and low NIF
What would EMG show in MG?
decremental muscle response to repetitive nerve stimulation
Why would you do a CT in MG?
to exclude thymoma
What is the treatment for acute severe for MG?
- 1st line: Intubation + mechanical ventilation + plasma exchange of IV Ig and corticosteroid
- 2nd line: Eculizumab or rituximab
What is the 1st line treatment for MG mild disease?
anticholinesterase inhibitors: pyridostigmine (max 1.2g/d)
How do anti cholinesterase inhibitors work?
- which stops breakdown of acetylcholine
- so increases concentration of acetylcholine
- so helps to counteract effect of acetylcholine receptor antibodies
What other management is used in mild MG?
- corticosteroid e.g. prednisone to recue production of autoantibodies
- thymectomy: reduce muscle weakness
What are possible cholinergic SE?
- Increase salivation
- Lacrimation
- Sweat
- Vomiting
- Miosis
- Diarrhoea
What is the management for moderate MG?
1st line: pyridostigmine
-thymectomy
-Plasma exchange or IV Ig
2nd line eculizumab or rituximab
What is the management of MG severe post crisis?
1st line: pyridostigmine plus immunosuppressant
- intermittent IV Ig
- thymectomy
- plasma exchange
What are possible complications of MG?
- Pyridostigmine-induced adverse reaction
- Respiratory failure
- Impaired swallowing
- Acute aspiration
- Secondary pneumonia
- Plasma exchange-induced adverse reaction
- IVIG-induced adverse reactions
What is the prognosis of MG?
normal life span
How common is a myasthenic crisis? What is this crisis?
15%-20% of patients with MG experience a myasthenic crisis: hard to differentiate from cholinergic crisis – decreased function of breathing muscles
What is the patho of MG?
autoantibodies against AChR at NMJ (some patients may have anti-MuSK antibodies)
What are thymus associations of MG?
- Thymic hyperplasia (70%)
- Thymoma (10%)
Why is the thymus involved in MG?
- adults with myasthenia gravis, the thymus has clusters of immune cells similar to lymphoid hyperplasia
- usually only happens in the spleen and lymph nodes during an active immune response
What can individuals with MG develop?
thymomas (tumours of the thymus gland) – usually harmless but can turn malignant
How does the muscles fatigue change in MG?
with use
What is the patho of Lamber eaton Myasthenic syndrome?
- autoantibodies against presynaptic VGCC at NMJ
2. Repeated Use leads to build up of synaptic Ca2+ so increased contraction
How does the muscle weakness change in LEMS?
muscles weakness improves with use
What are the symptoms of LEMS?
- Difficulty walking
- Weakness in upper arms and shoulder
- Similar symptoms to MG
- Autonomic: dry mouth, constipation, incontenece
What are signs of LEMS?
- Symptom improvement following use
2. Hyporeflexia
What are the associations of LEMS?
- Small cell lung cancer: paraneoplastic
2. Autoimmune disease
What test can be used for MG
Simpson test (shut eyes then ptosis as all Ach used up)
