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Myasthenia Gravis - Coggle Diagram
Myasthenia Gravis
Clinical Presentation
Look for ptosis (drooping of upper eyelid), diplopia (double vision) and myasthenic snarl on smiling
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Limb muscles (proximal), speech and facial expression are commonly affected
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Muscle groups affected in order - extra-ocular, bulbar, face, neck, trunk
Weakness is worsened by pregnancy, hypokalaemia, infection, emotion, exercise and drugs (opiates, beta blockers, gentamicin and tetracycline)
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Diagnosis
CT of thymus to look for hyperplasia, atrophy or tumour
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Tension test - IV edrophonium (short-acting anti-cholinesterase) given and muscle power increases within seconds - rarely used to diagnose due to side effects
Serum anti-AChR:
- Raised in 90%
- If negative then look for MuSK (muscle specific tyrosine kinase) antibodies
Pathophysiology
This is achieved by immune complex deposition of anti-AChR IgG and complement at the post-synaptic membranes, causing interference with and destruction of receptors
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Autoimmune disease mediated by antibodies to nicotinic acetylcholine receptor - anti-AChR antibodies, interfering with the neuromuscular junction via depletion of working post-synaptic receptor sites
This blocks the excitatory affect of ACh on the nicotinic receptors (as there is less) resulting in muscle weakness
Treatment
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Thymectomy - removal of thymus if onset < 50 yrs and disease poorly controlled with anti-cholinesterase's
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Aetiology/Risk Factors
If over 50 yrs, then MG is commoner in men and is associated with THYMIC ATROPHY or THYMIC TUMOUR, rheumatoid arthritis and SLE
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If under 50 yrs, then MG is commoner in women and is associated with other autoimmune disease e.g. pernicious anaemia, SLE and rheumatoid arthritis and thymic hyperplasia
Differential Diagnosis
MS, hyperthyroidism, acute Guillain-Barre syndrome
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