Myasthenia Gravis Flashcards
How does pyridostigmine work?
- Blocks AChE
- Too much = hypersalivation and bronchospasm
What is myasthenia gravis?
Autoimmune disorder where body makes antibodies against acetylcholine receptors on the post-synaptic motor end plate
Antibodies bind to ACh receptors and degrade them meaning muscular contraction is imapired
Epidemiology of myasthenia gravis
Annual prevalence of 5 in 100,000
Patients present in 20s-30s or 70+
Clinical features of myasthenia gravis
Fatiguability is classical - weakness worse with use
Starts in highly active ocular and bulbar muscles - within 1 year is all over body in 80% patients
‘Feel stronger in the morning, my speech slurs with talking, can’t finish a meal because I can’t swallow, double vision comes and goes’
Features of myasthenia based on muscle region
Ocular: Cant look up because eyelids droop, eye movement restricted
Bulbar: Trouble swallowing and speaking over time
Axial: Trunk and neck weakness
Distal muscles: weak arms and legs after use, can’t brush hair
Respiratory: due to central axial muscle weakness, impaired oxygenation and CO2 expiration
What are the first clinical features of myasthenia gravis?
Slurred speech, drooping eyelid, difficulty chewing
What is an acute myasthenic crisis?
Severe exacerbation of MG
Causes: infection, medication, pregnancy, stress
10-20% patients with MG
Management: IVIg, plasmapheresis, endotracheal intubation
How is myasthenia gravis diagnosed?
Clinical diagnosis: characteristic weakness + no sensory or UMN signs
90% have auto antibodies
Nerve confuction studies: decreased muscle response after repeated stimulation of nerve
CT mediastinum to exlcude lesion in thymus
A tumour in which organ is a known cause of myasthenia gravis?
Thymus
10-15% of cases of MG are due to thymic tumours and removing them relieves severity or is curative
The idea is that the thymoma produces antibodies that block acetylcholine receptors
Investigations in myasthenia gravis
Bloods: Acetylcholine receptor antibodies found in 80-90%
Electromyography
Management of myasthenia gravis
Symptomatic treatment: acetylcholinesterase inhibitors e.g. pyridostigmine
Immunosuppression: prednisolone or IVIg/ plasma exchange if patients don’t respond to other treatment
Steroid saving agent: azathioprine
Surgery: if thymoma
Crisis: IVIg, intubation, plasma exchange
What is a steroid saving agent?
Used to make the body more responsive to steroids meaning lower doses can be used
They lower the body’s immune response making it easier for steroids to work
Prognosis of myasthenia gravis
Prognosis = good but steroids can reduced QoL
80% patients develop generalised symptoms within 1 year
Most have normal life expectancy
Steroids can cause weight gain, osteoporosis and cataracts and can impair QoL
What is Lambert-Eaton myasthenic syndrome?
Autoimmune NMJ disease where antibodies target voltage gated Ca2 channels on presynaptic terminal
Lambert-Eaton vs Myasthenia Gravis
Lambert Eaton - antibodies eat calcium channels, pre-synaptic, arms and legs
Myasthenia Gravis - antibodies attach acetylcholine receptors, post-synaptic, ocular and bulbar
What is amifampridine?
For Lambert Eaton
K+ channel blocker - prolongs depolarisation and allows more time for Ca2 to enter pre-synaptic terminal
