NMJ Flashcards
What is the pathophysiology of Myasthenia Gravis:
Most common neuromuscular junction disorder
It is autoimmune antibody mediated decrease in the number of available achetylcholine receptors at the postsynaptic neuromuscular junction.
What are the clinical features of Myasthenia Gravis:
Weakness and Fatigability of skeletal muscles – often ptosis and proximal muscle weakness, can have bulbar weakness, diplopia, dysphagia, limb weakness.
>50% of people present with ptosis or diplopia
Normal reflexes, tone, sensation
Rapid decrease in amplitude of evoked responses on repetitive nerve stimulation
There is associated hyperplastic thymus in 65% of patients and 10% have thymic tumours
How is Myasthenia Gravis diagnosed? What % are AB +ve
Tensilon test can be used in obvious ptosis to diagnose
Can order immunological tests (Abs to acetylcholine receptors or MuSK (muscle specific tyronise kinase)
80% are Ab +ve for AchR. Only 1% are negative for both AChR and MuSK
NCS show decreased compound muscle action potential amplitude. EMG may show jitter and conduction block
How is Myasthenia Gravis treated?
Treated with acetylcholinesterase inhibitors – pyridostigmine as well as immunosuppression (glucocorticoids / cyclophosphamide).
Thymoma removed can improve sx even without a tumour present.
What drugs can unmask MG?
Aminoglycosides, B blockers, CCB.
What is the pathophysiology of Lambert Eaton Myasthenic Syndrome?
Lambert Eaton Syndrome is a NMJ disease.
Antibodies against P/Q type calcium channels. These interfere with the normal calcium flux needed to release acetylcholine pre-synaptically.
It can be sporadic or part of a paraneoplastic syndrome most often associated with small cell lung cancer
What are the clinical features of Lambert Eaton Myasthenic Syndrome
Proximal weakness greater in the legs than the arms. Can have ocular / facial involvement but not as common.
Autonomic dysfunction occurs in 75% of patients – dry mouth, blurred vision, constipation and orthostatic hypotension
Clinically reflexes are depressed or absent and there is an incremental response on repetitive nerve stimulation
There can be improvement with exercise for a short time as ach levels can return to normal (true of both weakness and reflexes)
How is Lambert Eaton Myasthenia diagnosed?
NCS show reproducible post exercise increase in compound muscle action potential amplitude. Abs against VGCC (voltage gated calcium channels)
How is Lambert Eaton Myasthenia treated?
Treated with pyridostigmine, IVIg, Plasmaphoresis and immunosuppression, but less responsive to pyridostigmine than MG.
What is a myasthenia crisis ?
ventilator support.
1/5 to 1/3 of all patients with MG
Caused by severe weakness of respiratory muscles or upper airway muscles
Typically precipitated by poorly controlled disease, muscle relaxants, certain ABx, benzo, beta blocker
How do you treat Myasthenia Crisis?
Both IVIg and Plasmapheresis are used comparatively
Prednisolone should be tapered slowly as can cause / worsen crisis.
Pred given at 1mg / kg for several months after recovery until alternative immunosuppression working
IV pyridostigmine is controversial and can cause coronary vasospasm
