Myasthenia gravis and Lambert Eaton Flashcards
What is myasthenia graves?
MG is an autoimmune disease mediated by antibodies to nicotinic acetylcholine receptors on the post-synaptic side of the NMJ
Both B and T cells are implicated
How does MG present? Signs?
More common in women
Slowly increasing or relapsing muscular FATIGUE
Muscles become progressively weaker during periods of activity and slowly improve after periods of rest
Extraocular muscle wakness: diplopia
Bulbar muscle weakness: dysphagia, chewing weakness
Face, neck, limb gridle - ptosis
Signs: Ptosis Diplopia Myasthenix snarl on smiling Peek sign of orbiculares fatigamility (eyelids begin to separate after manual opposition to sustained closure)
What exacerbates symptoms in MG?
Pregnancy Hypokalaemia Ifnection OVer treatment Change of climate Emotion Exercise Opiates Gentamicin Phyenytroin Penicillamine Beta-Blockers
What are association of MG?
Autoimmune disease: pernicious anaemia, autoimmune thyroid disease, RA, SLE
Thumic atrophy
Thymic tumours
What investigation sin MG?
Antibodies:
Anti AChR antibodies raised
MUSK (muscle specific tyrosine kinase)
Sing fibre electromyography (EMG): decremental muscle response to repetitive nerve stimulation ± single fibre jitter
CT to exclude thymoma
What is seen on EMG in MG?
Decremental muscle response to repetitive nerve stimulation ± single fibre jitter
What is management of MG? Side effects?
Long-acting acetylcholinesterase inhibitors
E.g. pyridostigmine
Cholinergic side effect: salivation, lacrimation, sweating, vomiting, mitosis, diarrhoea
Immunosuppression:
Prednisolone with osteoporosis prophylaxis and stomach protection
Thymectomy
What is a myasthenia crisis?
Life threatening weakness of respiratory muscles during a relapse
Can be difficult to differentiate from a cholinergic crisis (over treatment)
Treat with plasmapheresis (removes AChR antibodies from the circulation)
IV immunoglobulins and identify and treat trigger for relapse (infection, drugs)
What is Lambert Eaton syndrome?
Paraneoplastic syndorme (associated with small cell lung cancer, breast/ovarian) Or autoimmune
Antibodies to pre-synpatic voltage gated calcium channels in the peripheral nervous system
What are features of lambert eaton syndrome?
Gait difficulty before eye signs
Repeated muscle contractions lead to increased muscle strength
Limb girdle weakness (lower limbs affected first)
Hyporeflexia and weakness improves after exercise
Autonomic symptoms: dry mouth, impotence, difficulty micturating, constipation)
Ophthalmoplegia and ptosis do not commonly feature unlike MG
What is seen on EMG in LES?
Incremental response to repetitive electrical stimulation
Amplitude increase greatly post-exercise
What is management of lambert eaton syndrome?
Treatment of underlying cancer
Immunosuppression with prednisolone or azathioprine
3.4-diaminopyridine - works by blocking K channel efflux in the nerve terminal so that action potential duration is increased allowing Ca channels to be open longer allowing more ACh relapse
IV immunoglobulin
Do regular CXR/high resolution CT as symptoms may precede cancer
Where does botulinum toxin act?
Interferes with the fusion of synaptic vesicles with the pre-synaptic membrane, reducing release of Each thus reducing contraction
