Myasthenia Gravis Flashcards
What is myasthenia gravis?
Autoimmune disorder mediated by antibodies against nicotinic acetylcholine receptors on the post synaptic side of the neuromuscular junction as well as loss of junctional folds. Involving both B and T cells.
What other conditions is myasthenia gravis associated with?
Often associated with other autoimmune disease such as RA, SLE, pernicious anaemia and thyroid and 80% of patients have abnormalities of the thymus either thymic hyperplasia or thymomas.
At what ages does myasthenia gravis most commonly occur in the different sexes?
Women 20-30s
Males 60s-80s (most common in men)
What are the clinical features of myasthenia gravis?
Slowly increasing or relapsing muscular fatigue starting with eyelid drooping
Muscle groups affected in a particular order: extraocular, bulbar (chewing and swallowing), face, neck, limb girdle and trunk (respiratory)
Ptosis, diplopia and peak sign (eyelids begin to separate when put under pressure) importantly pupils unaffected
Counting to 50 – voice fades
Myasthenic snarl on smiling
Reflexes are normal
What can exacerbate myasthenia gravis?
Exertion, pregnancy, low potassium, Infection, over-treatment, change of climate, emotion, exercise, and Drugs – gentamicin, tetracycline, quinine, opiates, procainamide, phenytoin, lithium, and beta blockers.
How is suspected myasthenia gravis investigated?
Antibody test – raised anti AChR (seen in 90%).
If Anti AChR -ve then look for MUSK antibodies (muscle specific tyrosine kinase)
Single Fibre Electromyography (EMG) – decremental muscle response to repetitive nerve stimulation
CT to exclude thymoma
Creatinine Kinase levels normal
Tensilon test – inject 10mg of edrophonium, if MG will see an improvement in symptoms, if not there will be no change. Note inject 2mg first to check for adverse reaction and have 2mg atropine ready in case of cholinergic crisis. Not common anymore due to cardiac risk.
What is the Tensilon Test
Tensilon test – inject 10mg of edrophonium, if MG will see an improvement in symptoms, if not there will be no change. Note inject 2mg first to check for adverse reaction and have 2mg atropine ready in case of cholinergic crisis.
How is myasthenia gravis managed?
Symptoms control with Anticholinesterase such as pyridostigmine (oral) or neostigmine (oral or IV) Cholinergic SE – salivation, lacrimation, sweats, vomiting, miosis. Other SE include diarrhoea and colic
How are relapses of myasthenia gravis managed?
Relapses treated with high dose prednisolone to dampen the immune system. Give osteoporosis prophylaxis. Steroid sparing drugs such as azathioprine, cyclosporine and mycophenolate can also be used.
When is a thymectomy useful in myasthenia gravis?
Thymectomy – beneficial even in those without a thymoma. Consider in young patients especially <5yrs and with poor response to medical treatment.
How does a myasthenic crisis present and what is the management?
Myasthenic crisis - slack facial muscles weak neck, drooling, nasal speech, generally unwell and unsafe swallow.
Management of myasthenic crisis – plasmapheresis and intravenous immunoglobulins.
What is a cholinergic crisis?
Cholinergic crisis can also occur, this looks similar to a myasthenic crisis. Excess of cholinesterase inhibitors causes excess stimulation of striate muscle causing a flaccid paralysis. This can progress to respiratory failure and will present with miosis and SSLUDGE (salivation, sweating, lacrimation, urinary incontinence, diarrhoea, GI upset, hypermobility and emesis).
What is Lambert Eaton Syndrome?
Lambert-Eaton myasthenic syndrome is seen in association with small cell lung cancer and to a lesser extent breast and ovarian cancer. It may also occur independently as an autoimmune disorder. Lambert-Eaton myasthenic syndrome is caused by an antibody directed against presynaptic voltage-gated calcium channel in the peripheral nervous system.
How does Lambert Eaton Syndrome present differently to myasthenia gravis?
Repeated muscle contractions lead to increased muscle strength (in contrast to myasthenia gravis). In reality, this is seen in only 50% of patients and following prolonged muscle use muscle strength will eventually decrease.
Limb-girdle weakness (affects lower limbs first)
Hyporeflexia
Autonomic symptoms: dry mouth, impotence, difficulty micturating
Ophthalmoplegia and ptosis not commonly a feature (unlike in myasthenia gravis)
How is the lambert eaton EMG different to MG?
EMG shows an incremental response to repetitive electrical stimulation
