Myasthenia Gravis Flashcards
What is Myasthenia Gravis
An acquired autoimmune disorder causing weakness and fatiguability
What is the pathophysiology of Myasthenia Gravis
Antibodies against ACh receptors at the post-synaptic junction
Which muscles are most commonly affected in Myasthenia Gravis
Proximal limb, ocular and bulbar muscles
What other conditions are associated with Myasthenia Gravis
Thymic hyperplasia - 75%
Thymoma - 25%
Also associated with hyperthyroidism, RA, SLE, pernicious anaemia
What are some of the signs of Myasthenia Gravis
Fatigability of muscles Diplopia Ptosis Myasthenic snarl - uneven smile Difficulty swallowing Slurred speech Normal reflexes
*If bilateral ptosis - very likely to be MG
What exacerbates the weakness found in Myasthenia Gravis
Pregnancy Low potassium Infection Emotion Exercise Drugs- "anti" drugs: Antibiotics e.g. gentamicin, anti-pain e.g. opiates, anti-seizure e.g. phenytoin, anti-psychotics e.g. lithium
What is Myasthenia crisis
Paralysis of respiratory muscles - patients require ventilation
What may trigger a Myasthenic crisis
Infection
Fever
Adverse drug reactions
Emotional stress
What blood tests would you do to investigate Myasthenia Gravis
anti-ACh receptor antibodies
Antibodies to muscle specific receptor tyrosine kinase
What imaging would you do to investigate Myasthenia Gravis
CXR - widened mediastinum if thymoma
CT thorax - look for thymoma
What other tests would you do to investigate Myasthenia Gravis
Vital capacity (assesses severity)
Single fibre EMG
Repetitive nerve stimulation
Tensilon test - ONLY if resuscitation facilities and atropine to hand
“ice pack test”- v sensitive and specific
What drug is used in the Tensilon test
10mg edrophonium - v. short acting anticholinesterase
How would you interpret the Tensilon test
If edrophonium improves power in 1 min, test is positive and indicates MG
What medications would you give a patient with MG
Anticholinesterase - eg. pyridostigmine (symptomatic but do not alter progression of disease)
Immunosuppression - prednisolone
What medications would you give to treat a patient in Myasthenic Crisis
Plasma Exchange
IVIG
What surgical options are there for patients with MG
Thymectomy - remission in 25%, benefits in 50%
What are some other causes of widened mediastinum
5 Ts
Thoracic dissection Thymoma Thyroid - retrosternal TB Terrible lymph nodes
Is Myasthenia Gravis a LMN or UMN disorder
LMN - as it affects the neuromuscular junction
What is Lambert Eaton Myasthenic Syndrome
An autoimmune disorder characterized by muscle weakness of the limb due to antibodies to presynaptic voltage gated calcium channels
What disease is Lambert Eaton associated with
Small cell lung cancer
How does Lambert Eaton differ from MG
Weakness reduces with repeated muscle contraction (EMG- “seoncd wind” phenomenon)
Eyes are rarely affected - ie. no ptosis
Hyporeflexia
Lower limbs more than upper
What symptoms might patients with MG present with?
Intermittent diplopia
What are characteristic findings on CN examination of MG?
Fatiguable weakness that improves with rest
Compex extra-ocular palsies
Ptosis, bulbar palsy
Myasthenic snarl on attempting to smile
