Myasthenia Gravis

Question 1

What is Myasthenia Gravis

Answer 1

An acquired autoimmune disorder causing weakness and fatiguability

Question 2

What is the pathophysiology of Myasthenia Gravis

Answer 2

Antibodies against ACh receptors at the post-synaptic junction

Question 3

Which muscles are most commonly affected in Myasthenia Gravis

Answer 3

Proximal limb, ocular and bulbar muscles

Question 4

What other conditions are associated with Myasthenia Gravis

Answer 4

Thymic hyperplasia - 75%
Thymoma - 25%

Also associated with hyperthyroidism, RA, SLE, pernicious anaemia

Question 5

What are some of the signs of Myasthenia Gravis

Answer 5

Fatigability of muscles
Diplopia
Ptosis
Myasthenic snarl - uneven smile
Difficulty swallowing
Slurred speech
Normal reflexes

*If bilateral ptosis - very likely to be MG

Question 6

What exacerbates the weakness found in Myasthenia Gravis

Answer 6

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

Question 7

What is Myasthenia crisis

Answer 7

Paralysis of respiratory muscles - patients require ventilation

Question 8

What may trigger a Myasthenic crisis

Answer 8

Infection
Fever
Adverse drug reactions
Emotional stress

Question 9

What blood tests would you do to investigate Myasthenia Gravis

Answer 9

anti-ACh receptor antibodies

Antibodies to muscle specific receptor tyrosine kinase

Question 10

What imaging would you do to investigate Myasthenia Gravis

Answer 10

CXR - widened mediastinum if thymoma

CT thorax - look for thymoma

Question 11

What other tests would you do to investigate Myasthenia Gravis

Answer 11

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

Question 12

What drug is used in the Tensilon test

Answer 12

10mg edrophonium - v. short acting anticholinesterase

Question 13

How would you interpret the Tensilon test

Answer 13

If edrophonium improves power in 1 min, test is positive and indicates MG

Question 14

What medications would you give a patient with MG

Answer 14

Anticholinesterase - eg. pyridostigmine (symptomatic but do not alter progression of disease)
Immunosuppression - prednisolone

Question 15

What medications would you give to treat a patient in Myasthenic Crisis

Answer 15

Plasma Exchange

IVIG

Question 16

What surgical options are there for patients with MG

Answer 16

Thymectomy - remission in 25%, benefits in 50%

Question 17

What are some other causes of widened mediastinum

Answer 17

5 Ts

Thoracic dissection
Thymoma
Thyroid - retrosternal
TB
Terrible lymph nodes

Question 18

Is Myasthenia Gravis a LMN or UMN disorder

Answer 18

LMN - as it affects the neuromuscular junction

Question 19

What is Lambert Eaton Myasthenic Syndrome

Answer 19

An autoimmune disorder characterized by muscle weakness of the limb due to antibodies to presynaptic voltage gated calcium channels

Question 20

What disease is Lambert Eaton associated with

Answer 20

Small cell lung cancer

Question 21

How does Lambert Eaton differ from MG

Answer 21

Weakness reduces with repeated muscle contraction (EMG- “seoncd wind” phenomenon)
Eyes are rarely affected - ie. no ptosis
Hyporeflexia
Lower limbs more than upper

Question 22

What symptoms might patients with MG present with?

Answer 22

Intermittent diplopia

Question 23

What are characteristic findings on CN examination of MG?

Answer 23

Fatiguable weakness that improves with rest

Compex extra-ocular palsies
Ptosis, bulbar palsy

Myasthenic snarl on attempting to smile