Week 5 - Myasthenia Gravis

Question 1

What is the Aetiology for myasthenia gravis?

Answer 1

• Autoimmune disorder
• Formation of acetylcholine receptor site antibodies
• The antibodies prevent acetylcholine from binding and reduce the effectiveness of the neurotransmitter
•Acetylcholine continues to be released
• This maintains the striated muscle contracture until the stores are drained.
• This then shows the classic early muscle fatigue

Question 2

What are the two divisions of Myasthenia Gravis?

Answer 2

• Systemic
• Ocular

Question 3

What are the features of Myasthenia Gravis?

Answer 3

• Excessive fatigue of striated muscle
- Affects EOM, facial, bulbar, neck, limb girdle, distal limp and trunk muscles

• 80-90% of general MG have receptor site antibodies in blood, where as 40-50% in ocular MG

• If respiratory muscles affected can be fatal

• EOM may be most affected due to high concentration of receptors/increased sensitivity at neuromuscular junction

Question 4

Symptoms - Myasthenia Gravis?

Answer 4

• Generally symptoms of MG increase as the day goes on.
• Patients may be symptom free in the morning and only complain of fatigue by the evening

Question 5

What are the ocular symptoms of myasthenia gravis?

Answer 5

• Ptosis
• Diplopia
• Inadequate lid closure

Question 6

What is Ptosis related to MG?

Answer 6

• Usually first presenting sign of MG
• Bilateral but asymmetrical
• Ptosis increases throughout day

Question 7

What is found during assessment of ptosis?
( Ptosis increased? Cogans lid twitch?)

Answer 7

• Ptosis will increase on continued elevation/repeated up and down gaze, or in extreme cases lids may drop in primary
• +ve Cogan’s lid twitch
- px should look down for 15s, then refixate the PP, twitch can be seen in upper lid as overshoots the midline and returns to its ptotic position

Question 8

What is found during assessment of ptosis?
(Flutter and holding eye open?)

Answer 8

• Flutter type upper lid movements can be observed due to lid twitches
• If hold most affected eyelid open, innervational drive to both upper eyelids is reduced, ptosis on less affected eye increases
• Frontalis over-action in attempt to raise eyelids can give rise to apparent upper-lid retraction

Question 9

Diplopia with myasthenia gravis?

Answer 9

• Dipopia can be Hor/Vert/Both and will vary throughout the day
• MG may cause any type of muscle palsy

Question 10

What type of palsies are present in MG?

Answer 10

• Limited elevation most common
• Pseudo INO
• Isolated LR Palsy
• Pseudo gaze palsy
• Pseudo 3rd, 4th and 6th nerve palsy

Question 11

Orbicularis with MG?

Answer 11

• Orbicularis weakness
• Ask px to close eyes tightly and then examiner tries to open them, whilst they keep them shut

Question 12

Systemic signs of MG?

Answer 12

• Dependent on which muscle group affected
• Ensure that asks for all signs of general MG in case history
• Difficulty chewing/swallowing (jaw muscles)
• Difficulty speaking (bulbar muscles)
•Breathlessness (Respiratory Muscles)
• Fatique climbing stairs or holding arms up high shows a weakness of the (limb girdle muscle)
• Lack of facial expression (Facial Muscles)

Question 13

What are the 4 age classifications of MG?

Answer 13

• Neonatal - rare
• Congenital - infants may be affected with both Ocular and Systemic MG
• Juvenile - from Birth to Puberty similar to the adult cases
• Adult

Question 14

What are the 4 classifications for adults

Answer 14

• Ocular - that does not become generalised after 2 years since onset
• Mild/Mod Generalised MG - Ocular signs before the disease spreads to skeletal and bulbar muscles
• Acute Fulminating MG - Rapid onset with early involvement of respiratory muscles
• Late Severe MG - Can develop in the ocular or mild group, 2 years after onset

Question 15

What tests can be done for MG?

Answer 15

• Ice pack test
- Lowering temp can improve symptoms
- Ice pack applied to the eyelid can improve the ptosis (Sethi et al 1987)

• Sleep test (Odel et al 1991)
- Ask the patient to go out into the waiting room or treatment room bed and have a nap for 30 minutes
- Lid position will improve if MG

Question 16

What is checked for in the Blood Test?

Answer 16

• Serum blood testing for acetylchoine receptor site antibodies
•80-90% seen in General MG
• 40-50% seen in Ocular MG
• Do not exclude MG if -ve blood result

Question 17

How is Electromyography done?

Answer 17

• EMG is carries out to record electrical activity of the skeletal muscles
• Single or multiple muscle fibres may be tested
• Nerve supply to muscles are electronically stimulated and muscle activity is recorded

Question 18

What is the Tensilon test?

Answer 18

•Edrophonium is a short acting anticholinesterase
• Injected intravenously
• If +ve MG than muscle function should improve
•Risk of reaction

Question 19

What are the dosages for the Tensilon Test?

Answer 19

• First a test dose of 2 mg is delivered into the vein and flushed through with saline
• If the patient becomes brachycardic due to hypersensitivity than atropine shoud be given to reverse the effects
• If no reaction, then the further 8mg is delivered and the patient is assessed

Question 20

How is MG Managed with drugs by opthalmologist?

Answer 20

• Ophthalmologist may trial longer acting antichoinesterase

• Immuno-suppression with systemic steroids (starting with 10mg prednisolone)
- Azathioprine/Myophenolate can enhance

Question 21

How is MG Managed with surgery by opthalmologist

Answer 21

• CT scan of thymus gland, as thymus can be enlarged in MG
- If gland is enlarged (Thymoma) can be removed - eliminating B-cells that produce acetylcholine receptor antibody

• Plasmaphoresis ( removal of plasma and antibodies and replacement with plasma substitute)

Question 22

how can MG be ocular managed?

Answer 22

• MG that remains confined to the EOM”s for more than 2 years is unlikely to progress into generalised MG
- Conversion rate to GMG was 20.9%

• Fresnel prisms
• Ptosis props
• Occlusion
•Botox
• Strabismus surgery