myasthenia Gravis Flashcards
aetiology of MG
- Autoimmune disorder
- Formation of acetylcholine receptor site antibodies
- The antibodies prevent acetylcholine from binding and reduce the effectiveness of the neurotransmitter
mechanism of MG
- Acetylcholine continues to be released
- This maintains the striated muscle contracture until the stores are drained.
- This then shows the classic early muscle fatigue
Can be systemic or ocular - systemic is worse than ocular, most px will have ocular and may go on to have systemic
features of MG
- 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 due to more acetylcholine in system when they wake up
ocular symptoms of MG
- Ptosis - SUSPECT MG
→ Usually the first presenting sign of MG
→ Bilateral but Asymmetrical
→ Ptosis increases throughout the day - measure palpebral aperture first - normal = 10/11 mm - Diplopia - may be variable due to which muscle is being affected - can seem like a 6NP onr week and a 3rd the next week
- Inadequate lid closure - due to weakness of orbicularis muscle and cant keep it closed
Assessment of Ptosis
REMEMBER COGANS AND GORELICK SIGNS
Ptosis should increase on continued elevation or repeated up and down gaze, or in extreme cases the lids may drop on continued gaze in the primary position - do for 30 secs/ 1 min to look at sustained elevation
+ve Cogan’s Lid Twitch (Cogan 1965) - KEY SIGN OF MG px looks at torch, looks down (relaxes muscle) and then look in pp - surge of acetylcholine makes lid go higher
Patients should look down for 15 secs, then refixate in the PP. A twitch can been seen in the upper lid as it overshoots the midline and then returns to it’s ptotic position
“Flutter type” due to surge of acetylcholine upper lid movements can be observed due to lid twitches
If you hold the most affected eyelid open, the innervational drive to both upper eyelids is reduced and the ptosis on the other, less affected eye increases (Gorelick 1981)
Frontalis overaction in an attempt to raise the eyelids can give rise to apparent upperlid retraction
Diplopia in MG
- Diplopia can be Hor/Vert/Both and will vary throughout the day
- MG may cause any type of muscle palsy
- Limited elevation is the most common ( Cleary et al 2008)
- Pseudo INO
- Isolated IR Palsy
- Pseudo gaze palsy
- Pseudo 3rd, 4th, 6th nerve palsies
- In fact pseudo anything!!!
Obicularis weakness
* Ask the patient to close their eyes tightly and then examiner tries to open them, whilst they keep them shut
systemic signs of MG
- All dependent on which muscle groups are affected
- Ensure that you ask for signs of general MG in case History
- Difficulty chewing/swallowing (Jaw Muscles)
- Difficulty speaking (Bulbar Muscles)
- Breathlessness (Respiratory Muscles)
- Fatigue climbing stairs or holding arms up high shows a weakness of the (limb girdle muscle)
- Lack of facial expression (Facial Muscles)
classification of MG
- Neonatal – rare
- Congenital – infants may be affected with both Ocular and Systemic MG
- Juvenile – from Birth to Puberty similar to the adult cases
MG in adults
- 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
investigations for MG
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
blood test investigation for MG
- 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
EMG for MG
Usually used for botox
- Electromyopgraphy is carried out to record the electrical activity of the skeletal muscles - Electrodes on muscle and get px to look in direction
- Single or multiple muscle fibres may be tested
- Nerve supply to the muscles are electronically stimulated and muscle activity is then recorded
tensilon itest - investigation for MG
Edrophonium is a short acting anticholinesterase
* Injected intravenously
* If +ve MG than muscle function should improve
* Risk of reaction
- 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
→ Usually performed on the HESS
→ Observation of Ptosis
→ EMG
management of MG
→ The Ophthamologist may trial a longer acting antichoinesterase drug for example pyridostigmine/mestinon
→ CT scan of the thymus gland, as the thymus can be enlarged in MG
→ If the gland is enlarged (Thymoma) it can be removed. Thus eliminating the B-cells that can produce the acetylcholine receptor antibody
→ Immunosupression with systemic steroids – beginning with 10mg Prednisolone (steroid)
→ Azathioprine or Myophenolate can enhance the steroids
→ Plasmaphoresis ( removal of plasma and antibodies and replacement with plasma substitute)
ocular management of MG
MG that remains confined to the EOM”s for more than 2 years is unlikely to progress into generalised MG
→ Prognosis of Ocular Myasthenia Gravis (Retrospective Multicentre Analysis) was published in Ophthalmology 2015. Nagia et al reviewed the case notes of OMG patients between 1993 and 2012
→ In 158 patients they found that the conversion rate to GMG was 20.9%
· Fresnel prisms - due to changing deviation as MG is variable
· Ptosis props
· Occlusion - if deviation changing too often - aim is BSV, if not possible with prism resort to occlusion
· Botox
· Strabismus surgery under local anaesthesia with adjustables
