Myasthenia Gravis Flashcards
Myogenic
MG aeitiology
Autoimmune disorder (Dia, Rheumatoid Arthritis)
Formation of acetylcholine receptor site antibodies
The antibodies prevent acetylcholine from binding and reduce the effectiveness of the neurotransmitter
Acetylcholine continues to be released which maintains the striated muscle contracture until the stores are drained.
This then shows the classic early muscle fatigue
MG features
Characterised by excessive fatiguability of striated muscle
EOM, facial, bulbar, neck, limb girdle, distal limb and trunk muscles
80-90% of patients with general MG have receptor site antibodies in their blood serum, compared to only 40-50% in Ocular MG
When the respiratory muscles are severely affected it can be fatal
EOM’s may be mostly affected as they have a very high concentration of receptors and an increased sensitivity of the neuromuscular junction
It may be associated with other autoimmune diseases Diabetes, Graves’ Orbitopathy and Rheumatoid arthritis
MG symptoms
Generally, symptoms of MG increase as the day goes on, and worse when ill.
Patients may be symptom free in the morning and only complain of fatigue by the evening (diurnal)
MG Ocular symptoms:
Ptosis (if newly acquired, gets worse throughout the day)
Diplopia (may/may not, asymmetry between eyes) – variable and often vertical
Inadequate lid closure (try to pry eyelids open, if open easily = weak orbicularis muscle) = scleral show
Limited elevation is the most common ( Cleary et al 2008)
MG ptosis + assessment
Usually the first presenting sign of MG
Bilateral but Asymmetrical (unlike CPEO which is more symmetrical)
Ptosis increases throughout the day
Assesemnt: 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 = suspended lid elevation = measure palpebral apertures before and after testing.
+ve Cogan’s Lid Twitch : Patients should look down for 15-30 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” upper lid movements can be observed due to lid twitches (Ach being absorbed or multiple 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’s)
Frontalis overaction in an attempt to raise the eyelids can give rise to apparent upperlid retraction
Orbicularis Weakness, when px attempts to ‘bury’ their lashes and examiner attempts to open eyelids, might show a ‘peek’ sign where lid margins open or might be too weak to keep them closed.
MG classification
Neonatal MG – rare
Congenital MG– infants may be affected with both Ocular and Systemic MG
juvenile MG – from Birth to Puberty similar to the adult cases
Drug-related: d-penicillamine-induced myasthenia.
Systemic signs MG
All dependant 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) à very serious!!
Fatigue climbing stairs or holding arms up high shows a weakness as you can push down easily
Lack of facial expression (Facial Muscles)
MG why occur in ocular first
Ocular – that does not become generalised after 2 years since onset
Mild/Mod Generalised MG – Ocular signs before disease spreads to skeletal + 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
Because EOM striated!!
MG investigations further (rarely done)
Investigations :
Ice Pack Test
Lowering temp can improve symptoms
Ice pack applied to the eyelid can improve the ptosis
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
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
Electromyopgraphy is carried out to record the electrical activity of the skeletal muscles
Single or multipe muscle fibres may be tested (muscle biopsy)
Nerve supply to the muscles are electronically stimulated and muscle activity is then recorded
Tensilon test
A short-acting anticholinesterase that is injected intravenously. It improves the function of myasthenic muscles within 1 minute following injection and is effective for approximately 5 minutes.
Serious reactions can occur to the drug including bradycardia, respiratory depression so resuscitation team must be available.
MG management systemic
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 (reduce antibody production) – beginning with 10mg Prednisolone
Azathioprine or Myophenolate can enhance the steroids (it may take6-12 months to start working)
Plasmaphoresis ( removal of plasma and antibodies and replacement with plasma substitute)
Thymectomy (/thymus surgery) – only if have generalized, not ocular alone unless have
ocular mx for MG
MG that remains confined to the EOM”s for more than 2 years is unlikely to progress into generalised MG
In 158 patients they found that the conversion rate to GMG was 20.9%
Fresnel prisms – might be ineffective due to variability of deviation
Ptosis props – have to be careful of dry eye
Occlusion of one eye may be preferred in some cases rather than risk the systemic side-effects of medical treatment. R
Botulinum toxin has a role in confirming binocular potential in patients prior to embarking on surgical treatment. Successful outcomes using botulinum toxin and/or surgery were reported by Bentley et al. (Strabismus surgery under local anaesthesia with adjustables
strab sx for mg
Surgery is contraindicated in the active stage of the disease because of the variability of the deviation. However, once the condition is quiescent, patients may benefit from strabismus surgery.
Surgery performed under general anaesthesia has a risk of causing a myasthenic crisis; intensive care support must therefore be available prior to embarking on surgery under general anaesthesia.
We prefer to operate on adult patients under local anaesthesia, using adjustable sutures where possible.
what condition is associated with MG that is alarming
Lambert–Eaton myasthenic syndrome (LEMS) :
This is a disorder of neuromuscular transmission that differs from myasthenia in that the transmission defect is presynaptic rather than postsynaptic.
There is a strong association between LEMS and malignancy, predominantly small-cell carcinoma of the lung. Present for > than 2 years without evidence of malignancy the risk ofits subsequent development declines sharply. Autonomic symptoms are common, particularly a dry mouth.
