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| Myasthenia Gravis |
 | Epidemiology
 | Incidence 1:20,000. |
| 5 - 7% familial |
|
| Age of onset
| May start at any age. |
| Rare before 15 or after 70. |
| 60 per cent has onset between the ages of 20
and 40 |
|
| Due to the increased proportion of older persons,
more older patients are found to have MG
| Early onset: onset before 50 yo
 | Predominantly female |
|
| Late onset: onset after 50 yo
| likely to be more severe |
| myopathy is more common |
| Effect of acetylcholinesterase inhibitors
is often temporary. |
| Plasma exchange has more complications in
the elderly |
| Result of thymectomy is poorer |
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| Classification after Ossreman & Genkins
| Adult MG
| Group I: Ocular (20%) |
| Group IIA: Mild generalized (30%) |
| Group III: Acute fulminating (11%), rapid onset, early respiratory
involvement, high mortality. |
| Group IV: Late Severe (9%), > 2 years after onset. |
|
| Transient Neonatal MG: 1/6 born to MG mother. Last a few weeks. |
| Congenital Myasthenic Syndrome: |
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| Testing |
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| Thymoma
| 15% of patient has thymoma, 50% has thymic hyperplasia.
|
| Antiskeletal muscle Ab are detected in 90% of patients with thymoma. |
| CT Chest detect over 85% of thymoma. |
| Removal of thymoma produces a delayed improvement of MG 6 - 24
months later. Sustained improvement in > 50%, probably less in older
patients. No known long-term side effects. |
|
|
| Tensilon test (Edrophonium)
| Given in incremental doses. Start with 2 mg, observe the
response for 45 to 60 seconds, followed by doses of 3 and 5 mg and observation for a
clinical response for 1 to 2 minutes following each dose. |
| During the injection, patients often experience 1 to 3 minutes of
increased salivation, mild sweating, perioral fasciculations and mild nausea. |
| Hypotension and bradycardia are extremely rare but precautions should be
taken. Atropine sulfate (0.6 mg intramuscular or intravenously) should be available in
case of an emergency. |
| Positive means unequivocally improvement of weakness. |
| Slight to moderate improvement in muscle strength must be interpreted
with extreme caution. |
| Mild to moderate clinical improvement after tensilon has been reported
in: brain stem lesions, oculomotor palsy due to cerebral artey aneurysm, diabetic
abducens paresis and even in normal control subjects. |
|
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| Electrophysiology: Repetitive stimulation at 3 hertz.
>10% decrement of compound action potential. |
| Single fiber EMG: Increased jitter: jitter is the varying time interval
between the triggered muscle action potential in 2 muscle fibers within the same motor
unit. Positive in over 90%. |
|
| Treatment |
| Principles of treatment
| Onset before age 60
| Thymectomy, pretreat with plasmapheresis, cholinesterase inhibitors |
| If response unsatisfactory before or after thymectomy, consider high dose daily prednisone and/or other immunosuppressive agents |
|
| Onset after age 60
| Cholinesterase inhibitors with prednisone, azathioprine or other immunosuppressant |
| Plasmapheresis for severe exacerbations |
| Consider thymectomy |
|
| Anticholinesterase useful in all forms |
| For patient with thymoma, thymectomy is indicated in all ages. They may
spread in the mediastinum. |
| Plasmapharesis is effective, but practical only on a short term basis. |
|
| Pyridostigmine (Mestinon)
| Tab: 60 mg, half life 4 hrs, take 1 q 4 while awake
Time Span: 180 mg, irregular absorption, use at night only. |
| IV: 2 mg = 60 mg of oral dose |
| Side effect: diarrhea, treat with Lomotil or Imodium |
|
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| Steroid treatment
| Indications:
| Insufficient control w Mestinon |
| Diplopia rarely respond to Mestinon alone |
| Older male |
|
| Start at 100 mg to avoid treatment failure. After remission obtained,
switch to alternate day dose, slow taper over 6 to 12 months. Patient may
get weaker initially. |
| Exacerbation is common. |
|
| Azathioprine (Imuran)
| Initial dose 2-3 mg/kg/day. |
| Mertens: Complete remission 40%, partial remission 51%, minimal
improvement 6.4%, no effect in 2.6%. |
| Improvement begins in 2-3 months, peaks in 6-15 months. |
| Keep WBC above 3000/ml. |
| Monitor liver function weekly X 3 months, then 2x/month. |
| Sometimes used in combination with steroid. |
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|
| If above treatment fails, consider Cyclosporine (Sandimmune)
| 5 mg/kg/day, in bid dose. Monitor BP, renal function, Cyclosporine level,
Amylase, Cholesterol. |
| May cause nephropathy, hypertension, hirsutism, liver function
abnormality, opportunistic infection, may increase risk of malignancy. |
|
| Plasma Exchange: short term improvement |
| Human Immune Globulin: Probably effective |
|
| Drugs that may adversely affect MG |
| Antibiotics
| Aminoglycosides: Neomycin, Gentamycin |
| Peptide: Polymyxin B, Colistin |
| Other: tetracycline, Clindamycin, Erythromycin, Ampicillin |
|
| Neuromuscular blockers: Botulinum Toxin |
| Cardiac drugs: Quinine, Quinidine, Procanamide, Lidocaine, Beta blockers,
Calcium Channel Blockers |
| Miscellaneous: Dilantin, Oxytocin, Lithium, Magnesium, Diazapam, D
penicillamine, Cloroquine, Interferon |
| Corticosteroids may initially produce worsening of MG. |
|
| Other causes of
exacerbation |
| Febrile illness |
| Thyroid disease |
| Heat |
| Pregnancy |
| Major Physical Stress |
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| Further reading |
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| Patient resources |
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