Main Tests Used
| Test | Where used |
|---|---|
| Edrophonium* (Tensilon test) | Cholinesterase inhibitor. Side effects: bradycardia and/or hypotension |
| Ice test | Not commonly used, applicable only when ptosis present |
| Acetylcholinreceptor antibody in serum | May be the diagnostic “gold standard”, titres do not always correspond with the severity of MG [14] |
| Repetitive nerve stimulation | Sensitive 75%, not specific, uncomfortable to patient |
| Single fibre electromyography | Most sensitive, not specific |
| Anti-MuSK antibodies | For subgroup of seronegative MG |
| Chest CT/MRI | For associated thymic tumours |
| Brain MRI | Where structural brain stem lesion is possible |
The main tests used in the diagnosis of MG are shown in the table.
The edrophonium test
The ‘edrophonium test’ is performed to identify MG infrequently; its application is limited to the situation when other investigations do not yield a conclusive diagnosis.
This test requires the intravenous administration of edrophonium chloride (Tensilon, Reversol) or neostigmine (Prostigmin) drugs that block the breakdown of acetylcholine by cholinesterase (cholinesterase inhibitors) and temporarily increases the levels of acetylcholine at the MNJ. In people with MG involving the eye muscles, edrophonium chloride will briefly relieve weakness [10].
In the past, the edrophonium test was routinely performed to confirm the diagnosis and distinguish undertreatment from overtreatment of MG, this practice has been largely abandoned as it is potentially dangerous in impending cholinergic crisis.
Radioimmunoassay test to detect the antibodies that bind to AchR is considered the diagnostic ‘gold standard’ [2]. AchR antibody titers correlate poorly with severity of disease.
Muscle fatigability tests
Muscle fatigability can be tested for in many muscles [8].
Examples include:
- Looking upward and sideways for 30 seconds to check for ptosis and diplopia
- Looking at the feet while lying on the back for 60 seconds
- Keeping the arms stretched forward for 60 seconds
- The ‘peek sign’: after complete, initial apposition of the lid margins, they quickly (within 30 seconds) start to separate and the sclera starts to show [3]
Learning Bite
The ‘edrophonium test’ is performed to identify MG infrequently; its application is limited where the patient has obvious ptosis. Diagnosis should be confirmed by electrophysiologic and/or immunologic tests.