Tests for myasthenia gravis: Pearls

1. Tensilon test should be unequivocal and not subjective. In one study all patients who had a positive test responded in the first 7 mg.

2. False positive tensilon tests occurred in LEMS< GBS, compressive cranial neuropathies and brain stem lesions (Seminars of Neurology 2003, author Pascuzzi).

3. Muscarinic effects (tearing, sweating,cramping, salivating and nausea) are common. Relative contraindications are cardiac arrythmias and asthma. Cardiac monitoring may not be universally required.

4. A neostigmine methylsulfate injection can be used in children with a longer onset and offset (latter is 30 minutes) but may be harder to interpret.

5. The ice pack test of placing the ice pack on for 2-5 minutes has reported high sensitivity and specificity but may be hard to tolerate for patients (Golnick AC et al. Opthalmology 1999 186:1282-6). The mean improvement with this test is 4.5 mm.

6. The rest test (close eyes for 3 minutes) results in a mean improvement of 2 mm of ptosis.

7. The sleep test (30 minutes) shows improvement in all Tensilon positive patients plus two others (out of 42 tested).

8. Lab tests of binding Ach receptors is positive in 50 % of OM patients (v. 90 % of MG patients overall); blocking antibodies add only another 1 percent sensitivity, and modulating antibodies actually do improve sensitivity slightly but also have more false positives. see Howard FJ et al. NY Acad Sci 1987; 505: 526-538).

9. False positive conditions with ACH receptor antibodies (again from Howard et al.) are: AI hepatitis, SLE, ALS, inflammatory neuropathies, LEMS, thyroid opthalmopathy, 1 degree relatives of MG patients, thymoma, RA, and patients taking penicillamine.

10. Striated muscle ab's as a marker for thymoma: they predict thymoma in patients with MG under 40 (80%) and thymoma in nonmyasthenics (positive in 25 % of those). False positives are seen in RA treated with penicillamine, LEMS, Bone marrow graft recipients, GVHD, and paraneoplastic disease (see Lennon, Neurol suppl 5, 1997 S23-27).

11. De Graefe's test 12. Mary Walker phenomenon-- inflation of cuff above normal systolic pressure causes fatigue. Deflation of the cuff causes exacerbation of MG in rest of the body

13. Fatigue-ability is the hallmark of MG. Test for with sustained upgaze test 30-60 sec (especially check medial rectus muscle> superior and lateral recti); sustained abduction of the arms (120 s); sustained elevation of the leg while lying supine (90 sec); repeated rising from a chair without using arms, up to 20 times, look for bfm's; counting aloud to 50.

14. The rare patient that presents with isolated respiratory involvement may have orthopnea, ie SOB while lying down.

15. Enhanced ptosis of contralateral side may occur with manual elevation of one ptotic lid

16. Unilateral frontal hypercontraction is a sign of weak lids.

17. Specific muscles to assess in suspected MG include eom's, oropharyngeal, facial, respiratory, axial and limb

18 Pseudo INO occurs when adducting eye does not adduct and abducting eye has nystagmus

19. Cover - uncover test can reveal weakness if done repeatedly.

20. Beware of holding target to fixate too close, will reveal a convergence abnormality rather than a true EOM abnormality.

21. Look for snarl with attempt to smile, inability to pucker to kiss

22. Pattern of weak jaw closure and strong jaw opening is expected in MG. Test temporalis/pterygoid separately. Weak jaw opening with pterygoid weakness is rarely found in MG.

23.  Fatigue of swallowing does occur in MG

24.  Slurp test touted in May, 2010 Neurology podcast for children with MG that parents can do to test for weakness.   fill 4 oz cup with water with a straw and have child slurp after finishing the drink.  Time it, use patient as own control.