Myasthenia gravis and LEMS

Myasthenia gravis (MG) can mimic many of the aforesaid conditions. 75 % of patients present with ptosis or diplopia.

Diagnostic pearls-- a painless pupil sparing third nerve palsy suggests the possibility of a "pseudo-third nerve palsy" which may be due to MG. Variability, alternating quality between eyes, and presence of orbicularis oculi face weakness helps to make the diagnosis. Face weakness rules out CN palsy as a cause. Saccade velocities are preserved in the setting of limited eye movements especially with smaller eye movements (larger ones may induce fatigue). Diagnostic tests include Tensilon test, sleep test, or the ice test, or serum antibody tests or neurodriagnostic tests.

EMG/NCS pearls in MG-- typically with rep stim four to five stimuli is best; with more repair occurs and effect is harder to see. You should see a smooth tapering response with a best effect on fourth or fifth stimulus, and it should be confirmed in a second nerve. Mestinon and timespan should be held for about twelve hours prior to testing. After exercise, rest elicits repair followed by post exercise exhaustion. You may need to repeat post exercise test every 30 seconds for up to four or five minutes. Picture on NCS can be indistinguishable from LEMS. Recall though that anti VGCC is found in up to 90 % of LEMS patients. Clinical PEARL-- can check for facilitation of a DTR after exercise--if DTR returns, favoos LEMS. LEMS oftenhas a flat CMAP that returns to normal after exercise. Instability is seen on needle examination of affected muscles on EMG. At the Mayo, Tensilon is rarely done except with the LR green eye exam. Usually 3-4 nerves are tested including facial, spinal accessory. Look for finger extensor weakness in MG and if found, check radial nerve at EIP and spiral groove.