37: NMJ disorders Flashcards
What Ab’s would be present if a patient with myasthenia clinically does not have Ab’s to AChR?
Muscle-specific tyrosine kinase (MuSK);
low-density lipoprotein receptor-related protein 4 (LRP4)
What are the most common types of weakness in MG?
Extra-ocular;
bulbar (difficulty with swallowing, chewing, and speaking) with nasal and slurred speech
What is seen in folks with anti-MuSK Ab-associated MG? Who does it tend to affect?
Younger females;
look for severe oculobulbar weakness along with tongue and facial atrophy, with tongue fasciculations;
look for neck, shoulder, and respiratory weakness with little or no ocular weakness;
or you see a pattern similar to anti-AChR Ab-associated MG
What test can improve ptosis with MG?
Ice pack test
In what condition can MG be seen as a rare complication?
Cancer patients treated with immune checkpoint inhibitors
What can immune checkpoint inhibitors lead to?
Myasthenia, myositis, GBS, CIDP
How to treat MG provoked by ICPI’s?
Plasmapharesis and high-dose steroids; need to stop the drug
Other causes of autoimmune MG?
Transient neonatal MG and penicillamine treatment
Major difference between LEMS and MG on NCS?
Low CMAP amplitude for LEMS typically
Diagnostic yield of MG on RNS increases with what nerves stimulated?
Proximal nerves, as proximal muscles usually involved more than distal
What should be done if there is no significant decrement on RNS studies at baseline?
Exercise testing (1 min of exercise, then RNS at 1-min intervals for the next 3-4 mins, looking for CMAP decrement with post-exercise exhaustion)
What do MUAP’s look like with NMJ disorders?
Unstable, small and short-duration, or both
What is the clinical correlate of blocking?
Muscle weakness
What is an advantage of SFEMG or RNS?
Can show increased jitter even without clinical weakness
What filter settings change with SFEMG? What is the distance single-fiber muscle action potential need to be to be recorded for SFEMG?
Low freq filter increased to 500-1000 Hz;
200-300 um, to only catch single muscle fibers close to the needle
What is necessary to deem the upper limit of jitter of a muscle abnormal? How to make a diagnosis of NMJ disorder?
More than 10% of the pairs studied must exceed the upper limit of normal;
Mean jitter must be abnormal, or ULN jitter must be abnormal in more than 10% of individual pairs
Increased _____ is consistent with an NMJ disorder;
when does blocking typically occur?
jitter;
mean consecutive difference (measure of jitter) is more than 80-100 us
What is the most sensitive test to demonstrate impaired NMJ transmission?
SFEMG
Pathogenesis of LEMS?
Immune-mediated;
look for IgG Ab’s directed at presynaptic P/Q and N-type voltage-gated Ca channels
What is seen with LEMS clinically not usually seen with MG?
DTR’s usually reduced or absent
With slow RNS before and after exercise in LEMS, what happens with decrement and the baseline CMAP?
Decrement in both places;
however, baseline CMAP larger post-exercise than pre-exercise
What trick can be used if patient has low or borderline low CMAP amplitude at rest and you suspect LEMS?
Repeat the distal motor stim after 10 seconds of maximal exercise, looking for post-exercise facilitation of the CMAP
What are CMAP amplitudes in botulism? How does the increment compare after brief exercise or fast RNS?
Low, like LEMS;
increment is present, but not as much as LEMS
What is seen spontaneous activity wise and on SFEMG for botulism?
Fibs and PSW’s;
Increased jitter and blocking
