NMJ Disorders Flashcards
MC NMJ disorder
Myasthenia Gravis
Three types of myasthenia
- generalized
- ocular
- bulbar
Age/Sex MC seen in myasthenia
females: teens-30s
males: 50-70
(Bimodal distribution)
Three pathophys mechanisms that lead to myasthenia
1) Direct blocking of the nicotinic AchR site
2) Accelerated internalization of the AchR by crosslinking of IgG
3) Complement mediated lysis of the muscle end plate
How commonly is thymus involved in myasthenia?
50% have thymic hyperplasia
10-15% have thymoma
(Thymocytes produce the AchR Ab)
Classic symptom description for myasthenia:
- fatigable weakness
- diurnal variation (worse at night)
**Normal sensation, reflexes
Most common Ab assc with MG
AchR BINDING Ab
1st screening test for myasthenia, may not ALWAYS be present especially in ocular MG
Myasthenia Ab that is assc with thymoma
Anti-striated muscle
seen in 75-80% of those with thymoma, also sometimes those without
In addition to screening for Ab’s, what other tests are diagnostic for MG:
- repetitive nerve stimulation
- edrophonium/tensilon test
How sensitive/specific is tensilon test for MG?
“71-95%” sensitive
not very specific
not reliable in anti-MuSK MG
Side effects assc with the tensilon test
bradycardia, hypotension (treat with atropine)
What must be done to ensure there is no bias in tensilon testing?
-patient+ docs blind, only nurse may know in which order patient receives placebo vs tensilon
MC treatment for symptom reduction in myasthenia
- pyridostigmine (mestinon)
- AchEi
Effect of mestinon on end plate potential
increases size and length of end plate potential
ADRs assc with mestinon
liquid out of all the places.
diarrhea, runny nose, sweating, vomiting etc
Short term therapies to suppress immune system in MG?
- plasma exchange
- IVIG
Long term therapies to suppress immune system in MG?
- thymectomy
- corticosteroids
- azathioprine/ mycophenolate
Why must plasma exchange be done every OTHER day for 3-6 days as opposed to EVERY day?
-It depletes clotting factors
When are Plasma exchange and IVIG used for MG
-exacerbations, resistant cases
What patients should have prophylactic thymectomy in MG?
-Most patients under 50
Lambert Eaton:
- MC sex
- paraneoplastic of what cx?
- how commonly is LEMS paraneoplastic?
- males=females
- small cell, half of LEMS patients have small cell, 3% of small cell patients have LEMS
**IF YOU DIAGNOSE LEMS YOU HAVE TO SCREEN FOR SMALL CELL!!!!!!!
Non small cell LEMS is most commonly assc with?
-younger females with other autoimmune disease
What antibody is responsible for lambert eaton?
-IgG antibody against voltage gated calcium channel
Ca is responsible for the release of Ach at NMJ
How does LEMS effect the motor end plate potential?
-decreased –> failed transmission –> low CMAP
Typical initial presentation of LEMS
- proximal leg/arm weakness that improves w/ exercise
- MILD CN sx
- dry eyes/mouth, metallic taste
- absent reflexes (UNLIKE MG)
- normal sensation
Subtle signs of LEMS (2)
- weakness if given CCB for HTN
- prolonged paralysis following intubation
How does repetitive stimulation testing vary in LEMS vs MG?
- increased end plate potential over time in LEMS
- decreased EPP over time in MG
Three possible MG antibodies
- antiMUSK
- antiAchR
- anti striated muscle
