Terms in this set (7)
Myasthenia gravis (MG)
An autoimmune disorder caused by an immunoglobulin F (IgG) antibody directed attack on the NMJ, aimed specifically at the nicotinic acetylcholine receptor in most cases.
A form of myasthenia gravis where the antibody present is directed at muscle-specific tryosine kinase (MUSK), which is a surface receptor involved in the clustering of ACHRs during developement. 40-50% of MG cases have this type.
Clinical features of MG
Muscle fatigue and weakness, worse with use
Extraocular, bulbar or proximal limb involvement
DTRs are spared
Clinical features of anti-MUSK MG
Severe presentation at younger age
Prominent bulbar, neck, shoulder and respiratory involvement
Iceberg test in MG
When ice is applied over the forehead muscles for several minutes, ptosis may improve markedly temporarily.
MG on NCSs
CMAP amplitudes normal
Slow RNS shows DECREMENTAL response >10% (this will be normal if the MG form is strictly ocular)
EMG may show small, short duration MUAPs
SFEMG shows jitter
The variation in the time interval between the firing of adjacent single muscle fibers from the same motor unit. Primarily reflects NMJ transmission time.
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