Flashcards in Electromyography II Deck (13)
What do neuropathic potentials on an EMG voluntary motion exam?
large amplitude and long duration. often polyphasic- reflect large motor units due to reinnervation of previously denervated muscle fibers
What do myopathic potentials look like on an EMG voluntary motion exam?
smaller motor unit size due to drop-out of individual muscle fibers
What is volume conduction?
spread of current from a potential source through a conducting medium (body tissue). results in poorly defined motor unit and muffled sound. implies that the electrode is incorrectly positioned in the muscle fiber wrt the portion of the fiber being activated
What is recruitment?
this describes the initiation of firing of additional motor units as the active ones increase their rates of discharge. ask the pt to contract their muscles as hard as they can. this will increase the rate of firing for fibers that were firing before, and will also recruit more motor fibers to the task.
When do you see recruitment abnormalities?
chronic myopathies and neuropathies
What do you see in terms of recrutiment in chronic myopathies? Why?
Rapid recruitment. brain tries to compensate for loss of muscles by early recruitment.
What do you see in terms of recruitment in chronic neuropathies?
recruitment is reduced- total number of motor units has decreased. in severe neuropathoies, it is possible only one rapidily firing motor unit will be observed
What EMG abnormalities will be seen in active inflammatory myopathies?
fibrillations and possible waves with muscle at rest.
What EMG abnormalities will be seen in chronic myopathies?
voluntary motor unit potentials are of low amplitude and short duration and are often polyphasic. recrutiment may be rapid
What EMG abnormalities will be seen in an acute denervation?
fibrillations and positive waves. indicate spontaneous discharge of individual muscle fibers
What EMG abnormalities will be seen in a chronic neuropathy?
voluntary motor unit potentials are of large amplitude and long duration
often polyphasic becuase motor units are enlarged as a result of reinnervation of adjacent previously denervated musc,e fibers. recruitment is reduced
How is EMG useful for myopathies?
distinguish btw active myopathies (aka inflammatory myopathies) and chronic myopathies