Questions frequently arise concerning timing of EMG/NCS studies following an acute injury.  Though it would seem appropriate to test the nerves and muscles of an injured patient in close proximity to an injury, certain physiologic phenomena dictate the need to delay a study following an injury.  Different rules apply to the EMG and nerve conduction portions of the examination.

Surprisingly, following a complete nerve transsection which results in the nerve being non-functional, nerve conduction values can be obtained for that nerve for up to seven days following the actual death of the nerve.  Following that time, no response will be seen.  As a result, nerve conduction studies are not valid if performed before that seven-day mark. 

A different set of principles determines the appearance of acute changes seen on EMG.  Acute changes which appear following a nerve injury are seen most commonly as fibrillations, increased insertional activity, or positive sharp waves in reporting. 

Once again, using the example of a complete nerve transsection which results in a nerve becoming nonfunctional and a muscle being paralyzed, acute changes are not seen when studying the non-functional muscle for approximately two-and-a-half weeks.  As a result, needle EMG performed prior to the two-and-a-half week mark following an acute injury fails to reveal signs of an acute injury.  A study performed after the two-and-a-half week mark reveals significant signs of denervation manifest by increased insertional activity, fibrillations, and positive sharp waves.  As a result, a study performed before the two-and-a-half week mark would likely underestimate the degree of damage and, in some cases, might result in a normal needle exam.

Just remember, given the underlying physiology, following an acute injury, studies should be scheduled no sooner than two-and-a-half weeks following an injury.

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SOURCES: AANEM.  Guidelines and Practice Parameters.  Online.  Internet Accessed 27/2/02; 20/8/04; 13/12/04; 22/08/07 ; 30/04/2010.  Available at http://www.aanem.org