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A Silence!

Topic: Adult

Created on Saturday, November 8 2008 by rednucleus

Last modified on Saturday, November 8 2008.

You are doing an EMG for a 7-year-old male with progressive proximal weakness and waddling gait and greatly raised serum CPK. The EMG needle is inserted into the left calf, but no insertional activity is being recorded; what is the likely explanation of this observation?

 
        A) The needle is in the belly of the muscle
 
        B) The needle is within inflamed muscle fibers
 
        C) The needle is near an end-plate
 
        D) The needle is broken
 
        E) The needle is an area of fibro-fatty infiltration
 

 


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This question was created on November 08, 2008 by rednucleus.
This question was last modified on November 08, 2008.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ANSWERS AND EXPLANATIONS




A) The needle is in the belly of the muscle

This answer is incorrect.


If you get an insertional activity (which is seen as a volley of action potentials, reflecting miniature end-plate potentials), this means that you are in an excitable tissue and the needle is in an area near an end-plate; If no insetional activity is obtained, this means either you are not in an excitable tissue (like you are still in the subcutaneous tissue) or the muscle fibers are replaced by an unexcitable tissue (like fibro-fatty infiltration of the muscle as in muscular dystrophies, scarred muscle from previous trauma…etc). The needle is usually inserted into the belly of the muscle (which is the thickest part of it).  (See References)

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B) The needle is within inflamed muscle fibers

This answer is incorrect.


This would be seen as a prolonged insertional activity. In normal individuals, the insertional activity should disappear as soon as a the needle stops; if this activity persists for an appreciable interval after the needle has stopped, this means that you are entering a hyper-excitable tissue (myositis, myotonia, and muscle denervation)  (See References)

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C) The needle is near an end-plate

This answer is incorrect.


If you get an insertional activity (which is seen as a volley of action potentials, reflecting miniature end-plate potentials), this means that you are in an excitable tissue and the needle is in an area near an end-plate; If no insetional activity is obtained, this means either you are not in an excitable tissue (like you are still in the subcutaneous tissue) or the muscle fibers are replaced by an unexcitable tissue (like fibro-fatty infiltration of the muscle as in muscular dystrophies, scarred muscle from previous trauma…etc). The needle is usually inserted into the belly of the muscle (which is the thickest part of it).  (See References)

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D) The needle is broken

This answer is incorrect.


A broken needle would not be used in the first place!  (See References)

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E) The needle is an area of fibro-fatty infiltration

This answer is correct.


If you get an insertional activity (which is seen as a volley of action potentials, reflecting miniature end-plate potentials), this means that you are in an excitable tissue and the needle is in an area near an end-plate; If no insetional activity is obtained, this means either you are not in an excitable tissue (like you are still in the subcutaneous tissue) or the muscle fibers are replaced by an unexcitable tissue (like fibro-fatty infiltration of the muscle as in muscular dystrophies, scarred muscle from previous trauma…etc). The needle is usually inserted into the belly of the muscle (which is the thickest part of it). Our patient most likely has Duchenne's muscular dystrophy.  (See References)

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References:

1. Victor, M., and Ropper, A.H. (2001). Adams and Victor's Principles of Neurology, 7th Edition. McGraw-Hill, New York. (ISBN:0070674973)Advertising:
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adult
A Silence!
Question ID: 110808102
Question written by rednucleus. (C) FrontalCortex.com 2006-2009, all rights reserved. Created: 11/08/2008
Modified: 11/08/2008
Estimated Permutations: 120

User Comments About This Question:

1 user entries
 

jdmiles
adult Specific Reference Nov 09, 2008 @ 12:04
Thanks to rednucleus for this good question.

For the reader interested in learning more, details can be found on page 1361 in Adams & Victor's 7th Edition, or page 1103 in the 8th Edition.


 
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