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Lower Extremity EMG 01

Topic: Anatomy

Created on Tuesday, February 13 2007 by jdmiles

Last modified on Tuesday, February 13 2007.

A patient presents with lower extremity weakness. Needle EMG study of the gastrocnemius and gluteus maximus reveals large, long, polyphasic motor unit action potentials. Needle EMG study of the vastus medialis, vastus lateralis, extensor digitorum longus and tibialis anterior are normal. Motor and sensory NCS are normal. Of the following options, which is the most likely lesion?

A) Acute L2 radiculopathy

B) Chronic S1 radiculopathy

C) Chronic L2 radiculopathy

D) Acute L5 radiculopathy

E) Chronic L4 radiculopathy

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This question was created on February 13, 2007 by jdmiles.
This question was last modified on February 13, 2007.

ANSWERS AND EXPLANATIONS

A) acute L2 radiculopathy

This answer is incorrect.

The abnormal MUAPs found were long, large, and polyphasic, suggesting a chronic denervation. There is no other evidence to suggest an acute denervation. The abnormal MUAPs were found only in gastrocnemius (S1, S2), and gluteus maximus (L5, S1, S2), neither of which has innervation from L2. The muscles innervated by L2 - vastus medialis (L2, L3, L4) and vastus lateralis (L2, L3, L4) - had normal needle studies. Thus, L2 is not the location of the lesion. (See References)


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B) chronic S1 radiculopathy

This answer is correct.

The abnormal MUAPs found were long, large, and polyphasic, suggesting a chronic denervation. The muscles involved all had S1 innervation: gastrocnemius (S1, S2), and gluteus maximus (L5, S1, S2). The other muscles are normal, suggesting no chronic denervation: vastus medialis (L2, L3, L4), vastus lateralis (L2, L3, L4), extensor digitorum longus (L4, L5), and tibialis anterior (L4, L5). This makes S1 a more likely location than the other options. Note that S2 could also be involved. (See References)


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C) chronic L2 radiculopathy

This answer is incorrect.

The abnormal MUAPs were found only in gastrocnemius (S1, S2), and gluteus maximus (L5, S1, S2), neither of which has innervation from L2. The muscles innervated by L2 - vastus medialis (L2, L3, L4) and vastus lateralis (L2, L3, L4) - had normal needle studies. Thus, L2 is not the location of the lesion. (See References)


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D) acute L5 radiculopathy

This answer is incorrect.

The abnormal MUAPs found were long, large, and polyphasic, suggesting a chronic denervation. There is no other evidence to suggest an acute denervation. Abnormal MUAPs were found in gluteus maximus (L5, S1, S2), but not in other muscles innervated by L5 - extensor digitorum longus (L4, L5) and tibialis anterior (L4, L5). Also, abnormal MUAPs were found in gastrocnemius (S1, S2), which does not receive innervation from L5. S1 or S2 would be more likely locations for the lesion than L5. (See References)


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E) chronic L4 radiculopathy

This answer is incorrect.

The abnormal MUAPs were found only in gastrocnemius (S1, S2), and gluteus maximus (L5, S1, S2), neither of which has innervation from L4. The muscles innervated by L4 - vastus medialis (L2, L3, L4), vastus lateralis (L2, L3, L4), extensor digitorum longus (L4, L5), and tibialis anterior (L4, L5) - had normal needle studies. Thus, L4 is not the location of the lesion. (See References)


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References:

1. Preston, D.C., and Shapiro, B.E. (2005). Electromyography and Neuromuscular Disorders: Clinical-Electrophysiologic Correlations, 2nd Edition. Elsevier, Philadelphia.
2. Guarantors of Brain. (2000). Aids to the Examination of the Peripheral Nervous System, fourth edition. W.B. Saunders, Edinburgh.
3. Moore, K.L. (1992). Clinical Oriented Anatomy, 3rd Edition. Williams & Wilkins, Baltimore.


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anatomy
Lower Extremity EMG 01
Question ID: 021307133
Question written by J. Douglas Miles, (C) 2006-2009, all rights reserved.
Created: 02/13/2007
Modified: 02/13/2007
Estimated Permutations: 50400