EMGs and Sciatica

[Mike__0__0]

  I have been diagnosised with sciatic.  Quick rundown, SLR test was positive for L5 nerve root involvement in the left leg.  Two MRIs report Disk material intrusion into the left neural structures at the L4-5 level, and central disk protusion into the L5-S1 level into both left and right neuroforeman with out causing severe nerve root compression at this level.  My question:  I resently had an EMG and nerve conduction velocity tests which were normal.  Although NCVT were on the slow side of normal in both legs.  I'm glad I'm not showing nerve damage, but my attending physician told me that an EMG should not be taken as a sole basis of diagnosis.  I spoke with a friend of mine that went through this before.  She told me that an abnormal report is very accurate in determining sciatic involvement, but a normal report does not rule out a problem because of a somewhat high false negative error rate.  Is this true?  Thank you for your response.
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Thanks for your question.  The EMG/NCS studies are capable of demonstrating
two types of neuronal lesions: demyelination, and axon-loss/denervation.
Demyelination refers to the loss of the myelin sheath (fatty insulation coverings)
surrounding the nerve fibers.  A loss of myelin can be detected by NCS
(Nerve Conduction Studies, the part using electric stimuli) if the affected
segment is tested during the exam.  It does have the important caveat that
certain proximal (closer to the body trunk) segments of peripheral nerves
are technically difficult to test appropriately.
An axon-loss/denervation type lesion, as the name indicates, reflect the
loss of innervation of nerve fiber to the muscles.  This type of lesion
can be detected by either the NCS and the NEE (Needle Electromyographic
Exam).  Furthermore, NEE can often provide a differentiation between an
acute vs. chronic denervation by the examination of the muscle spontaneous
activity and the morphology of the recorded muscle electric activity.
The occurrence of false-negative in both NCS and NEE parts of the EMG
study are dependent on the severity of the spinal cord or peripheral nerve
lesion, and the number of peripheral nerves, and muscles studied during
the exam.  The bottom line is, if the nerve lesion is moderate to severe
and the appropriate nerves and/or muscles are examined, the test is RARELY
negative.  The exam is frequently such more sensitive and specific than
a clinical exam.
I hope this information is helpful.  Best of luck.
This information is provided for general medical education purposes only.
Please consult your doctor regarding diagnostic and treatment options.