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By:
Tennyson W. Lee, M.D.,
Center for Orthopaedic Surgery and Sports Medicine
Think of EMG's as an extension of the physical
examination. When someone has the clinical symptoms and physical
findings for carpal tunnel, you may use it to tell you how severe the
carpal tunnel is, as well as, how the disorder progresses over time,
and, if the diagnosis is in doubt, prove or disprove it.
EMG's are actually composed of two parts: the
nerve conduction studies (NCS) as well as the electromyogram (EMG).
The NCS part involves studying the nerves
themselves. Electricity is sent from one part of the nerve to a
different part of the nerve. The time it takes to travel from point A to
point B is divided over the measured distance between the two points
(mm/msec) and a conduction velocity calculated.
Also, the amount of nerve fibers transmitting can
be measured by the size (amplitude), of the electrical pulse received.
Thus, the health of the nerve can be measured objectively.
The EMG part involves studying the
muscles. A pin is inserted into different muscles and the electrical
activity in the muscles is picked up.
When there is muscle damage, as from a
muscle disorder (myopathy), the electrical signals are altered. The
normal triphasic wave (similar in appearance to the QRS wave of an EKG),
becomes shorter and more narrow.
Also, when there is damage to the nerves
innervating the muscle (ie. radiculopathies), the normal triphasic
wave is again altered. Depending on how long the damage has occurred,
different changes can be seen.
These include increased wave
complexity, wave size, and number of waves.
Moreover, when either nerve or muscle
are damaged, there will be spontaneous electrical activity
(fibrillations), not normally seen.
Common reasons that physicians order
EMG's include numbness, paresthesias (pins/needles or other strange
sensations), weakness, and pain that are otherwise unexplained and/or
need further evaluation.
Examples include: carpal tunnel
syndrome (median neuropathy at the wrist), cubital tunnel syndrome (ulnar
neuropathy at the elbow), cervical, thoracic, or lumbar radiculopathies,
peripheral neuropathy, myopathies, cranial nerve palsies, brachial
plexopathies, neuromuscular disorders (ALS, myotonic conditions, etc.)
Please feel free to
contact me if you have any question I can help with.
317-888-1051
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