When to order an EMG/NCS?
Electromyogram (EMG) and Nerve Conduction Studies (NCS) provide
information about the function of the peripheral nervous system
above and beyond what can be gleaned from a neurological exam.
Specifically, it can determine:
- a particular
- focal neuropathy
horn cell disease
(e.g. axonal vs. demyelinating neuropathy)
- particular nerve
fibers involved, even if subclinical (sensory/motor/autonomic)
- occasionally a very
specific diagnosis can be rendered (e.g. distal myotonic discharges and myopathic changes would be pathognomonic of myotonic dystrophy)
of a diagnosis can be rendered with an EMG/NCS?
For the most part, EMG/NCS identifies the localization of the
pathology (e.g. polyneuropathy vs. myopathy) rather than the
etiology (diabetic neuropathy vs. uremic neuropathy, etc.).
However, occasional pathognomonic findings are present (see
Question 1 above).
I order an EMG, an NCS, or both?
For nearly all indications, both EMG and NCS are required to
provide the most accurate assessment of the situation. For example,
many physicians think it is sufficient to simply order just
NCS in the evaluation of carpal tunnel syndrome (CTS). Although
slowing of median conduction across the wrist is the most specific
electrodiagnostic finding in CTS, EMG is helpful to provide
additional information such as the degree of axonal involvement,
the presence of active vs. chronic denervation, and distinguishing
CTS from other disorders that may clinically look the similar
(cervical radiculopathy, proximal median neuropathy, thoracic
outlet syndrome, etc).
the difference between NCS and NCV?
NCS (Nerve Conduction Study) and NCV (Nerve Conduction Velocity)
both refer to the same procedure. “Nerve Conduction Velocity”
is an older term, used originally when slowed conduction was
the only parameter being evaluated. The more modern term “Nerve
Conduction Study” reflects the notion that there are many
other parameters besides just conduction velocity that are being
the sensitivity of the test?
The sensitivity of detecting any given condition with these
tests varies depending on several factors, such as severity,
timing, presence of co-existent neuromuscular abnormalities,
and the particular studies chosen by the electromyographer.
For example, the sensitivity for detecting carpal tunnel syndrome
is excellent (above 95%), but only if the person performing
the analysis takes the time to do additional procedures (e.g.
median-vs.-ulnar comparison studies) in addition to the routine
tests. For radiculopathies, the EMG/NCS can be entirely normal
if only the sensory root is involved.
case of radiculopathies, does an EMG/NCS need to be ordered
if an MRI has already been done?
Imaging studies, like MRIs, show the anatomy of the relevant
structures whereas EMG and NCS are functional tests. As such,
they provide complementary types information, and often both
imaging and EMG/NCS may be required to arrive at the most accurate
assessment of a particular situation.
the best time to order an EMG/NCS after an abrupt onset of symptoms?
Following an acute axonal injury, a variety of different abnormalities
typically appear in a relatively characteristic sequence. Some
abnormalities can appear immediately (e.g. abnormal F-waves)
whereas other changes can take longer (e.g. fibrillations after
several weeks, neurogenic motor units after several months).
In general, waiting at least 3 weeks following an acute injury
is recommended in order to allow time for the appearance of
findings that are most helpful in localizing the lesion.
can be ordered in the evaluation of neuromuscular junction disorders
(e.g. myasthenia gravis)?
Repetitive nerve stimulation (RNS) takes a few minutes to perform
but only has a sensitivity of approximately 70% in myasthenia
gravis, depending on the severity of the weakness; RNS serves
as a useful quick screen for these conditions. “Single
fiber EMG” (SF-EMG) is generally considered to be the
most sensitive test for myasthenia gravis, but requires a needle
to be in the tested muscle for approximately 2 hours. The anti-acetylcholine
receptor antibody assay is more specific than SF-EMG and only
requires venipuncture. In general, SF-EMG is most useful if
myasthenia gravis is clinically suspected but both antibodies
and RNS are negative. The Tensilon test can also provide a diagnosis
of myasthenia gravis, but only if the patient has a reliable,
objective deficit, such as ptosis or ophthalmoparesis. Tensilon
tests are performed with continuous EKG monitoring at the Cedars-Sinai
Neurophysiology Laboratory (310-423-6841).
Routine nerve conduction studies require brief electrical stimulations
on the skin surface. Most patients tend to find them a little
startling at first, and describe the sensation as mild discomfort.
The EMG requires and thin sterile needle (usually 28 gauge)
to be placed for around 30 seconds in different muscles; it
is also described by most as mild discomfort. However, every
patient tolerates these procedures differently. People always
have the option of immediately stopping the test, and there
should be no lasting discomfort. Most patients do not require
pretreatment with medication, but some do benefit from their
referring physician prescribing them a mild analgesic or anxiolytic
to be taken an hour before the test (assuming they have a ride
to and from the office).
EMG be done if the patient is anticoagulated?