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ORIGINAL ARTICLE
EMG: Myths and Facts
Joseph Feinberg, MD, MS
Published online: 13 February 2006
* Hospital for Special Surgery 2006
Electrodiagnostic testing, more commonly known as theEMG test,
to many is a Bblack box.^ It is a commonlyordered test that can
provide very definitive informationthat is often not well
understood. A large amount ofinformation is usually provided, yet
most clinicians relyprimarily on the concluding statements, which
may or maynot be well substantiated. Added insight into this
testdemands understanding some basic facts about the BEMG^test and
dispelling some of the myths.
The timing for ordering an EMG study
One of the most common myths about EMG tests is that onemust
wait 23 weeks following a nerve injury before reliableinformation
can be obtained. It is true that the degree ofmuscle denervation
that occurs after nerve injury can not bedetermined until Wallerian
degeneration is complete and thiscan take as short as 1 week or as
long as 4 weeks [1]. This is alength-dependent process so that the
longer the length of the
injured axon, the longer Wallerian degeneration will
take.However, when there is significant axonal injury or even
asevere neuropraxia (conduction block), a decreased rate ofmotor
unit recruitment will instantaneously become abnor-mal and a
skilled electromyographer can often determinethis. These early
findings can help to localize the site ofinjury and to some extent
the degree of injury.
Nerve conduction studies across an injured segment ofnerve will
also be abnormal immediately. The limitation hereis that one can
not differentiate a conduction block where thenerve/axon remains
intact from axonal degeneration or fromnerve/axon transection.
However, electrodiagnostic studiescan nevertruly tell whether there
is axon transection or not.
Determining neurological recovery
EMG testing can be used to demonstrate neurologicalrecovery.
Peripheral neurological recovery occurs in twoforms: true axonal
regeneration and the reinnervation ofdenervated muscle fibers by
terminal collateral sprouting.
True axonal regeneration occurs at the site where the nervehas
been injured and where axonal degeneration begins [2].For axonal
regeneration to occur, there must be an intactanterior horn cell
also known as the motor neuron cell bodyand an intact channel for
regeneration, the endoneurialtube. When axonal injury occurs, the
axon distal to the siteof injury undergoes Wallerian degeneration,
whereas theproximal axonal component usually remains intact.
Theproximal axon forms a bud that begins to regeneratedistally
through the endoneurial tube toward the denervatedmuscle. This
process occurs at a rate of 34 mm/day soaxonal regeneration is
length-dependent [3, 4].
Neurological recovery is also dependent on healthyviable muscle
tissue. This is necessary for the release ofnerve growth factors
from denervated muscle. Thesefactors act as a catalyst to stimulate
the axon to regenerate.If the denervated muscle becomes fibrotic,
these factorsmay no longer be released. Muscle tissue must also
remainviable and electrically active if a regenerating axon isgoing
establish a connection with a functional neuromus-
cular junction. Chronically denervated muscle will eventu-ally
become fibrotic and electrically inactive. This usuallyhappens
somewhere between 18 and 24 months.
The second form of peripheral neurological recovery isterminal
collateral sprouting. This is the reinnervation ofdenervated muscle
fibers by small nerve sprouts that arisefrom nearby uninjured and
intact axons [5]. The mostterminal portion of these axons generates
sprouts that growuntil they innervate the denervated muscle fibers.
Thisprocess occurs much earlier than true axonal
regenerationbecause the distances are much shorter. A motor unit
isdefined as all the muscle fibers innervated by one anteriorhorn
cell and its axon. There is a limit to how much
terminal collateral sprouting a single motor unit canundergo. It
has been estimated that a motor unit canreinnervate denervated
muscle tissue until it has innervatedfive times the amount of
muscle tissue it had originallyinnervated [6, 7].
EMG testing can identify and distinguish between thesetwo
processes. Electrophysiologically, a normal motor unithas a di- or
triphasic configuration. Polyphasic potentialsare abnormal
electrical configurations of a motor unit andcan be identified
following axonal injury. Two types ofpolyphasic potentials can form
following axonal degener-ation: (1) nascent potentials (Fig. 1a)
and (2) motor units
HSSJ (2006) 2:1921DOI 10.1007/s11420-005-0124-0
J. Feinberg, MD, MS (*)Physiatry Department at the Hospital for
Special Surgery,Hospital for Special Surgery,535 East 70th Street,
New York, NY 10021, USAe-mail: [email protected]
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formed from terminal collateral sprouting (Fig. 1b). Trueaxonal
regeneration leads to the formation of nascentpotentials, which are
usually low in amplitude, polyphasicin configuration, and can have
a short or normal andsometimes even long duration. Terminal
collateral sprout-ing always leads to the formation of
long-duration poly-phasics. Although both are polyphasic, their
configuration
is distinctly different and it is important to remember thatonly
the nascent potentials represent true axonal regener-ation. This
important distinction is often overlooked inmany EMG reports.
Understanding the difference betweenthe two and performing a
careful critical analysis shouldusually enable an electromyographer
to determine whattype of polyphasic is being identified and what it
indicates.
Prognosticating outcome
Frequently, after a neurological injury an individual willshow
signs of improved strength and function. Although
this could represent a form of neurological recovery,
otherfactors can lead to similar results. Adaptive
biomechanicalchanges, functional compensation, and muscle
substitutionpatterns can all lead to improved strength and
function.These may give the appearance of neurological recoverywhen
in fact there may be none. EMG needle testing candefinitively
answer this question. If there are no active
motor units in a given muscle, then neurological recoveryhas not
yet occurred.
Conversely, an individual may show no improvement instrength
several months after a nerve injury. The clinicalimpression is that
there has been no recovery. Thesefindings would increase the
suspicion of a nerve transec-tion. The decision of surgical
intervention must then beentertained. When axonal regeneration
occurs, the initialimmature nascent potentials are small and may
beincapable of generating a significant or a detectable force.EMG
testing at this stage may identify these earlyimmature motor units.
This not only indicates neurologicalrecovery when clinically there
appeared to be none, but
also prognosticates that additional functional recoveryshould
occur in time. This also establishes the fact thatthere is axonal
continuity.
Differentiating acute vs chronic injuries
The question of whether neurological deficits are the resultof
an acute or chronic injury often arises. Although nerveconduction
studies are very sensitive for localization, theyare not effective
for determining chronicity. It is the needleexam or EMG component
that can help define whether thedeficits are the result of acute
and/or chronic injury.
The earliest manifestation of axonal denervation is thepresence
of positive sharp waves and fibrillation potentials.As noted
earlier, it takes between 1 and 4 weeks after anerve injury before
they can be identified. These earlydenervation potentials will be
large in size on the EMGscreen. Months later, the fibrillation
potentials and positivesharp waves become smaller. Initially,
fibrillation poten-tials may be as larger as 1 mV [1] in size,
whereas themean fibrillation amplitude within the first 2 months
is600 mV [8]. At 6 months the mean amplitude drops by 50%to 300 mV,
and at 1 year will be 100 mV or smaller [9]. Asa rule, larger
amplitudes suggest nerve injury within thepast few months, whereas
the smaller amplitudes suggest
that injury occurred at least 6 months or a year ago.Motor unit
configuration can also be used to determinewhether a nerve injury
is acute or chronic. Polyphasicmotor units usually indicate
chronicity except in myopathicdisorders. As discussed earlier, it
usually takes severalmonths before nascent potentials appear. Based
on thelength of the distal component of the injured nervenecessary
for axonal regeneration, one can often roughlyestimate the amount
of time regeneration will take. Forexample, injury to the lower
trunk of the brachial plexusresulting in denervation of the hand
intrinsics may take ayear and a half before one would expect to see
a nascentpotential. An injury to the axillary nerve leading to
Fig. 1. a A nascent potential from axonal regeneration. b A
longduration polyphasic potential from terminal collateral
sprouting
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denervation of the deltoid may take less than 6 monthsbefore
nascent potentials could be identified.
The polyphasic motor units that result from terminalcollateral
sprouting as mentioned earlier look very differentfrom nascent
potentials and form much sooner than nascentpotentials. Generally,
the time frame for the sprouts toreinnervate denervated muscle
tissue should roughly be the
same regardless of the length of the injured nerve. This
isbecause the sprouts form from the most distal portion ofuninjured
axons and must only grow the short distancenecessary to connect to
the neuromuscular junction of thatsame muscle. The distance is
always short and so thesepolyphasics may be seen as early as 68
weeks. As the myelinof terminal collateral sprouts mature, these
long durationpolyphasics shorten and the main motor unit becomes
biggerin time as they begin to summate. Eventually, the motorunit
may grow extremely large and if enough muscle tissueis denervated
and reinnervated by sprouts from the samemotor unit, it may grow
large enough to become a Bgiantmotor unit.^The giant motor units
are greater than 8 mV and
usually take a long time to reach their final size, often
takingseveral years. They are classically seen in postpolio
patients,in slowly progressive inherited conditions, and in some
verychronic acquired axonal neuropathies.
Precise localization of the nerve injury
Localization of a peripheral nerve injury is one of the
mostcommon reasons a study is being performed. Both thenerve
conduction studies and EMG part of the exam help tolocalize the
site but the nerve conduction studies are moreprecise. This is
because one can usually stimulate the nerveat multiple sites. This
is known as segmental stimulation
[10 12]. At the point of injury there will be either
focalslowing or a sudden change in the waveform configuration.Nerve
conduction studies can often localize injury in early/mild
neuropraxias.
Localization via the needle exam is usually based onidentifying
muscles with the denervation potentials (fibril-lation potentials
and positive sharp waves). By identifyingmuscles with and without
the denervation potentials, onecan usually map out and
approximately estimate where theinjury is. The limitation here is
that most neuropraxias do notyield denervation potentials. However,
as mentioned previ-ously, a significant change in motor unit
recruitment can alsobe detected on the needle exam. Severe
neuropraxias can lead
to marked decreases in motor unit recruitment that can also
beused to map out and estimate the level of nerve injury.
Judging the quality of an EMG report
A good-quality EMG report should always include theactual
waveforms of sensory, motor, and F wave tests. Thisis in addition
to the numerical data that are usuallyincluded. The exclusion of
this prevents a more completeinterpretation of the report.
Another important indicator of a good-quality study is
theinclusion of temperature data. Temperature can have an im-
portant effect on the nerve conduction studies. Cold
temper-atures will slow down nerve conduction velocities and
couldlead to falsely prolonged distal latencies or slow
conductionvelocities. Skin temperature is monitored with a
temperatureprobe that connects to the EMG machine. Temperature
isusually measured alongside the recording electrodes. It isusually
recommended that temperature be at least 30-C in the
lower limbs and 32-C in the upper limbs. A heat lamp orwarm
compresses are used to warm the limb, but there areoccasions where
the desired temperature can not be obtainedand in these cases one
can use a conversion factor to accountfor the cold temperature.
Whether or not the ideal temperaturecan be obtained, an EMG report
without a temperature re-corded offers incomplete and possibly
inaccurate information.
Electrodiagnostic studies should always be considered
anextension of the physical exam. The final diagnosis and
sub-sequent clinical decisions made should reflect that. Studiesof
the contralateral asymptomatic side may sometimes beindicated for
comparison when there are abnormal findings.
Conclusion
Electrodiagnostic studies provide important
diagnosticinformation often essential for optimal patient
manage-ment. A clinician requesting these studies will benefit
fromknowing the proper timeframe for ordering them, whatinformation
can be obtained and is specifically needed, andwhether a study is
of good quality.
References
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