evoked potential in multiple sclerosis

8/12/2019 Evoked Potential in Multiple Sclerosis

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Evoked Potential in Multiple Sclerosis

What are evoked potentials?

Evoked potentials are used to measure the electrical activity in certain areas of the brainand spinal cord. Electrical activity is produced by stimulation of specific sensory nervepathways. These tests are used in combination with other diagnostic tests to assist in the

diagnosis of neurological disorders.

Evoked potentials test and record how quickly and completely the nerve signals reach the

brain. Evoked potentials are used because they can indicate problems along nervepathways that are too subtle to show up during a neurologic examination or to be noticed

by the patient. The disruption may not even be visible on MRI exam.

These tests can be helpful in making the diagnosis of multiple sclerosis M!" and other

neurological disorders. Evoked potentials are used to show abnormalities in the functionof nerve pathways that can be caused by neurological disorders.

Types of evoked potentials

#ou are scheduled for one or more of the following tests. $lease wear loose%fittingclothing and have your hair clean. &o not apply hair sprays' oils' or gels.

Visual Evoked Potentials (VEP): This test will require that you observe a

flashing checkerboard pattern pro(ected on a monitor. If you have eyeglasses'

please bring them to use during testing. Electrodes will be placed on your scalpand shoulder. This procedure may take up to )* minutes to complete. $atients

unable to focus on the screen' such as young children' will be given special

goggles.

Auditory Evoked Potentials (AEP): This examination involves listening to

clicking noises generated in a set of headphones. The test requires the application

of a few electrodes to the scalp. This exam may take )* minutes to complete.

Median erve Sensory Evoked Potentials (MSEP): This test requires the

stimulation of your median nerve located near your wrist. The test requires the

application of electrodes on the scalp' neck' chest' and wrists. This exam may take

up to + to +,hours to complete.

Posterior Ti!ial erve Sensory Evoked Potentials (PTSEP): This test

requires stimulation of your posterior tibial nerve located near your ankle. This

test requires the application of electrodes on the scalp' back' hips' knees' and

inner ankles. This exam may take up to +, hours to complete.

Evoked Potential "ack Avera#in#: This test involves recording EE- and

multiply (erking movements +**%**". This test requires the application of

electrodes to the scalp and electrodes where the (erking movements occur. The

test can take up to / hours' depending on the frequency of the (erking movements.


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$lease contact your physician the day prior to testing if you are not having (erking

movements.

Evoked potentials are used to assist with the diagnosis of M! and other neurologicaldisorders. !pecially trained neurologists or neurophysiologists will interpret the results.

Information provided by these tests will be considered' along with other findings fromclinical history' neurological examination' MRI' and other clinical or laboratory

information when diagnosing a medical condition.

EVOKED POTENTIAL STUDY

0isual Evoked $otential !tudy 0E$"

1rainstem 2uditory Evoked $otential !tudy 12E!$"

!omatosensory Evoked $otential !tudy !!E$"

Electronic averaging has permitted the recording of low amplitude potentialsevoked by different types of sensory stimulus. These responses are commonly

used in the diagnosis of Multiple !clerosis M.!"' a relapsing and remitting

condition which is characteri3ed by patchy inflammation affecting the myelin

sheath of the central nervous system. The ob(ect is to demonstrate

abnormalities in regions of the nervous system not known from clinical

manifestations to be involved %silent lesions. 4or example' the finding of

abnormal 0E$s or 12E$s in a patient with paraparesis would demonstrate

abnormalities in at least sites of the central nervous system characteristic of

M.!.

Visual Evoked Potentials (VEPs)
http://www.friendswithms.com/evoked_potential_study.htm#vephttp://www.friendswithms.com/evoked_potential_study.htm#brainstemhttp://www.friendswithms.com/evoked_potential_study.htm#somatosehttp://www.friendswithms.com/evoked_potential_study.htm#brainstemhttp://www.friendswithms.com/evoked_potential_study.htm#vephttp://www.friendswithms.com/evoked_potential_study.htm#brainstemhttp://www.friendswithms.com/evoked_potential_study.htm#somatosehttp://www.friendswithms.com/evoked_potential_study.htm#brainstem


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The visual evoked potential is agross electrical signal recordedfrom the occipital cortex inresponse a systematic change in

some visual event such as aflashing light or an alternatingchequered pattern. The recordingelectrode is placed over theoccipital cortex and the amplitudeand latency of the waveformgenerated can be measured.

This method currently provides themost sensitive means of detecting

subclinical lesions of the opticnerve and may enable a diagnosisof M.S. to be made at an earliertime. Abnormality in the V! isalso encountered withcompressive lesions of the anteriorvisual pathways. "n the paediatricage group# the flash V! may beused as a screening test for the

visual pathway.

xamples of normal V!s in adultswith $%&$% vision.

Brainste Auditor! Evoked Potentials (BAEPs)

"f one could record directly from several different levels of thesubcortical auditory pathway# one would see during the first '%milliseconds following an appropriate acoustic stimulus a series

of potentials corresponding to the sequential activation ofperipheral# pontomedullary# pontine and midbrain portions ofthe pathway. (hen these acoustic nerve and brainstempotentials are volume conducted to surface recordingelectrodes at the vertex and earlobe# they form a compositeseries of waves known as brainstem auditory evokedpotentials. Since the amplitude of these responses are very


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small )about '&'%% of spontaneous * activity+# specialcomputer averaging technique is required.

The neurologic applications of ,A!s have been proven usefulin the diagnosis of M.S.. "t may be used as a screening test forearly detection of acoustic neuromas and in the assessment ofcomatose patients. -ike the V!s# it may also be a usefulscreening test for hearing in the paediatric age group such asneonatal screening and those who do not cooperate sufficientlywith behavioural testing.

Soatosensor! Evoked Potentials (SEPs)

S!s can be recorded from

electrodes placed over thesomatosensory cortex of thecerebral hemispherecontralateral to the appliedstimulus. Automaticaveraging techniques areused to facilitate recording ofthese small potentials. "n theupper limb# the stimulus is

usually applied to the medianor ulnar nerve whereas in thelower limb the tibial nerve ismost frequently used.Stimulation of the mediannerve at the wrist elicits aresponse at the brachialplexus at about msecs)/+# the cuneate nucleus '$msec )/'$+ and

somatosensory cortex at$%msec )/$%+.

/ormal S!s elicited bystimulation of the mediannerve and recording at rb0spoint )!+# the 1th and $nd


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cervical spine )2V1 and 2V$+and somatosensory cortex)23+.

"lini#al uses o$ SEPs

The S!s findings may help in the detection and locali4ation oflesions of the central somatosensory pathways but are notpathognomonic of specific diseases. "n Multiple Sclerosis )MS+the presence of S! abnormalities may reveal subclinicallesions involving the central somatosensory pathways thusaiding in early diagnosis. "n patients with definite MS# theincidence of S! abnormality is about 5%6 whereas in the

category of possible MS# the yield is only about 7%6. Theinterpretation of electrophysiologic results must thereforealways be taken in a clinical context.

Evoked potential

2n evoked potentialor 5evoked response5" is an electrical potentialrecorded from the

nervous systemof a humanor other animalfollowing presentation of a stimulus' as

distinct from spontaneous potentials as detected byelectroencephalographyEE-" or

electromyographyEM-".

Evoked potential amplitudestend to be low' ranging from less than amicrovoltto several

microvolts' compared to tens of microvolts for EE-' millivolts for EM-' and often close

to a volt for E6-. To resolve these low%amplitude potentials against the background of

ongoing EE-' E6-' EM- and other biological signals and ambient noise' signalaveragingis usually required. The signal is time%locked to the stimulus and most of the

noiseoccurs randomly' allowing the noise to be averaged out with averaging of repeated

responses.7+8

!ignals can be recorded fromcerebral cortex'brain stem'spinal cordandperipheral

nerves.9sually the term 5evoked potential5 is reserved for responses involving either

recording from' or stimulation of' central nervous system structures. Thus evokedcompound motor action potentials 6M2$" or sensory nerve action potentials !:2$" asused in nerve conduction studies:6!" are generally not thought of as evoked potentials'

though they do meet the above definition.
http://en.wikipedia.org/wiki/Electrical_potentialhttp://en.wikipedia.org/wiki/Nervous_systemhttp://en.wikipedia.org/wiki/Nervous_systemhttp://en.wikipedia.org/wiki/Humanhttp://en.wikipedia.org/wiki/Humanhttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Stimulus_(physiology)http://en.wikipedia.org/wiki/Electroencephalographyhttp://en.wikipedia.org/wiki/Electroencephalographyhttp://en.wikipedia.org/wiki/Electroencephalographyhttp://en.wikipedia.org/wiki/Electromyographyhttp://en.wikipedia.org/wiki/Electromyographyhttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Microvolthttp://en.wikipedia.org/wiki/Microvolthttp://en.wikipedia.org/wiki/Microvolthttp://en.wikipedia.org/wiki/Electrocardiogramhttp://en.wikipedia.org/wiki/Averaginghttp://en.wikipedia.org/wiki/Averaginghttp://en.wikipedia.org/wiki/Noisehttp://en.wikipedia.org/wiki/Evoked_potential#cite_note-0http://en.wikipedia.org/wiki/Cerebral_cortexhttp://en.wikipedia.org/wiki/Cerebral_cortexhttp://en.wikipedia.org/wiki/Cerebral_cortexhttp://en.wikipedia.org/wiki/Brain_stemhttp://en.wikipedia.org/wiki/Spinal_cordhttp://en.wikipedia.org/wiki/Spinal_cordhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Nerve_conduction_studyhttp://en.wikipedia.org/wiki/Electrical_potentialhttp://en.wikipedia.org/wiki/Nervous_systemhttp://en.wikipedia.org/wiki/Humanhttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Stimulus_(physiology)http://en.wikipedia.org/wiki/Electroencephalographyhttp://en.wikipedia.org/wiki/Electromyographyhttp://en.wikipedia.org/wiki/Amplitudehttp://en.wikipedia.org/wiki/Microvolthttp://en.wikipedia.org/wiki/Electrocardiogramhttp://en.wikipedia.org/wiki/Averaginghttp://en.wikipedia.org/wiki/Noisehttp://en.wikipedia.org/wiki/Evoked_potential#cite_note-0http://en.wikipedia.org/wiki/Cerebral_cortexhttp://en.wikipedia.org/wiki/Brain_stemhttp://en.wikipedia.org/wiki/Spinal_cordhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Peripheral_nervous_systemhttp://en.wikipedia.org/wiki/Nerve_conduction_study


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Sensory evoked potentials

!ensory evoked potentials !E$" are recorded from the central nervous systemfollowing

stimulation of sense organsfor example' visualevoked potentials elicited by a flashinglight or changing pattern on a monitor;78auditoryevoked potentials by a click or tone

stimulus presented through earphones" or by tactile or somatosensoryevoked potential!!E$" elicited by tactile or electrical stimulation of a sensory or mixed nerve in theperiphery. They have been widely used in clinical diagnosticmedicine since the +


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. Intraoperative somatosensory evoked potentials !!E$s" performed either alone'

or in combination with motor evoked potentials ME$s" are considered medically

necessary for monitoring the integrity of the spinal cord to detect adverse changesbefore they become irreversible during spinal' intracranial' orthopedic' or vascular

procedures' when the following criteria are met?

+. 2 specially trained physician who is not a member of the surgical teamcontemporaneously interprets the intraoperative evoked potentials during

the operation; and

. The evoked potential monitoring is performed in the operating room bydedicated trained technician; and

A. The physician who performs the interpretation is monitoring no more than

three procedures at the same time; and

/. The physician who performs the interpretation provides direct' immediatecommunication of intraoperative evoked potential results to the technician

and surgeon during the operation.

Intra%operative !E$ monitoring' with or without ME$s' may be appropriate forthe following types of surgery not an all%inclusive list"?

. !pinal !urgeries?

+. 6orrection of scoliosis or deformity of the spinal cord involving

traction on the cord. &ecompression of the spinal cord where function of the spinal cord

is at risk

A. Removal of spinal cord tumors

/. !urgery as a result of traumatic in(ury to the spinal cord. !urgery for arteriovenous 20" malformation of the spinal cord

). Intracranial !urgeries?

+. 6orrection of cerebral vascular aneurysms

. &eep brain stimulationA. Endolymphatic shunt for MeniereBs disease

/. Microvascular decompression of cranial nerves e.g.' optic'

trigeminal' facial' auditory nerves". >val or round window graft

). Removal of cavernous sinus tumors

=. Removal of tumors that affect cranial nerves

F. Resection of brain tissue close to the primary motor cortex andrequiring brain mapping


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=. 0ascular !urgeries?

+. 2rteriography' during which there is a test occlusion of the carotid

artery. 6irculatory arrest with hypothermia does not include surgeries

performed under circulatory bypass such as 621-' and ventricularaneurysms"

A. &istal aortic procedures' where there is risk of ischemia to thespinal cord

/. !urgery of the aortic arch' its branch vessels' or thoracic aorta'

including carotid artery surgery' when there is risk of cerebralischemia.

Intra%operative !!E$s with or without ME$s are considered experimental and

investigational for all other indications.

:ote? &epending on the clinical condition being investigated' it may be medicallynecessary to test several nerves in one extremity and compare them with the

opposite limb.

:ote? Intraoperative evoked potential studies have no proven value for lumbar

surgery below distal to" the end of the spinal cord; the spinal cord ends at @+%@.

:ote? $ost%operative !E$ or ME$ monitoring is not considered medicallynecessary for individuals who have undergone intra%operative !E$ or ME$

monitoring.

:ote? The :IM%!pine !ystem received +*k" clearance from the 4&2 in Gune**A. It offers types of monitoring modalities? i" electromyography' and ii"ME$.

:ote on documentation requirements? The physicianBs !E$ report should note

which nerves were tested' latencies at various testing points' and an evaluation ofwhether the resulting values are normal or abnormal.

http?DDwww.aetna.comDcpbDmedicalDdataD+**H+

evoked potential in multiple sclerosis

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