Editing Evoked potential (section)

== Sensory evoked potentials ==
Sensory evoked potentials (SEP) are recorded from the [[central nervous system]] following stimulation of [[sense organ]]s, for example, [[visual]] evoked potentials elicited by a flashing light or changing pattern on a monitor,<ref name=osheaetal>O'Shea, R. P., Roeber, U., & Bach, M. (2010). Evoked potentials: Vision. In E. B. Goldstein (Ed.), Encyclopedia of Perception (Vol. 1, pp. 399-400, xli). Los Angeles: Sage. {{ISBN|978-1-4129-4081-8}}</ref> [[Auditory system|auditory]] evoked potentials by a click or tone stimulus presented through earphones), or tactile or [[somatosensory]] evoked potential (SSEP) elicited by tactile or electrical stimulation of a sensory or mixed nerve in the [[peripheral nervous system|periphery]].  Sensory evoked potentials have been widely used in [[clinical diagnosis|clinical diagnostic]] medicine since the 1970s, and also in intraoperative neurophysiology monitoring (IONM), also known as surgical neurophysiology.

There are three kinds of evoked potentials in widespread clinical use: auditory evoked potentials, usually recorded from the scalp but originating at [[brainstem]] level; visual evoked potentials, and [[somatosensory evoked potentials]], which are elicited by electrical stimulation of peripheral nerve. Examples of SEP usage include:<ref name=Kwasnica2011 />
* SSEP can be used to locate lesions such as peripheral nerve or spinal cord.
* VEP and BAEP can supplement [[neuroimaging]] as part of workups to diagnose diseases such as [[multiple sclerosis]].
* Short latency EPs such as SSEP, VEP, and BAEP can be used to indicate prognosis for traumatic and anoxic brain injury.  Early after anoxic brain injury, no response indicates mortality accurately.  In traumatic brain injury, abnormal responses indicates failure to recover from coma.  In both types of injury, normal responses may indicate good outcome.  Moreover, recovery in responses often indicates clinical recovery.

Long and Allen<ref>{{cite journal |vauthors=Long KJ, Allen N | year = 1984 | title = Abnormal Brainstem Auditory Evoked Potentials Following Ondine's Curse | journal = Arch. Neurol. | volume = 41 | issue = 10| pages = 1109–1110 | pmid = 6477223 | doi=10.1001/archneur.1984.04050210111028}}</ref> were the first investigators to report the abnormal brainstem auditory evoked potentials (BAEPs) in an alcoholic woman who recovered from [[acquired central hypoventilation syndrome]].  These investigators hypothesized that their patient's [[brainstem]] was poisoned, but not destroyed, by her chronic alcoholism.

===Visual evoked potential===
Visual evoked potential (VEP) is an evoked potential elicited by presenting light flash or pattern stimulus which can be used to confirm damage to visual pathway<ref>{{cite book | year = 2013 | title = visual-evoked potential (VEP) | edition = 9th | work = Mosby's Medical Dictionary | publisher = Elsevier Mosby | editor-last1 = O’Toole | editor-first1 = Marie T | isbn = 978-0-323-08541-0 | pages = 1880 }}</ref>
including [[retina]], [[optic nerve]], [[optic chiasm]], [[optic radiations]], and [[occipital cortex]].<ref name=HammondGrafton2011>{{cite book | last1 = Hammond | first1 = Flora | last2 = Grafton | first2 = Lori | title = Visual Evoked Potentials | year = 2011 | work = Encyclopedia of Clinical Neuropsychology | editor-last1 = Kreutzer | editor-first1 = Jeffrey S | editor-last2 = DeLuca | editor-first2 = John | editor-last3 = Caplan | editor-first3 = Bruce  | publisher = Springer | isbn = 978-0-387-79947-6 | doi = 10.1007/978-0-387-79948-3 | pages = 2628}}</ref>
One application is in measuring infant's visual acuity.  Electrodes are placed on infant's head over [[visual cortex]] and a gray field is presented alternately with a checkerboard or grating pattern.  If the checker's boxes or stripes are large enough to be detected, VEP is generated; otherwise, none is generated.  It's an objective way to measure infant's visual acuity.<ref>{{cite book | year = 2013 | last1 = Goldstein | first1 = E Bruce | chapter = Chapter 2: The Beginning of Perceptions | edition = 9th | title = Sensation and Perception | publisher = WADSWORTH: CENGAGE Learning | isbn = 978-1-133-95849-9 | at = Method: Peferential looking, p. 46}}</ref>

VEP can be sensitive to visual dysfunctions that may not be found with just physical examinations or MRI, even if it cannot indicate etiologies.<ref name=HammondGrafton2011 />
VEP may be abnormal in [[optic neuritis]], [[optic neuropathy]], [[demyelinating disease]], [[multiple sclerosis]], [[Friedreich’s ataxia]], [[vitamin B12 deficiency|vitamin B<sub>12</sub> deficiency]], [[neurosyphilis]], [[migraine]], ischemic disease, tumor compressing the optic nerve, [[ocular hypertension]], [[glaucoma]], [[diabetes]], [[toxic amblyopia]], aluminum neurotoxicity, [[Manganism|manganese intoxication]], [[retrobulbar neuritis]], and [[brain injury]].<ref>{{harvp | Hammond | Grafton | 2011}} cited {{cite web | last1 = Huszar | first1 = L | year = 2006 | title = Clinical utility of evoked potentials | url = http://www.emedicine.com/neuro/topic69.htm | publisher = eMedicine | access-date = 2007-07-09 }}</ref>
It can be used to examine infant's visual impairment for abnormal visual pathways which may be due to delayed maturation.<ref name=HammondGrafton2011 />

The P100 component of VEP response, which is the positive peak with the delay about 100 ms, has a major clinical importance.  The visual pathway dysfunction anterior to the optic chiasm maybe where VEPs are most useful.  For example, patients with acute severe optic neuritis often lose the P100 response or have highly attenuated responses.  Clinical recovery and visual improvement come with P100 restoration but with an abnormal increased latency that continues indefinitely, and hence, it maybe useful as an indicator of previous or subclinical optic neuritis.<ref name=Aminoff2001>{{cite book | work = Harrison's Principles of Internal Medicine | year = 2001 | edition = 15th | last1 = Aminoff | first1 = Michael J | editor-last1 = Braunwald | editor-first1 = Eugene | editor-last2 = Fauci | editor-first2 = Anthony S | editor-last3 = Kasper | editor-first3 = Dennis L | editor-last4 = Hauser | editor-first4 = Stephen L | editor-last5 = Longo | editor-first5 = Dan L | editor-last6 = Jameson | editor-first6 = J Larry | publisher = McGraw-Hill | isbn = 0-07-007272-8 | title = 357. ELECTROPHYSIOLOGIC STUDIES OF THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS | at = EVOKED POTENTIALS }}</ref>

In 1934, Adrian and Matthew noticed potential changes of the occipital EEG can be observed under stimulation of light. Ciganek developed the first nomenclature for occipital EEG components in 1961. During that same year, Hirsch and colleagues recorded a visual evoked potential (VEP) on the occipital lobe (externally and internally), and they discovered amplitudes recorded along the [[calcarine fissure]] were the largest. In 1965, Spehlmann used a checkerboard stimulation to describe human VEPs. An attempt to localize structures in the primary visual pathway was completed by Szikla and colleagues. Halliday and colleagues completed the first clinical investigations using VEP by recording delayed VEPs in a patient with retrobulbar neuritis in 1972. A wide variety of extensive research to improve procedures and theories has been conducted from the 1970s to today and the method has also been described in animals.<ref>{{Cite journal|last1=Strain|first1=George M.|last2=Jackson|first2=Rose M.|last3=Tedford|first3=Bruce L.|date=1990-07-01|title=Visual Evoked Potentials in the Clinically Normal Dog|journal=Journal of Veterinary Internal Medicine|language=en|volume=4|issue=4|pages=222–225|doi=10.1111/j.1939-1676.1990.tb00901.x|pmid=2401969|issn=1939-1676|doi-access=free}}</ref>

====VEP Stimuli====

The diffuse-light flash stimulus is rarely used nowadays due to the high variability within and across subjects. However, it is beneficial to use this type of stimulus when testing infants, animals or individuals with poor visual acuity. The checkerboard and grating patterns use light and dark squares and stripes, respectively. These squares and stripes are equal in size and are presented, one image at a time, via a computer screen.

====VEP Electrode Placement====

Electrode placement is extremely important to elicit a good VEP response free of artifact. In a typical (one channel) setup, one electrode is placed 2.5&nbsp;cm above the [[inion]] and a reference electrode is placed at Fz. For a more detailed response, two additional electrodes can be placed 2.5 &nbsp;cm to the right and left of Oz.

====VEP Waves====

[[File:VEP-normal.gif|thumb|Normal visual evoked potential]]

The VEP nomenclature is determined by using capital letters stating whether the peak is positive (P) or negative (N) followed by a number which indicates the average peak latency for that particular wave. For example, P100 is a wave with a positive peak at approximately 100 ms following stimulus onset. The average amplitude for VEP waves usually falls between 5 and 20 microvolts.

Normal values are depending on used stimulation hardware (flash stimulus vs. [[cathode-ray tube]] or [[liquid crystal display]], checkerboard field size, etc.).

====Types of VEP====

Some specific VEPs are:
* Monocular pattern reversal (most common)
* Sweep visual evoked potential
* Binocular visual evoked potential
* Chromatic visual evoked potential
* Hemi-field visual evoked potential
* Flash visual evoked potential
* LED Goggle visual evoked potential
* Motion visual evoked potential
* [[Multifocal visual evoked potential]]
* Multi-channel visual evoked potential
* Multi-frequency visual evoked potential
* Stereo-elicited visual evoked potential
* [[Steady state visually evoked potential]]

===Auditory evoked potential===
Auditory evoked potentials (AEP) can be used to trace the signal generated by a sound through the ascending auditory pathway.  The evoked potential is generated in the cochlea, goes through the [[cochlear nerve]], through the [[cochlear nucleus]], [[superior olivary complex]], [[lateral lemniscus]], to the [[inferior colliculus]] in the midbrain, on to the [[medial geniculate body]], and finally to the [[auditory cortex|cortex]].<ref>{{cite book|author1=Musiek, FE|author2=Baran, JA|name-list-style=amp|year=2007|title=The Auditory system|location=Boston, MA|publisher=Pearson Education, Inc.}}</ref>

Auditory evoked potentials (AEPs) are a subclass of [[event-related potentials]] (ERPs). ERPs are brain responses that are time-locked to some "event", such as a sensory stimulus, a mental event (such as recognition of a target stimulus), or the omission of a stimulus. For AEPs, the "event" is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus, such as different tones, speech sounds, etc.

[[Brainstem auditory evoked potential]]s are small AEPs that are recorded in response to an auditory stimulus from electrodes placed on the scalp.

AEPs serve for assessment of the functioning of the [[auditory system]] and [[neuroplasticity]].<ref name="Kumar2016">{{cite journal | last1=Sanju | first1=Himanshu Kumar | last2=Kumar | first2=Prawin | title=Enhanced auditory evoked potentials in musicians: A review of recent findings | journal=Journal of Otology | volume=11 | issue=2 | year=2016 | issn=1672-2930 | pmid=29937812 | pmc=6002589 | doi=10.1016/j.joto.2016.04.002 | pages=63–72}}</ref>
They can be used to diagnose learning disabilities in children, aiding in the development of tailored educational programs for those with hearing and or cognition problems.<ref>{{cite journal|title=Auditory evoked potential: a proposal for further evaluation in children with learning disabilities|first1=Ana C. F.|last1=Frizzo|journal=Frontiers in Psychology|doi=10.3389/fpsyg.2015.00788|date=10 June 2015|volume=6|page=788|pmid=26113833|pmc=4461809|doi-access=free}}</ref>

===Somatosensory evoked potential===
[[File:SEPmedL.gif|thumb|Normal somatosensory evoked potential (tibial nerve)]]
[[Somatosensory evoked potential|Somatosensory evoked potentials]] (SSEPs) are EP recorded from the brain or spinal cord when stimulating peripheral nerve repeatedly.<ref name=McElligott2011>{{cite book | last1 = McElligott | first1 = Jacinta  | title = Somatosensory Evoked Potentials | year = 2011 | work = Encyclopedia of Clinical Neuropsychology | editor-last1 = Kreutzer | editor-first1 = Jeffrey S | editor-last2 = DeLuca | editor-first2 = John | editor-last3 = Caplan | editor-first3 = Bruce  | publisher = Springer | isbn = 978-0-387-79947-6 | doi = 10.1007/978-0-387-79948-3 | pages = 2319–2320 }}</ref> SSEPs are used in [[neuromonitoring]] to assess the function of a patient's [[spinal cord]] during [[surgery]].  They are recorded by stimulating peripheral nerves, most commonly the [[tibial nerve]], [[median nerve]] or [[ulnar nerve]], typically with an [[electrical]] stimulus.  The response is then recorded from the patient's [[scalp]].

Although stimuli such as touch, vibration, and pain can be used for SSEP, electrical stimuli are most common because of ease and reliability.<ref name=McElligott2011 />
SSEP can be used for prognosis in patients with severe traumatic head injury.<ref>{{harvp | McElligott |2011}} cited {{cite book | last1 = Lew | first1 = HL | last2 = Lee | first2 = EH | last3 = Pan | first3 = SS L | last4 = Chiang | first4 = JYP  | year = 2007 | title = Electrophysiological assessment techniques: Evoked potentials and electroencephalography | editor-last1 = Zasler | editor-first1 = ND | editor-last2 = Katz |editor-first2 = DL | editor-last3 = Zafonte | editor-first3 = RD | work = Brain Injury Medicine. Principles and Practice }}</ref>
Because SSEP with latency less than 50 ms is relatively independent of consciousness, if used early in comatose patient, it can predict outcome reliably and efficiently.<ref>{{harvp | McElligott |2011}} cited {{cite journal | last1 = Lew | first1 = HL | last2 = Dikman | first2 = S | last3 = Slimp | first3 = J | last4 = Temkin | first4 = N | last5 = Lee | first5 = EH | last6 = Newell | first6 = D | display-authors = etal | year = 2003 |title = Use of somatosensory evoked potentials and cognitive event related potentials in predicting outcome in patients with severe traumatic brain injury | journal = American Journal of Physical Medicine & Rehabilitation | volume = 82 | issue = 1 | pages = 53–61| doi = 10.1097/00002060-200301000-00009 | pmid = 12510186 | s2cid = 45096294 }}</ref>
For example, comatose patients with no responses bilaterally has 95% chance of not recovering from coma.<ref>{{harvp | McElligott |2011}} อ้างอิง {{cite book | last1 = Robinson | first1 = L. R. | year = 2004 | editor-last1 = Kraft | editor-first1 = GL | editor-last2 = Lew | editor-first2 = HL  | title = Somatosensory evoked potentials in coma prognosis | work = PM&R clinics of North America | volume = 15 | issue = 1 | pages = 43–61 | location = Philadelphia | publisher = WB Saunders | doi = 10.1016/s1047-9651(03)00102-5 | pmid = 15029898 }}</ref>
But care should be taken analyzing the result.  For example, increased sedation and other CNS injuries such as the spinal cord can affect SEP.<ref name=McElligott2011 />

Because of the low [[amplitude]] of the signal once it reaches the patient's scalp and the relatively high amount of electrical noise caused by background [[EEG]], scalp muscle [[Electromyography|EMG]] or electrical devices in the room, the signal must be averaged.  The use of averaging improves the [[signal-to-noise ratio]].  Typically, in the operating room, over 100 and up to 1,000 averages must be used to adequately resolve the evoked potential.

The two most looked at aspects of an SSEP are the amplitude and latency of the peaks.  The most predominant peaks have been studied and named in labs.  Each peak is given a letter and a number in its name.  For example, N20 refers to a negative peak (N) at 20ms.  This peak is recorded from the cortex when the median nerve is stimulated.  It most likely corresponds to the signal reaching the [[somatosensory cortex]].  When used in intraoperative monitoring, the latency and amplitude of the peak relative to the patient's post-intubation baseline is a crucial piece of information.  Dramatic increases in latency or decreases in amplitude are indicators of neurological [[:wikt:dysfunction|dysfunction]].

During surgery, the large amounts of [[anesthetic]] gases used can affect the amplitude and latencies of SSEPs.  Any of the [[halogenated]] agents or [[nitrous oxide]] will increase latencies and decrease amplitudes of responses, sometimes to the point where a response can no longer be detected.  For this reason, an anesthetic utilizing less halogenated agent and more intravenous hypnotic and narcotic is typically used.

====Clinical Uses====

SEP findings do not by themselves lead to a specific diagnosis, and organic diseases cannot necessarily be excluded with normal SEP findings. Findings must be interpreted in the context of the patient’s clinical presentation. Evaluating the peripheral responses with SEPs could contribute to the diagnosis of peripheral nerve damage.

Furthermore, SEPs could be abnormal in different pathologies such as [[multiple sclerosis]] (MS), hereditary spinocerebellar degenerations, hereditary spastic paraplegia, AIDS and vitamin B<sub>12</sub> or vitamin E deficiency. In patients with MS, evoked potential findings often complement findings on MRI.

In the acute stage after a traumatic spinal injury or brain trauma, the absence of SEP responses do not correlate with prognosis. However, an early return to normal or preserved cortical responses in the subacute stage correlate with a positive outcome.

SEPs can be helpful to evaluate subcortical and cortical function in comatose patients and are less sensitive to sedative drugs than EEG. SEP´s and BAEP´s together are the best tools to assist in the confirmation of brain death in comatose patients

====Clinical consideration in children====

As in the adult, SEP findings in combination with the clinical assessment and EEG findings can contribute to the determination of prognosis in comatose children. In high risk newborns, tracking SEP findings over time can be helpful for outcome prognostication. Several neurodegenerative disorders have abnormal findings in spinal and cortical SEP components. Moreover, compressive lesions on the spine (e.g. Arnold-Chiari malformation or mucopolysaccharidosis) are associated with abnormal SEPs, which may precede abnormalities on MRI.

====Laser evoked potential====

Conventional SSEPs monitor the functioning of the part of the somatosensory system involved in sensations such as touch and vibration. The part of the somatosensory system that transmits pain and temperature signals is monitored using laser evoked potentials (LEP). LEPs are evoked by applying finely focused, rapidly rising heat to bare skin using a laser. In the central nervous system they can detect damage to the [[spinothalamic tract]], lateral [[brain stem]], and fibers carrying pain and temperature signals from the [[thalamus]] to the [[cerebral cortex|cortex]]. In the peripheral nervous system pain and heat signals are carried along thin ([[C fiber|C]] and [[A delta fiber|A delta]]) fibers to the spinal cord, and LEPs can be used to determine whether a [[neuropathy]] is located in these small fibers as opposed to larger (touch, vibration) fibers.<ref name = Treede>{{cite journal |vauthors=Treede RD, Lorenz J, Baumgärtner U |title=Clinical usefulness of laser-evoked potentials |journal=Neurophysiol Clin |volume=33 |issue=6 |pages=303–14 |date=December 2003 |pmid=14678844 |doi= 10.1016/j.neucli.2003.10.009|s2cid=18486576 }}</ref>