Editing Evoked potential (section)

===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>