Evoked potentials (2. LF UK)

= Evoked Potential Practical: =

Introduction:
Evoked potentials are electrical activities that occur in the neutral pathways and structures as a response to external stimulations by light, sound, electric or other stimulations. Evoked potentials are polyphasic waves (having more than two phases) that often presents with an amplitude between 0.1-20 microA which are formed within 2-500 ms. Examining evoked potentials can help us to understand if the impulses is reaching the destination on time? When a person receives a visual stimulus such as light, the EEG system records concerning the occipital region demonstrates waves that are called ‘photic driving response’.

The three major types of evoked potentials used in clinical studies are visual (VEP), brainstem auditory (BAEP) and somatosensory (SSEP) evoked potentials.

From the physical point, one of the most important reaction is a changing of electrical potential of the membrane. The process of depolarization is entirely dependent upon the intrinsic electrical nature of most cells. When a cell is at rest, the membrane potential which it has in that moment is known as a resting potential. After a cell has established a resting potential, that cell has the capacity to undergo depolarization. During depolarization, the charge within the cell rapidly shifts from negative to positive. Formation of the action potential can be explained as a result of irritation of opened ion channels, thereby changing the permeability of the membrane to certain ions.

The difference between the threshold and the resting potential is usually from 5 to 15 mV. Thus, if the resting potential e.g. -70 mV, the threshold potential may be -60 mV. Formation of the action potential can be explained as a result of the membrane irritation, which is a reason for ion channels to open, thereby changing the permeability of the membrane to certain ions. The permeability for ions K + increases only a little, but the permeability for ions Na + increase 600 times. This results in a substantially faster flow of positive ions Na +from the extracellular medium into the cell than the ions K + in the opposite direction. This adjusts the negative charge inside the cell and potential rises from the original negative resting potential to zero and even occurs trans-polarization  (inside the cell becomes positive).

In the next phase (decreasing) the membrane again becomes permeable to ions K + than for ions of Na + and membrane potential returns towards resting level. Action potential originates in excitable cells in cases where the stimulus was enough big to reduce the absolute value from the resting potential to the threshold potential (threshold stimulus), or any serious stimulus (stimulus supraliminals).

Physiologists express this fact as the force of law "all or nothing". The quantity of ions, which expired the membrane during the action potential is very low, for about 3 · 10 -12 to 4 · 10 -12 moles per 1 cm2 of membrane area. The process of creating the action potential is associated with minimum energy requirements. Also, the period for which the ion channels are open is very short (less than 1 ms).Therefore, the action potential value of its maximum only approaches the equilibrium trans-membrane potential difference for Na +, which is not enough to fully establish.

After the action potential is followed by a very short time (about 1 ms), so called “absolute refractory period”, during which there isn’t any new action potential causes. This is followed by a relative refractory period (lasting from 10 to 15 ms) that may cause potential, but only by supraliminal stimulus.

Purpose of the Evoked Potential Tests:
are under general anesthesia
 * To assess the function of the nervous system
 * To aid in the diagnosis of nervous system lesions and abnormalities
 * To monitor the progression or treatment of degenerative nerve diseases
 * To monitor brain activity and nerve signals during brain or spine surgery, or in patients who
 * To assess brain function in a patient who is in a coma

Somatosensory Evoked Potentials:
They are used for evaluating the synaptic terminals extending towards cortex, by stimulation the peripheral sensory pathways via delivery of an electric current.

On the ‘early component’ we see potentials with very small amplitude (10-15ms) which arise from sub cortical structures (cerebellum, basal ganglia, spinal medulla and etc) and longer component, which starts nearly 20ms after the stimulation ( cortical structures ).

SSEP test is applied for testing the peripheral sensory fibers, spinal cord.

It helps us in diagnosis of plexus injuries, carpal/tarsal syndromes, nerve roots traumas, cervical and back pain, neuritis, neuropathies and etc.

Auditory Evoked Potentials:
This evoked potential is used for checking the auditory canals up to the primary cortex.

The auditory signal is applied through a classic earphone by delivering a wave of 100-200 micro-seconds duration for 10 times a second. The intensity of the sound applied for the stimulus is generally above the hearing threshold and range between 65-70dB. On the graph you can see waves, which were detected from these anatomical areas:

1 wave: distal action potential of acoustic nerve

2 wave: ipsilateral proximal acoustic nerve(or/and cochlear nerve activity)

3 wave: ipsilateral superior nucleus olivarius activity

4 wave: lateral lemniscal nuclear or axonal activity

5 wave: inferior colliculus activity (mesencephalic)

6 wave: medial geniculate body activity (thalamic)

7 wave: thalamo-cortical projection activity (cortical)

Using BAEP we can indicate hearing loss, balance disorders, headaches, head traumas, tinnitus and etc.

Visual Evoked Potentials:
Visual evoked potentials are obtained from the optic tract by recording the evoked potentials generated by retinal stimulation (flash-light with regular flashing intervals and etc). Mostly used method is a monitor with checkerboard, showing constant luminance at which the patient has to look. The checks alternate from black to white and white to black every 1-2 seconds. Each alternation forms a stimulation at the occipital lobe.

Main pathologies that may be seen in VEP are as follows:

Multiple sclerosis, demyelinating disease, optic nerve damage/neuritis, headaches, brain tumors/aneurysm and etc.

Literature Review:
A brainstem auditory evoked potential (BAEP) is caused by an acoustic stimulus, ie some kind of a sound, usually a series of clicks. The acoustic stimuli is transmitted from an acoustic transducer in the form of either an insert earphone or standard noise cancelling headphones. The stimuli generates a response, ie electrical signals, from the basilar region of the cochlea. The signal then travels along the auditory pathway from the cochlear nuclear complex to the colliculus (mid-brain). The cochlear nuclear complex carry information from the inner ear, the cochlea, and transmits it to higher regions of the auditory brainstem. The elicited waveform response is measured by surface electrodes typically placed at the vertex of the scalp and ear lobes. The amplitude, ie micro-voltage, of the signal is averaged and graphed against the time, in milliseconds. The graphs (waves) are recorded on an electroencephalogram (EEG) and examined. The waveform peaks, typically labeled I-V, usually occur within a 10-millisecond time period after an acoustic stimulus. Waveform I-II correspond to true action potentials, later waves may reflect activity in major brainstem auditory centers. The positive waveform peaks reflect combined sensory activity from axonal pathways in the auditory brainstem.

BAEP is a useful tool to evaluate conduction through the brainstem and the auditory nerve pathways that are not accessible by the means of other testing procedures. For example BAEP is used to assist in the diagnosis of hearing loss as well as detecting tumors and multiple sclerosis. BAEP also has the ability to inform the patient about lesions affecting the auditory nervous system.

Despite the advantages that Evoked potential provides, advancements in visual imaging limits the use of evoked potentials. Magnetic resonance imaging (MRI) today provide more benefits than Evoked potential (EP) in imaging and structural issues, EG provides a more functional aspect and thus provides information regarding the physiology of the anatomical pathways. It also provides a more cost effective choice and BAEP can also be used in the monitoring of patients during neurosurgery and sometimes in the intensive care unit (ICU).

The BAEP testing procedure is painless and side-effects are very rare. The most common side effect is skin irritation, often by either the gel or the electrodes themselves. The degree of the irritation depends on the patient. But many factors may influence the evaluation of the response given, factors like hearing loss, assymmetry of hearing loss, test parameters, distractions and other patient factors. These factors must be factored in when performing and analyzing BAEP results.

Listed below are the equipment necessary for a Brainstem Auditory Evoked Potential test:
1) Alcohol 2) Eletrocondutor creme 3) Machine to record brainactivity (Path Medical Sentiero// Advanced SOH06) 4) Headphones and electrodes to record brain activity 5) Example of the examination

Methodology:
The Equipment that is used for the Evoked Potential experiment is the Path Medical Sentiero // Advanced SOH06. '

1) The Electrodes will be attached as shown: [[File:M1.png|none|thumb|438x438px|Red Electrode: Mastoid Process

Black Electrode: Cheek

White Electrode: Forehead ]]

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i) Prepare the skin by wiping the areas where the electrodes will be placed with alcohol.

ii) Then apply a layer of gel/Vaseline.

iii) Place electrode patches on these areas.

iv) Attach the electrode clips to the electrode patches, according to their color.

3)

i) Plug the ABR cable into the white jack of the machine.

ii) Place the electro-acoustic transducers (headphones) on the patient, making sure the red side is on the patients right ear and the blue side is on the left ear.

4)

i) Turn the monitor on.

ii) Enter patients name and details.

iii) Advise the patient to lay as comfortably as possible and to avoid any muscle activity.

iv) Press “ABR”(Auditory Brainstem Response).

v) Then press “BAEP” (Brainstem Auditory Evoked Potential).

vi) Choose which ear will be tested. vii) Electrode Impedance is checked before BAER measurement is checked. When this is complete the play button will appear. Press this button to start the measurement.

5)

i) When the BAER has been measured, the results will be displayed as shown below: ii) Press “Enter” to save results and to go back to regular ABR screen display. Record results.

iii) Take off electrodes and turn the monitor off.

Conclusion:
The Brainstem Auditory Evoked Potential (BAEP) proves to be a noninvasive and a less costly method of assessing the functionality of the auditory nerve, cochlear nucleus, superior olive, and inferior colliculus of the brainstem compared to its counterpart MRI. The test is commonly conducted by the use of simple auditory stimuli or "clicks." Although this test has been exceptionally successful in detecting complications such as hearing loss, acoustic neuroma, multiple sclerosis, sub-arachnoid inflammation, and many other neurological disorders, it doesn't provide any solid information on how speech sounds are actually processed. A new method of testing undergoing research, Complex Auditory Brainstem Response (cABR), involves the usage of speech noises or music instead of the old-fashioned "clicks." This new method opens the door to a deeper understanding of the effects of peripheral hearing loss on speech processing in people with auditory processing problems and a stronger understanding of acoustic-based hearing complications.

Reference List:
Oraii, 2012.
 * “Electrophysiology - From Plants to Heart” edited by Saeed

“Clinical Biophysics”, Warren H. Green, 1991.
 * Anbar, Michael & Spangler Robert A & Scott Peter

edition”, Lippincott-Raven Publishers, 1997
 * Chiappa, Keith H. “Evoked Potentials in Clinical Medicine 3d


 * Walsh, P., Kane, N., Butler, S.. (2005, June). The clinical role of evoked potentials in Journal of Neurology, Neurosurgery & Psychiatry. Vol. 76,↵suppl. 2. ISSN: 1468-330X http://jnnp.bmj.com/content/76/suppl_2/ii16.full
 * Kouni, Sophia N, Constantinos Koutsojannis, Nausika Ziavra, and Sotirios Giannopoulos. "A Novel Method of Brainstem Auditory Evoked Potentials Using Complex Verbal Stimuli." NCBI. U.S. National Library of Medicine, 6 Aug. 2014. Web. Dec. & jan. 2016. 