Engineering in Medicine: Evoked Potentials
Today, there are many forms of imaging available to the medical profession. Among the more well-known types include x-rays, ultrasounds, CTscans, PETscans, and the most groundbreaking, the MRI. One lesser known form of imaging is the technique of evoked potentials. The evoked potential uses stimulation of the body to force activity in the brain. Using electrodes, a clinician can take signals directly from the brain without any intrusive methods. The electrodes remain on the surface of the skin and unlike many other imaging techniques, evoked potentials do not involve any type of ionizing radiation that would be dangerous to the body.
Since evoked potentials involve the stimulation of the body, they are directly related to the arousing of the sense. There are three main types of evoked potentials: visual, auditory, and somatosensory. Although there are also experimental studies being conducted with gustatory and olfactory evoked potentials, vision, hearing, and touch have shown the most successful clinical uses. In this paper, visual evoked potentials will be covered in depth and auditory and somatosensory evoked potentials will be reviewed in brief. The most common clinical uses of this technique are to obtain ideas of brain activity by monitoring the size of amplitudes and latencies and subsequently diagnosing diseases and disorders concerning neural activity.
The most common potential obtained is the visual evoked potential (VEP).
Before explaining how a VEP is obtained, however, the basic physiology of the visual system must be understood. Each individual eye receives light from both the right and left visual fields. Upon entering the eye, they hit the retina and then they are transferred to the optic nerve. The optic nerve is the eye’s connection to the brain. The optic nerve of each eye cross at the optic chiasm, where the information from both the right and left visual fields are separated. Thereafter, they are directed into the opposite hemisphere of the brain via the optic tract. The information is then brought to the occipital cortex at the rear of the brain. This is also called the primary visual cortex.
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Today, a more modern form of the VEP is the multifocal visual evoked potential, which through different testing techniques and forms of analysis can take signals from various locations, forming a more complete image of the brain’s activity. When conducting this test, a patient will be seated and will be facing a monitor. On the monitor a checkerboard pattern will be displayed and the squares on the pattern will reverse randomly from black to white. Typically, the stimulus is circular and consists of 60 sectors with each sector containing 16 checks, for a total of 960 checks. They will alternate very quickly (every few milliseconds) and will serve as the stimulus for the patient. Attached to the patient’s scalp will be five electrodes. The most important of these electrodes is placed at a landmark bump on the back of the head called the inion. This bump is approximately where a fold in the brain, called the calcarine fissure, is located. It is at the calcarine fissure where the primary visual cortex is located. In addition to the inion electrode, three other electrodes are placed on the scalp of the back of the head to create three channels with the inion and, altogether, six channels can be derived mathematically. Also, a ground electrode is placed on the forehead. One eye on the patient is covered and each eye is stimulated for approximately seven minutes. Two runs are typically made causing the entire procedure to last about 45 minutes. Running the test twice is necessary to increase the signal to noise ratio. Once the test is complete, a computer software using complicated algorithms is used to analyze the signals received through the electrodes. A computer can derive a VEP response in each eye for each of the corresponding 60 sectors found in the stimulus.
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The VEP does not have a standard response. Every individual will respond to the stimulus with different wave patterns based on sex, age, race, in addition to a number of other factors. Moreover, the same individual may give different responses if tested on two different days. A drawback to this means that one’s VEP cannot be compared to a standard response in order to measure normality. Nevertheless, the VEP for each patient’s left and right eyes are almost identical. This allows physicians to compare the two eyes and then evaluate subjects based on abnormalities from eye to eye.
There are a few very important applications to the VEP. The most important application deals with the condition of Optic Neuritis. In optic neuritis, the optic nerve becomes inflamed and a patient’s vision can become blurry to partially or completely blind. Optic neuritis is usually a temporary condition and most patients recover their vision after a week. The cause of optic neuritis can be attributed to multiple problems; however, the most prevalent syndrome found in relation to optic neuritis is Multiple Sclerosis (MS).
In MS, the myelin, a substance that insulates the nerves in the brain and spinal cord, is destroyed. This is the possible reason for the inflammation that causes optic neuritis. During an episode of optic neuritis, the VEP is very obviously abnormal. After it has subsided, however, a patient with MS would still show some abnormalities in the latency of the VEPs. The eye that previously had optic neuritis would show signals that are slower than the other eye. This difference in time indicates demyelination and therefore multiple sclerosis.
There are many other diseases and conditions for which the VEP can be used for. Besides comparing time difference between the left and right eyes, one can also measure the difference in amplitude. This can lead to diagnoses of glaucoma or tumors that are pressing on the optic nerve. In addition, the VEP has been able to diagnose different kinds of migraines as well as optic neuropathy.
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Although the MRI can be used to view most abnormalities in the brain, the VEP has its advantages. The VEP is more sensitive than the MRI, especially concerning disorders of the optic nerve. Moreover, the VEP costs substantially less, both in equipment and running of the test. Overall, the visual evoked potential is a leading imaging technique used in diagnosing disorders concerning the entire visual system as well as diagnosing diseases such as multiple sclerosis.
Another form of evoked potentials is the auditory evoked potential, otherwise known as the brainstem auditory evoked potential (BAEP).
This is mainly used in the clinical setting. To understand how the BAEP works, a basic understanding of the auditory system must be established. Sound enters through the outer ear, which consists of the lobe and the ear canal. The main purpose of the outer ear is to collect sound waves and send them to the middle ear. The most important part of the middle ear is the ear drum and three little bones called the ossicles. The ear drum receives the vibrations and sends the vibrations through the ossicles which bring the sound to the inner ear. The middle ear is also responsible for controlling sound pressure. In the inner ear, the vibrations lead to the cochlea where tiny hair cells cause signals that go to the vestibulocochlear nerve in the brain.
To obtain the BAEP, about three of four electrodes are used. They are placed on each earlobe and two are placed on the scalp of the patient. This setup creates two primary channels. The patient then wears headphones which provide stimuli which consist of clicks in each ear. These clicks produce five different signals for each eye. The first wave represents the signal from the vestibulocochlear nerve. The second and third are from the cochlear nuclei and the superior olivary nucleus, respectively. These are both part of the brainstem section of the auditory pathway. The fourth and fifth waves are received from the lateral lemniscus and inferior colliculcus. These are found further up in the brain, closer to the auditory cortex. The first, third, and fifth waves are the most important when it comes to clinical use. When the BAEP is taken, it is similar to the VEP in that there is not a set normal standard as the response depends on the attributes of the patient.
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There are important clinical applications for the BAEP as well. The most obvious use is to measure the extent of hearing loss based on the amplitudes of the signals (fifth wave).
With this information, physicians can decide how to treat the hearing loss and to what degree. A person who is completely deaf would exhibit flat-line responses. The BAEP can also detect lesions in the auditory cortex or any place in the auditory pathway. These abnormalities could also be attributed to tumors or acoustic neuromas, which are non-cancerous growths on the vestibulocochlear nerve in the brain. The BAEP is especially useful in detecting brain tumors on the brainstem; however, the BAEP will only recognize tumors once they are larger than one centimeter. For this reason, the MRI is more useful when attempting to detect tumors in the brain.
For infants, the BAEP is extremely useful. Since it is difficult to tell when an infant has any hearing loss, the BAEP is an objective method to discover if a child is partially or completely deaf. Furthermore, it can determine if a child needs to have special hearing aids. If a child grows up with impaired hearing, they can develop irregular speaking voices. The BAEP test will prevent children from developing these speech disorders.
When children have sustained serious injury to their head, they fall into a post-traumatic coma. The BAEP can be used to determine the likelihood of survival. Although the normal and abnormal BAEP signals varied with survival rates, all children who do not show any responses to the stimuli have died.
When a person undergoes general anesthesia, he/she loses consciousness; however, there can be different degrees to the depth of anesthesia. Recently, the BAEP has been being used to determine the level of anesthesia. An Auditory Evoked Potential Index (AEPex) is being used to set a system to determine depth of anesthesia and it is being programmed into the anesthesia monitors in operating rooms. This method is still in its experimental stages. Altogether, the brainstem auditory evoked potential is a very important clinical tool as well as a method to understand other conditions of the body.
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A third type of evoked potential is the somatosensory evoked potential (SSEP).
In this test, the body is stimulated by touch or electrical signal and electrodes are able to record the signals created by the brain. The basic physiology of the somatosensory system is slightly more complicated. While the eyes and ears send their nerves directly to the brain, the somatosensory system must travel farther to reach its destination. The peripheral receptors in the body pick up stimuli and these nerves lead to the spinal cord. From the spinal cord, the response is then directed to the brain where it reaches the primary somatosensory cortex. In the body, there are different kinds of receptors for different kinds of stimuli. There are thermoreceptors which respond to changes in temperature, nociceptors which monitor pain, proprioceptors which monitor the position of the body, chemoreceptors which are sensitive to chemicals, and mechanoreceptors which deal with mechanical changes in the body.
There are two main types of somatosensory evoked potential tests: the upper limb SSEP and the lower limb SSEP. In the upper limb SSEP, an electrode is placed on the wrist as well as electrodes on the head and back. The patients’ fingers would be stimulated with low voltage stimulation. Since placing the electrode on the wrist would only monitor from one point on the spinal cord, the electrode is also placed on other parts of the arm including closer to the elbow to read signals going to the lower spinal cord as well. The lower limb SSEP is done by placing the electrode on the lower leg and the stimulation on the ankle. To test other parts of the spinal cord, the electrode can be placed on the thigh and stimulation can occur at the knee. Although the thought of sending a current into a patient may seem extreme, the voltage is kept very low and the electric pulse only causes the patient a small twitch in the limb that was stimulated.
Many of the clinical motivations of SSEP are similar to those of auditory and visual evoked potentials. The SSEP can be used to detect lesions in the entire pathway leading up to the brain including those in the spinal cord. Furthermore, it can trace the location of a tumor if the amplitudes of an SSEP are depressed.
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One of SSEP’s primary advantages are found inside the operating room. The SSEP can monitor the status of the spinal chord through stimulation. During surgery on the spinal chord, the surgeon can keep track of the condition of the spinal chord. If the SSEP begins to depress, the surgeon can retrace and repair. This is a common technique in scoliosis repair surgery and any other surgeries directly affecting the spinal cord.
The SSEP has a second role in the operating room. When operating on the brain, the exact location of the primary somatosensory cortex needs to be established. This is done by placing electrodes directly on the brain. The lips or fingers are used as points of stimulation. By monitoring the responses, surgeons can determine where the somatosensory cortex is. In addition to its functions in the operating room, the SSEP has an assortment of other diseases and syndromes in which it can be used as a form of diagnosing. Although many of the SSEP’s functions have been replaced by the MRI, it still has many valuable functions in the medical field.
The disease that is associated with all three types of evoked potentials is multiple sclerosis. Although the visual evoked potential is the most useful in diagnosing MS, somatosensory and auditory evoked potentials are not far behind. When the three methods are used together, they can become very useful. The three tests are usually in sync with each other when determining if a patient has MS. Unfortunately, each test takes about an hour to perform, so conducting all three tests would take a very long time (as they are rarely done together).
In conclusion, the evoked potential is a very useful form of imaging. In addition to the many other types of imaging today, the evoked potential is crucial for diagnosing such things as multiple sclerosis as well as lesions on nervous tissue. The fundamental basis for the entire technique is the neuron and its ability to transmit signals. Using electrodes and a form of stimulating the senses, the evoked potential is able to accurately determine the response of the brain to certain stimuli by recording the signals of neurons. Visual evoked potentials being the most prevalent, they are the most capable for helping to diagnose multiple sclerosis because of the visual episodes of optic neuritis. The brainstem auditory evoked potentials are very important for diagnosing hearing loss in infants and determining levels of anesthesia in the operating room. Lastly, the somatosensory evoked potential, although useful in diagnosing lesions, is most useful in the operating room to navigate the brain and to maintain the spinal cord’s vitals. The three techniques are useful both in diagnosing disorders as well as serving as a useful tool in surgery. In conclusion, the evoked potential is an enormously valuable technology that is widely used in today’s medical profession.
Works Cited
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4.Hood, Donald C., Greenstein, Vivienne C. “Multifocal VEP and Ganglion Cell Damage: Applications and Limitations for the Study of Glaucoma.” Progress in Retinal and Eye Research 22 (2003).
5.Hood, Donald C., Odel, Jefferey G. “Tracking the Recovery of Local Optic Nerve Function after Optic Neuritis: A Multifocal VEP Study.” Investigative Ophthalmology & Visual Science November 2000, vol. 41, no. 12.
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