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The Motor Evoked Potential MEP Test Explained

Medically reviewed by Heidi Moawad, M.D. — By Jill Seladi-Schulman, Ph.D. on October 13, 2022Your nerves transmit messages within the brain, as well as among the brain, spinal cord, and the body. In order to do this, they use a combination of electrical and chemical signaling. Motor neurons are nerve cells located in the brain, spinal cord, and peripheral nerves. The neurons throughout the pathway travel from the motor areas of the brain to the muscles telling your muscle cells to contract or shorten, leading to movement. Motor evoked potentials (MEPs) are electrical signals that are produced when the motor regions of the brain or spinal cord are naturally simulated. A doctor can also stimulate these areas and measure the MEPs for a variety of different purposes. Below, we’ll go over what exactly an MEP test is and why it may be done. Then, we’ll cover what to expect from the procedure and how results are interpreted. Keep reading to discover more.

What is a motor evoked potential MEP test

An MEP test measures electrical signaling through the motor pathways of your nervous system. It does this by stimulating the area of the brain that controls movement. This area is called the motor cortex. When the motor cortex is stimulated, it makes an electrical signal. This signal can then be detected in muscles at other points in the body. An MEP test gives information on electrical signals moving from your brain to a target muscle group. Because of this, it can help a doctor to know if there are problems with specific motor pathways.

Why is a motor evoked potential MEP test done

A doctor can use the MEP test to monitor the motor pathways of your nervous system. There are a couple of reasons why this may be important. First, MEP monitoring can be used during surgery affecting the spinal cord. This type of surgery carries the risk of damage to the spinal cord and nearby nerves. MEP testing is sometimes done during epilepsy surgery and during surgical procedures that involve the treatment of Parkinson’s disease as well. Since these surgeries carry a risk of brain damage, this test is used so the neurosurgeon will know exactly where the motor areas of the brain are located. As such, MEP monitoring can help assure a surgeon that motor pathways are still functioning as usual. MEP readings are typically taken at critical moments of the procedure. The second use for MEP testing is in the diagnosis and monitoring of certain neurological conditions. Evaluating the results of MEP testing can help determine if a condition has caused damage along certain motor pathways.

How do you prepare for a motor evoked potential MEP test

Overall, minimal preparation is necessary for an MEP test. Some good general rules of thumb are:Make sure that your hair is clean and free of styling products.Remember to remove all jewelry.Try to wear loose, comfortable clothes. If the MEP test is being done during surgery, a doctor will give you specific directions on how to prepare for your surgery. This can include things like fasting or adjusting medications prior to your procedure. Be sure to follow these directions carefully.

What happens during a motor evoked potential MEP test

During an MEP test, the motor cortex is stimulated. This can happen in one of two ways — transcranial electric or magnetic stimulation. In transcranial electric stimulation, electrical impulses are applied to electrodes placed at specific places on your scalp. These impulses then pass through the skull and stimulate the motor cortex. In transcranial magnetic stimulation, an electromagnetic coil is placed next to a specific area on your head. The magnetic field made by this coil is able to stimulate the motor cortex. In both of these methods, the electrical signal made in response to stimulation moves from the brain to the spinal cord. From there, it travels to the peripheral nervous system, where it eventually makes its way to the target muscle group. The MEP reading is typically measured using an electrode placed over the target muscle group. Electromyography (EMG) testing or nerve conduction velocity (NCV) testing might also be done to help assess nerve function.

Are there risks from a motor evoked potential MEP test

MEP tests are generally safe. However, there are some side effects and risks to be aware of. The electrical impulses used in the electrical stimulation method can be painful. As such, this method may be used more often during surgery when you’re under anesthesia. Magnetic stimulation is more likely to be used when you’re awake. Muscle contractions that happen during the test have to potential to lead to injury. Some of the most common types of injuries in this scenario is biting the lips or tongue. Other potential risks of the MEP test include things like:skin irritation from the placement of electrodesscalp burnsarrhythmiaseizuresnerve cell damage

How are the results reported

The results of an MEP test are reported on a type of graph called a waveform. There are two important aspects of the waveform:Amplitude: The amplitude measures the intensity of the electrical signal, typically measured in millivolts (mV). Amplitude is measured on the vertical part of the graph.Latency: The latency is the speed of the electrical signal, often measured in milliseconds (ms). It shows how long it takes the signal to travel from the brain to the target muscle group and is measured on the horizontal part of the graph. A normal amplitude indicates a normal signal, while a lower amplitude indicates a weaker signal. Lower amplitudes can mean that damage has happened to a specific motor pathway. Meanwhile, a longer latency means that the electrical impulse took a longer time to travel from the brain to the target muscle group. This can mean that the motor pathway under consideration is disrupted or damaged in some way.

Frequently asked questions

Now let’s look into the answers to some of the additional questions that you may have about MEP testing.

Can certain factors impact the results of MEP testing

Certain medications and physiological changes can interfere with MEP measurements. When this happens, the results obtained from MEP may not be as accurate. Medications that may interfere with MEP are:anesthetics, both inhaled and intravenousmuscle relaxantsbenzodiazepinesopioids The physiological changes that can impact MEP readings are:significant increases or decreases in body temperaturelow blood pressurelow blood oxygen levelselectrolyte imbalances

Are there other types of evoked potential tests

Yes. Your sensory nerves send sensory information, such as that collected through sight, hearing, and touch, to your brain. As such, there are also evoked potential tests that assess how your brain responds to sensory information. These include the:visual evoked potential (VEP) testbrainstem auditory evoked potential (BAEP) testsensory evoked potential (SEP) test

Bottom line

MEP testing measures electrical signaling in a specific motor pathway when your brain’s motor cortex is stimulated. The results are given on a type of graph called a waveform, which shows the intensity and latency of the signal. You may receive MEP testing as a part of can be used during spinal cord surgery and brain surgery to help monitor for complications. It may also be used to diagnose or monitor certain neurological conditions, such as amyotrophic lateral sclerosis (ALS). Overall, MEP testing is generally safe. A doctor will give you a detailed breakdown of the different benefits and risks associated with MEP measurement prior to your test. Last medically reviewed on October 13, 2022

How we vetted this article

SourcesHistoryHealthline has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations. We avoid using tertiary references. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.Doyal A, et al. (2022). Motor evoked potential.
ncbi.nlm.nih.gov/books/NBK580548/Evoked potential tests. (2015).
swbh.nhs.uk/wp-content/uploads/2012/07/Evoked-potential-tests-ML3910.pdfKim G-W, et al. (2015). Can motor evoked potentials be an objective parameter to assess extremity function at the acute or subacute stroke stage?
ncbi.nlm.nih.gov/pmc/articles/PMC4414972/Legatt AD, et al. (2016). ACNS guideline: Transcranial electrical stimulation motor evoked potential monitoring.
acns.org/UserFiles/file/ACNS_Guideline___Transcranial_Electrical.9_v1.pdfMotor evoked potential. (n.d.).
sciencedirect.com/topics/neuroscience/motor-evoked-potentialStrommen JA, et al. (2016). Motor evoked potentials.
academic.oup.com/book/35514/chapter-abstract/305237381Triccas LT, et al. (2018). Measurement of motor-evoked potential resting threshold and amplitude of proximal and distal arm muscles in healthy adults: A reliability study.
ncbi.nlm.nih.gov/pmc/articles/PMC6453085/Yperman J, et al. (2022). Motor evoked potentials for multiple sclerosis, a multiyear follow-up dataset.
nature.com/articles/s41597-022-01335-0Zoccolella S, et al. (2020). Motor-evoked potentials in amyotrophic lateral sclerosis: Potential implications in detecting subclinical UMN involvement in lower motor neuron phenotype.
link.springer.com/article/10.1007/s00415-020-10073-5Our experts continually monitor the health and wellness space, and we update our articles when new information becomes available. Current Version Oct 13, 2022 By Jill Seladi-Schulman, PhD Edited By Debbie Nurmi Medically Reviewed By Heidi Moawad, MD Copy Edited By Christina Baswell Share this articleMedically reviewed by Heidi Moawad, M.D. — By Jill Seladi-Schulman, Ph.D. on October 13, 2022

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