Intraoperative neurophysiological monitoring techniques assess the function of the brain, brainstem, spinal cord, cranial nerves, and peripheral nerves during the procedure. They are immensely valuable in the detection and prevention of neurological insult. Intraoperative monitoring is now becoming part of standard medical practices and routinely used during many surgical procedures, including the risk of neurological injury.

IONM employs a wide variety of physiological principles, each with a unique application and frequently used together in the same surgery, leading to improved patient outcomes. As the benefits of monitoring become apparent, the use of different neuromonitoring techniques during the additional surgical procedure has expanded. This activity reviews the different modalities of neurophysiological monitoring, their indications, and contraindications. This activity highlights the interprofessional team’s role in evaluating and improving care for patients in providing high-quality peri-operative care to detect and prevent neurologic injuries.

Each technique of intraoperative neurophysiological monitoring monitors a specific neural pathway.

01. Somatosensory Evoked Potential (SSEP): SSEP monitors the dorsal column–medial lemniscus pathway, which mediates tactile discrimination, vibration, and proprioception. Stimulation of sensory receptors in the skin initiates peripheral sensory nerves, which extend through the nerve root to the Ipsilateral  dorsal root ganglia at spinal levels. The projections from these first-order neurons form fasciculi gracilis and cuneatus, which carry impulse from the lower and upper extremities, respectively. The first synapse occurs in the lower medulla, then the impulses cross over at the level of the brainstem and form medial lemniscus. The impulse then ascends to the contralateral thalamus and relay information to the primary sensory cortex in the parietal lobe. In the upper extremities, the median and ulnar nerve are monitored, whereas, in the lower extremities, the posterior tibial and peroneal nerve are monitored.

02. Motor Evoked Potential (MEP): MEP monitor motor pathways, trans-cranial electrical stimulation elicits excitation of corticospinal projections at multiple levels. Depending on the intensity of stimulation and the placement of electrode, motor evoked potentials are generated at different levels of the brain, including superficial white mater just underneath the motor cortex, the deep white matter of the internal capsule, and pyramidal decussation. The electrical potential is recorded at the spinal cord or muscles. MEP is generated and transported via the pyramidal tract.

03. Visual Evoked Potential (VEP): VEP measures the functional integrity of the optic pathways from the retina to the brain’s visual cortex in response to light stimulus. Visual stimulus is converted into nerve signals in the retina. These signals are transmitted via the optic pathway to the brain, from the retina to the optic nerve, optic chiasma, optic tract, lateral geniculate body, optic radiation, and visual cortex occipital lobe.

04. Brainstem Auditory Evoked Potential (BAEP): BAEP monitors the function of the auditory nerve and auditory pathways in the brainstem. The auditory signal travels from the cochlear hair cell to the primary auditory cortex via the vestibulocochlear nerve, superior olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate body.

05. Electromyography (EMG): EMG monitors somatic efferent nerve activity and evaluates the functional integrity of individual nerves. EMG monitors intracranial, spinal, and peripheral nerves during surgeries. Depolarization of a motor nerve produces electrical potential within the muscles innervated by that specific nerve, and the generated electrical activity is monitored using subdermal or intramuscular electrodes.

06. Electroencephalography (EEG): The electrical activity measured by EEG is generated by groups of pyramidal neurons, which has cell bodies in the 3rd and 5th layer of the cerebral cortex.

EEG BRAIN ACTIVITY

An electroencephalogram (EEG) is a test that detects electrical activity in your brain using small, metal discs (electrodes) attached to your scalp. Your brain cells communicate via electrical impulses and are active all the time, even when you’re asleep. This activity shows up as wavy lines on an EEG recording. An EEG is one of the main diagnostic tests for epilepsy. EEG is a test that detects abnormalities in your brain waves, or in the electrical activity of your brain. During the procedure, electrodes consisting of small metal discs with thin wires are pasted onto your scalp. The electrodes detect tiny electrical charges that result from the activity of your brain cells. The charges are amplified and appear as a graph on a computer screen, or as a recording that may be printed out on paper.

Why it’s done

An EEG can determine changes in brain activity that might be useful in diagnosing brain disorders, especially epilepsy or any other seizure disorder. The EEG is used to evaluate several types of brain disorders. When epilepsy is present, seizure activity will appear as rapid spiking waves on the EEG. People with lesions of their brain, which can result from tumors or stroke, may have unusually slow EEG waves, depending on the size and the location of the lesion. The test can also be used to diagnose other disorders that influence brain activity, such as Alzheimer’s disease, certain psychoses, and a sleep disorder called narcolepsy. The EEG may also be used to determine the overall electrical activity of the brain (for example, to evaluate trauma, drug intoxication, or extent of brain damage in comatose patients). The EEG may also be used to monitor blood flow in the brain during surgical procedures.

Types of EEG

An EEG might also be helpful for diagnosing or treating the following disorders and different EEG have different recording timelines than others:

• Routine EEG: Routine EEG scans take 45 minutes. Your EEG technologist may ask you to breathe differently or look at flashing lights during the procedure. This is usually the standard for  all patients before graduating
• Prolonged EEG: A prolonged EEG test usually takes one hour and 30 minutes, but some types can last several days. A prolonged EEG gives your healthcare provider more information than a routine EEG. Your provider may use a prolonged EEG test to diagnose or manage seizures disorders. Prolonged EEGs use video.
• Ambulatory EEG: Ambulatory EEGs last one to three days. Ambulatory EEGs take place at home or at an EEG monitoring unit. During an ambulatory EEG, electrodes connect to a small EEG recorder. You can do most of your daily activities while the machine tracks your brain activity. You or family member can press a button if you have a seizure or event that your healthcare provider is trying to capture.
• Video EEG: The technician makes a video recording of you during your EEG. Video recording helps your healthcare provider see and hear what you’re doing when you have a seizure or other brain event. Your provider may also call this test EEG monitoring, EEG telemetry or video EEG monitoring.
• Sleep EEG:   A technician performs an EEG test while you sleep. Healthcare providers may order sleep EEGs if a routine EEG doesn’t offer enough information. You might have a sleep study to test for sleep disorders with a sleep disorders center.

When will I get my EEG results?

You will find out the results of your EEG at a follow-up appointment. Your healthcare provider will explain your EEG results to you.

What do the EEG results mean?

Your healthcare provider will review the brain wave patterns that your EEG identified. The test results describe patterns as normal or abnormal.

Abnormal patterns have different causes, such as:

• Alcoholism or substance use disorders (drug abuse).
• Bleeding in the brain.
• Brain swelling (edema).
• Brain tumor.
• Head injury.
• Migraines.
• Seizure disorder like epilepsy.
• Sleep disorder, such as sleep apnea or narcolepsy.
• Stroke.

What happens if I have an abnormal EEG reading?

Your healthcare provider may refer you to a specialist, like a neurologist. A specialist can diagnose, treat or manage your condition.

An EEG (electroencephalogram) is a safe, painless test that measures brain activity. An EEG can help your healthcare provider learn the cause of symptoms like seizures, confusion or memory loss. With a diagnosis, your provider can treat and manage a brain-related condition appropriately.

Electromyography– (EMG)

What are electromyography (EMG) and nerve conduction studies?

Electromyography (EMG) is a test that measure the electrical activity of muscles and nerves. Nerves send out electrical signals to make your muscles react in certain ways. As your muscles react, they give off these signals, which can then be measured.

• An EMG test looks at the electrical signals your muscles make when they are at rest and when they are being used.

EMG tests and nerve conduction studies can both help find out if you have a disorder of your muscles, nerves, or both. These tests can be done separately, but they are usually done at the same time.

Other names: electro diagnostic study, EMG test, electromyogram, NCS, nerve conduction velocity, NCV

What are they used for?

EMG and nerve conduction studies are used to help diagnose a variety of muscle and nerve disorders. An EMG test helps find out if muscles are responding the right way to nerve signals. Nerve conduction studies help diagnose nerve damage or disease. When EMG tests and nerve conduction studies are done together, it helps providers tell if your symptoms are caused by a muscle disorder or a nerve problem.

Why do I need an EMG test and a nerve conduction study?

You may need these tests if you have symptoms of a muscle or nerve disorder. These symptoms include:

• Muscle weakness
• Tingling or numbness in arms, legs, hands, feet, and/or face
• Muscle cramps, spasms, and/or twitching
• Paralysisof any muscles

What happens during an EMG test and nerve conduction study?

For an EMG test:

• You will sit or lie down on a table or bed.
• Your provider will clean the skin over the muscle being tested.
• A small needle electrode is inserted into the target muscle. A needle electrode is a special wire that a mild electric current flows through. You may have slight pain or discomfort when the electrode is inserted.
• The machine will record the muscle activity while your muscle is at rest.
• Then you will be asked to tighten (contract) the muscle slowly and steadily. The machine will record this activity.
• The electrode may be moved to record activity in different muscles.
• The electrical activity is recorded and shown on a video screen. The activity is displayed as wavy and spiky lines. The activity may also be recorded and sent to an audio speaker. You may hear popping sounds when you contract your muscle.

Nerve Condition Study – (NCS/NCV)

For a nerve conduction study:

• You will sit or lie down on a table or bed.
• Your provider will attach one or more electrodes to a certain nerve or nerves using tape or a paste. The electrodes, called stimulating electrodes, deliver a mild electrical pulse.
• Your provider will attach different types of electrodes to the muscle or muscles controlled by those nerves. These electrodes will record the responses to the electrical stimulation from the nerve.
• Your provider will send a small pulse of electricity through the stimulating electrodes to stimulate the nerve to send a signal to the muscle.
• This may cause a mild tingling feeling.
• Your provider will record the time it takes for your muscle to respond to the nerve signal.
• The speed of the response is called the conduction velocity.

If you are having both tests, the nerve conduction study will be done first.

Will I need to do anything to prepare for these tests?

Tell your health care provider if you have a pacemaker or cardiac defibrillator. Special steps will need to be taken before the test if you have one of these devices.

Wear loose, comfortable clothing that allows easy access to the test area or can be easily removed if you need to change into a clinic gown.

Make sure your skin is clean. Don’t use lotions, creams, or perfumes for a day or two before the test.

Are there any risks to the tests?

You may feel a little pain or cramping during an EMG test. You may have a tingly feeling, like a mild electric shock, during a nerve conduction study.

What do the results mean?

If your results were not normal, it can indicate a variety of different conditions. Depending on which muscles or nerves are affected, it may mean one of the following:

• Carpal tunnel syndrome, a condition that affects nerves in the hand and arm. It’s usually not serious, but can be painful.
• Herniated disc, a condition that happens when a part of your spine, called a disc, is damaged. This puts pressure on the spine, causing pain and numbness
• Guillain-Barre syndrome, an autoimmune disorder that affects the nerves. It can lead to numbness, tingling, and paralysis. Most people recover from the disorder after treatment
• Myasthenia gravis, a rare disorder that causes muscle fatigue and weakness.
• Muscular dystrophy, an inherited disease that seriously affects muscle structure and function.
• Charcot-Marie-Tooth disease, an inherited disorder that causes nerve damage, mostly in the arms and legs.
• Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. This is a progressive, ultimately fatal, disorder that attacks nerve cells in your brain and spinal cord. It affects all the muscles you use to move, speak, eat, and breathe.

If you have questions about your results, talk to your health care provider.

Nerve Condition Study – (NCS/NCV)

For a nerve conduction study:

• You will sit or lie down on a table or bed.
• Your provider will attach one or more electrodes to a certain nerve or nerves using tape or a paste. The electrodes, called stimulating electrodes, deliver a mild electrical pulse.
• Your provider will attach different types of electrodes to the muscle or muscles controlled by those nerves. These electrodes will record the responses to the electrical stimulation from the nerve.
• Your provider will send a small pulse of electricity through the stimulating electrodes to stimulate the nerve to send a signal to the muscle.
• This may cause a mild tingling feeling.
• Your provider will record the time it takes for your muscle to respond to the nerve signal.
• The speed of the response is called the conduction velocity.

  If you are having both tests, the nerve conduction study will be done first.

  Will I need to do anything to prepare for these tests?

  Tell your health care provider if you have a pacemaker or cardiac defibrillator. Special steps will need to be taken before the test if you have one of these devices.

  Wear loose, comfortable clothing that allows easy access to the test area or can be easily removed if you need to change into a clinic gown.

  Make sure your skin is clean. Don’t use lotions, creams, or perfumes for a day or two before the test.

  Are there any risks to the tests?

  You may feel a little pain or cramping during an EMG test. You may have a tingly feeling, like a mild electric shock, during a nerve conduction study.

  What do the results mean?

  If your results were not normal, it can indicate a variety of different conditions. Depending on which muscles or nerves are affected, it may mean one of the following:

  • Carpal tunnel syndrome, a condition that affects nerves in the hand and arm. It’s usually not serious, but can be painful.
  • Herniated disc, a condition that happens when a part of your spine, called a disc, is damaged. This puts pressure on the spine, causing pain and numbness
  • Guillain-Barre syndrome, an autoimmune disorder that affects the nerves. It can lead to numbness, tingling, and paralysis. Most people recover from the disorder after treatment
  • Myasthenia gravis, a rare disorder that causes muscle fatigue and weakness.
  • Muscular dystrophy, an inherited disease that seriously affects muscle structure and function.
  • Charcot-Marie-Tooth disease, an inherited disorder that causes nerve damage, mostly in the arms and legs.
  • Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig’s disease. This is a progressive, ultimately fatal, disorder that attacks nerve cells in your brain and spinal cord. It affects all the muscles you use to move, speak, eat, and breathe.

  If you have questions about your results, talk to your health care provider.

Evoked potentials are used to measure the electrical activity in certain areas of the brain and spinal cord. Electrical activity is produced by stimulation of specific sensory nerve pathways. These tests are used in combination with other diagnostic tests to assist in the diagnosis of neurological disorders

VISUAL EVOKED POTENTIALS: WHAT ARE THEY?
VEPs provide a sensitive indication of abnormal conduction in the visual pathway. Increases in retino-striate conduction time caused by processes such as demyelination can be detected by measuring the latency of this cortical response. Abnormalities in the amplitude and waveform of the VEPs may also be caused by the loss of axons in the pathway. VEPs are therefore widely used in the investigation of demyelinating disease, optic neuritis, and other optic neuropathies.

Brainstem auditory evoked potentials (BAEPs)
Brainstem auditory evoked potentials (BAEPs) are electrical field potentials generated by stimulation of the auditory pathways. With repetitive auditory stimulation, reproducible electrical potentials can be elicited and recorded from scalp electrodes. These waves are generated by specific brain regions and occur at predictable intervals. Clinically, this neurophysiological property is useful to evaluate the integrity of auditory pathways (plus, by extension, neighboring CNS structures) and to localize defective transmission. This chapter summarizes the methodology and clinical application of BAEPs in the investigation of disorders affecting auditory pathways and the surrounding brainstem

Somatosensory evoked response (SSER) test.
This test can detect problems with the spinal cord that cause numbness of the arms and legs. For this test, a healthcare professional attaches electrodes to your wrist, the back of your knee, or other locations. He or she will apply a mild electrical stimulus through the electrodes. Electrodes on your scalp then determine the amount of time it takes for the current to travel along the nerves to the brain.