Overview of Intraoperative Neurophysiological Monitoring
During a surgical procedure, a technique called intraoperative neurophysiological monitoring (IONM) is often used to check the health of the nerves and the patient’s level of consciousness. With this approach, doctors can closely watch the state of nerve tissue throughout surgery and even locate essential nerve structures. The goal of IONM is to quickly spot any harm to the nerves during surgery. This allows the surgeon to respond immediately, either to prevent or limit any lasting damage to the nerves, thereby avoiding issues with nerve functions after the surgery. To make sure this monitoring is effective and not confused by other factors, a specific kind of anesthesia technique is used that doesn’t interfere with the nerve signals.
There are several different types of IONM techniques, each of which is designed to monitor a particular nerve pathway. These include:
1. Evoked potentials, such as somatosensory evoked potential (SSEP), motor evoked potential (MEP), brainstem auditory evoked potential (BAEP), and visual evoked potential (VEP). These are techniques that track nerve responses to different types of stimuli.
2. Electroencephalography (EEG), which records the electrical activity in the brain.
3. Electromyography (EMG), which measures the electrical activities in the muscles.
Using a combination of all these monitoring techniques is encouraged because it has been found to be beneficial in preventing lasting nerve damage during surgical procedures.
Anatomy and Physiology of Intraoperative Neurophysiological Monitoring
There are various types of monitoring methods used during surgery to track the signals in specific nerve pathways in your body.
One of these, the Somatosensory Evoked Potential (SSEP), watches over the pathway in your nervous system that handles touch sensitivity, vibration sense, and body position awareness. It gets started when sensors in your skin send signals through nerve roots and up into the spinal cord. These signals then travel via different nerve pathways to various parts of your brain, allowing you to be aware of sensations from your arms and legs.
The Motor Evoked Potential (MEP) is another monitoring method that observes motor or movement pathways. Through the stimulation of specific brain areas, it tracks signals that control movement, which are generated at different levels of the brain depending on the intensity and location of stimulation.
Visual Evoked Potential (VEP) is a technique used to check the functionality of the pathway from your eye (retina) to the vision center of your brain (the visual cortex), in response to light. When light is detected by your eye, it is converted into nerve signals that are then sent along a pathway to various parts of the brain involved in vision.
The Brainstem Auditory Evoked Potential (BAEP) monitors the function of your hearing (auditory) nerve and the pathways within your brain related to hearing. It follows the path of an auditory signal from the hair cells in your ears all the way to the primary hearing center in your brain.
Electromyography (EMG) is a way to monitor nerve activity controlling voluntary muscle movement and to check the health of specific nerves. It keeps track of nerve activity in the brain, spinal cord, and other nerves in the body during surgery. It does this by measuring electrical signals produced by muscle fibers when a nerve stimulates them.
Last but not least, Electroencephalography (EEG) is used to record electrical activity from groups of nerve cells in the brain, giving information about the brain’s overall level of activity.
Why do People Need Intraoperative Neurophysiological Monitoring
Intraoperative neurophysiologic monitoring (IONM) is a technique used during surgery to monitor the nervous system, helping to reduce the risk of damage to the brain, nerves, and spinal cord. This monitoring is particularly recommended during surgeries that pose a higher risk of nerve injuries. IONM can help monitor and measure different neurological responses. Here are some situations where it’s used:
For somatosensory evoked potential (SSEP) or motor evoked potential (MEP) monitoring:
- During surgeries involving the spine and spinal cord such as for scoliosis and kyphosis (conditions that cause abnormal curvatures of the spine), tethered cord release, spina bifida correction, tumour removal and more.
- During surgeries on the brain and brain stem including tumor removal, aneurysm repair, and more.
- During cerebrovascular surgery, which involves the blood vessels in the brain.
- Stereotactic surgery on the brain stem, thalamus, and cerebral cortex (which involves the use of a computer and scan images to guide the surgeon to a specific area in the brain).
- Pelvic fracture surgery
- Thoracoabdominal aortic aneurysm repair (surgery to repair a widened part of the main artery that supplies blood to the body).
Some other procedures that may utilize IONM include repair of coarctation of the aorta (a congenital heart defect), brachial plexus and lumbosacral plexus surgery (surgeries involving a network of nerves in the shoulders or lower back), peripheral nerve repair, carotid endarterectomy (surgery to unblock a carotid artery), and thyroid surgery.
Brainstem auditory evoked potential (BAEP) is a method used to monitor the hearing pathway in the brainstem during surgery. It is most useful in surgeries such as acoustic neuroma resection (surgery to remove a tumor on the nerve that connects the ear to the brain), vestibular nerve section, brainstem tumor resection, and various others.
Visual evoked potentials or response (VEP) monitoring is used to examine the visual system during surgeries like optic nerve surgery, orbital surgery, or pituitary gland surgery.
An electroencephalogram (EEG), which monitors the electrical activity of the brain, is used during surgeries like carotid endarterectomy, cerebral aneurysm clipping, epilepsy surgery, and for monitoring depth of anesthesia.
Electromyography (EMG) is often used to monitor the function of nerves during various surgeries such as acoustic neuroma resection, skull base tumor resection, neck surgery, among others.
When a Person Should Avoid Intraoperative Neurophysiological Monitoring
There are no absolute reasons that would prevent anyone from undergoing techniques involved in the procedure known as intraoperative neurophysiological monitoring (IONM), which is a method used to track and record the functional integrity of certain neural structures (nerves, spinal cord and parts of the brain) during surgery.
However, there are some situations that might make it more challenging to use these techniques. These include: having vascular clips or intracranial electrodes (small devices placed inside the skull), having a pacemaker or other implanted bio-mechanical equipment, having lesions on the brain’s surface, having defects in the skull, having increased pressure inside the skull, or having a history of epilepsy.
The American Clinical Neurophysiology Society (ACNS) suggests that the procedure known as transcranial motor evoked potential (MEP), which is a type of monitoring where small electric pulses are used to stimulate nerve cells in the brain, could potentially trigger seizures. However, this happens very rarely, and so even having a history of epilepsy is not seen as a reason to avoid MEP monitoring.
Equipment used for Intraoperative Neurophysiological Monitoring
The Intraoperative Neurophysiological Monitoring (IONM) is a type of system used during operations, particularly those that involve the nerves and brain. This system is designed to keep track of and display various aspects of the patient’s bodily functions in real time during surgery. This includes trends and raw signals – basically, changes and patterns in how the body is responding to the surgery.
Additionally, the IONM system can stimulate specific nerves and muscles, recording their responses. This includes nerves that are in the periphery of the body, as well as cranial nerves, which are those involved with the head and neck.
Another useful feature of this system is that it records specifics like when different waveforms occurred during surgery, detailed data about these waveforms, and notes taken by the technician during the procedure. A waveform is just a visual representation of a signal – in this case, the responses from muscles and nerves. The time and date of these waveforms are specifically recorded for reference.
The system also comes with the ability to reject or ignore errors or “artifacts” that come from the use of electrosurgical procedures. Electrosurgical procedures are when the surgeon uses electricity to heat and cut or coagulate tissue. Sometimes, the electricity can interfere with the IONM system’s recordings, but it’s equipped to disregard these artifacts so they don’t influence the results.
Who is needed to perform Intraoperative Neurophysiological Monitoring?
During surgery, a special type of check called Intraoperative neurophysiological monitoring (IONM) is done. This means that a specialist called a neurophysiologist or an IONM technician is monitoring the activity of your nervous system while the surgery is going on. These professionals, who are trained and experienced in IONM, work under the strict guidance of other highly trained professionals. They are part of a larger team, which also includes the people putting you to sleep (anesthesia personnel), the surgeon who is doing the operation, and the other medical staff in the operating room. These professionals from different areas work together as a team to make sure you are safe and the operation goes well.
Preparing for Intraoperative Neurophysiological Monitoring
A team with extensive training in intraoperative neurophysiological monitoring (IONM), which is a method used to keep track of the nervous system’s functions during surgery, is appointed for each patient and surgery. Part of their job involves checking the patient’s health records, conducting physical exams, discussing with the surgeon about the patient’s brain and nerve systems, the planned surgery, and reviewing images like X-rays or scans.
The IONM team also decides what kind of IONM method will be most beneficial for the patient’s specific surgery. They talk with the patient about what the IONM procedures will involve and what the possible risks might be, making sure to record the patient’s details accurately. Additionally, the IONM team works closely with the nursing staff to figure out where to place the monitoring equipment in the operating room. They also ensure everything is set up and working correctly before the patient comes in for surgery.
Anesthesia, or the medicines which help you sleep and not feel pain, is another crucial aspect the team has to discuss before the surgery. This takes into account the type of surgery and specific IONM method being used. IONM itself involves the use of electrodes (small devices that measure electrical activity) which are safely placed on the skin after it is cleaned thoroughly to avoid infection.
Once everything is set up, the IONM team gets some starting measurements, chats with the surgical and anesthesia teams about any potential concerns, and coordinates the monitoring throughout the surgery. They will also discuss the anesthesia process to make sure it is maintained correctly during the operation. All important details such as timing of surgery events, communication between different teams, any alerts issued, anesthesia used, and it’s dosages are carefully documented. Important changes in heart rate, blood pressure, and temperature are also recorded during the procedure.
How is Intraoperative Neurophysiological Monitoring performed
When a surgeon performs an operation, they often use various techniques to monitor the nervous system in real time. These techniques help ensure the patient’s safety and effective outcome of the surgery. There are few standard techniques for this:
- Somatosensory sensory evoked potential (SSEP)
- Motor-evoked potential (MEP)
- Spontaneous and triggered electromyography (EMG)
These approaches involve applying a specific kind of stimulus that leads to a response from the nervous system. This response can then be shown as a graph, with the time on one axis and the strength of the response on the other. This graph helps the surgeon to monitor the function of the nervous system during the surgery.
SSEPs are generated when a peripheral nerve (a nerve outside the brain and spinal cord) is stimulated. This could be a nerve in your wrist or ankle for example. These signals then travel from the stimulated nerve to the brain, and their journey can be tracked using electrodes placed on the scalp and along the nerve pathway.
MEPs, on the other hand, are generated by stimulating the brain directly and measuring the resulting activity in the muscles or spinal cord. These are monitored using electrodes placed on specific muscles or nerve regions that the surgeon wants to monitor. These locations often include particular muscles in the hand or leg.
There are also auditory and visual evoked potentials. The former involves delivering a sound stimulus to one ear and measuring the response from electrodes placed on the scalp or ear. The latter involves measuring the electrical response initiated by visual stimuli from an electrode on the scalp over the visual cortex, the brain area responsible for processing visual information.
EMGs use electrodes directly placed on a muscle to record the muscle’s electrical activity. This technique involves either provoking a response with a stimulus (triggered EMG) or simply monitoring the muscle’s activity without any additional trigger (spontaneous EMG).
An electroencephalogram (EEG) is a technique that measures the brain’s electrical activity via electrodes placed on the scalp. This approach allows doctors to monitor the general activity of large portions of the brain during surgery. There are also invasive forms of EEG that involve inserting electrodes directly into or on the surface of the brain to monitor specific areas more precisely.
Possible Complications of Intraoperative Neurophysiological Monitoring
When a patient undergoes surgery, doctors sometimes use a process called intraoperative neurophysiological monitoring (IONM). This technology helps keep track of the patient’s nerve activity during the operation, and overall, the risks associated with using IONM are quite low.
One of the main concerns during surgery is ensuring that the electric equipment used is safe. As patients are under anesthesia and can’t feel pain or discomfort, doctors need to thoroughly check this equipment before starting the operation. If they don’t, there’s a risk that malfunctioning equipment could cause skin burns or other serious problems.
In some cases, using this equipment could also potentially lead to a seizure. This is because when the brain receives a high frequency of electric signals (between 50 to 60 Hertz to be exact), it can sometimes cause abnormal neuron (brain cell) activity, which may induce a seizure.
Another concern is the effect of muscle stimulation. Specifically, stimulation can cause the masseter muscles (which help us chew and close our mouths) to move forcefully. This unintended movement could potentially result in injuries such as a cut tongue, broken tooth, or cracked jawbone. However, doctors can avoid these risks by using a device called a bite block.
Some patients may also experience minor discomfort at the areas where needles have been inserted for monitoring, like tingling, bruising, swelling, or soreness.
In rare cases, when doctors use an invasive electroencephalogram (a specific type of brain monitoring device) during epilepsy surgery, there might be slight risks, such as minor bleeding in the brain, surface infections, brain infections, or increased brain pressure. However, these incidents are reported to be quite uncommon.
While it is important to be aware of these potential risks, remember that doctors and medical personnel take various steps to ensure the safety of patients during surgical procedures and the probability of these complications occurring is very low.
What Else Should I Know About Intraoperative Neurophysiological Monitoring?
People having spinal surgery can sometimes experience neurological deficits (problems related to the brain, spinal cord, and nerves). This can occur with or without any noticeable complications during the procedure. To detect any potential issues early, doctors use a technique called intraoperative neurophysiological monitoring (IONM) during the surgery. IONM tracks the functioning of the nerves by using motor-evoked potentials (MEPs – tests of the nerve pathways responsible for movement), somatosensory-evoked potentials (SSEPs – which assess the nerve pathways that receive bodily sensations), and electromyography (EMG – a test of the electrical activity of muscle tissue).
IONM helps predict the success of the operation by alerting doctors immediately if there are changes in nervous system signals during the procedure. If a doctor observes loss of IONM signals or changes from the normal baseline signal during surgery, this could mean that a nerve injury has occurred and there’s a risk of neurological problems after the operation.
There are several factors that can influence IONM signals, including the type of anesthetic used, blood pressure, body temperature, levels of oxygen, hypocapnia (a condition characterized by reduced carbon dioxide in the blood), and any technical issues that arise. Different classes of anesthetics can have varied effects on IONM. For example, intravenous anesthetics are less likely to affect IONM signals, whereas inhaled ones can impact signal strength and timing. Muscle relaxants are typically only used at the start of the operation since they can block nerve communication.
Mechanical stress on nerve tissue, as well as reduced blood flow, can also affect IONM readings. Maintaining the body’s temperature and carbon dioxide levels during surgery is also important for accurate IONM readings. For instance, at very low body temperatures (below 28 degrees Celsius) and very low carbon dioxide levels, changes in MEP and SSEP readings may occur due to restricted blood flow to brain tissue.
Research indicates that using IONM during spinal surgeries significantly enhances neurological outcomes, making it an important tool in optimizing patient care.
