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Spine surgery can be unpredictable and carries potential risks, particularly because it involves areas close to nerves and the spinal cord. There is a chance of nervous system damage during these procedures, especially when hardware is placed near nerves or when correcting spinal curvature. It is crucial for surgeons to have an experienced professional available to monitor nerve tissue throughout the operation. 

To enhance safety during spine surgery, the nervous system can be monitored. This process is known as intraoperative monitoring, or intraoperative neurophysiologic monitoring (IOM). Intraoperative monitoring is not only utilized in complex spinal surgeries but is also commonly applied in various other surgical procedures, including brain surgery, ear surgery, and surgeries involving arteries.

The spine consists of several sections. The upper part is known as the cervical spine, which is the neck area. The middle section is referred to as the thoracic spine, or the trunk. The lower part is called the lumbar spine, commonly known as the low back. The spine is made up of bony structures called vertebrae, which are stacked on top of each other. Between these vertebrae are discs that act as cushions. The spinal cord is safeguarded by the bony structure of the spine, resting within a hollow space known as the spinal canal. Nerves extend from the spinal cord, reaching out to the limbs, trunk, and internal organs. 

The section of the nerve that emerges from the spinal cord is called the nerve root. The spinal cord is responsible for sending and receiving messages between the body and the brain. The nerves that branch off from the spinal cord connect to different parts of the body, facilitating both sensation and movement. Nerve tissue is quite fragile and can be easily damaged. If the spinal cord or nerves sustain injury, it can lead to changes in sensation or motor function in the affected area, and this damage may be permanent.

Intraoperative neurophysiological monitoring (IONM) employs specialized equipment to assess the function of the spinal cord and nerves during spine surgery. Its primary purpose is to provide the surgeon with real-time feedback and alerts before any permanent nerve damage occurs. This approach has been shown to enhance safety and improve outcomes in complex spine surgeries. 

IOM initially utilized somatosensory evoked potentials (SSEPs) to measure sensory conduction above and below the surgical site. During spinal procedures, electrodes are placed on the limbs that may be impacted by the surgery, as well as on the skull over the corresponding area of the brain that processes the limb's sensory input. A machine monitors the brain's electrical activity, similar to an electroencephalogram (EEG), recording this activity as waves. When the surgeon or technician stimulates the limb with an electrical current, a response should be observed in the brain, which helps evaluate the sensory function of the nerves and spinal cord.

SSEPs are the most commonly used form of intraoperative neurophysiological monitoring during surgeries. However, it's important to note that SSEP monitoring may not always detect injuries to individual nerve roots.

In the 1980s, surgeons began incorporating electromyography (EMG) to monitor the motor function of the nerves during spinal surgeries, and its use has been increasing. While the patient is under anesthesia, needle electrodes are inserted into the muscle groups corresponding to the surgical area. A machine monitors the electrical activity from the muscles, recording it as waves, much like the SSEP monitor. This is also akin to the waves recorded from the heart during an electrocardiogram (ECG). Baseline recordings are taken before the surgery starts, and these recordings are repeated throughout the procedure. Any significant changes in the wave patterns alert the surgeon or technnologist.

Spine surgery frequently utilizes instrumentation, which includes hardware such as screws, rods, plates, and cages. The complication rate associated with instrumentation during lumbar spine fusion can range from 1% to 33%. Potential complications from hardware placement include damage to the spinal cord and nerve roots. In about 10% of cases, patients may experience pain and sensory changes in the affected limb, a condition known as radiculopathy. Additionally, there is a 1% chance of motor changes or weakness in the limb at the surgical level, referred to as myelopathy. These outcomes are not desirable following spine surgery.

Surgeons have traditionally depended on imaging techniques, primarily X-rays and computed tomography (CT) scans, to verify the correct placement of hardware during surgery. However, there are instances when these imaging methods fail to detect improperly placed hardware, spinal bone fractures, or nerve tissue damage.

Intraoperative neurophysiological monitoring (IOM) has proven to be more effective in detecting neurological damage and hardware misplacement. Some studies indicate that IOM is one-third more effective than imaging techniques like X-ray or CT scans.

Before the advent of intraoperative neurophysiological monitoring, surgeons typically used the wake-up test and the clonus test to assess nerve function during surgery. However, these methods can be challenging to repeat during the procedure, may extend surgery time, and do not provide a comprehensive evaluation of nerve function. The wake-up test involves waking the patient from anesthesia and asking them to move body parts that might be affected by the surgery. The clonus test consists of the surgeon quickly jerking the foot towards the shin to check if the stretch reflex is still functioning, which assesses spinal cord activity.

Intraoperative neurophysiological monitoring (IOM) is a quick, painless, and cost-effective method. Recordings can be taken multiple times during surgery or even throughout the entire procedure, causing no harm to the patient and providing real-time feedback.

After you are put to sleep by the anesthesiologist, small wire electrodes are placed in the muscles of your lower leg for lumbar surgery. In some cases, they may be positioned in the arms for neck surgery. For EMG monitoring, these electrodes are strategically placed in specific muscles that correspond to particular levels in the spinal cord. 

In terms of technicality, EMG monitoring is relatively straightforward, and the setup requires almost no extra time during the positioning for the surgical procedure. 

Intraoperative monitoring is typically conducted by a technician who remains in the operating room throughout the entire procedure. A neurophysiologist may also be involved in interpreting the recordings, usually from a remote location where they can view the same data as the technologist. This real-time interpretation by an experienced neurophysiologist is considered the standard of care in many areas of the United States. 

If the spinal cord or nerve roots sustain damage while drilling into bone, placing hardware in the spine, or decompressing the spine, the SSEP or EMG signal will change. This serves as an alert for the technician, who can then notify the surgeon. Some monitoring units come equipped with a loudspeaker that provides immediate audio feedback, allowing the surgeon to make adjustments at that moment to prevent damage to nerve tissue. 

When pedicle screws are utilized, the surgeon may employ a handheld probe that delivers electrical stimulation to the pedicle screw once it has been placed. If the screw is correctly positioned, it will require a specific amount of current to induce a muscle contraction and produce a wave on the monitor. Conversely, if it is misplaced, it will require less current. This phenomenon is known as an evoked EMG. 

The surgeon can then reassess the screw's placement to avoid harming the nerve root or spinal cord. SSEPs and EMG recordings can be taken multiple times during the surgery if needed, without causing any harm to the patient.

There are really no drawbacks. The surgery can proceed without unnecessary interruptions when neurophysiological monitoring is utilized, which enhances patient safety. Although the skin is punctured with needle electrodes, the risk of infection is very low since the skin is disinfected prior to needle insertion. The needles are sterile and disposed of after one use. Some mild soreness may occur at the insertion sites. Intraoperative monitoring is generally not expensive and is a worthwhile investment to prevent neurological complications during spinal surgery whenever feasible.