Neuromonitoring in Spinal Surgeries

Introduction

Spinal surgeries are medical procedures that address a wide range of spinal conditions, including herniated discs, spinal stenosis, deformities like scoliosis, and even spinal tumors. These surgeries aim to reduce pain, improve mobility, and correct structural issues in the spine. The intricate nature of the spinal column and its proximity to vital neural structures make these surgeries inherently complex and delicate.

What Is Neuromonitoring?

Neuromonitoring, or intraoperative neurophysiological monitoring (IONM), is an important adjunct in spinal surgeries. Neuromonitoring means the patient's neural functions are monitored closely during the surgical procedure using different methods. This live monitoring helps doctors check if the motor and sensory pathways are working well and quickly address any problems. This lowers the chance of nerve injuries and problems after surgery. Spinal surgeries with neuromonitoring show how surgical skills and advanced technology are used to make patients safer and improve surgical outcomes in this medical field.

What Are the Types of Neuromonitoring During Spine Surgery?

There are several types of neuromonitoring techniques employed in different surgical contexts.

Electromyography (EMG)

• Continuous EMG - In this technique, fine needle or surface electrodes are placed into specific muscles to monitor their electrical activity continuously. Continuous EMG helps identify abnormal muscle activity, such as muscle spasms or contractions, which can indicate nerve irritation or compression.

• Triggered EMG - Triggered EMG is used during surgeries involving nerve decompression or tumor resection. When the surgeon stimulates a nerve, triggered EMG records the muscle's response. This can help identify the proximity of neural structures to the surgical site and guide the surgeon's actions to prevent nerve injury.

Somatosensory Evoked Potentials (SSEPs)

SSEPs stimulate peripheral nerves (usually in the limbs) and record the electrical responses at various points along the neural pathway, including the spinal cord and brain. SSEPs provide information about sensory nerve function. During spinal surgeries, SSEP monitoring can detect changes in sensory nerve signals, which may indicate spinal cord compromise or nerve root compression.

Some of the features of SSEPs include:

• This was first employed in the 1970s for monitoring spinal cord function during scoliosis correction surgery.

• SEPs offer reliable monitoring of peripheral sensory pathways.

• SEPs indicate dorsal column activities, such as sensory tract function.

• Shifting the stimulation site helps identify the laterality of dorsal column lesions.

• This information is valuable in procedures like posterior myelotomy for intramedullary spinal cord tumor removal.

• Surgical procedures, mechanical factors, and ischemia can alter SEPs.

• Patient-specific factors like age, height, limb length, systemic hypotension, hematocrit levels, hypothermia, and anesthesia (volatile agents) can also impact SEPs.

• Body temperature is a common factor affecting spinal SSEP latency readings, emphasizing the importance of maintaining a stable body temperature during monitoring.

Motor Evoked Potentials (MEPs)

MEPs are used to assess the integrity of motor pathways. They involve transcranial electrical stimulation of the brain's motor cortex and recording muscle responses at various points along the motor pathway. In spinal surgeries, MEPs can help identify issues like spinal cord ischemia (lack of blood flow to the spinal cord) and guide the surgeon in minimizing the risk of motor deficits.

Different types of MEP in spinal surgery monitoring include

1. Direct Waves (D-Waves or Spinal MEPs)

• D-waves are initiated by direct axonal activation with a conduction velocity of approximately 50 m/s.

• These waves monitor motor pathways from the brain's cortex to the spinal electrode.

• Single transcranial electrical stimulation generates D-waves, providing real-time feedback during surgery.

2. Neurogenic MEP:

• Neurogenic MEPs are electrically stimulated at the spinal cord using epidural electrodes and then recorded from peripheral nerves.

• Stimulation occurs through flexible spinal electrodes in the epidural space near the surgical site.

• This technique allows monitoring of the overall spinal cord, combining sensory and motor pathway monitoring.

3. Muscle MEP (Myogenic MEP):

• Muscle MEPs are evoked using transcranial electrodes placed on the scalp over the motor cortex.

• Compound muscle action potentials (CMAP) are recorded with needle electrodes from specific muscles in all four limbs.

• Muscles selected depend on the surgical procedure and spinal levels involved.

What Are the Indications for Neuromonitoring?

Neuromonitoring in spinal surgeries is critical in enhancing patient safety and surgical outcomes. Several indications warrant the use of neuromonitoring during these procedures.

• Complex spinal surgeries.

• Spinal cord decompression.

• Pedicle screw placement.

• Tumor resection.

• Deformity correction.

• Pediatric spinal surgery.

• Revision surgeries.

• Pre-existing neurological conditions like neuropathy or myelopathy.

• Minimally invasive procedures.

What Are the Factors That Influence Neuromonitoring in Surgical Procedures?

Various factors can influence both SSEP and MEPs. This includes drugs used during surgery and the natural state of the body. These factors can change how the nerve signals appear in the monitoring.

• Sensitivity to Nerve Length - Nerves that are longer and have more connections are more sensitive to changes. This means that surgical procedures involving these nerves need careful monitoring to ensure they are not damaged during the operation.

• Upper vs. Lower Limbs - It is easier for doctors to collect nerve signals from the upper limbs compared to the lower limbs. This is because the part of the brain that controls the hands has a larger representation in the motor cortex.

• Anesthetic Effects - Inhaled or intravenous Anesthesia can affect SSEP and MEPs. Inhaled anesthetics have a greater impact on these signals compared to intravenous ones. They can alter the size and timing of the nerve signals.

• Muscle MEPs Sensitivity - MEPs are more sensitive to anesthesia's effects than SSEPs. Therefore, extra care is needed to maintain the signals when monitoring motor function.

• Muscle Relaxants and Anesthesia - During surgery, it is important not to use strong muscle relaxants, as this can interfere with MEP monitoring. Instead, a combination of propofol and fentanyl is commonly used for anesthesia.

• Monitoring Muscle Relaxation - To ensure that the muscles are relaxed enough, doctors continuously monitor the level of muscle relaxation by recording compound muscle action potentials (CMAP) in response to a series of stimuli. This helps maintain the right balance between relaxation and muscle function during surgery.

What Are the Advantages of Neuromonitoring in Spinal Surgeries?

• Neuromonitoring helps safeguard the nervous system during surgery, reducing the risk of neurological complications such as paralysis or sensory deficits.

• Surgeons receive immediate feedback on neural function, allowing them to make necessary adjustments during the procedure to protect neural structures.

• It enables surgeons to identify and avoid damage to critical neural elements precisely.

• It has been associated with reduced postoperative neurological deficits and improved patient outcomes in spinal surgeries.

• Surgeons can customize their approach based on real-time monitoring data.

• By minimizing intraoperative nerve damage, neuromonitoring can decrease the need for revision surgeries.

What Are the Limitations of Neuromonitoring in Spinal Surgeries?

• Neuromonitoring can produce false alarms or fail to detect nerve damage accurately.

• The effectiveness of neuromonitoring heavily relies on the expertise of the monitoring team, which may only be available in some healthcare settings.

• Implementing neuromonitoring can add to the overall surgery cost due to the need for specialized equipment and trained personnel.

• Surgical instruments, anesthetic agents, and patient positioning can interfere with monitoring signals, leading to inaccuracies.

• Neuromonitoring techniques can be complex, and not all surgeons know their intricacies.

• While neuromonitoring can detect changes during surgery, it may not accurately predict long-term neurological outcomes.

Conclusion

Neuromonitoring in spinal surgeries ensures safer procedures by continuously tracking nerve function. It provides real-time feedback to surgeons, helping prevent neurological complications and improve surgical outcomes.