What is Intraoperative Neuromonitoring (IONM)?
Intraoperative Neuromonitoring is testing and monitoring of the spinal cord and peripheral nerve roots to ensure that proper functions remain throughout surgery. Proper monitoring gives indication to the surgeon and operating room personnel if something has changed, if something should be investigated, or if something needs to be corrected.
The monitoring modalities chosen to protect the patient’s motor nerve pathways and spinal cord should answer the following criteria:
What is at risk?
When is it at risk?
How is it at risk?
Types of Tests Involved in Intraoperative Neuromonitoring
EMG – or electromyography, measures continual muscle activity in the arms, legs, and abdominal area. It is the electrical activity of the nerve root that causes the compound muscle action potential (CMAP). This is the activity that is measured.
We measure the compound muscle action potential by placing a pair of needle or surface electrodes into or on the muscles being monitored. Each nerve root innervates a specific group of muscles. For example, the S1 nerve root affects the Medial Gastrocnemius (medial portion of the calf muscle).
Valuable monitoring information for all spine and skull-based procedures is obtained with EMG responses and they measure electrical activity in the brain. The CMAP that is measured is displayed on a monitor as a “wave form.” Changes with baseline EMG activity indicate that something has happened to the spinal cord or nerve roots and should be investigated.
Characteristics of EMGs:
Nerve root specific; correlated to specific muscle groups.
Free running EMG is “passive” reading
Triggered EMG is “active” stimulation and reading
Most often used in lumbar spine procedures
SSEP - Stands for SomatoSensory Evoked Potentials and tests the Sensory pathway of the spinal cord which runs on the posterior track of the cord. The SSEP involves stimulating from the muscle and recording at the skull. The sensory is the signal coming from the muscle to the brain (Sensory - feeling heat on your finger.) Clinically, they are most valuable when taking a posterior approach, for deformity cases, tumor resections, dorsal root rhizotomies, and cauda equina surgeries.
SSEPs may help to prevent brachial plexus injuries during surgeries. These tests have to “recruit" responses and average them before any change is seen, therefore, feedback is delayed. SSEP’s are valuable and are specifically good for cases where the spinal cord is at risk and/or the patient may be at risk from operating room positioning injuries.
Characteristics of SSEPs:
Posterior cord and sensory pathway monitoring
Paralytic anesthesia may be used
TcMEP - Stands for Transcranial Motor Evoked Potentials and tests the Motor pathway of the spinal cord which runs on the anterior track of the cord. The motor evoked potential (MEP) involves stimulating through the skull and recording at the sight of the muscle. The motor potential is the signal from the brain that makes your muscle move (Motor – moving your finger.) MEP’s offer instant feedback and are especially valuable when using an anterior approach.
Specifically, MEP potentials will change when there is an injury or loss of blood flow to the motor tract of the spinal cord. These changes are early indicators of possible paralysis if immediate action is not taken. The speed with which changes can be seen is very helpful so that surgeons can make real-time adjustments.
Characteristics of TcMEP:
Anterior cord pathway
Mostly used in Thoracic and Cervical case
Other tests are involved in the world of intraoperative monitoring but usually are not used for spine cases. They are mostly used for cranial surgery. These tests are called electroencephalogram (EEG), visual evoked potential (VEP), and brainstem auditory evoked response (BAER) or auditory brainstem response (ABR), and they measure electrical activity in the brain.