Motor neurons of the spinal cord travel in the anterolateral spinal cord. Injuries to motor neurons can be detected by MEPs, which run at specific moments during the procedure at the request of the surgeon. MEPs are very sensitive to anesthetic choice and require anesthetics that lack any paralytic or muscle relaxant properties. If paralytics or muscle relaxants are required for the induction of anesthesia, they must be rapidly cleared and muscle twitches must be confirmed to have returned in order to establish a reliable preincisional baseline. MEPs are carried out by electrical stimulation of electrodes placed in the scalp above the motor cortex of the brain. Stimulation of these electrodes will then trigger a response from the associated motor group within the homunculus. A response of the associated muscle group is then recorded distally and compared to a reliable preincisional baseline. A disruption in the signaling of a specific muscle group with the MEPs should alert the surgeon that there is a lack of transmission somewhere within the neural pathway that would be inhibiting transmission of the signal. Aside from surgical misadventures causing a nerve injury, the anesthesiologist should check their anesthetic choice, blood pressure, and any point of external compression-related positioning. An assistant can also check to see if the recording electrode is still intact. Any of these variables can confound MEP recordings and result in the appearance of a nerve injury.

Sensory fibers of the spinal cord travel within the dorsal columns. Measurements of injuries to sensory nerves can be recorded by SSEP monitoring [11, 12]. Stimulation is managed by electrodes in different sensory distributions of the legs. They are recorded through scalp electrodes over the sensory cortex of the brain and are compared to a reliable preincisional baseline. SSEPs are less confounded by anesthetics and therefore can be a more reliable of a measurement of nerve injury. Because SSEPs measure the sensory component of neural function, anesthetics with paralytic properties do not confound their readings. However, inhalational agents given as part of anesthesia induction can attenuate results and hamper SSEP readings. SSEPs are averaged over a 15 min timeframe and therefore are not an accurate indicator of the injury at the time of its exact instance. Additionally, multiple nerve roots can contribute to a sensory deficit in a specific sensory distribution. The multiple nerve root contributions to a sensory domain would make it difficult to isolate exactly which nerve root was injured. Proposed alarm criteria are variable and preoperative injuries must be considered. A proposed 50 % reduction in amplitude and 10 % decrease in latency are generally considered to be significant events and a trigger for alarm from the neurophysiologist to the surgeon and anesthesiologist. This should be correlated with intraoperative events including correction maneuvers of the spine, hardware insertion, compression from retractors, and hypotension.

Spontaneous electromyography (EMG) allows for the passive evaluation of muscle group firing that might undergo mechanical stimulation by their associated nerve root. This type of EMG can also be referred to as “free-run” EMG due to its continuous, passive means of monitoring. spEMGs function by way of recording muscle groups in each nerve root category. A single electrode is placed distally in the muscle group of interest, and recordings are performed throughout the procedure without any need for proximal stimulus, hence the term “free run.” The role of spEMGs is to monitor the degree of distal responses to proximal surgical manipulation. Spontaneous EMG has also been used to document adequacy of decompression, showing a decrease in spontaneous firing of decompressed nerve roots intraoperatively. Such changes however may or may not occur during the time of surgery and are usual only in the minority of cases where there is an absence of chronic nerve root injury [13]. spEMG recordings are important for identifying nerve or muscle irritation [14]. For example, if there is evidence of the tibialis anterior firing on spEMG, this could be an indicator of mechanical stimulation of the L5 nerve root or downstream nerves, either from the retractors or the surgeon’s instrument. Cold irrigation in the operative site can also cause increased activity of spEMG. Increased activity seen on spEMG warns the surgeon that the nerve or nerve root is being stimulated. Stimulation of the nerve seen on spEMGs are an indicator for a nerve at risk for potential injury. It is important to point out that spEMGs that have returned to normal activity following a recorded stimulus could indicate that not only was the stimulus removed and the nerve returned to a normal resting state but could direly also indicate that the nerve injury has been completed and is no longer able to depolarize and transmit a signal to the recorded muscle group. In this case, it would be important to investigate this further through an MEP run.

Triggered electromyography (trEMG) has been demonstrated to be particularly useful for lateral access spine surgery [15]. Triggered EMG is the intentional stimulation of a nerve by a proximal source, which is recorded distally at an associated muscle group. This carries a high utility in measuring the proximity of nerve or nerves to the trigger source. An intentional electrical stimulus, measured in milliamps, is deployed through the queried point of contact (a trochar, retractor, etc.). The electrical stimulus starts at a low ampere and then moves up to the point of which there is an appreciable distal response from a muscle group. This is known as a threshold stimulus. The higher the recorded threshold stimulus, the higher amount of safe distance (or protective tissue) exists between the trigger source and the nerve. This function of IONM is particularly useful in investigating the surrounding structures upon entry and access through the psoas major muscle as the surgeon approaches the lateral lumbar spine. For example, a trEMG firing can be performed at the placement of the Jamshidi trocars, the retractor setup, or as a handheld probe that can investigate tissue within the operative corridor if there is a concern for a nearby nerve. Low-threshold firing of any downstream nerves would indicate that there is a nerve or nerve root within the proximity of the stimulus that is prone to injury. This technique carries high utility in “neural mapping,” which will be described later in this chapter.