Intraoperative Monitoring in Routine and Complex Surgery

Somatosensory-Evoked Potentials

Intraoperative monitoring using SSEPs came into use in the 1970s. ^, The methodology performs continuous assessment of the large-fiber somatosensory pathways through monitoring potentials evoked by stimulating peripheral nerves. The majority of the potentials are conducted via the dorsal column of the spinal cord, and therefore this monitoring is more sensitive to detecting dysfunction in the dorsal spinal cord. There is a small percentage of this stimulated pathway that travels up the ventral spinal tracts, but is not thought to be clinically significant when it comes to intraoperative assessment of the ventral spinal cord. ^, SSEP monitoring is the most commonly used mode of spinal cord monitoring. It is clear that SSEP monitoring can be useful in cases of deformity surgery, as reported by Nuwer et al. in their review of 500,000 monitored spinal surgeries. These cases included patients with scoliosis (60%), spinal fractures (7.5%), kyphosis (6.5%), and spondylolisthesis (5.5%). The study reported the false-negative rate associated with SSEP monitoring to be 0.063%. SSEP monitoring is sometimes used in isolation, without other methodologies such as TES-MEP or EMG, depending on the particular surgery being performed. The major limitation of SSEPs is the time delay between injury and changes in SSEPs, as well as the lack of definitive correlation with anterior cord dysfunction. ^, Although it has its sensitivities to certain anesthetics, SSEP monitoring can be performed successfully even if a muscle relaxant such as rocuronium is being used. In addition to the potential deleterious effect of anesthesia on SSEPs, electrical interference from operating equipment, hypocarbia, hypothermia, and hypotension can also affect SSEP monitoring. ^, Although SSEPs can provide valuable assistance on their own in a variety of spine procedures, it is important to emphasize that they only represent the function of the dorsal columns of the spinal cord and therefore may be unable to detect some neurological changes to the anterior (motor) pathways. ^{, ,}

Electrode Placement

For lower limb somatosensory monitoring, the posterior tibial nerve is most frequently used as the site of stimulation. A pair of either needle or adhesive surface electrodes are placed at the medial surface of the ankle, between the medial malleolus and the Achilles tendon. Recording electrodes such as electroencephalogram (EEG) disc electrodes, straight needles, or corkscrew needles are placed on the scalp at locations overlying the somatosensory cortex. These electrodes are denoted by the International 10-20 System used for EEG monitoring. The sites of electrodes placement are typically over midline and centroparietal regions that are in close proximity to the underlying somatosensory cortex. The channels using these electrodes will record the cortical potentials vital to assessing the sensory pathways’ function. Along with these scalp electrodes, recording electrodes are also typically placed at a number of additional locations along the somatosensory pathways, including the popliteal fossa behind the patient’s knee, the patient’s chin, and the anterior neck at C5. The intensity required for stimulation for lower limb SSEPs is relatively low (usually around 30–40 mA, with a pulse width of 300 μsec). The intensity of stimulation is often increased beyond the motor threshold so as to cause twitching of the patient’s toes.

For the upper extremities, either the median or ulnar nerve is stimulated with electrodes placed at the ventral side of the wrist. Electrodes are placed approximately 3 cm below the hand at the mesial side for the ulnar nerve or at the lateral side for the median nerve. As with the posterior tibial SSEPs, the cortical potentials are recorded at electrodes on the patient’s scalp. Additional electrodes are placed at the C5 spine, at the brachial plexus (Erb’s point). The required intensity for upper extremity SSEP stimulation is typically lower than that required of the lower limbs (15–25 mA, 300 μsec pulse width) ( Fig. 83.1 ).

System Settings

Continuous SSEP monitoring commonly uses interleaved stimulation, with each limb being stimulated in sequence at a rate of 3 to 5 Hz. To curtail 60 Hz artifact, a stimulation frequency not evenly divisible into 60 (for example, 3.1 Hz) may be used. The typical settings used for the evoked potential machine are a low-frequency filter of between 5 and 100 Hz and a high-frequency filter of 2000 to 3000 Hz.

SSEP monitoring allows for continuous assessment of the somatosensory pathways by continuous stimulation and recording the evoked potentials that are averaged over several hundred stimulation trials. Averaging is necessary for SSEPs, as the recorded neurological signals have a relatively low amplitude. Additionally, various electrical instruments in the operating room will produce electrical artifacts that will frequently obscure the desired recordings. By displaying an average of 300 to 1000 responses, the IOM machine is able to digitally average out all signals unrelated to the evoked stimulus, while reinforcing those signals that are temporally related to the stimulation. Although it stands to reason that a displayed average generated by a greater number of stimuli will theoretically yield a cleaner signal, the IOM technologist must keep in mind that the detection of neurological changes is highly time-sensitive. The technologist should use discretion in allowing his or her machine to acquire the number of averages appropriate to producing legible signals without unnecessarily increasing the time required to detect changes efficiently ( Fig. 83.2 ).