For the lower extremities, electrical stimulation to the PTN is applied to the ankle. That nerve is superficial and located just posterior to the medial malleolus. For some patients, the peroneal nerve is chosen instead. That nerve can be found superficially lateral to the knee just below the fibular head. That site is useful especially in patients with a peripheral neuropathy, such as diabetics, and in the elderly. Stimulation to the upper extremity is delivered to the median nerve or ulnar nerve. Both nerves are superficial at the wrist.

Peripheral nerves may be stimulated unilaterally to test left and right sides separately. For bilateral monitoring, left and right stimulation can be alternated during the same period of time in a method called asynchronous stimulation. Asynchronous stimulation allows for the rapid collection of data while retaining the ability to interpret data with side-to-side specificity. Modern IOM equipment can average simultaneously from several sites using programmable protocols with delays between different stimulation sites.

Bilateral upper and lower SEP monitoring is used for spine cases. Median or ulnar nerve stimulation is included in thoracic and lumbar cases as a means of monitoring for nonsurgical changes such as anesthesia-related changes. Ischemia secondary to hemodynamic events may also be detected by SEP monitoring from all extremities. Ulnar nerve monitoring also can help detect an incidental brachial plexus impairment resulting from patient positioning during long cases.

For cervical procedures, median or ulnar nerve pathways are the primary pathways monitored. Because the ulnar nerves enter the spinal cord at a lower level, ulnar nerve monitoring is preferred for cervical cases at and below the C6 spinal level. PTN channels are monitored in cervical cases for detection of a high thoracic or low cervical spinal cord injury. The additional limb coverage also provides greater spinal cord protection from perisurgical events such as hemodynamic changes.

SEP data are low amplitude, often <1 μV. This amplitude is less than the surrounding noise field, which includes cerebral activity EEG. For this reason, SEP data must be averaged. Averaging of low amplitude signals increases the signal-to-noise ratio (SNR) in a manner proportional to the number of trials. More trials result in better SNR. About 300 averaged recording trials often produce well-defined peaks.

The correct way to determine the optimum stimulus intensity is to determine the intensity that produces the largest amplitude peripheral response and then add 10 %. This is known as supramaximal stimulation and ensures that 100 % of the nerve fibers are being recruited and that small changes in electrode resistance won’t appreciably affect the recruitment percentage. Supramaximal stimulation will exceed the motor threshold and cause a 1–2 cm movement in the appropriate muscle groups in the absence of neuromuscular blockade. Median nerve stimuli produce thumb movement. Ulnar nerve stimuli produce fifth digit movement. PTN stimuli produce foot flexion, while peroneal nerve stimuli produce foot dorsiflexion. A stimulus artifact should be seen in the recording channels confirming that the stimulus is actually being delivered. Many modern IOM machines show current delivered and returned, and this is also used for confirmation of stimulus delivery.

Stimulation electrodes can be of various types including needles, discs, or adhesive electrodes. An electrode pair consisting of a cathode and anode is secured over the nerve. The resistance between the electrodes and the skin should be <5 kΩ to ensure adequate stimulus delivery and avoid large stimulation artifact. Needle electrodes provide a low resistance and avoid resistance changes over long cases. For disc or adhesive electrodes, skin preparation with an abrasive is used to reduce electrical impedances. Patients allergic to citrus fruit may have a reaction to the skin preparation gel containing lemon. If using an electrode paste, it should be free of calcium to avoid chemical burns from iontophoresis into the skin.

The repetition rate must strike a balance between rapid data collection and recording of a quality waveform. Typical repetition rates are between 2 and 5 stimulations per second. A complete data set can usually be obtained within a couple of minutes at these rates. Repetition rates >5 per second sacrifice data quality for more rapid collection. The amplitude of the peaks will decrease appreciably as rates are increased above 5 pulses per second due to refractory times of the individual nerve fibers. Stimulation rates should avoid exact multiples of 60 Hz (or 50 Hz) to avoid line noise artifact.

SEP recordings are made from different points along the DCML pathway. These points are chosen to provide recordings from peripheral, subcortical, and cortical potentials. In general, the active electrode is placed as near as possible to the anatomic generator, and a reference electrode is placed some distance away. The reference may be another scalp site or a non-cephalic site. Bipolar recording montages compare inputs between two nearby electrodes, while referential recording montages compare inputs between an active electrode near the anatomic generator and a much further placed reference electrode. The amount of electrical noise is proportional to the distance between the active and recording electrode as well as the distance between the anatomic generator and the active electrode. While the cervical potential is more susceptible to electrical noise because of the distance of the generator, these potentials are less affected by inhalation anesthetic concentrations (due to the lack of synapses up to this point in the pathway). For this reason they often are included in the recording montage despite their predisposition to noise.

The surgical field may make first choice recording sites inaccessible. When this happens, it is necessary to scout for alternate recording sites that will yield the highest possible recordings. Neurosurgical craniotomies may displace scalp sites. Cervical surgery may displace cervical recording sites. Several nearby alternate sites may be tried.

The International Federation of Clinical Neurophysiology’s 10–20 System provides the accepted naming convention for scalp recording sites. The 10 % extension of the 10–20 system [1] adds additional nomenclature. The EEG chapter in this book has further information on electrode nomenclature. For those unfamiliar with the naming conventions, a brief overview is given here. Electrode sites are named in a coordinate fashion with the first part of the binomial indicating the position along the anteroposterior axis and the second part of the name indicating mediolateral position. A series of anteroposterior lines are named according to their position relative to certain brain features. The C-line runs generally along the central sulcus. The P-line is at the level of the parietal lobe. The line in between the C-line and P-line is the CP-line. Mediolateral positioning is named relative to the lateral distance from the Z-line which runs along the vertex of the skull (midline). Odd numbers are to the left of the Z-line and even numbers to the right. The smaller the number, the closer to the Z-line. For example, an electrode placed over the right postcentral gyrus near the hand area (lateral) would be CP4. The midline position would be named CPz. The location halfway between CPz and CP3 is known as C1. The letters “i” or “c” can replace the numbers when referring to general positions as either ipsilateral or contralateral, respectively.

Estimating recording sites by visual gross inspection frequently instead of measuring locations according to the 10–20 system misplaces electrodes by a centimeter or two and may result in suboptimal recordings and poor ability to reproduce recordings if an electrode needs to be replaced after falling off.

SEP IOM also uses non-cephalic recording sites, e.g., over vertebral spines and at Erb’s point. Erb’s point is located above the clavicle, 2 cm lateral to the insertion of the sternocleidomastoid muscle. Sites over vertebrae are referred to with their spinal level, sometimes including the term Sp for spine. In that way, CSp5 is located over the fifth cervical spine’s posterior spinous process.

Some recommended technical parameters given in Table 6.1.

Lower extremity SEP recordings are made from CSp5 and the scalp. The CSp5 channel monitors the cervical-brain stem activity, and the scalp channels monitor cortically generated peaks.

There is no one correct scalp recording site for the cortically generated peak of the lower extremity SEP. The dipole of the generator is oriented differently in different patients and can change with depth of anesthesia. Principal sites for the active electrode include CPz, CP1, CP2, CP3, CP4, and CPz. The orientation of the neurons that generate the potential changes as the postcentral gyrus bends toward midline. The orientation of the midline neurons that generate the cortical potential in response to lower extremity stimulation causes the dipole to project across the midline. This dipole projection results in a “paradoxical localization” of the potential over the scalp ipsilateral to the site of stimulation. This is paradoxical in that the neurons generating the potential are located in the contralateral hemisphere (as indicated by DCML pathway anatomy). Common sites for the active electrode are CPi, CPc, and CPz.

Choosing a site for the reference electrode is no less important. Scouting among possible recording channels early in the case helps to find the best channels to monitor that patient, although time may not permit this exercise. References may include the forehead, ear, mastoid, or the scalp location contralateral to the active electrode. Short distances between the active and reference electrodes (e.g., CPi–CPc) reduce noise but also may reduce peak amplitudes.

The subcortical peaks may be recorded over the spinous process of C5 (CSp5) with an ear, forehead, or shoulder reference. The subcortical peaks are less affected by anesthesia due to the lack of synapses at this point of the DCML pathway. Peripheral recording sites include the popliteal fossa or over the lumbar and thoracic vertebrae such as TSp12 or LSp1. Older or obese patients may have no recordable lumbar potentials as a normal variant.

For upper extremity SEPs, recordings are made at the shoulder, cervical spine, and scalp. Scalp sites are generally optimum over the contralateral postcentral gyrus (CPc) with a forehead, ear, or mastoid reference. Subcortical peaks are popularly recorded from CSp5, earlobe, or mastoid with a reference located either at the forehead or contralateral Erb’s point. An Erb’s point channel (referenced to the contralateral Erb’s point) can be used to test peripheral conduction and is useful for monitoring changes secondary to positional issues.

The typical low-frequency filter is set to 30 Hz and high-frequency filter 1,500–3,000 Hz. This balances control over noise while maintaining most SEP peak characteristics. These settings reduce random amplitude fluctuations and some anesthetic-related variability [2]. Properly set filters will yield reproducible SEPs with minimum background variability in amplitude and latency.

Notch filters should not be used during SEP recording. The notch filter can cause a stimulus artifact with a decaying sinusoidal tail with peaks at 16.6, 33.3, and 50 ms. Those peaks easily can be mistaken for stable EPs at 16.6 or at 33.3 ms. This is called ringing artifact.