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SSEP
Notes:
The general consensus is that the dorsal or posterior column spinal pathways primarily mediate the SSEPs. Other pathways such as the dorsal spinocerebellar tracts and the anterolateral columns may contribute to the early SSEP responses that are used for monitoring purposes.
No synapses are encountered between the peripheral stimulation sites and the medullary nuclei (nucleus cuneatus and nucleus gracilis) where the responses arrive after ascending the posterior column of the spinal cord. These early responses are predominantly a reflection of the integrity of spinal cord white matter and provide little direct information about the condition of spinal cord gray matter. Therefore, the ascending SSEP responses up to the level of the medullary nuclei are affected only minimally by general anesthetics.
Blood Supply
To the Cord:
The blood supply for nourishing the posterior column pathways which mediate SSEPs is generally thought to be the posterior spinal arteries.
The anterior spinal artery is generally believed to provide the primary blood supply to the anterior and antero-lateral portions of the spinal cord which make up the remaining two-thirds of the spinal cord. Motor pathway function is mediated by spinal cord pathways which receive their blood supply from the anterior spinal artery.
Therefore, loss of motor function due to compromise of the blood supply to the anterior spinal artery may be associated with little or no loss of the sensory function which is mediated by the dorsal column pathways (anterior cord syndrome). However, the degree to which this is true is uncertain and may vary between individuals.
To the Brainstem and Brain:
The functional status of the portions of the brain which are responsible for mediating the SSEP responses are dependent upon the blood supply to the brain and brainstem and the specific arterial branches which provide this supply.
Perforating branches of the basilar artery and the vertebral artery supply the brainstem.
The middle cerebral artery provides the blood supply to the area of the cortex which mediates the upper extremity SSEPs
The anterior cerebral artery provides the blood supply to the area of the brain which mediates the lower extremity SSEPs.
Blood Pressure and Cerebral Perfusion
Decreasing blood pressure may significantly affect cerebral perfusion.
In a normothermic individual, when cerebral perfusion drops to about 18 cc/min/100 grams of tissue, electrical activity of the brain decreases and SSEPs begin to diminish in amplitude.
When perfusion drops to 15 cc/min/100 grams of tissue, electrical activity of the brain drops still further and SSEPs are generally not recordable.
Further drops in blood flow to the brain, particularly if they are sustained, will result in cellular damage and irreversible changes in electrical activity.
Stimulation:
The use of constant current stimuli will compensate for any change in electrical conductivity as long as the electrodes remain securely in place.
Current spread to the underlying nerves is the effective stimulus and the use of a constant current stimulus is meant to compensate for any changes in contact resistance.
Use of a constant voltage stimulus provides a constant stimulus intensity only if the contact resistance does not change. For this reason, the use of constant current stimulation is recommended.
An electrical stimulus is typically presented as a series of rectangular pulses with a certain pulse duration and frequency of presentation. The intensity of the stimulus is dependent on its amplitude, pulse duration and frequency. An increase in any of these parameters will normally cause an increase in stimulus intensity because the amount of current flow will increase.
It is suggested that a pulse duration of 200-300 microseconds be used for eliciting both SSEPs and DSSEPs.
Controlling the stimulus rate is essential in obtaining high quality evoked responses.
Stimulation rates between 2 and 5 Hz are recommended. However, lower stimulation rates (between 1.5 and 3 Hz) can sometimes improve lower extremity responses, particularly when compromise of neurological function is present, whereas upper extremity SSEPs may demonstrate little or no change at stimulation rates as high as 9 Hz.
Increasing the stimulus rate beyond 9 Hz for the upper extremity SSEPs and 5 Hz for the lower extremity typically results in a substantial degradation of the SSEPs, particularly the cortical responses
Often, there are other sources of noise in the evoked response and sometimes minimally changing the stimulus rate (for example from 4.7 to 4.9 Hz) may change the quality of the recorded evokedpotentials in the setting of high amplitude rhythmic noise.
Supramaximal stimulation intensities should be utilized
Generally, it should not be necessary to utilize stimulation intensities which exceed 50 mA in order to elicit repeatable SSEPs or DSSEPs and to provide effective monitoring unless pathology is present or the current spread from the stimulating electrode is not reaching the underlying neural tissue at a sufficient intensity to cause excitation such as in patients with large or edematous extremities
Stimulus intensities to as high as 100 mA may be necessary to produce an effective stimulus but the monitorist should consider other options as well such as repositioning the stimulation electrodes, changing to needle rather than surface stimulation electrodes, or selecting an alternate stimulation site.
Although concerns may exist regarding the possibility of tissue damage resulting from high current densities at the stimulation sites, these concerns appear to be unfounded and there is no evidence in the literature or otherwise to support them if the stimulus parameters available on commercially available devices are utilized.
Different concerns apply to the acquisition of DSSEP responses. For these responses, high stimulation intensities may result in current spread and the contamination of the desired DSSEP responses from a single dermatome with the responses from adjacent dermatomes or from neural structures located beneath the skin surface such as muscle stretch receptors.
In addition, the latencies of DSSEP responses have been shown to be related to stimulus intensities. For these reasons, attention must be paid to stimulation intensities. Minimally effective stimulation intensities should be utilized to elicit DSSEP responses and elevated stimulation intensities should be avoided.
Another method for eliciting SSEPs is to simultaneously present stimuli to a pair of extremities. Historically, this methodology has been discouraged because it was felt that the resulting responses could mask significant unilateral functional changes. However, bilateral stimulation may be of value when the responses that result from the stimulation of a single extremity are too small and/or variable to use for monitoring purposes.
When the neural tissues at risk are nerve roots, DSSEPs have been shown to be sensitive to changes in nerve root function.
In patients with large edematous legs, peroneal nerve stimulation may provide better responses than posterior tibial nerve stimulation.
Recording
Reliance only on the cortical responses can result in false positive changes because they are significantly affected by general anesthesia and blood pressure.
Because far fewer synapses are associated with mediating the subcortical response, anesthetic effects are far less pronounced than on the cortical responses. However, reliance on only subcortical responses can also result in false positive findings due to the quality of the subcortical responses, their generator sites, and other factors. As a result, it is advisable to utilize both cortical and subcortical recording sites.
When the spinal recording site is not available, such as during a posterior cervical procedure, the subcortical response can be recorded from one earlobe or linked earlobes.
Two peaks are generally used to define the amplitude of the cortical SSEP responses.
These two peaks, labeled N20 and P22, which result from median nerve stimulation, are considered to be waves of thalamic and cortical origin.
Two derivations have been suggested for recording these waves.
One is CPc (cortex contralateral to the stimulus---i.e. CP3 if the right arm is stimulated and CP4 if the left arm is stimulated)-Fz (midline frontal electrode).
The other is CPc-CPi (contralateral to ipsilateral--- i.e. CP3-CP4 or CP4-CP3 depending upon which arm is being stimulated).
Each of these is acceptable and each laboratoryshould choose what is appropriate. It is however critical to be able to record in either way.
In some patients with neurologic injury, the cortical responses may be of extremely low amplitude and hence a cortical response may be recorded using one derivation and not the other.
There are many ways in which to record the far-field subcortical potentials.
The P14 and N18 far field potentials are most likely generated in the caudal medial lemniscus and multiple generator sources in brainstem and thalamus, respectively, and are best recorded by using a derivation that includes ipsilateral (to the side of stimulation) centro-parietal cortex to a non-cephalic reference such as CP3-right Erb’s point for left median nerve stimulation.
Another subcortical response which is typically recorded is known as the cervical or N13 response. There is more than one method to record this response. One method is to use a cervical to Fz recording derivation. However, since the N13 has two components, one at the cervico-medullary junction and one generated in the cord, the placement of the cervical electrode could be critical depending on the surgical procedure being monitored.
Another method to record this response is to use Fz or Cz to linked ears. This recording montage has the advantage of recording responses with components from generators in the medulla or higher.
The peripheral potentials at the
brachial plexus are best recorded with electrodes over Erb’s point which is just 2 cm above the midpoint of the clavicle and at the angle between the clavicle and the posterior border of the head of the sternocleidomastoid muscle. The responses ipsilateral to the stimulation site are referenced to the opposite Erb’s point.Lower Extremity Stim
After stimulation of the posterior tibial nerve or the peroneal nerve, because of the anatomy of the somatosensory cortex, the major positive and negative peaks, P37 and N45, of the cortical responses are often of highest amplitude at CPz so that one derivation for recording the cortical responses is CPz-Fz (frontopolar electrode). However, because of “paradoxical lateralization” resulting from the lateral orientation of the dipole generator of the P37 peak, high quality cortical responses can also be recorded using CP3-CP4 or a CPz-CPc derivation.
The ability to record other derivations decreases the chance that a low amplitude cortical response will be overlooked.
Subcortical responses consisting of P31 and N34 waves originating from the brainstem (and analogous to the upper extremity N13 peaks) can be recorded either from CPi (ipsilateral with respect to the side of stimulation)-linked ears or a cervical-Fz derivation. Peripheral responses can be recorded using two electrodes on each leg; one placed at the popliteal fossa and the other placed three to four centimeters proximal to the popliteal fossa electrode.
Grounding
Although a ground electrode can be placed anywhere on the body, to reduce the amount of noise pickup, it is best to place it nearer rather than farther from the other recording electrodes. Placing it on a shoulder is generally a good site.
Multiple reference grounds are never used because they introduce ground loops which may introduce excess noise in the recordings and an earth ground should never be used for safety reasons because it provides an alternate path for the bovie current.
Keeping the recording input leads short and the electrode impedance values at 5 kohms or lower for gold disc or subdermal electrodes will help to minimize the amount of stimulus artifact and other electrical noise that is recorded.
However, the acquisition of some stimulus artifact can be useful because it demonstrates that the stimulators are functional when troubleshooting is necessary.
Recording Technique
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