Somatosensory-Evoked Potentials

SSEPs have been used as monitoring tools during surgical procedures since the 1960s. SSEPs are the most widely used monitoring modality and are routinely used during brain, spinal, and peripheral surgical procedures. SSEPs are optimal for protection of the patient’s ascending sensory spinal pathways (dorsal column) during high-risk surgical procedures. These ascending dorsal column pathways mediate stereognosis, proprioception, tactile discrimination, vibration sensation, and form recognition.

Upper and lower extremity SSEP monitoring is usually performed during all surgical procedures of the cervical spine. Median nerve (C5 to T1 roots) and ulnar nerve (C8 to T1 roots) SSEPs are frequently performed in the upper extremities, whereas in the lower extremities, posterior tibial nerve (L4 to S2) and peroneal nerve (L4 to S1) SSEPs are typically performed. SSEPs are generated by a low-intensity (≈25 mA) electrical stimulation of peripheral nerves in the hands and feet ( Table 11-1 ). Recordings made at multiple locations along the sensory pathway (brachial plexus and popliteal fossa, brainstem, and somatosensory cortex) can determine the anatomic and functional integrity at different locations as the signal travels from the periphery to the cortex. SSEPs are also useful in detection of mechanical and ischemic changes in the peripheral nerves, posterior spinal cord, and cerebral cortex.

Electrical stimulation of nerves in upper and lower extremities produces major positive and negative deflections as signals travel along the somatosensory pathway. The negative potential (N20) is recorded from the somatosensory cortex at the scalp corresponding to upper extremity stimulation (median and ulnar nerves). Conversely, a positive potential (P37) is recorded from the somatosensory cortex at the scalp corresponding to lower extremity stimulation (posterior tibial and peroneal nerves) ( Fig. 11-1 ).

From left to right, Somatosensory-evoked potential (SSEP) data showing left median, right median, left posterior (post.) tibial, and right posterior (post.) tibial nerve SSEP responses. Cortical responses are C4′-Fpz, C3′-Fpz, Cz′-Fpz, C3′-C4′, and C4′-C3′. Subcortical responses are CV5-Fpz. Peripheral responses are Erb (brachial plexus) point (Erb). C3 ′, Placed at CP3; C4 ′, placed at CP4; Cz ′, placed at CPz; Cv5, placed at the fifth cervical spine; Lt., left; Rt., right.

Adequacy of stimulation, peripheral limb perfusion, and peripheral nerve compression can be monitored by a peripheral response recorded at the level of the brachial plexus (for the upper extremities) or the popliteal fossa (for the lower extremities). In SSEPs, peripheral and subcortical responses are less sensitive to anesthesia and are frequently used to differentiate SSEP monitoring changes resulting from anesthesia and surgical manipulation. The SSEP responses are transmitted by the fasciculus cuneatus (upper extremity) and the fasciculus gracilis (lower extremity). Therefore, the spinothalamic pathways for pain and temperature are not covered by SSEP monitoring. SSEP uses three orders of neurons. First order neurons carry the signal from the stimulation site, enter the spinal cord through the dorsal root entry zone (DREZ), and travel upward in the dorsal columns forming the fasciculus cuneatus (upper extremity) and the fasciculus gracilis (lower extremity). These fibers make the first synapse with the second order neurons at the nucleus cuneatus (upper extremity) and the nucleus gracilis (lower extremity) in the lower medulla. This is followed by decussation at the medial lemniscus and the making of another synapse, with third order neurons in the thalamus. The third order neurons travel to the somatosensory cortex and terminate in the postcentral or somatosensory gyrus.

Usually, any changes of 50% or more in amplitude or 10% or more in latency of SSEP responses are considered significant and must be reported to the surgeon. Such changes should prompt a search for an intervention to reverse the procedure to reduce the risk of permanent damage.

Transcranial Electrical Motor-Evoked Potentials

Since their discovery in the 1980s, TCeMEPs have emerged as extremely useful tools for activation and monitoring of the corticospinal tracts. Many research studies have proven that the evaluation of functional integrity of the descending corticospinal tracts by TCeMEPs during high-risk neurosurgical and orthopedic procedures makes a significant difference in the motor outcome of patients.

TCeMEPs assess in real time the function of motor pathways in the spinal cord and reduce the risk of paralysis. They also help in detecting ischemic changes in the motor cortex, spinal cord, and peripheral motor nerves. Since 2000, further research has refined the use of TCeMEP techniques during high-risk spinal procedures. The Stagnara wake-up test was the only way to assess the functional integrity of the motor pathways during spine operations before the introduction of TCeMEP monitoring. The Stagnara wake-up test involved waking patients up during surgical procedures and asking them to move their feet. This technique can be difficult in noncompliant patients (e.g., language issues, hearing impairment) and can also delay the surgical procedure. Although patients do not feel any pain and have no memory of the test postoperatively, this method does not give continuous feedback to the surgeon during critical surgical steps, and the surgeon must stop the procedure to perform the test.

When the Stagnara wake-up test result is positive and the patient cannot move one or both feet, the period between incurring the injury and performing the test may be prolonged. Because of this potential delay between the injury and the test, a risk exists of missing the critical period during which an intervention may have been performed to reverse the neurologic insult.

In the past, SSEPs comprised the only modality performed during spine procedures. The assumption was that any damage to the spinal cord would affect both ascending sensory and descending motor pathways in the spinal cord. In some reported cases, no change in SSEP data was noted, and the patient woke up with motor deficits. TCeMEPs are very sensitive in predicting postoperative motor deficits as compared with SSEPs. This difference may be related to the disparate blood supplies to the ventral and dorsal spinal cord. The blood supply for dorsal columns that supply SSEPs is through the posterior spinal artery. However, the anterior spinal artery is the major source of perfusion to the anterior and lateral corticospinal tracts. If the anterior spinal artery is compressed, it may result in loss of TCeMEP responses while leaving SSEPs unaffected. Intraoperative loss of muscle MEPs indicates some postoperative impairment of voluntary motor control with a specificity of approximately 90% and a sensitivity of 100%.

During spinal surgical procedure, an anode and a cathode are placed on the patient’s scalp. Multipulse electrical anodal stimulation transcranially through these electrodes transcranially results in activation of upper motor neurons. The signals are transmitted to lower motor neurons at anterior horn cells in the spinal cord that synapse with distal muscle fibers through the neuromuscular junction, with resulting compound muscle action potentials ( Fig. 11-2 ). The downward volley can be recorded over the spinal cord by placing an epidural electrode. These responses are generated by direct activation (direct or D waves) and indirect activation (indirect or I waves).

Transcranial electrical motor-evoked potentials (TCeMEPs). Left, Responses from muscles of the left (L) upper and lower extremity muscles. Right, Responses from muscles of the right (R) upper and lower extremity muscles. AH, Abductor hallucis; APB, abductor pollicis brevis; ADM, abductor digiti minimi; BR, brachioradialis; BICEP, biceps brachii; DELT, deltoid; EHB, extensor hallucis brevis; FCU, flexor carpi ulnaris; PL, peroneus longus; TA, tibialis anterior.

For TCeMEP stimulation, electrodes are placed at the C3 and C4 position or the C1 and C2 position. The recording electrodes are placed in all four extremities. Multiple stimulation and recording sites should be selected especially in patients in whom TCeMEPs are difficult to elicit, such as patients with myelopathy. Increasing the stimulation intensity results in spatial summation, whereas increasing the number of stimulation trains results in temporal summation. The stimulation rate is set between 200 and 500 Hz ( Table 11-2 ). The latency of responses in upper extremity muscles is earlier than in lower extremity muscles. TCeMEP responses are very sensitive to neuromuscular blocking agents and are somewhat sensitive to inhalational anesthetic agents and infused agents. Appropriate anesthetic technique must be used for adequate monitoring. The alarm criteria for TCeMEP consist of various factors, including all or none response, changes in signal morphology, changes in stimulation intensity, and changes in amplitude.

Table 11-2

Transcranial Electrical Motor-Evoked Potential Setup

Stimulus Parameters
Pulse	Electric monopolar rectangular
Duration	50 μsec
Intensity	100-1000 V
Interstimulus interval (ISI)	1.1-9.0
Trains	1-9
Averages	1
Recording Parameters
Low cut	10 Hz
High cut	3000-10,000 Hz
Sweep	10 msec/Div
Gain	100 μV/Div
Reject	Off
Upper extremity	Delt, FCU, BR, APB, ADM
Lower extremity	Quads, TA, MG, AH, EHB

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