Intraoperative Neurophysiologic Monitoring

Somatosensory Evoked Potentials (SSEPs)

SSEPs are elicited by the averaging of neural responses to electrical stimuli that are applied to peripheral nerves. Averaging is necessary for sensory evoked potentials because the response to each stimulus is small relative to background electrical activity. Typical stimulation sites for intraoperative SSEPs are the median or ulnar nerve at the wrist and the posterior tibial nerve at the ankle or knee. Recordings are typically made at both peripheral sites (to verify that the stimulation is constant) and central sites in the spinal cord and brain.

The pathway for upper limb SSEPs includes the ulnar (or median) nerve, the brachial plexus, the tractus cuneatus in the posterior column of the cervical cord, the nucleus cuneatus, the medial lemniscus, the ventral posterior nucleus of the thalamus, the thalamocortical radiations, and the primary somatosensory cortex along the lateral convexity of the anterior parietal lobe. Recordings are typically made at Erb's point (located on the side of the neck, 2 to 3 cm above the clavicle, at the lateral root of the brachial plexus), the cervical spine, and the scalp overlying the somatosensory cortex.

The pathway for lower limb SSEPs includes the posterior tibial nerve (usually stimulated at the ankle), the tractus gracilis in the posterior column of the thoracic and cervical spinal cord, the nucleus gracilis, the medial lemniscus, the ventral posterior nucleus of the thalamus, the thalamocortical radiations, and the primary somatosensory cortex along the midline. Potentials are typically recorded at many locations, including the popliteal fossa, lumbar spine, and scalp overlying the leg region of the somatosensory cortex.

SSEP monitoring is widely used, because it is sensitive to ischemia; responsive to physical manipulation of peripheral nerves, spinal cord, and parietal cortex; and can be elicited easily. SSEPs and EEGs are the two neurophysiologic recordings used most widely during surgery.

Auditory Brain Stem–Evoked Responses (BAERs)

BAERs are elicited by the presentation of brief auditory clicks to the ear. The auditory pathway includes the cochlea, the cochlear nerve, and multiple pathways through the lower pons, the lateral lemniscus, and the inferior colliculus of the midbrain. Auditory potentials above the midbrain cannot be recorded in the operating room because these potentials are suppressed by general anesthesia. BAERs are very sensitive to both stretch and compression of the cochlear nerve. Mild to moderate stretch results in increased latency with smaller effects on amplitude. Compression typically reduces the amplitude of the BAERs in a graded manner.

Visual Evoked Responses (VEPs)

VEPs are elicited by either flashes of a strobe light positioned 20 to 30 cm in front of a patient's eyes or by flashes from light-emitting diodes (LEDs) embedded in eye goggles. The flashes activate, sequentially, the layers of the retina (yielding an electroretinogram), the fibers in the optic nerve and tract, the lateral geniculate nucleus in the thalamus, and the visual cortex in the occipital lobe. Although VEPs are a staple of diagnostic neurophysiology laboratories, they are infrequently used in the operating room. Reasons for this include the low number of cases in which the visual system is at risk, the high sensitivity of VEPs to changing levels of anesthesia, and the substantial variability of their amplitudes intraoperatively. Nevertheless, VEPs can be useful in cases of tumor resection in or near visual structures and in cases involving the posterior circulation of the brain.

Motor Evoked Potentials (MEPs)

MEPs are elicited by transcranial electrical stimulators that deliver trains of high energy, which are closely spaced electrical pulses. In normal patients, such multi-pulse transcranial electrical stimulation can evoke muscle contractions (despite the general anesthesia) by facilitating the motor neurons in the spinal cord. In patients with significant paresis , however, monitoring of the motor system may require more invasive epidural recording, if it can be accomplished at all.

MEPs are typically recorded bilaterally from muscles in both hands and both legs during spine surgery even if the surgical site is below the cervical spine. The MEPs from the hand muscles serve as controls for systemic factors during thoracic or lumbar surgery. MEPs do not require signal averaging, because MEPs are large relative to ongoing background muscle activity. Direct spinal cord recordings are used much less frequently than are MEPs from muscle, but direct recordings are extremely helpful during invasive spinal cord surgeries (e.g., resection of intramedullary spinal cord tumors ).