Brainstem evoked potentials are obtained by stimulating the auditory system with click stimuli. The neural action potentials between the cochlea and the medial geniculate body are evaluated based on the brainstem evoked response. Brainstem evoked responses are sensitive to disturbances of the auditory nerve and to retractor disturbances of the brainstem.
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Near-infrared absorption measurements are used to calculate an index of cerebral venus oxygen saturation
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Language, motor, and sensory areas of the exposed cortex can be identified by direct brain recording or stimulation. The somatosensory response is recorded from the surface of the brain. Language areas are determined by verbal responses during brain stimulation in the awake patient. Motor areas of the cortex and the internal capsule are determined from EMG and evoked by electric brain stimulation.
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On going EEG activity from the scalp is recorded from a multi-electrode array and subjected to a fast Fourier transform (FFT) analysis producing a frequency/power spectrum that can be monitored to assess changes in the metabolic state of cortical structures.
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Spinal Nerve Roots: Spontaneous EMG monitoring warns of potential nerve damage during manipulation. Evoked EMG stimulation of the pedicle screws can indicate a misdirected screw placement.
Cranial Nerves: The evaluation of spontaneous and evoked electromyogenic activity can be extremely useful in monitoring cranial nerve function. Spontaneous activity in the distribution of these nerves can signal mechanical disturbance of the cranial nerve.
Peripheral Nerves: The function of peripheral motor nerves can be evaluated during surgery by means of spontaneous and evoked electromyographic activity.
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Somatosensory evoked responses are obtained by stimulating a peripheral nerve at the wrist and/or ankles. The propagation of the sensory action potentials is evaluated along their pathways to the cortex. This modality gives information about the integrity of the sensory pathways. It is also highly correlated with preservation of the spinal cord motor pathways.
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Blood flow velocity is measured from intracranial arteries using Doppler ultrasound. This technique is used to detect emboli, vasospasm and inadequate blood flow to cortical structures. Microvascular Doppler can also be used to directly assess blood flow within and through an aneurysm.
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Motor Evoked Potentials are obtained by transcranial electric stimulation of the cortical motor areas. Monitoring the evoked muscle responses or the descending potentials from the spinal cord (The “D” wave) can assess the integrity of the motor pathways.
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All Spinal Surgeries that Place the Spinal Cord at Risk Including:
Visual evoked potentials are obtained from flash stimulation of the eye, and are measured from the occipital region. They may provide useful information regarding the function of the pre-chiasmal visual pathways, but are labile and require careful anesthetic control. Additionally, retrochiasmal functional changes are difficult to assess using flash visual evoked potentials.
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