To obtain a clinically useful method of intraoperative monitoring of visual evoked potentials (VEPs), the authors developed a new light-stimulating device and introduced electroretinography (ERG) to ascertain retinal light stimulation after induction of venous anesthesia.
The new stimulating device consists of 16 red light-emitting diodes embedded in a soft silicone disc to avoid deviation of the light axis after frontal scalp-flap reflection. After induction of venous anesthesia with propofol, the authors performed ERG and VEP recording in 100 patients (200 eyes) who were at intraoperative risk for visual impairment.
Stable ERG and VEP recordings were obtained in 187 eyes. In 12 eyes, stable ERG data were recorded but VEPs could not be obtained, probably because all 12 eyes manifested severe preoperative visual dysfunction. The disappearance of ERG data and VEPs in the 13th eye after frontal scalp-flap reflection suggested technical failure attributable to deviation of the light axis. The criterion for amplitude changes was defined as a 50% increase or decrease in amplitude compared with the control level. In 1 of 187 eyes the authors observed an increase in intraoperative amplitude and postoperative visual function improvement. Of 169 eyes without amplitude changes, 17 manifested improved visual function postoperatively, 150 showed no change, and 2 worsened (1 patient with a temporal tumor developed a slight visual field defect in both eyes). Of 3 eyes with intraoperative VEP deterioration and subsequent recovery upon changing the operative maneuver, 1 improved and 2 exhibited no change. The VEP amplitude decreased without subsequent recovery to 50% of the control level in 14 eyes, and all of these developed various degrees of postoperative deterioration of visual function.
With the strategy introduced here it is possible to record intraoperative VEPs in almost all patients except in those with severe visual dysfunction. In some patients, postoperative visual deterioration can be avoided or minimized by intraoperative VEP recording. All patients without an intraoperative decrease in the VEP amplitude were without severe postoperative deterioration in visual function, suggesting that intraoperative VEP monitoring may contribute to prevent postoperative visual dysfunction.
"IOM of the visual pathway is necessary in diverse types of surgeries, including transsphenoidal surgery, aneurysm clipping of the posterior circulation, and removal of tumors that lie near the optic radiation. Recently, it has gained attention with advances in both monitoring and anesthetic techniques (32, 33). However, the usefulness of intraoperative VEP has only been demonstrated in a limited number of studies (32, 33), and there seems to be some debate regarding the correlation of intraoperative VEP changes and postoperative visual outcomes (34, 35). "
[Show abstract][Hide abstract] ABSTRACT: The recent developments of new devices and advances in anesthesiology have greatly improved the utility and accuracy of intraoperative neurophysiological monitoring (IOM). Herein, we review the basic principles of the electrophysiological methods employed under IOM in the operating room. These include motor evoked potentials, somatosensory evoked potentials, electroencephalography, electromyography, brainstem auditory evoked potentials, and visual evoked potentials. Most of these techniques have certain limitations and their utility is still being debated. In this review, we also discuss the optimal stimulation/recording method for each of these modalities during individual surgeries as well as the diverse criteria for alarm signs.
Journal of Korean medical science 09/2013; 28(9):1261-1269. DOI:10.3346/jkms.2013.28.9.1261 · 1.27 Impact Factor
"Monitoring of the prechiasmatic visual pathways in anesthetized patients undergoing surgical and endovascular procedures, presents a particular challenge to the monitoring team. Transcranial VEP is unreliable for monitoring these visual pathways and is associated with poor reproducibility and clinical correlation. The reason for this poor correlation is that the visual fibers from each retina travel to both occipital poles such that nonspecific illumination of the entire retina currently makes it impossible to reliably detect small changes in a specific anatomical region of the retina until there is irreversible retinal damage. "
[Show abstract][Hide abstract] ABSTRACT: Intraoperative neuromonitoring (IONM) is used for real-time evaluation of neuronal tracts and reflexes in the anesthetized patient, when a neurologic exam is not possible. Changes in IONM signals forewarn of possible neurological deficit. This real-time feedback allows for immediate alterations in therapeutic technique by the treating physician. Transcranial visual evoked potentials are not reliable for evaluating the integrity of the prechiasmatic visual system. Electroretinography (ERG) has been used in animal models for monitoring retinal ischemia and can be used in humans as well to monitor for prechiasmatic ischemia of the retinae and optic nerves.
We present a case where ERG signal amplitude and latency changed during ophthalmic arteriovenous fistula embolization, resulting in an intraprocedural decision to refrain from embolization of additional arterial pedicles to preserve vision. After awakening from general anesthesia, the patient had no deficits in visual acuity or field testing, but did complain of transient pain with eye movement that resolved the next day and worsened with episodes of blood pressure elevation.
ERG may be helpful for detecting prechiasmatic ischemic changes during endovascular procedures and may provide early warning signs to the surgeon before the onset of permanent retinal damage. Further investigation is needed to assess the utility of ERG monitoring during the treatment of orbital and periorbital vascular lesions.
Surgical Neurology International 03/2013; 4:40. DOI:10.4103/2152-7806.109653 · 1.18 Impact Factor
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