The exact mechanism underlying POVL is poorly understood, but nearly all cases seem to involve ischemic optic neuropathy (ION). Posterior ION is more commonly the result of spinal procedures. The spine surgery associated risk factors include: prone position, Trendelenburg positioning, blood loss, prolonged procedure, use of vasoconstriction agents to correct blood pressure, direct ocular compression, and hypotension \[1,2\]. Given the fact that the suspected mechanism is a gradual infarction of the optic nerve, POVL could theoretically be prevented based on early detection of reversible optic nerve dysfunction. Visual evoked potentials (VEP) are a highly sensitive method for detecting optic nerve dysfunction, including ischemia.
During intraoperative neurophysiological monitoring (IONM), an emitted light passes through the lens, reaching the retina and causing hyperpolarization. The signal generated by the retina in response to a flash of light is known as the electroretinogram (ERG). This response may be recorded using conventional neurophysiological equipment and reflects overall retinal function. The VEP peaks are generated during the passage of electrical signal from the retina via the optic pathways to the primary visual cortex, and can also be registered during IONM. Measurements of VEP peaks under anesthesia can vary depending on type of anesthetic regimen, some producing more interference with the electrical impulses than others.
The problems encountered during VEP monitoring are numerous: a reusable device must be cleaned in between patients, tightly fitting goggles pose a risk of damaging the eyes, goggles may fall off or move intraoperatively and be difficult to reposition once the procedure is underway. Also, the light emitting diodes tend to be too weak to produce an adequate stimulus. This pilot study presents a new different approach from current methods. The SightSaverTM visual stimulator design consists of the following key advantages over the current technologies: hygienically superior to re-usable goggles, provides physical protection to the ocular region, use of higher intensity diodes and adhesive foam padding shaped to contours of periocular region.
This pilot, single center, prospective, randomized, two-arm study will involve 20 subjects at The Ohio State University Wexner Medical Center who are scheduled to undergo spine surgery that requires prone position and at least two hours of general anesthesia or TIVA and intraoperative neurophysiological monitoring. Eligible subjects that provide voluntary and written informed consent will be included in the study with a 1:1 randomization ratio of two arms, n=10 patients per each of two anesthesia regimen groups: general anesthesia and TIVA. All patients will wear the SightSaverTM visual stimulator. IONM of VEP with include insertion of half-inch needle electrodes placed just under the skin on either side of the eye, at the top of the forehead and three along the back of the head. Flashes of light, delivered at continuous intervals, will be emitted from diodes mounted in the SightSaverTM visual stimulator. After the baseline VEP is established for the patient, monitoring of VEP will occur at every 30 minutes during surgery. If during the monitoring process, a significant VEP waveform change has occurred, an alert will trigger the surgical staff to check four different parameters: technical issues with goggles, anesthetic changes, significant blood loss or a blood pressure change. Analysis of data collected will be used to evaluate the efficacy of SightSaverTM visual stimulator for intraoperative VEP-based detection of significant visual changes during prone spine surgeries.
