FEATURED ARTICLE

Spinal Cord Fiber Optic Monitoring with Diffuse Optics

Thomas F. Floyd, MD
Professor, Anesthesiology & Pain Management, Cardiothoracic Surgery, and Radiology
The University of Texas Southwestern

David R. Busch, PhD
Assistant Professor, Anesthesiology & Pain Management, Neurology and Neurotherapeutics
The University of Texas Southwestern

Dr. Sharma
Thomas F. Floyd, MD

Clinical Background
Iatrogenic neurological injuries arise during vascular procedures1, during resection of spinal cord tumors,2 during correction of spinal deformities,3 and during stabilization of spine fractures.4. Spinal deformity surgery carries a risk of spinal cord injury (SCI) that approaches 1%, 5, 6 with vascular compromise due to stretch/vasospasm contributing to ischemia.3,7 The majority of patients with SCI present with injuries that are potentially responsive to surgical intervention.8 Spinal cord ischemia or infarction after descending thoracic and thoracoabdominal aortic aneurysm repair is a devastating complication9 that has not been eliminated by intra-aortic stenting approaches. These injuries result in an impaired quality of life and significant financial burden related to long-term care. Finally, spinal cord injury is associated with a marked decrease in life expectancy, loss in quality of life, and disability, again with staggering associated long-term care costs.

Current Technology
Monitoring for spinal cord ischemia is conducted frequently in multiple clinical arenas as discussed above. Current methods available for assessment of spinal cord ischemia are indirect. They are based upon neuroelectrophysiological principles, especially somatosensory and motor evoked potentials (SSEP, MEP) which monitor the integrity of posterior spinal sensory pathways and the anterior and lateral spinal motor tracts, respectively. When combined, these modalities can help to identify injury, and can offer the surgeon insight into the impact of interventions and opportunities to limit or reverse injury. Accurate and timely interpretation of these data however, requires the presence of a skilled neurologist with expertise in neuroelectrophysiological monitoring. Neuroelectrophysiological monitoring, in turn, may be influenced by anesthetics, patient temperature, ischemia (cord and limb), and mechanical mechanisms.  “False negatives”, wherein patients awaken with important deficits in spite of “normal” evoked potentials, as well as “false positives”, wherein patients awaken without deficits in spite of loss or degradation of signal, have been reported with both SSEP10 and MEP11 monitoring,  even when used in a complimentary fashion.12 Further, neuroelectrophysiological alerts may be temporally delayed relative to the inciting event,13 this lack of temporal sensitivity diminishes the chance for rescue of threatened tissue.  To complicate the situation further, recovery of signals after rescue attempts are also markedly delayed, leaving the surgeon in a quandary as to how to proceed. MEPs can only be performed in heavily sedated patients due to the pain involved with the application of high currents to the scalp.  SSEPs can be performed in awake patients, but the electrical stimulation is still painful. Therefore, no spinal cord monitoring modality currently exists for monitoring patients continuously in the ICU setting.

A 2012 evidence-based guideline update, entitled, “Intraoperative Spinal Monitoring with Somatosensory and Transcranial Electrical Motor Evoked Potentials: A Report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology and the American Clinical Neurophysiology Society,” noted that only 16%–40% of the intraoperative monitored patients with electrophysiological changes judged to be significant actually developed postoperative-onset paraparesis, paraplegia, or quadriplegia.14 Nevertheless, despite the well documented deficiencies of this methodology, MEP and SSEP monitoring are still considered the “gold standard” for functional monitoring of the spinal cord for spine, spinal cord, and aortic surgery, and their use is strongly supported by professional societies such as the Scoliosis Research Society15 and liability insurers.  Demand for the use of a monitoring technology with so many deficiencies speaks to the perceived risk involved with the related surgical procedures. No technology is therefore currently available to directly, immediately, reliably, and continuously monitor for spinal cord ischemia, representing a critical gap in current neurological monitoring capabilities. Neurologists, surgeons, anesthesiologists, neuro-intensivists, and neurocritical care nurses simply need a better tool.

Diffuse Optical Monitoring
Diffuse Optical Spectroscopy (DOS) techniques have been widely employed to facilitate real-time monitoring of hemoglobin concentrations. A simple version of this technology is now commonly used in “NIRS” cerebral oximeters. The technique is relatively inexpensive, portable, and has other advantages such as high temporal resolution, non-invasiveness, and the ability to probe deep tissues. DOS measurements with modulated sources (~100 MHz) relate the wavelength-dependent detected light intensity and phase to the concentration of tissue chromophores, such as oxy-hemoglobin (Hgb-O2) and deoxy-hemoglobin (Hgb), in the microvasculature. The relative changes in DOS light signals can then be used to determine relative changes of Hgb-O2 and Hgb concentrations.

Diffuse Correlation Spectroscopy (DCS) is a newer technique (e.g., compared to DOS/NIRS) for continuous non-invasive measurement of microvascular blood flow in deep tissues.17 This technique quantifies the temporal fluctuations of detected NIR light which are due to the motion of red blood cells (RBCs). Specifically, the temporal intensity autocorrelation function of the transmitted speckle is measured to characterize these fluctuations and derive a blood flow index. Larger red blood cell flux (due to an increase in velocity and/or RBC concentration) produces more rapid fluctuations of light intensity, and faster decay of the autocorrelation function. The ability of DCS to detect blood flow changes has been validated extensively in brain,17-19 and in a wide variety of other clinical scenarios,20 and through comparison to a variety of other technologies (ASL-MRI, Xe-CT).19,21  DOS and DCS signals, when employed together, facilitate continuous measurements of both blood flow and tissue oxygenation, which can be combined to measure relative changes in tissue oxygen metabolism. The complementary applications of both DOS and DCS have therefore been exploited in multiple neurocritical care and operating room settings.19, 22-25 In addition, though it is based on a qualitatively different signal, DCS shares many attractive features of DOS (NIRS), including portability and non-invasiveness. In important contrast to laser Doppler flow approaches, DCS can probe and resolve deep tissues.

Spinal Cord Diffuse Optical Monitoring
Readers are quite familiar with surface probes employed for cerebral oximetry employing similar physical principles. Somewhat problematically, these surface probes sample blood not only in the cortical brain, but also from underlying skin and bone, potentially impacting the accuracy and reliability of the data. Over the past several years we have developed and conducted extensive pre-clinical testing with thin and highly flexible epidural fiber optic probes allowing for the real-time and continuous monitoring of spinal cord oxygenation and blood flow. These probes can be placed within the epidural space at the time of surgery via laminotomy, or percutaneously using methods familiar to anesthesiologists and surgeons which carry exemplary safety profiles. The epidural position of the probes avoids the potentially contaminating interference of the skin and bones seen with surface probes and allows for the continuous monitoring of the spinal cord, a vulnerable organ that heretofore has frustrated our need for timely and reliable monitoring.

Initial work in this area tested the application of probes capable of monitoring a single level in the spinal cord, using a single light source and detector series. Early results testing these probes demonstrated that the probes were highly sensitive to changes in spinal cord blood flow induced by physiological interventions such as hypercarbia, hypoxia, and acute hypertension and hypotension, as well as aortic occlusion.26  Further work demonstrated that the single-level probes were reasonably accurate in comparison to microsphere measurements of blood flow changes, were capable of precise measurement in repeatability studies, able to axially discriminate levels of ischemia within the cord in response to aortic occlusion at varying levels.27 Critically, the probes were able to detect ischemia immediately, within seconds, while motor evoked potentials changes were delayed beyond 10 minutes.27 These probes have additionally demonstrated their utility in simulated spine-distraction surgery.28 Finally, these single-level probes were not found to elicit spinal cord injury in both histopathological and neurological evaluations.29

Subsequently we have developed a fiberoptic probe capable of monitoring multiple levels within the spinal cord simultaneously. 30  This probe may also be placed both percutaneously and via laminotomy and is undergoing extensive pre-clinical testing. This new probe has demonstrated the ability to successfully monitor multiple regions of the spinal cord and the ability to axially discriminate alterations in flow and the onset of ischemia. See figure below.

Figures a, b, c

Summary
The ability to measure spinal cord blood flow and oxygenation using DOS and DCS fiber optics may: 1) facilitate reliable, expedient, and easily interpretable monitoring for spinal cord ischemia, 2) enable continuous bedside monitoring and continuity between the operating room and the neuro-intensive care unit, 3) provide an earlier opportunity to therapeutically address spinal cord ischemia and offer immediate and reliable feedback as to the success of the intervention in restoring blood flow and tissue oxygenation, 4) allow for preoperative or pre-stent deployment of mapping of regional spinal cord vulnerability, and 5) provide critical data in the laboratory and clinic for assessment of the efficacy of therapeutic approaches to ameliorate ischemia.

Acknowledgements

We wish to acknowledge the contributions of our many collaborators over the years to include Arjun Yodh (University of Pennsylvania), Wei Lin, Chia Chieh Goh, Robert Galler, James Barsi, and Tom Bilfinger (Stony Brook), Rickson Mesquita (University of Campinas, Brazil), and Matthias Pelz, Feng Gao, and Nick Larson (University of Texas Southwestern).  This work has been supported by the Craig H. Neilsen Foundation and NIH U01-NS095761.

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