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July 1, 2013 Volume 77, Number 7
Subspecialty News: New Developments in Neuroanesthesiology Jeffrey J. Pasternak, M.S., M.D.


The practice of contemporary neuroanesthesiology demands that the anesthesiologist have a fundamental understanding of the primary implications of neurologic diseases as well as the secondary changes that neurologic diseases can have on systemic physiology, or alternatively that systemic diseases can have on neurologic function. Additionally, our fundamental understanding is evolving for many disease processes, factors affecting patients’ risk for injury and options for reducing risk. In this overview, I will briefly review four topics currently on the forefront of research and innovation in neuroanesthesiology: anesthetic-induced neurotoxicity, perioperative blindness, brain monitoring, and neuroanesthesiology subspecialty training.

Is it possible that certain anesthetic drugs that protect neurons from injury in some instances can cause neuronal injury in others? In the past, much research focused on the neuroprotective effects of anesthetic drugs. Although that area of research is still flourishing, new lines of investigation have identified potentially neurotoxic effect of anesthetics. This research suggests that the brains of the very young and very old are possibly the most susceptible to anesthetic neurotoxicity.1 In children, exposure to multiple anesthetics correlates with an increased incidence of long-term developmental, learning and behavioral disorders, although it is not clear if this association results from a direct toxic effect of anesthetics or occurs because “sicker” children require more surgery and more anesthetics (i.e., thus chronic illness is a confounding factor in interpreting the data).2,3 In elderly patients, anesthetic neurotoxicity may manifest as postoperative cognitive dysfunction.4 Although the mechanisms for these associations are not clear, current data suggest that, depending on the age group, anesthetic activation of neuronal apoptotic pathways, alterations in synaptogenesis and synaptic plasticity, neurogenesis and differentiation, and, more recently, anesthetic-induced neuronal accumulation of tau protein and inflammation may all play a role. For readers interested in anesthetic neurotoxicity, I refer you to the March 2013 issue of Anesthesiology that contains a collection of recent articles describing novel findings and accompanying editorials relevant to this topic.

Postoperative visual loss (POVL) is a rare but devastating complication following a variety of surgical procedures, but is best known for its association with major spine surgery. Earlier published data describing POVL were limited to case reports and series, making it difficult to identify contributing factors. In the January 2012 issue of Anesthesiology, the Postoperative Visual Loss Study Group compared perioperative factors between a group of 80 subjects with vision loss following spine surgery who were enrolled in the ASA Postoperative Visual Loss Registry and a reference group of 315 matched patients who underwent spine surgery but who did not develop POVL.5 Data from reference patients were obtained from 17 major academic medical centers and all procedures were performed in the prone position. Following multivariate logistic regression analysis, male sex, obesity, use of the Wilson surgical frame, anesthesia duration and estimated blood loss were associated with an increased risk of POVL. In contrast, use of colloid was associated with a reduced risk of POVL (Table 1). The authors theorized that venous congestion within the optic system may contribute to POVL. Additionally, they note that anesthesia duration was used as a surrogate marker of surgical duration as this metric was more reliably acquired from the medical record than the duration of surgery.

In recent years, there have been many advances in bedside brain monitoring devices. These include noninvasive techniques such as cerebral near infrared oximetry, and invasive modalities, including intraparenchymal oxygen partial pressure monitoring and cerebral microdialysis. When used in conjunction with older technology, such as intracranial pressure monitoring and transcranial Doppler sonography, these tools have advanced our understanding of human cerebral physiology in both health and disease. For example, there has been speculation on the nature of the lower limit of cerebral autoregulation (LLA). By calculating the Pearson correlation coefficient between mean arterial blood pressure and other variables such as intracranial pressure, cerebral blood flow velocity, cerebral hemoglobin oxygen saturation or brain oxygen partial pressure, one can identify the point where blood flow becomes inadequate and brain metabolism suffers.6 This technique holds promise for individually tailoring clinical management in a single patient.

Cerebral microdialysis involves the continuous sampling of small amounts of brain extracellular fluid. Quantification of brain lactate-to-pyruvate ratio, glycerol concentration and glutamate concentration can indicate the extent of anaerobic metabolism, cell membrane lysis and excitotoxicity, respectively. Future technology may expand the type and number of biomarkers monitored (e.g., inflammatory markers), increasing our understanding of brain pathophysiology and allowing individually tailored clinical management.

Clearly, taking advantage of the aforementioned technical and conceptual issues will require the training and education of tomorrow’s neuroanesthesiology providers. To address this issue, in the January 2013 Journal of Neurosurgical Anesthesiology, Mashour and other members of the Society for Neuroscience and Critical Care published guidelines to help structure neuroanesthesiology fellowship training programs in the United States.7 These recommendations were made based on a review of current fellowship programs and a survey of program directors, and discussions with program directors and among task force members. A strong consensus was achieved to not seek accreditation by the American College of Graduate Medical Education at this time. The task force designed a program that consisted of four-week modules of training devoted to the activities outlined in Table 2. Within each of these modules, the group provided minimum recommendations for learning objectives, goals and expectations. The team also provided standards for program directors and faculty, including a minimal requirement for clinical and scholarly activity.

The topics of anesthetic neurotoxicity, perioperative blindness, innovations in brain monitoring and neuroanesthesiology subspecialty training represent only a small subset of the new developments in neuroanesthesiology. For readers interested in a more thorough and comprehensive review of the literature relevant to the perioperative care of patients with neurologic diseases, I refer you to the April issue of the Journal of Neurosurgical Anesthesiology, where literature relevant to neuroanesthesiology is annually reviewed in an article titled “Neuroanesthesiology Update.”



Jeffrey J. Pasternak, M.S., M.D. is an Associate Professor of Anesthesiology, and Chair of the Division of Neurosurgical Anesthesiology, Mayo Clinic College of Medicine, Rochester, Minnesota.

References:
1. Brambrink AM, Orfanakis A, Kirsch JR. Anesthetic neurotoxicity. Anesthesiol Clin. 2012;30(2):207-228.
2. Wilder RT, Flick RP, Sprung J, et al. Early exposure to anesthesia and learning disabilities in a population-based birth cohort. Anesthesiology. 2009;110(4):796-804.
3. Sprung J, Flick RP, Katusic SK, et al. Attention-deficit/hyperactivity disorder after early exposure to procedures requiring general anesthesia. Mayo Clin Proc. 2012;87(2):120-129.
4. Terrando N, Brzezinski M, Degos V, et al. Perioperative cognitive decline in the aging population. Mayo Clin Proc. 2011;86(9):885-893.
5. Postoperative Visual Loss Study Group. Risk factors associated with ischemic optic neuropathy after spinal fusion surgery. Anesthesiology. 2012;116(1):15-24.
6. Joshi B, Ono M, Brown C, et al. Predicting the limits of cerebral autoregulation during cardiopulmonary bypass. Anesth Analg. 2012;114(3):503-510.
7. Mashour GA, Avitsian R, Lauer KK, et al. Neuroanesthesiology fellowship training: curricular guidelines from the Society for Neuroscience in Anesthesiology and Critical Care. J Neurosurg Anesthesiol. 2013;25(1):1-7.