Andres A. Gonzalez

Intraoperative neurophysiological monitoring during spine surgery: a review.

Spinal surgery involves a wide spectrum of procedures during which the spinal cord, nerve roots, and key blood vessels are frequently placed at risk for injury. Neuromonitoring provides an opportunity to assess the functional integrity of susceptible neural elements during surgery. The methodology of obtaining and interpreting data from various neuromonitoring modalities-such as somatosensory evoked potentials, motor evoked potentials, spontaneous electromyography, and triggered electromyography-is reviewed in this report. Also discussed are the major benefits and limitations of each modality, as well as the strength of each alone and in combination with other modalities, with regard to its sensitivity, specificity, and overall value as a diagnostic tool. Finally, key clinical recommendations for the interpretation and step-wise decision-making process for intervention are discussed. Multimodality neuromonitoring relies on the strengths of different types of neurophysiological modalities to maximize the diagnostic efficacy in regard to sensitivity and specificity in the detection of impending neural injury. Thorough knowledge of the benefits and limitations of each modality helps in optimizing the diagnostic value of intraoperative monitoring during spinal procedures. As many spinal surgeries continue to evolve along a pathway of minimal invasiveness, it is quite likely that the value of neuromonitoring will only continue to become more prominent.

Operative strategies for minimizing hearing loss and other major complications associated with microvascular decompression for trigeminal neuralgia.

OBJECTIVE To retrospectively assess the surgical outcomes and complication rates following microvascular decompression (MVD) for trigeminal neuralgia, using a targeted, restricted retrosigmoid approach. METHODS During the period 1994-2009, a total of 119 patients underwent MVD for trigeminal neuralgia. A retrospective review was conducted in order to assess pain outcomes following surgery and at most recent follow-up. The intraoperative findings, Barrow Neurologic Institute (BNI) pain scores, medication usage, brainstem auditory evoked potential records, and complication rates (including postoperative hearing status) were reviewed and subsequently analyzed. RESULTS Of the 119 patients who underwent MVD, 61 (51%) were male and 58 (49%) were female. The mean age was 60 years (range 22-86 years). Operative findings included 94 patients (79%) with arterial compression, 16 patients (13%) with isolated venous compression, 1 patient (1%) with a small arteriovenous malformation, and 8 patients (7%) with no obvious source of compression. No perioperative deaths or major complications, including hearing loss, occurred in any patients. Minor complications occurred in 9 patients (8%), including a transient trochlear nerve palsy in 1 patient, transient nystagmus in 1 patient, cerebrospinal fluid leak requiring revision in 1 patient, wound infections requiring revision in 3 patients, and wound infections requiring antibiotics alone in 3 patients. Follow-up data were available for 109 patients, of whom 88 (81%) had excellent outcomes (BNI Score I-II). Ninety-eight patients (90%) had good outcomes (BNI scores I-IIIb), 7 patients (6%) had persistent pain that was not controlled with medications (BNI Score IV), and 4 patients (4%) experienced no relief following surgery (BNI Score V). CONCLUSION The use of a small craniectomy (<20 mm) in conjunction with a restricted retrosigmoid approach, inferolateral cerebellar retraction, and maintenance of the vestibular nerve arachnoid may minimize complications and optimize surgical outcomes associated with microvascular decompression for trigeminal neuralgia.

The Use of Transcranial Magnetic Stimulation for Monitoring Descending Spinal Cord Motor Function

This report describes our initial clinical experience using transcranial magnetic stimulation for monitoring spinal cord motor function during surgical procedures. Motor evoked potentials were elicited using a cap shaped coil placed on the scalp of 27 patients while recording peripheral motor responses (compound muscle action potentials – CMAPs) from the upper (N=1) or lower limbs (N=26). Wherever possible, cortical somatosensory responses (SEPs) were also monitored by electrically stimulating the left and right posterior tibial nerve (N=25) or the median nerve (N=1). The judicious choice of anesthetic regimens resulted in successfully obtaining motor evoked responses (MEPs) in 21 of 27 patients and SEPs in 26 of 27 patients. Single pulse TMS resulted in peripheral muscle responses having large variability, whereas, the variability of SEPs was much less. Criteria based on response variability for assessing clinically significant changes in both MEPs and SEPs resulted in two false negative predictions for SEPs and none for MEPs when evaluating postoperative motor function. We recommend monitoring both sensory and motor pathways during procedures where placing the spinal cord at risk of damage.

Prevention and Management of Pupil Block After Descemet Membrane Endothelial Keratoplasty.

Purpose: To assess frequency, associations, and outcomes of full air fill after Descemet membrane endothelial keratoplasty (DMEK). Methods: This retrospective study reviewed 368 consecutive DMEK cases performed to treat Fuchs dystrophy, bullous keratopathy, or failed keratoplasty. Primary outcomes were air bubble status, intraocular pressure, and incidence of pupil block and air reinjection. Results: Using an air management algorithm, slit-lamp assessment 1 hour after DMEK identified complete air fill in 80/368 eyes (22%). This spontaneously resolved in 45 eyes. Air was removed from 35 eyes (9%). Eyes that required air removal were more likely to have occluded iridotomy than did eyes in which the full air fill spontaneously resolved (23% vs. 6.6%, respectively), P = 0.05. Although full air fill was more likely after DMEK combined with cataract surgery (P = 0.001), air removal was more likely after DMEK-only procedures (P = 0.01). Eyes that underwent air removal tended to have lower rebubble rates, although this did not reach statistical significance (P = 0.06). No cases of pupillary block glaucoma occurred, and full air fill did not significantly affect 6-month postoperative vision or endothelial cell density. Conclusions: A careful air management strategy successfully prevented pupil block in eyes that developed full air fill. DMEK-only procedures were more likely to require air removal than those combined with cataract surgery, which suggests that decreased zonular flexibility may prevent the air bubble from taking a more spherical shape and more easily allow occlusion of iridotomy.

Overview of Intraoperative Neurophysiological Monitoring During Spine Surgery.

Summary: Intraoperative neurophysiologic monitoring has had major advances in the past few decades. During spine surgery, the use of multimodality monitoring enables us to assess the integrity of the spinal cord, nerve roots, and peripheral nerves. The authors present a practical approach to the current modalities in use during spine surgery, including somatosensory evoked potentials, motor evoked potentials, spinal D-waves, and free-run and triggered electromyography. Understanding the complementary nature of these modalities will help tailor monitoring to a particular procedure to minimize postoperative neurologic deficit during spine surgery.

Posterior circulation cerebral hyperperfusion syndrome after high flow external carotid artery to middle cerebral artery bypass

We present the first report, to our knowledge, in which revascularization of the middle cerebral artery (MCA) with a high flow extracranial-intracranial procedure resulted in symptomatic hyperemia of the posterior circulation. Cerebral hyperperfusion syndrome (CHS) is a poorly understood phenomenon that is classically seen in the distribution of a revascularized artery. A 37-year-old woman presented with a 3 month history of cognitive and speech difficulties, persistent headaches, weakness, numbness, and paresthesia which was worse in the right extremities and face. She was found to have bilateral watershed infarcts worse in the left cerebral hemisphere, severe bilateral stenosis of the supraclinoid internal carotid artery, and a small left superior hypophyseal aneurysm. The patient underwent left cerebral hemisphere revascularization with a high flow external carotid artery to MCA bypass with aneurysm trapping. During skin closure, significant changes were seen in her bilateral upper extremity motor-evoked potentials. The patients postoperative exam was noted for an intermittent inability to follow commands, bilateral upper extremity weakness, vertical nystagmus, and alogia that all dramatically improved with strict blood pressure control. Postoperative perfusion imaging revealed posterior circulation hyperemia. This patient highlights the potential for hyperemic complications outside the revascularized arterial territory. Strict blood pressure control is recommended in order to prevent and manage hyperemia-associated symptoms. Improving our understanding of CHS may assist in identifying at risk patients and at risk arterial territories in order to optimize CHS prevention and management strategies.

Perioperative Use of Transcranial Magnetic Stimulation

Abstract:A practical means of noninvasively stimulating the cortex was developed in the mid-1980s. Both electrical and magnetic stimulating pulses applied transcranially were shown to be capable of exciting motor cortex. Transcranial magnetic stimulation (TCMS), compared with transcranial electrical

Immunologic Rejection Episodes After Deep Anterior Lamellar Keratoplasty: Incidence and Risk Factors

Purpose: To assess the rejection episode rate after deep anterior lamellar keratoplasty (DALK) and to identify associated risk factors. Methods: This retrospective review of 251 primary DALK procedures performed by 14 surgeons at a single center between February 2008 and November 2015 evaluated the rejection episode rate and associated risk factors using Kaplan–Meier survival and proportional hazards analyses, which took the length of follow-up into consideration. Results: Transplant indications were keratoconus or ectasia after laser refractive surgery (n = 170, 68%), corneal opacity (n = 72, 28%), and other anterior corneal disease (n = 9, 4%). The median recipient age was 46 years. The overall rejection episode rate was 14% with 18-month median follow-up and a 7-week median postoperative corticosteroid duration. In univariate analysis, increased risk of rejection episodes was associated with younger recipient age [relative risk (RR): 2.1, 95% confidence interval (CI): 1.4–5.2], African American race (RR: 2.1, 95% CI: 1.1–4.1), and use of manual trephination (compared with the femtosecond laser) for the side-cut incisions (RR: 2.7, 95% CI: 1.4–5.2). In multivariate analysis, the combined effect of patient age and race (P = 0.0012) and the side-cut method (P = 0.021) were each significant risk factors. Conclusions: This study demonstrates the substantial rate of rejection episodes that can be induced by corneal stroma in DALK and suggests that postoperative topical corticosteroids should be continued longer than the studys 7-week median and that young African Americans need higher-dose, longer-duration topical corticosteroids. The association between the side-cut method and rejection risk merits further investigation.

Magnetic cortical stimulation techniques

Publisher Summary This chapter reviews the use of transcranial magnetic stimulation (TCMS) for monitoring the functional integrity of the descending motor systems during surgery. It also discusses a potential role in the preoperative and postoperative period in conscious patients. It reviews that intraoperative use of somatosensory evoked potentials (SEPs) for monitoring the central nervous system function has been a popular technique. The chapter discusses that the appearance of several case reports describing new postoperative motor deficits with unchanged intraoperative SEP responses served as the stimulus for finding a direct motor system monitor. It also explores that two stimulation approaches have been developed for selectively exciting descending motor pathways. They are transcranial electrical stimulation (TCES) and TCMS. Both excite corticospinal tract (CST) neurons of origin using a transcranial technique. The chapter also focuses on the anatomy, physiology, and anesthetic considerations important to the use of TCMS for monitoring the descending motor system function. After establishing this foundation, it reviews the clinical experience using TCMS for monitoring the functional integrity of the CST-anterior horn cell pathway during spinal cord surgery.

Intraoperative Cortical Function Localization Techniques

This article describes two functional techniques for localizing eloquent cortical areas during surgery. The motivation for their use is derived from the inherent variability in cortical surface blood vessel and sulci pattern between patients; the distortion of the cortical surface from cortical masses; limited craniectomy size, which alters the perspective gained from surrounding cortical anatomy; and cortical plasticity of function, in which some functions relocate within the cortex when invaded by pathologic processes. The advantage of this approach is that maximum surgical removal of diseased tissue can occur while minimizing the risk of incurring new neurologic deficits. The first technique involves discrete electrical stimulation of the cortical surface. This approach has been used for localizing sensory, motor, and cortical areas important for speech function. The technique in most cases requires that the neurosurgical procedure be carried out on sedated patients. The second technique can be used to localize the central sulcus in both anesthetized and awake patients. It involves a morphologic analysis of the cortical surface somatosensory evoked potentials acquired when electrically stimulating contralateral peripheral nerves. The neurophysiologic basis for locating the central sulcus by finding the cortical surface where a phase reversal occurs in the distributed somatosensory evoked potentials is presented. Finally, practical aspects important for the successful use of both techniques are reviewed.