Parastou Shilian

Tactile spatial acuity differs between fingers: a study comparing two testing paradigms.

Tactile spatial acuity (TSA) is a reliable and reproducible measure of somatosensory system function that has been used to study a broad range of subject populations. Although TSA is most often assessed at the fingertip, published studies employing identical stimuli disagree on whether TSA differs between the fingers of neurologically normal subjects. Using a validated grating orientation discrimination task, we determined TSA bilaterally at the index and ring fingers of 16 healthy young adults. Motivated by earlier work, we utilized two stimulus presentation paradigms, the method of constant stimuli (MCS) and a staircase (SC) method. We found that TSA was significantly higher (the discrimination threshold was lower) at the index than at the ring finger, which was consistent with a prior study. Although mean thresholds at both fingers were higher when measured with the SC than with the MCS paradigm, this difference did not reach statistical significance ( p =.14). These findings should facilitate both design and interpretation of future studies investigating TSA.

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.

T93. When should monitoring end? A case report

Introduction Transcranial motor evoked potentials (TcMEP) is the preferred modality for monitoring corticospinal tract integrity during spine surgery. In practice, final TcMEPs are obtained immediately after rod placement, during washout, or during early stages of closing before administration of inhalational agents. This raises the question: “When should monitoring conclude?” We present a case of a 63-year male who underwent T8-L4 posterior fusion, T12-L2 laminectomy and L1 partial corpectomy. In this surgery, we performed intraoperative neurophysiological monitoring during the entire case through suturing of the skin. During the final stages of skin closure, both TcMEP and somatosensory (SSEP) signals decreased significantly in the bilateral lower extremities. The surgeons subsequently re-opened the incision site and discovered a large blood clot compressing the dura. Methods Patient presented with severe back pain and workup revealed a pathological L1 fracture with cord compression. At the onset of the surgery, anesthesia administered total intravenous anesthesia and avoided neuromuscular blockade . We obtained SSEPs and TcMEPs continuously throughout the case. Evoked potentials were unchanged until the stapling of skin during final stages of closure when TcMEPs were suddenly lost in the bilateral tibialis , gastocnemius and intrinsic feet muscle and significantly decreased in the bilateral vastus muscles. Upper extremity TcMEPs were stable. Upon further monitoring, bilateral lower extremity SSEPs were lost as well. There were no changes in anesthetic regimen. After reporting these findings, the surgical team completed staple closure and performed a clinical examination after extubation. Results During clinical examination, the patient demonstrated loss of motor function in bilateral lower extremities with preservation of movement in bilateral upper extremities. The patient was reintubated and repositioned to the prone position for wound exploration. Upon re-initiation of neuromonitoring, the bilateral lower extremity TcMEPs and SSEPs remained absent. Surgeons discovered the epidural blood clot compressing the ventral dura during wound exploration. After immediate evacuation of the hematoma, bilateral TcMEP signals returned in lower extremities, right greater than left. SSEPs also demonstrated partial recovery. Clinically, the patient woke up without deficit. Conclusion Loss of bilateral TcMEPs alone or in combination with SSEP changes warrant immediate due diligence. In this case, early intervention prevented further damage to nerve roots affected by the expanding hematoma. Because significant changes in signals can occur at any time, neurophysiological monitoring should be performed until final closure.

T141. Nerve root monitoring with TcMEPs during lumbar spine cases

Introduction Lumbar spine discectomy and fusion surgeries are performed with the goal of improving low back pain, radiculopathy, or weakness. One study analyzed 792 patients undergoing open lumbar discectomy and showed 0.05% rate of post-operative root injury (Desai et al., 2012). Another review of minimally invasive lumbar spine surgeries demonstrated a root injury rate of up to 23.8% (Epstein, 2016). Transcranial motor evoked potentials (TcMEPs) is one modality used to monitor the integrity of the spinal cord. However, the utility and the warning criteria of TcMEPs for detecting root injury have not been definitively established. Amplitude warning criteria can range widely from 50% to 80% reduction (MacDonald et al., 2012). Our goal is to explore potential TcMEP criteria that may warn of root injury. Methods This is a retrospective chart review of surgeries performed during 2016. We identified 102 patients undergoing lumbar spine surgeries with anatomical risk for injury to the nerve roots below the conus medullaris. We analyzed TcMEPs prior to incision and at skin closure. We excluded data from cases where the effect of neuromuscular blockade was present. For each TcMEP, we analyzed changes in stimulation voltage, % latency, % duration, % peak-to-peak (PTP) amplitude, % secondary PTP (sPTP) amplitude, and % area under the curve (AUC). We then compared the data to changes in the neurologic Medical Research Council (MRC) scale in the quadricep (quad), tibialis anterior (TA), and gastrocnemius muscles. We used gastrocnemius strength as a surrogate for foot strength as they share similar root innervations. We stratified MRC changes as either “mild to severe” (mild), “moderate to severe” (moderate), or “severe only” (severe) based on a reduction of greater than 0.5, 2, or 4 points, respectively. Results Baseline TcMEP data were available for 135 quads, 194 TAs, and 200 foot muscles. Post-operative deficits were seen in 3 quads, 5 TA, and 4 foot muscles. Measures of diagnostic accuracy (% sensitivity, % specificity, % positive predictive value, and % negative predictive value) were calculated using the following parameters: (A) 10% latency increase - mild (33/96/17/98), moderate (25/95/4.2/99), severe (100/95/4/100). (B) 67% decrease in PTP - mild (29/98/25/98), moderate (25/97/6/99), severe (100/97/6.3/100)

F119. Presence of TcMEPs in the setting of acute clinical paraplegia

Introduction In surgeries where neural structures are at risk, Intraoperative neurophysiological monitoring (IONM) provides insight into the integrity of the nervous system. Each patient serves as his or her own control, and baselines obtained at the beginning of the surgery provide a comparative standard for subsequent signals. The process of establishing controls is necessary to determine the presence and degree of potential intraoperative changes. Unfortunately, adequate baselines are sometimes unattainable due to preexisting neurological deficits. Patients with acute spinal cord injury and complete motor loss often demonstrate absence of transcranial motor evoked potentials (TcMEPs) below the level of the lesion. We are presenting a case of a 62-year-old man with traumatic L1 burst fracture and subsequent acute lower extremity paralysis (ASIA B) who underwent emergent surgical decompression and spinal fusion. TcMEPs were present despite absent of clinical movement in the bilateral lower extremities. Methods Total intravenous anesthesia was administered via propofol and remifentanil drip. No neuromuscular blockade was given. The patient was turned to the prone position for surgery. Intraoperative monitoring was performed using the Cascade Elite system by Cadwell. We placed 13 mm paired subdermal disposable needle electrodes in the intrinsic hand muscles, quadriceps femoris , tibialis anterior , gastrocnemius and intrinsic foot muscles. We placed corkscrew electrodes at C1 and C2 to deliver transcranial stimulation. Baseline TcMEPs were obtained using the following stimulation parameters: 7 pulses, 3 ms inter-stimulus interval, 400-volt intensity. The MEP recording bandpass was 30–2000 Hz and gain was set at 500 mcv/div. Results Baseline TcMEPs were obtained three hours after initial injury. Despite compelete paraplagia on preoperative exam, baseline TcMEP responses were present and robust in the bilateral quadriceps and tibialis anterior muscles, while small and variable in the bilateral intrinsic feet muscles. These aforementioned potentials remained unchanged throughout the procedure. On subsequent post-operative physical examinations, the patient regained motor strength in lower extremity muscles that were initially present on the baseline. Specifically, recovery of motor strength was greater in the proximal lower extremity muscles compared to their in distal counterparts. Conclusion TcMEPs can be present in the acute spine injury patient despite absent of motor response on clinical examination. We postulate that intraoperative TcMEP baselines in acute spinal cord injury patients with severe motor deficits may potentially be used as a predictor of post-operative clinical outcome.

T94. Stable “Crossover” TcMEP with post-op weakness in CEA

Introduction Carotid endarterectomies can be performed in patients with carotid artery stenosis in order to prevent future strokes. Electroencephalogram and somatosensory evoked potentials are intraoperative neuromonitoring modalities used to provide the surgeon with real-time feedback in detecting decreases in cerebral blood flow during carotid artery clamping. This may help the surgeon decide whether to shunt the carotid artery, which has potential complications. Transcranial motor evoked potentials have been used as an adjunct to EEG and SSEPs to monitor carotid endarterectomies, with Malcharek et al. reporting 0.4% false-negative TcMEPs in a review of 264 patients. We report a case where TcMEPs remain unchanged with unilateral SSEP changes that was associated with new post-operative deficits. Methods This is a case report of a 54 year old female with multiple vascular risk factors and a prior stroke three months prior with trace right upper (RUE) and lower (RLE) weakness. She presented with acute worsening of weakness and numbness, due to severe narrowing of the proximal left internal carotid artery. She underwent left carotid endarterectomy for symptomatic carotid stenosis. Three-channel EEG, SSEP with stimulation at the median nerve and posterior tibial nerve, and TcMEPs of the upper and lower extremities were setup for intraoperative neuromonitoring. The patient was anesthetized with propofol and fentanyl. Results Baseline SSEPs in all four extremities were adequate. TcMEPs were reproducible in all four extremities, except the left foot had variable responses. Of note, there were TcMEP responses ipsilateral to the hemisphere of stimulation. Seventy minutes post-incision, EEG showed burst suppression and the surgeons clamped the left carotid artery. Two minutes post-clamping, the right sided SSEP began to decrease in amplitude. The RUE and RLE SSEPs became absent at 10 min and 19 min post-clamping, respectively. TcMEPs run after the SSEPs changes were unchanged from baseline and the surgeons moved forward without shunting. At 42 min post-clamping, surgeons completed the CEA and removed the clamp. The RLE SSEP returned 3 min after unclamping, and the RUE SSEP 29 min after unclamping. The RLE SEP improved to baseline, but the RUE SEP amplitude remained significantly diminished. Left sided SSEPs were stable and bilateral TcMEPs remained unchanged throughout the entire surgery. EEG demonstrated burst suppression without any notable asymmetry. Pre-operative exam showed RUE drift and 4+/5 strength on RLE. Post-operative exam revealed flaccid RUE. Her RLE motor strength was anti-gravity, but she did not follow commands for individual muscle groups. Conclusion TcMEPs may be an unreliable measure of the nervous system when crossover responses are present in the limbs ipsilateral to the hemisphere of stimulation. In this left CEA, right-sided SSEP changes correctly heralded potential injury immediately after clamping, while right-sided TcMEPs remained unchanged.

Spinal cord mapping.

Over the past century, there has been significant expansion of surgical techniques and indications in spine surgery. Indications for spine surgery in the modern era include treatment for degenerative spine disease, traumatic spine injury, spinal deformity, and spinal neoplasms. A fundamental goal of spine surgery regardless of the indication is to preserve or improve neurological functions. Unfortunately, a well-known risk of spine surgery is neurological injury. Any tools or modalities that can help to minimize iatrogenic neurological injury are greatly beneficial to both the patient and the surgeon. Although the risk of neurological injury exists in essentially all spine surgeries, the risk can vary significantly depending on the location and type of surgery. Whereas the risk of significant iatrogenic neurological injury for surgical treatment of degenerative spine disease is well less than 1%, the risks for neurological injury for spine deformity correction with 3-column osteotomy is quite high, ranging from 11.1% to 29% (Daubs et al., 2007; Deyo et al., 2005; Sciubba et al., 2009; Suk et al., 2002, 2005; Yadla et al., 2010). Postsurgical neurological deterioration is a particular concern in cases with significant baseline myelopathy or patients with intrinsic spinal cord pathology. Particularly in surgical treatment of intramedullary spinal cord tumor, the direct surgical dissection within the cord is a great concern for postsurgical neurological deterioration. Even with the standard approach to access the spinal cord with a posterior midline myelotomy for tumor excision, patients often experience significant functional compromise from the resultant proprioception and sensory deficits (Constantini et al., 2000; Epstein et al., 1993; Jallo et al., 2001). The value of intraoperative monitoring techniques to surveillance neurological function during these high-risk cases cannot be understated (Kothbauer et al., 1998; Morota et al., 1997). The most common intradural intramedulary tumors are ependymomas, astrocytomas, hemangioblastomas, oligodendrogliomas, and lipomas (Fig. 1) Surgery is considered as the definitive treatment for the majority of intramedullary spinal cord tumor. Surgical goal for the treatment for most intramedullary spinal cord tumors is to achieve gross total tumor excision to maximize oncological outcome and survival without significantly compromising the patients’ neurological functions. Surgery in the spinal cord for intramedullary tumor excision dates back to the late 1800s (Sciubba et al., 2009). In 1887, Sir Victor Horsley is credited with the first successful spinal cord tumor resection (Tan and Black, 2002). However, the early efforts in spinal cord tumor surgery was quite limited by high rates of morbidity and mortality (Sciubba et al., 2009). A number of technical advances have led to improved outcomes for intramedullary tumor resection in the modern era, including the operative microscopy, bipolar cautery, ultrasonic aspirator, and intraoperative monitoring. Although most of the advancements improved the surgical techniques by refinement of surgical tools, intraoperative monitor is unique in its ability to provide surveillance of the neurophysiology functions of the central and peripheral nervous systems. Intraoperative monitoring can provide almost real-time feedback to the surgeon regarding the potential hazard from their surgical dissection. Any significant finding during intraoperative monitoring can provide surgeons an opportunity for a “change of course” to avoid postsurgical neurological injury. In this article, we will review the relevant anatomical consideration and techniques used for spinal cord mapping.