Tod B. Sloan

Multimodality monitoring of the central nervous system using motor-evoked potentials.

Purpose of review This review was conducted to examine the role of motor-evoked potential monitoring in spine and central nervous system surgery to determine whether other monitoring modalities such as the wake-up test or somatosensory-evoked potentials can be eliminated. Recent findings The current literature suggests that motor-evoked potential, despite some advantages, still requires that other monitoring modalities such as somatosensory-evoked potentials or electromyography be used to provide optimal monitoring. Summary The literature supports the use of multimodality monitoring using all of the electrophysiological techniques that can provide intraoperative information about the neural structures at risk during the surgery.

Anesthesia and Motor Evoked Potential Monitoring

Publisher Summary This chapter explores anesthesia and motor evoked potential (MEP) monitoring. Anesthesia used to conduct surgery where MEP monitoring is used has marked effects on the ability to record responses. This chapter focuses on both, the theory and the practical aspects of the effects of anesthetic agents. In theory, the type of interaction should be predictable, based on the mechanism of action of the drugs involved. Halogenated inhalational agents and ketamine appear to hinder axonal conduction. As a result, the major anesthetic impact on neurological pathways used for monitoring appears to be at the synaptic connections, with an additional minor component based on the length of the pathway. Varying locations of action as well as differences related to drug dosage make marked differences between agents when related to motor-evoked responses. Further, neurological disease appears to make the responses more difficult to record under anesthesia, increasing the challenge of monitoring in the very patients where it may be most important. Anesthesia for the monitoring of peripheral muscle responses to spinal cord nerve root stimulation should also be unaffected by anesthetic choice, with the sole exception of neuromuscular blockade.

Anesthetics and the brain

The action of anesthetics on the nervous system can be understood by considering their possible interactions with neuronal function. Anesthesia may be produced by a change in the balance of inhibitory synapses (notable via GABAa receptors) and excitatory synapses (notably glutamate receptors). Our knowledge of the specific mechanisms of anesthetic drugs and the structures in the CNS remains inadequate to explain the anesthetic state by one mechanism. The action of anesthetics can also be considered based on the action of the drugs on cerebral physiology, notably CMR, CBF, metabolic coupling, and autoregulation. Some specific anesthetic recommendations can be made for certain neurosurgical procedures and pathology based on the effects on physiology.

SCOLIOSIS SURGERY: APPROPRIATE MONITORING

Correction of scoliosis is largely an elective cosmetic procedure in the young population, who account for the largest portion of the surgical population. Associated with the correction, however, is a very real possibility of major neurological injury, including paralysis. Although the incidence of spinal cord injury was estimated as 0.5% in 1979, 75 newer instrumentation methods have markedly increased the risk. More recent reviews have reported markedly higher figures (e.g., 17% neurologic complications with 4% paralysis 29,40,84 ). The risks are highest for patients with kyphosis, congenital scoliosis, preexisting neurological impairment, and patients in traction preoperatively. As such, there has been a major impetus to develop monitoring methods that could warn the surgeon of impending spinal damage so that the procedure could be altered to improve outcome. Electrophysiologic techniques have been developed to meet this need, and scoliosis surgery serves as an excellent model to evaluate the application of these techniques. This article reviews these techniques and the results of their application to scoliosis surgery.

EVOKED POTENTIAL MONITORING OF THE CENTRAL NERVOUS SYSTEM INTRAOPERATIVELY

Electrophysiologic monitoring of the cerebral cortex and brainstem has become an important adjunct during several neurosurgical procedures. In these procedures it has become indispensable to monitor specific neural tracts, and in others measure general cerebral well-being. In several studies monitoring has demonstrated correlation with patient outcome. A National Institutes of Health (NIH) consensus panel has made one form of monitoring a standard of care for certain procedures. This article reviews applications of this type of monitoring for surgical procedures on the cerebral cortex and brainstem.

PROTECTING THE INJURED BRAIN AND SPINAL CORD

Traumatic injury is a major health problem in the United States, both in terms of human death and disability as well as financial costs. It is estimated that approximately 40% of the health care dollar is consumed by direct or indirect medical costs of injury. Injuries to the head and spinal cord result in disabilities that supercede, particularly in social impact, injuries to any other system. 20 Most of these injuries are preventable. For example, over 70% of motor vehicle accidents are due to human factors and greater than 90% of the spinal cord injuries (SCI) that result from motor vehicle accidents occur in unrestrained passengers. 20 Alcohol is also a major contributing factor. Preventative efforts, therefore, are the most logical approach to this problem. In the absence of prevention, we must attempt to limit further injury by interrupting the cascade of cellular ischemia and destruction and also prevent damage by complicating factors such as hypoxia, inadequate perfusion pressure, and intracranial hypertension. This article reviews the pathophysiology of central nervous system (CNS) injury, including the primary and secondary injury processes, and discusses treatment modalities aimed at preventing further neurologic injury.

Chapter 49 Anesthesia effects and evoked potentials

Publisher Summary During electrophysiological monitoring, anesthesia and physiological changes can alter the responses. Anesthesia management involves the choice of a favorable drug combination and maintenance of a steady state. Physiological effects can simulate neural dysfunction if they hamper the stimulated tracts. Anesthetic effects can be divided based on whether the responses recorded are sensitive or insensitive to anesthesia and whether they are helped or hindered by muscle relaxants. In Group I responses, synaptic participation in the response generation likely explains the reason these anesthetics reduce the amplitude, because these agents affect synaptic function. Thus, anesthetic effects are prominent with increased latency and decreased amplitude. As anesthetic agents differ in their mode of action and potency, agents differ in their specific effects on the evoked responses and neural location. Group I responses are less affected by intravenous anesthetic agents. Muscle relaxants may improve response amplitude. Group II responses are characterized by pathways that are less dependent on synaptic function such that the anesthetic effects of inhalational agents are less marked. Stimulation and recording from the spinal cord or recording of the peripheral nerve, from spinal stimulation, appears to be a group II type. Stimulation with multipulse spinal stimulation is employed to overcome the anesthetic effects. Group III responses are unable to use muscle relaxation and inhalational agents, and so requires total intravenous anesthesia. Group IV responses are easily recorded, because, although muscle relaxation is limited, the freedom to use inhalational agents makes anesthesia less challenging. Typical responses here are stimulation of the cranial or the peripheral nerves and recording of the peripheral muscle responses.