Continuous automated monitoring of somatosensory evoked potentials in posttraumatic coma

Using a microcomputer system, 36 patients were continuously monitored with somatosensory evoked potentials (SSEPs) following closed head injury. The mean duration of monitoring was five days. The patients' GCS scores at the start of monitoring ranged from 3-10. Mortality was 44%. Of 14 patients who had initial absence of SSEP activity after the N20 wave or lost this activity during the period of monitoring, 12 died and two remained vegetative. Eighteen of 22 patients who had preservation or enhancement of this activity had a functional survival (3-month outcomes, p less than 0.001). The best outcomes were observed in the patients with the greatest number of peaks and highest amplitudes in the SSEPs, often in spite of increased intracranial pressure (ICP). The management of three patients was altered as a result of SSEP monitoring. We conclude that continuous monitoring of SSEPs is a useful adjunct in the management of comatose head-injured patients.     

Clinical course of post-traumatic coma

The Authors describe the clinical course of 331 cases of post-traumatic coma, studied during the two-year period 1970-1971 at the 2nd Division of Neurosurgery (Ospedale Maggiore, Bologna). Among them, 42% died from trauma-related causes; 40% survived and had a complete reintegration into society; 18% survived at a lower social level or remained totally excluded from society. The quality of survival does not depend on the presence and degree of organic sequelae only, but also on psycho-social factors.     

Evaluation of cervical cord function using spinal evoked potentials from surface electrode.

Spinal evoked potentials from cervical skin surface (surface spinal evoked potentials) were measured to evaluate spinal cord function as a convenient method that precludes inserting electrodes into the epidural space, and results were compared with those of the former epidural recording method. Surface spinal evoked potentials were obtained from cervical skin surface over the C3, C5, and C7 spinous processes after median nerve stimulation in 18 normal subjects and 37 patients with a cervical lesion. In normal subjects, surface spinal evoked potentials consisted of three negative waves (N1, N2, N3). Abnormal N2 (80%) and abnormal N3 (100%) were observed in cervical myelopathy, and abnormal N2 was observed only in radiculopathy; this allows for differentiation between myelopathy and radiculopathy. Comparing preoperative and postoperative surface spinal evoked potentials, it was seen that improvement of clinical symptoms was proportional to that of surface spinal evoked potentials.     

Effect-site modelling of propofol using auditory evoked potentials

Auditory evoked potentials (AEP) were used to monitor central nervous system effects during induction and recovery from anaesthesia produced by infusion of propofol 30 mg kg-1 h-1 in 22 healthy male patients. Non- parametric and parametric modelling techniques were used successfully to calculate the parameter keo which linked pharmacokinetic with pharmacodynamic aspects of drug action in only 15 of the study patients. In the non-parametric analysis, keo was found to have a mean value of 0.2 (range 0.1-0.36) min-1. Estimation of keo allowed calculation of the effect-site concentration (Ce50) associated with 50% of AEP effect for the population (2.08 micrograms ml-1; 95% confidence limits 1.7-2.45). There were no significant differences between keo values calculated by non-parametric and individual parametric modelling techniques. During recovery, 50% of patients demonstrated evidence of waking at an effect-site concentration of 2.28 micrograms ml-1.      

Auditory evoked potentials

This chapter will focus on the two auditory evoked potentials (AEP) most commonly used to assess the effects of general anesthetics on the brain, the auditory middle latency response (AMLR) and the 40 Hz auditory steady-state response (40 Hz-ASSR). We will review their physiological basis, the recording methodology, the effects of general anesthetics, their ability to track changes in level of consciousness and their clinical applications. Because of space constraints, this review will be limited to human studies.     

Evoked potentials and post-traumatic evolution

Visual, somatosensory, and brainstem auditory evoked potentials provide functional quantitative assessment of the cerebral cortex and brainstem. Their contribution at the acute stage of coma concerns diagnosis, prognosis, and follow-up. Four patterns are observed in traumatic coma: pattern 1=dysfunction of the cerebral cortex, brainstem integrity: good prognosis in more than 80% of cases; pattern 2=midbrain dysfunction: prognosis depends on both the reversibility of midbrain dysfunction and the extent of associated axonal lesions in the hemispheric white matter; pattern 3=pontine dysfunction due to transtentorial herniation: ominous prognosis, this pattern must be early detected by continuous monitoring; pattern 4=brain death: we currently use evoked potentials at the only brain-death confirmatory test, even in sedated patients. The contribution of evoked potentials in vegetative or minimally responsive states concerns the identification of these patients whose state is determined by midbrain dysfunction and the evaluation of persisting cognitive abilities in individual cases.     

Auditory evoked potentials

Auditory evoked potentials (AEPs) are an electrical manifestation of the brain response to an auditory stimulus. Mid-latency auditory evoked potentials (MLAEPs) and the coherent frequency of the AEP are the most promising for monitoring depth of anaesthesia. MLAEPs show graded changes with increasing anaesthetic concentration over the clinical concentration range. The latencies of Pa and Nb lengthen and their amplitudes reduce. These changes in features of waveform are similar with both inhaled and intravenous anaesthetics. Changes in latency of Pa and Nb waves are highly correlated to a transition from awake to loss of consciousness. MLAEPs recording may also provide information about cerebral processing of the auditory input, probably because it reflects activity in the temporal lobe/primary cortex, sites involved in sounds elaboration and in a complex mechanism of implicit (non declarative) memory processing. The coherent frequency has found to be disrupted by the anaesthetics as well as to be implicated in attentional mechanism. These results support the concept that the AEPs reflects the balance between the arousal effects of surgical stimulation and the depressant effects of anaesthetics. However, AEPs aren't a perfect measure of anaesthesia depth. They can't predict patients movements during surgery and the signal may be affected by muscle artefacts, diathermy and other electrical operating theatre interferences. In conclusion, once reliability of the AEPs recording became proved and the signal acquisition improved it is likely to became a routine feature of clinical anaesthetic practice.     

Auditory evoked potentials

Auditory evoked potentials (AEPs) are an electrical manifestation of the brain response to an auditory stimulus. The waveform represents the passage of electrical activity provoked by auditory stimuli from the cochlea to cortex. The waves represented by I-VII are generated mainly in the brainstem. These waves are called the brain stem auditory evoked potentials (BAEPs) or the auditory brain stem response (ABR). The middle latency AEPs (MLAEP) are generated from the medial geniculate and primary auditory cortex. The long latency AEPs (LLAEP) are generated from the frontal cortex and association areas. The BAEPs appear to be an exquisitely sensitive monitor for pathological events during surgery. Anesthetics and mild hypothermia have minimum effect, if any, on the BAEPs. The BAEPs are useful during the microvascular decompression of the fifth or seventh cranial nerve, resection of acoustic neuroma and posterior fossa operations. Because the auditory pathway occupies a small area in the brainstem, combined use of other evoked potentials such as short latency sensory evoked potentials is recommended. The MLAEPs are most promising evoked responses for monitoring awareness or depth of anesthesia. When the concentration of anesthetics is increased, the amplitudes of the MLAEP's peaks are decreased and their latencies are elongated. Commercially developed A-line AEP monitor or aepEX can extract the AEPs waveform in a short period and automatically analyze the changes in the MLAEPs. These AEP based monitors may be superior to bispectral index (BIS) in detecting the transition from unconsciousness to consciousness.     

Motor evoked potentials

Motor dysfunction following spinal surgery and thoracoabdominal aortic surgery remain as one of their devastating complications. Since the development of postoperative motor dysfunction can deteriorate quality of life of patients, the prevention of such complications is an important clinical challenge. Recently, advances in stimulation technique using multipulse made intraoperative monitoring of functional integrity of motor pathways possible by recording myogenic motor evoked potentials (MEPs). However, myogenic MEPs can be affected by most of anesthetic agents and muscle relaxants. Anesthesiologists are therefore required to properly understand MEPs and to manage anesthesia carefully. We summarize the methods for monitoring of myogenic MEPs and anesthetic techniques during intraoperative MEP monitoring.