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Electrophysiologic monitoring in neurointensive care

Procaccio, Francesco MD*†; Polo, Alberto MD*; Lanteri, Paola MD†; Sala, ISSN: Author(s): Francesco MD† 1070- 5295 Issue: Volume 7(2), April 2001, pp 74-80 Accession: Publication Type: [] 00075198- Publisher: © 2001 Lippincott Williams & Wilkins, Inc. 200104000- University and City Hospital Neuroanesthesia and Intensive Care, Department 00004 of Neurological Sciences and Vision, Divisions of * and Full †, Verona, . Institution(s): Text Correspondence to Francesco Procaccio, MD, Neuroanesthesia and Intensive (PDF) Care, University and City Hospital, Pz Stefani, 1, 37124 Verona, Italy; e-mail: 69 K [email protected] Email Jumpstart Table of Contents: Find ≪ Neurologic complications in intensive care. Citing ≫ Pediatric neurologic emergencies. Articles ≪ Abstract Table Links of Cumulative evidence of potential benefits of Contents Abstract (EEG) and evoked potentials in About Complete Reference the management of patients with acute cerebral this ExternalResolverBasic damage has been confirmed. Continuous EEG Journal Outline monitoring is the best method for detecting ≫ nonconvulsive and is strongly recommended for the treatment of . Continuously displayed,

● Abstract validated quantitative EEG may facilitate early detection of secondary

● Electroencephalographic cerebral insults and may play a decision-making role in the management of monitoring in the neurointensive patients with head injury, , or . Long-latency care unit auditory evoked potentials and cognitive components constitute a new field

❍ Rationale for of interest for the progress of comatose patients. Motor evoked potentials

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electroencephalographic may become clinically important both in acutely injured and elective monitoring postoperative patients. In the neurointensive care units adequate techniques can

❍ Rationale for continuous be selected to answer targeted clinical questions. The efficacy can be improved electroencephalographic by implementing educational projects based on ad hoc training of nurses monitoring and neurointensive care specialists.

❍ Quantitative electroencephalography

❍ Technical guidelines Abbreviations:BIS Bispectral Index, EEG electroencephalography, EP

● Clinical applications of , MMN mismatch negativity, NICU neurointensive care unit, SE electroencephalographic monitoring in the neurointensive status epilepticus, TMS transcranial magnetic stimulation care unit

❍ Seizures Is electrophysiologic monitoring useful to improve the neurointensive care of

❍ Nonconvulsive status patients with acute cerebral damage? Management strategies and epilepticus monitoring guidelines have been extensively reviewed in recent years in parallel ❍ Stroke and subarachnoid hemorrhage with developing concepts of evidence-based [1]. As yet, cerebral

❍ Head injury electrical activity is not routinely included with physiologic parameters for ❍ Control of the level of sedation monitoring in the acute phase. Scepticism due to the supposed vulnerability ❍ Prognosis: to artifacts has outweighed considerations as to its potential utility electroencephalographic and noninvasiveness [2]. With the advent of digital processing and variability and reactivity

❍ Impact of continuous automated analysis, including expert systems, more positive evidence has emerged electroencephalography on [3–6] in favor of the use of electroencephalographic (EEG) monitoring. A management new methodologic approach is increasingly being adopted in pilot centers with

● Evoked potentials and event- diffuse application of continuous EEG monitoring and evoked potentials

related potentials (EPs). Cumulative evidence of its potential benefits has been ❍ Acoustic evoked potentials retrospectively confirmed, and targeted prospective studies on its ❍ Somatosensory evoked potentials efficacy, efficiency, and cost-effectiveness are in progress [7–9].

● Motor evoked potentials: This short review focuses on practical EEG and EP applications based on prognostic value

❍ clinically relevant questions arising in the neurointensive care unit (NICU). Postoperative motor evoked potentials

❍ Motor evoked potentials in Electroencephalographic monitoring in the neurointensive care acute cerebral lesions unit Rationale for electroencephalographic monitoring

● Conclusions

● References and recommended Secondary injury frequently occurs and negatively influences the outcome of

reading patients with acute cerebral damage [10]. Thus, prevention and early detection ● Section Description of cerebral insults are major aims of neurointensive care. Despite

http://ovidsp.tx.ovid.com/spb/ovidweb.cgi (2 of 14) [6/12/2008 11:43:39 AM] Ovid: Electrophysiologic monitoring in neurointensive care. increasingly sophisticated monitoring, subclinical seizures may be detected only by EEG, which provides invaluable information on the cerebral blood flow to metabolism ratio and may serve as a warning of ischemic insults, as a guide to treatment, and as a prognostic factor. EEG is sensitive to ischemia and can detect neuronal dysfunction at a reversible stage.

Rationale for continuous electroencephalographic monitoring

In the past, obvious limitations of analog recordings made EEG impractical for continuous monitoring. Nevertheless, a number of techniques for assessing EEG changes over time became very popular more than 20 years ago. The compressed spectral array [11,12], the cerebral function monitor [13], and the cerebral function analyzing monitor [14] have been extensively used.

On-line continuous digital EEG recording and automatic processing have radically changed our perspective. In a recent review, techniques, evidence-based clinical recommendations, and caveats regarding misleading use have been addressed [15].

Quantitative electroencephalography

Quantitative EEG transforms the EEG signal into a wide range of frequency and amplitude measurements. These continuous quantitative EEG outputs are easier for non-EEG experts and more “user friendly” than raw EEG. The advantages include data compression and visual display capability as bar graphs, compressed spectral arrays, or scalp maps [9].

Most data are derived from the fast Fourier transform (FFT) and displayed as measurement trends, including the percentage of alpha range activity (relative alpha), the total power or overall amplitude, and the alpha:delta ratio. The clinical scenario dictates the type of information that may be most significant; combinations of formats with raw EEG are possible.

If clinical decisions are made on the basis of quantitative EEG, raw EEG and expert supervision must be available at the same time for quantitative EEG validation and artifact discrimination [15].

Technical guidelines

Team training and continuous education are key factors. Continuous EEG monitoring should become part of the curricula of clinical neurophysiologists and neurointensive care specialists [16]. Particular methodologic and logistic strategies are needed

[17•]. Standards of monitoring must be strictly observed [18]. When time is critical, “stick-on” electrocardiographic electrodes can be applied in a “subhairline” bipolar montage by house staff [19]. Artifacts can be avoided or detected by trained nurses

[20]. Most artifacts are related to responses to and other interventions; automatic methods for context-related artifact detection in prolonged EEG recordings have been tested with good performance against human observers [21,22••].

Nursing notes are of paramount importance [16,23]. Continuous EEG should be incorporated in the single bedside monitor or personal computer; thus, relationships between physiologic parameters and EEG could be obtained at a glance.

Clinical applications of electroencephalographic monitoring in the neurointensive care unit

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Seizures

Seizures constitute a common and probably underestimated complication in the NICU. Continuous EEG is necessary to detect subtle seizures and titrate drugs and should probably be considered standard in the treatment of status epilepticus (SE) [15].

On the other hand, continuous EEG can help to discriminate seizures from involuntary movements, spasms and tremors, eye deviations, and posturing that are common clinical features in the NICU [9].

Nonconvulsive status epilepticus

Nonconvulsive SE includes three clinical situations: complex partial SE, absence SE, and electrographic SE [24]. Nonconvulsive

SE represents an epileptic state lasting more than 30 minutes with two principal components: (1) some clinically evident alteration in mental status or behavior versus baseline, and (2) activity on the EEG [25], including comatose patients in the NICU [26].

The reported proportion of SE patients who have nonconvulsive SE ranges from 4 to 20%. The incidence of absence SE compared with complex partial SE ranges from 3:1 to 1:3 [27–29]. A group of 127 patients with illnesses of various etiologies had an NICU continuous EEG; nonconvulsive seizures were documented in about 30%[30]. Ninety-four patients with underwent continuous EEG monitoring over 14 days post-admission to the NICU; seizures were detected in 21 patients (22%), and 6 patients had a nonconvulsive SE [31••]. A total of 326 patients with and no overt clinical seizure activity were also assessed by EEG monitoring; 8% of them met the criteria for the diagnosis of nonconvulsive SE. The percentage is lower than in previous studies because comatose patients were monitored regardless of suspicion of nonconvulsive SE [32••].

Nonconvulsive seizures are EEG and not clinical findings [30,33]. Although there is no established consensus, EEG correlates of nonconvulsive seizures (NCSs) may be interpreted on the basis of primary and secondary criteria (at least one primary and one or more secondary criteria). Multichannels of digitized, real time EEG are used: (1) traces are continuously displayed at the bedside; frequency analysis is performed (FFT 2 seconds epoch, 2 minutes averaging) and trends of total power are reviewed twice daily and screened for suspected seizure activity (automatic program for seizure detection) [31••]; (2) nurses are trained to recognize EEG abnormalities; (3) bedside display and review stations in the NICU are linked by cables to the main workstation in the electroencephalographer’s office [30].

Nonconvulsive status epilepticus can cause enduring cerebral dysfunction, affecting and other cognitive functions and behavior [34,35]. It has been difficult to confirm that nonconvulsive SE can cause brain damage because it is the underlying illness that first damages the brain [36,37]. The goal should be to show that nonconvulsive SE causes brain damage without confusing comorbidity with consequence [25]. Nonconvulsive SE and acute cerebral injury produce synergic and not simply additive effects on the brain with ongoing neuroexcitotoxic damage [38].

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Stroke and subarachnoid hemorrhage

In acute focal cerebral ischemia, continuous EEG changes occur early, when findings on a computed tomographic scan may be negative, and correlate with the decrease in cerebral blood flow; early detection of impending ischemia might facilitate timely thrombolytic [9].

Jordan [9] suggests that widespread attenuation of all frequencies without an accompanying delta (RAWOD) is a distinctive

EEG pattern in extensive acute focal cerebral ischemia, suggesting a high risk of malignant .

In patients with aneurysmal subarachnoid hemorrhage, Vespa et al. [39] found that trend variability in the percentage of alpha activity may predict changes in transcranial Doppler velocities and clinical deterioration due to vasospasm.

Head injury

After severe head injury, seizures occur in more than 20% of patients [31••]. Seizures may be underestimated and inadequately treated in the absence of continuous EEG monitoring, particularly if relaxant drugs are used.

During management of increased , continuous EEG may offer significant help in therapy based on an understanding of the underlying pathophysiology [40]. Jordan [9] has suggested multiple beneficial effects of contintuous

EEG monitoring when algorithms for targeted treatment are used. The EEG sensitivity to ischemia may provide (1) feedback on therapy based on maintenance of the cerebral perfusion pressure [41]; and (2) an early warning of hypocapnic ischemia and impending vasospasm, which is a common complication in severe brain injuries [42]. At the University of California at

Los Angeles (UCLA) [31••], EEG-based guidelines have been defined to guide multifaceted management. An important objection to quantitative EEG monitoring in head injuries is the frequent occurrence of abnormalities and asymmetries due to edema of the scalp and skull lesions.

Because the EEG provides an index of overall cerebral metabolic activity, concomitant monitoring of cerebral oxygen extraction by Sao2 and jugular venous oxygen saturation (Sjvo2) may facilitate identification of the different relationships between global cerebral blood flow and metabolic demand (CMRo2). High-dose barbiturate infusion can be a useful treatment for acutely reducing intracranial pressure in selected patients, particularly if cerebral metabolism is not already severely reduced

[43]. Thus, EEG may predict the efficacy of barbiturates in the individual patient, as already demonstrated by continuous cerebral function monitor recording for etomidate [44], avoiding undesirable iatrogenic hypotension and adverse effects. Quantification and monitoring of EEG suppression can be considered the best modality for achieving and maintaining maximum effects on CMRo2 and intracranial pressure with the lowest dose [45].

Control of the level of sedation

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Because frequent neurologic examinations must be performed, predictable sedation and rapid recovery are crucial.

Different sedative and anesthetic agents produce variable effects in individual patients, and consequently the level of sedation is empirically or symptomatically titrated. Recently, Albrecht et al. [46•] used a computer-controlled closed-loop EEG median frequency feedback system to adjust propofol infusion and maintain a predetermined level of sedation in a mixed population of 21 ventilated patients (including 5 with head injuries).

A relatively new device, known as the Bispectral Index (BIS), has yielded good results in continuous monitoring of the level of and is now being evaluated in intensive care. The BIS is a complex but empirical measurement, statistically derived from a large database of EEGs [47]. An initial validation of the BIS against the revised Sedation-Agitation Scale in

63 ventilated patients in general intensive care has been published [48]. Because EEG alterations due to cerebral lesion are expected, investigations are needed to assess whether BIS may be useful in guiding sedation/analgesia in the NICU, avoiding misleading extrapolations and unreliable results.

Prognosis: electroencephalographic variability and reactivity

Spontaneous variability of the EEG pattern [49,50] and reactivity to external stimuli [51] are well recognized factors associated with a favorable outcome. Published data on the prognostic role of EEG in severe head injury have recently been reviewed

[52]. During continuous EEG monitoring, changeable patterns and reactivity to auditory or painful stimuli can be confirmed over days to obtain more reliable prognostic data.

Recent data and meta-analyses of “alpha”[53•], “alpha-theta”[54•], and “spindle”[55•] coma have been published. These studies confirm that the absence of EEG reactivity is an independent factor associated with poor outcome [56,57].

Impact of continuous electroencephalography on management

At UCLA, trained nurses correctly identified generalized seizures, burst suppression, and reduced percentage alpha variability. Thus, the EEG was used daily to guide one or more decisions at the bedside in more than 90% of patients (n = 300), with a decisive impact on 54% of occasions, suggesting that prospective studies will prove cost saving by reducing complications and acute hospitalization [31••]. Other important controversial issues, such as clinical indications, duration of continuous

EEG monitoring, type of quantitative EEG in different clinical situations, and role in decision making, should be addressed in prospective studies.

Evoked potentials and event-related potentials

Vision, hearing, and somatic and motor function assessment is not precluded in comatose patients; thus, neurophysiologic tests are established monitoring tools in the NICU alongside clinical evaluation [3,58].

Acoustic evoked potentials

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The importance and utility of testing hearing in patients at risk of peripheral damage (temporal bone fractures, antibiotics, ) have received much attention. The integrity of peripheral structures (normal wave I) and absence of brain stem auditory EPs or progressive worsening during monitoring strongly correlate with poor outcome. Effects of drugs commonly used in NICUs on brain stem auditory EPs are not relevant enough to preclude functional assessment during high-dose barbiturate therapy for increased intracranial pressure [59].

Mid-latency and long-latency auditory EPs, reflecting early cortical processing of sounds, have been used for assessing the level of sedation [60].

Additional prognostic indicators predicting awakening from coma have recently been suggested by studying auditory event related potentials: P300 and mismatch negativity (MMN) are considered to represent information processing that is related to the task-relevant stimulus involving pre-perceptual memory-based cortical networks. Nielsen-Bohlman et al.[61] observed differential responses to deviant sounds during waking and all stages, thus supporting the theory that selective processing of auditory stimuli persists during sleep. Fischer et al.[62] suggested a diagnostic protocol for evaluating the predictive value of MMN in comatose patients. Regardless of the delay in latency or reduction in amplitude, the presence of MMN has been considered the main predictive index of awakening from coma. MMN provides earlier information than the

Glascow Coma Scale in predicting outcome [63]. The presence of MMN in comatose patients should indicate that certain pre-attentive sensory memory processes are active in these patients. In patients with traumatic and anoxic injuries, the

P300 complex to rare stimuli was waveformed by a frontocentral negativity (N330) followed in some patients by a frontocentral positivity (P430). Recovery of consciousness occurred in all patients with a preserved later positivity, and to a lesser extent when only N330 was present [64]. Jones et al.[65,66••] tried to apply more realistic sounds such as musical tones to study auditory processing in coma. This area of research is at an early stage of development, and further investigation may lead to new insights into the higher processes of auditory perception in unconscious patients.

Somatosensory evoked potentials

In patients with anoxic coma, bilateral absence of cortical somatosensory EPs more than 24 hours after the onset of coma is associated with death or the vegetative state. Although the preservation of the N20 is significantly correlated with a better outcome, it is not an absolute predictor in individual cases, as a poor outcome was observed in 25 to 50% of anoxic cases with preserved cortical somatosensory EPs [17•]. However, Berkhoff et al.[54•] and Madl et al.[67•] agree that the absence of cortical somatosensory EPs remains the main indicator in predicting poor individual outcome in postanoxic patients.

In patients with severe head trauma, any delay in central conduction time (studied by somatosensory EP monitoring) is variably associated with outcome; bilateral absence of the cortical component of the somatosensory EP is always associated with a poor outcome [68••]. The intracortical components P27 and N33 had a better predictive value than later components.

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Schwartz et al.[69] revealed a number of exceptions to this rule.

Motor evoked potentials: prognostic value Postoperative motor evoked potentials

Motor EPs represent a sensitive and specific tool for monitoring descending motor tracts. Besides their intraoperative use, motor EPs may also have a prognostic value in early neurosurgical postoperative management. For example, despite the preservation of corticospinal tracts documented by intraoperative preservation of muscle motor EPs, patients operated on for lesions involving the supplementary motor area may wake up from anesthesia with hypokinesia, which is occasionally so severe as to resemble hemiplegia [70]. In the past, postoperative single-stimulus transcranial magnetic stimulation (TMS) failed to elicit muscle motor EPs before evidence of clinical recovery [71]. Recently, multipulse TMS has proved effective in eliciting muscle motor EPs from akinetic limbs before any clinical recovery [72]. Unfortunately, preservation of muscle motor

EPs may fail to predict motor recovery in the case of bilateral supplementary motor area lesions [73].

Motor evoked potentials in acute cerebral lesions

Motor evoked potentials by TMS have not been systematically employed in the NICU setting, and most studies have dealt with stroke patients. Occasionally, TMS can provoke seizures and therefore should be avoided in patients with [74]. In contrast to somatosensory EP and brain stem auditory EP recordings, sedative or analgesic drugs change the cortical threshold and significantly affect motor EPs [75]. Nevertheless, motor EP recording can be safely performed in neurointensive patients, providing both diagnostic assistance in assessing motor function and information predictive of outcome [76,77••].

Conclusions

Current EEG and EP techniques are compatible with the NICU environment and provide different types of information that can be selected and used to answer targeted clinical questions in the treatment of patients with acute brain damage.

Continuous quantitative EEG is, at present, only a flexible practice option in neurointensive care, considering that little has been published on how quantitative EEG may affect the diagnosis and treatment of individual patients and that its use for treatment is unacceptable without personnel skilled in clinical EEG interpretation [15,78].

If EEG and EP methods are not supported by diffuse ad hoc investments in NICUs, favorable cost-benefit results and evidence of improved outcomes will be difficult to produce.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • Of special interest •• Of outstanding interest

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Section Description

Edited by Peter JD Andrews

Copyright (c) 2000-2007 Ovid Technologies, Inc.

Version: OvidSP_UI01.01.02, SourceID 35095

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