Evaluation and prognostication in coma

An automatic detection method for 40-Hz auditory steady state response and its application in prognosis of comatose patients

OBJECTIVE

We proposed a 40-Hz auditory steady-state response (ASSR) automatic detection method, and studied the prognosis of comatose patients by combining the 40-Hz ASSR detection results of multiple paradigms of auditory stimulation.

METHODS

The 40-Hz ASSR elicitation experiments were carried out on 32 comatose patients, with the detection results used as prognosis predictors. To achieve automatic detection, the detection was modeled as a binary hypothesis test for a sinusoidal waveform with unknown amplitude and phase, based on the generalized likelihood ratio test (GLRT). The patients were followed up for 6 months, and each patient's outcome was classified as either favorable outcome (severe disability, moderate disability or good recovery) or unfavorable outcome (vegetative state/unresponsive wakefulness syndrome or death) according to the Glasgow outcome scale (GOS). The performance of the prognosis predictors was assessed using the area under the receiver operating characteristic curve (AUC-ROC).

RESULTS

The largest AUC in univariate analysis involving a single stimulation paradigm was 0.849, while the AUC obtained by combining multiple predictors was increased to 0.966.

CONCLUSIONS

For comatose patients, the absence of 40-Hz ASSR in multiple stimulation paradigms may indicate an unfavorable prognosis. Furthermore, the combination of multiple auditory stimulation paradigms may increase the outcome prediction accuracy.

SIGNIFICANCE

The combination of multi-paradigm 40-Hz ASSR automatic detection results may provide a feasible automatic outcome prediction method for comatose patients.

Development of a point of care system for automated coma prognosis: a prospective cohort study protocol

INTRODUCTION

Coma is a deep state of unconsciousness that can be caused by a variety of clinical conditions. Traditional tests for coma outcome prediction are based mainly on a set of clinical observations. Recently, certain event-related potentials (ERPs), which are transient electroencephalogram (EEG) responses to auditory, visual or tactile stimuli, have been introduced as useful predictors of a positive coma outcome (ie, emergence). However, such tests require the skills of clinical neurophysiologists, who are not commonly available in many clinical settings. Additionally, none of the current standard clinical approaches have sufficient predictive accuracies to provide definitive prognoses.

OBJECTIVE

The objective of this study is to develop improved machine learning procedures based on EEG/ERP for determining emergence from coma.

METHODS AND ANALYSIS

Data will be collected from 50 participants in coma. EEG/ERP data will be recorded for 24 consecutive hours at a maximum of five time points spanning 30 days from the date of recruitment to track participants' progression. The study employs paradigms designed to elicit brainstem potentials, middle-latency responses, N100, mismatch negativity, P300 and N400. In the case of patient emergence, data are recorded on that occasion to form an additional basis for comparison. A relevant data set will be developed from the testing of 20 healthy controls, each spanning a 15-hour recording period in order to formulate a baseline. Collected data will be used to develop an automated procedure for analysis and detection of various ERP components that are salient to prognosis. Salient features extracted from the ERP and resting-state EEG will be identified and combined to give an accurate indicator of prognosis.

ETHICS AND DISSEMINATION

This study is approved by the Hamilton Integrated Research Ethics Board (project number 4840). Results will be disseminated through peer-reviewed journal articles and presentations at scientific conferences.

TRIAL REGISTRATION NUMBER

NCT03826407.

EEG and autonomic responses to nociceptive stimulation in disorders of consciousness

Since behavioral responses to external stimuli of patients presenting disorders of consciousness (DoC) are often difficult to qualify, covert physiological correlates of responsivity are deemed as potentially valuable tools to help assessment procedures. While noxious stimuli seem good candidates to explore DoC patients' responsivity, autonomic and electrophysiological correlates of pain detection in DoC patients are still debated. This research aims at investigating autonomic and cortical activation as covert measure of residual somatosensory and nociceptive processes in patients in vegetative state. Twenty-one patients received touch- and pain-related stimulations while autonomic and cortical measures were recorded, with minimal stress for them. Results showed an increased frontal and parietal activation in response to both touch and pain stimuli. Pain-related stimulation was however associated with greater delta parietal response, lower left frontal activation, and increased electrodermal and heart rate measures. Present findings suggest that both somatic stimulations could induce measurable central responses, which might mirror basic attention orientation and perceptual processes. Nonetheless, the nociceptive stimulation in particular seemed to induce a more consistent and informative pattern of covert response even if we used a mild pain-induction procedure.

Prognostic Value of Evoked Responses and Event-Related Brain Potentials in

The behaviourally unresponsive patient, unable to exhibit the presence of cognition, constitutes a conundrum for health care specialists. Prognostic uncertainty impedes accurate management decisions and the application of ethical principles. An early, reliable prognosis is highly desirable. In this review investigations studying comatose patients with coma of different etiologies were selected. It is concluded that objective prognostication is enhanced by the use of electrophysiological tests. Persistent abnormalities of brainstem auditory evoked potentials and short-latency somatosensory evoked potentials reliably indicate the likelihood of irreversible neurological deficit or death. Meanwhile, the presence of “cognitive” event-related brain potentials (e.g., P300 and mismatch negativity) reflects the functional integrity of higher level information processing and, therefore, the likelihood of capacity for cognition. An approach that combines clinical and electrophysiological values provides optimal prediction of outcome and level of disability.

The assessment of routine electroencephalography in patients with altered mental status

PURPOSE

Changes in electroencephalography (EEG) patterns may offer a clue to the cause of altered mental status and suggest the prognoses of patients with such mental status. We aimed to identify the EEG patterns in patients with altered mental status and to correlate EEG findings with clinical prognoses.

MATERIALS AND METHODS

We included 105 patients with altered mental status who underwent EEG. EEG and clinical chart reviews with ongoing patient follow-ups were performed to determine the clinical prognosis of the patients. Clinical data were sorted using the Glasgow Coma Scale (GCS) and Glasgow Outcome Scale (GOS). EEG findings were classified according to a method suggested by Scollo-Lavizzari. The EEGs were analyzed to find out whether any correlation existed with the prognoses of patients.

RESULTS

Nonconvulsive status epilepticus (NCSE) was detected in only three patients (2.9%). Specific EEG patterns were observed in 28 patients. Twenty-nine (27.6%) patients expired, and 45 (42.9%) patients were in a vegetative state. EEG grade and GCS significantly correlated with GOS. EEG grade alone had a correlation with GCS. Patients with a severe EEG finding had a poor prognosis.

CONCLUSION

EEG findings reflect the mental status of patients, and EEG grades are correlated with the clinical prognosis of patients. Although EEG is not frequently performed on patients with altered mental state, it can play a supplemental role in establishing a prognosis. Thus, the use of EEG should be emphasized in clinical setting.

Assessment of comatose patients: a Portuguese instrument based on the Coma Recovery Scale - revised and using nursing standard terminology

AIMS

To translate and adapt the Coma Recovery Scale - Revised to Portuguese using the ICNP® (International Classification of Nursing Practice) terminology and to determine if it can be administered reliably across examiners.

BACKGROUND

Assessment tools for the person in a coma can contribute to the planning, implementation and evaluation of care. It also strengthens the autonomy and responsibility of nurses, contributing to the safety, quality and satisfaction of those who deliver and receive care. This allows the sharing of information amongst healthcare professionals and supports decision-making within a multidisciplinary team.

METHODS

A convenience sample of 20 patients admitted to an intensive care unit constituted the study participants. The data were collected during 2009. The instrument was administered by the same two raters in all the patients on two consecutive days. The total and subscale score agreement was then examined, using inter-rater and test-retest analyses. The intercorrelation dependencies between the subscales were also analysed.

RESULTS

The results of the analyses suggest that the instrument can be used reliably, even when there are some patient fluctuations. The correlation of the subscale scores was high and better than the results presented for the original Coma Recovery Scale - Revised, indicating that this scale is a homogeneous measure of neurobehavioural function.

CONCLUSION

The new instrument can be administered reliably by trained examiners and produces a high degree of reproducibility in scores between raters over repeated assessments. We believe that assessment tools that can assess the communication ability of patients will be relevant to evaluating the continuity of care, and promote the effectiveness of care.

Improving the clinical assessment of consciousness with advances in electrophysiological and neuroimaging techniques

In clinical neurology, a comprehensive understanding of consciousness has been regarded as an abstract concept - best left to philosophers. However, times are changing and the need to clinically assess consciousness is increasingly becoming a real-world, practical challenge. Current methods for evaluating altered levels of consciousness are highly reliant on either behavioural measures or anatomical imaging. While these methods have some utility, estimates of misdiagnosis are worrisome (as high as 43%) - clearly this is a major clinical problem. The solution must involve objective, physiologically based measures that do not rely on behaviour. This paper reviews recent advances in physiologically based measures that enable better evaluation of consciousness states (coma, vegetative state, minimally conscious state, and locked in syndrome). Based on the evidence to-date, electroencephalographic and neuroimaging based assessments of consciousness provide valuable information for evaluation of residual function, formation of differential diagnoses, and estimation of prognosis.

Early somatosensory evoked potential grades in comatose traumatic brain injury patients predict cognitive and functional outcome

OBJECTIVES

To relate early somatosensory evoked potential grades from comatose traumatic brain injury patients to neuropsychological and functional outcome 1 yr later; to determine the day (within the first week after traumatic brain injury) that somatosensory evoked potential grade best correlates with outcome; to determine whether somatosensory evoked potential grade improvement in the first week after traumatic brain injury is associated with improved outcome.

DESIGN

Prospective cohort study.

SETTING

Critical care unit at a university hospital.

PATIENTS

Median nerve somatosensory evoked potentials were obtained from 81 comatose patients with traumatic brain injury. Somatosensory evoked potential grades were calculated from results obtained on days 1, 3, and 7 after traumatic brain injury. Glasgow Outcome Scale, Barthel Index, Rivermead Head Injury Follow-up Questionnaire, General Health Questionnaire, Stroop Color-Word Test, Paced Auditory Serial Addition Task, and Symbol-Digit Modalities Test scores were obtained 1 yr after injury.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Somatosensory evoked potential grade on days 1, 3, and 7 related significantly with Glasgow Outcome Scale and Barthel scores (day 3 better than day 1) but did not relate with Rivermead Head Injury Follow-up Questionnaire or General Health Questionnaire scores. Day 3 and day 7 somatosensory evoked potential grades related significantly with Stroop scores. Day 3 somatosensory evoked potential grades related significantly with Symbol-Digit Modalities Test scores. Patients with bilaterally present but abnormal somatosensory evoked potentials, whose somatosensory evoked potential grade improved between days 1 and 3, had marginally better functional outcome than those without somatosensory evoked potential grade improvement.

CONCLUSIONS

Day 3 somatosensory evoked potential grade related to information-processing speed, working memory, and the ability to attend to tasks 1 yr after traumatic brain injury. Day 3 somatosensory evoked potential grade had the strongest relationship with functional outcome. Somatosensory evoked potential grades were not related to emotional well-being.

Brain dysfunction in critically ill patients--the intensive care unit and beyond

Critical care physicians often find themselves prognosticating for their patients, attempting to predict patient survival as well as disability. In the case of neurologic injury, this can be especially difficult. A frequent cause of coma in the intensive care unit is resuscitation following cardiac arrest, for which mortality and severe neurologic disability remain high. Recent studies of the clinical examination, of serum markers such as neuron-specific enolase, and of somatosensory evoked potentials allow accurate and specific prediction of which comatose patients are likely to suffer a poor outcome. Using these tools, practitioners can confidently educate the family for the majority of patients who will die or remain comatose at 1 month. Delirium is a less dramatic form of neurologic injury but, when sought, is strikingly prevalent. In addition, delirium in the intensive care unit is associated with increased mortality and poorer functional recovery, prompting investigation into preventative and therapeutic strategies to counter delirium. Finally, neurologic damage may persist long after the patient's recovery from critical illness, as is the case for cognitive dysfunction detected months and years after critical illness. Psychiatric impairment including depression or post-traumatic stress disorder may also arise. Mechanisms contributing to each of these entities are reviewed.

Autonomic reactivity to sensory stimulation is related to consciousness level after severe traumatic brain injury

OBJECTIVE

To examine changes in the activity of the autonomic nervous system (ANS) that are related to recovery to consciousness in the post-acute phase after severe traumatic brain injury (sTBI).

METHODS

Skin conductance and heart rate reactivity to sensory stimulation were recorded every 2 weeks for an average period of 3.5 months in 16 adolescent patients, during the assessment of their level of consciousness (LoC), and their cognitive and functional behaviour.

RESULTS

Both heart rate variability (HRV) and skin conductance level (SCL) in reaction to sensory stimulation changed with recovery to consciousness. Indices of HRV and SCL that represent sympathetic activity of the autonomic nervous system (ANS) increased with recovery, whereas indices that represent parasympathetic activity decreased. In addition, we observed an increase in sympathovagal balance of the ANS with recovery.

CONCLUSIONS

Recovery to consciousness determined by clinical observation in sTBI in the post-acute phase is related to changes in SCL and HRV during sensory stimulation. ANS reactivity to environmental stimulation can therefore give objective supplementary information about the clinical state of sTBI patients, and can contribute to decision-making in the treatment policy of unresponsive patients.

SIGNIFICANCE

These findings demonstrate that autonomic reactivity can be informative concerning how a severely damaged nervous system reacts to environmental stimulation and how, in a recovering nervous system, this reactivity changes.

Over-Complete Discrete Wavelet Transformation of the Normal Auditory Brainstem Response Improves Prediction of Outcome following Severe Acute Closed Head Injury

Previous research has shown that complex statistical analysis (discriminant function analysis) of a 'normal' auditory brainstem response (ABR) result can improve this measure's ability to predict subject outcome following severe acute closed head injury (ACHI). We hypothesized that adding the ABR's time-frequency information to such an analysis would improve this predictive value even further. 'Normal' ABR results were sampled from 69 severe ACHI subjects (22 of whom died and 47 of whom lived) and their time-frequency information extracted using an over-complete discrete wavelet transformation (OCDWT). A series of logistic regression analyses then showed correct predictions of death and survival as follows: ABR measures only 72 and 89% (respectively), ABR OCDWT measures only 82 and 89% (respectively), and ABR and ABR OCDWT measures combined 86 and 93% (respectively). These results showed that the addition of time-frequency information can improve the ability of the 'normal' ABR result to predict outcome following severe ACHI.

The interpretation of the EEG in stupor and coma

This review discusses a variety of causes of stupor and coma and associated electroencephalographic (EEG) findings. These include metabolic disturbances such as hepatic or renal dysfunction, which are often characterized by slowing of background rhythms and triphasic waves. Hypoxia and drug intoxications can produce a number of abnormal EEG patterns such as burst suppression, alpha coma, and spindle coma. Structural lesions, either supra- or infratentorial, are reviewed. EEGs in the former may show focal disturbances such as delta and theta activity, epileptiform abnormalities, and attenuation of faster frequencies. In infratentorial lesions, the EEG may appear normal, particularly with a pontine lesion. Some patients may be encephalopathic because of ongoing epileptic activity with minimal or no motor movements. This entity, nonconvulsive status epilepticus (NCSE), is difficult to diagnose in obtunded/comatose patients, and an EEG is required to verify the diagnosis and to monitor treatment. Several EEG patterns and their interpretation in suspected cases of NCSE such as periodic lateralized epileptiform discharges (PLEDs), bilateral independent periodic lateralized epileptiform discharges (BIPLEDs), generalized periodic epileptiform discharges (GPEDs), and triphasic waves are reviewed. Other entities discussed include the locked-in syndrome, neocortical death, persistent vegetative state, brainstem death, and brain death.

Continuous EEG monitoring in comatose intensive care patients: epileptiform activity in etiologically distinct groups

INTRODUCTION

It is unclear whether patients or subpopulations of patients might benefit from EEG monitoring.

METHODS

We conducted a prospective trial of continuous electroencephalogram monitoring (CEEG; 48 hours).

RESULTS

Eleven of 55 (20%) patients who underwent CEEG monitoring recorded seizures. Of patients with acute structural brain lesions (ASBLs), 10 of 31 (32%) patients experiences recorded seizures, whereas only 24 (4%) patients with metabolic encephalopathies experienced recorded seizures (p < 0.01). Six patients with ASBLs (11%) and one patient with metabolic encephalopathy (4%) had spikes/interictal epileptiform discharges (p = 0.087).

CONCLUSION

Our study suggests that CEEG monitoring may be more valuable for detection of seizures in patients with ASBLs than in patients with metabolic encephalopathies.

Continuous or emergent EEG: can bedside caregivers recognize epileptiform discharges?

OBJECTIVE

When emergent or continuous EEG monitoring is carried out to detect subclinical seizures, expert interpretation of the results is usually not immediately available. We assessed the ability to recognize epileptiform discharges among various nonexpert bedside caregivers. A second objective was to determine whether such skill improves with a simple educational intervention.

DESIGN AND SETTING

A prospective cohort study with an educational intervention in a university-based tertiary medical center.

PARTICIPANTS

All 125 neurology and neurosurgery residents, intensive care fellows, critical care and neurological floor nurses, and EEG technicians were offered participation. Of these, 50 completed the study and provided 2,398 EEG responses.

INTERVENTIONS

A computer-based PowerPoint lecture designed to teach recognition of epileptiform discharges.

MEASUREMENTS AND RESULTS

(a) Average number of correct responses in identifying epileptiform discharges in a computer-based pre-test consisting of 24 EEG questions. (b) Impact of the educational intervention as measured by change in the mean average score in a similar post-test. The rate of correct responses by group ranged from 46% for medical critical care nurses to 94% for EEG technicians. Overall mean correct response rate was 14.72 (61%) for the pre-test, and increased to 16.06 (67%) in for post-test (p=0.002).

CONCLUSIONS

Recognition of epileptiform discharges by bedside caregivers is disturbingly low. A simple educational intervention only modestly improved such ability. Therefore we should not rely on nontrained personnel to interpret emergent or continuous recordings. EEG technicians are an exception, and their skill could be utilized during emergent studies.

The prognostic value of evoked responses from primary somatosensory and auditory cortex in comatose patients

OBJECTIVE

To evaluate somatosensory and auditory primary cortices using somatosensory evoked potentials (SEPs) and middle latency auditory evoked potentials (MLAEPs) in the prognosis of return to consciousness in comatose patients.

METHODS

SEPs and MLAEPs were recorded in 131 severe comatose patients. Latencies and amplitudes were measured. Coma had been caused by transient cardiac arrest (n=49), traumatic brain injury (n=22), stroke (n=45), complications of neurosurgery (n=12) and encephalitis (n=3). One month after the onset of coma patients were classified as awake, still comatose or dead. Three months after (M3), they were classified into one of the 5 categories of the Glasgow outcome scale (GOS).

RESULTS

At M3, 41.2% were dead, 47.3% were conscious (GOS 3-5) and 11.5% had not recovered consciousness. None of the patients in whom somatosensory N20 and auditory Pa were absent did return to consciousness and in the post-anoxic group, reduced cortical amplitude too was always associated with bad outcome. Conversely, N20 and Pa were present, respectively, in 33/69 and 34/69 patients who did not recover.

CONCLUSIONS

The prognostic value of SEPs and MLAEPs in comatose patients depends on the cause of coma. Measurement of response amplitudes is informative. Abolition of cortical SEPs and/or cortical MLAEPs precludes post-anoxic comatose patients from returning to consciousness (100% specificity). In any case, the presence of short latency cortical somatosensory or auditory components is not a guarantee for return to consciousness. Late components should then be recorded.

[Traumatic brain injuries in adults: from coma to wakefulness. Neurophysiological data]

OBJECTIVE

To analyse relevant literature and to express an expert point of view concerning the interest of electroencephalography and evoked potentials recordings in the evaluation of severe head trauma in adults in the context of a consensus conference.

MATERIAL AND METHODS

Scientific databases have been checked on the Internet using key-words. The summaries of 340 papers have checked out. Consequently 94 papers have been thoroughly analysed. Fifty-nine of them are cited in the text of this paper.

RESULTS

Electroencephalography (EEG) and evoked potentials (Eps) evaluate the functional status of the brain. They augment the clinical examination. They are non invasive and easy to perform at patient's bedside. The EEG evaluate globally the functional status of the brain but it is very sensitive to sedative and anaesthetic drugs. It can disclose subclinical or electroclinical epileptic seizures. When reactivity to sensory stimulations can be elicited, this can be considered a prognostic indicator for a good outcome. Evoked potentials are less influenced by sedative drugs. There are several types of evoked potentials, each one with a different localizing value. Brainstem auditory evoked potentials (or short-latency Eps) evaluate the auditory nerve and brainstem. When normal they have no specificity. When abnormal they are an indicator of a poor or bad outcome. Somatosensory and auditory middle-latency Eps evaluate the primary cortex. In coma due to traumatic brain injury the presence of primary cortex components is an indicator of a good outcome and its absence is an indicator of a poor outcome at least when there is no focal brain lesion as to have the primary cortex component to be absent. Event-related potentials evaluate associative brain areas. When they are present in a comatose patient they favor the idea that some cognitive processes are active and they have a high positive predictive value for a return to consciousness. The electrophysiological evaluation can help to identify atypical situations and pathologies close to coma, disclose nonconvulsive seizures and localize certain complications or dysfunctions in atypical cases.

Improving the prediction of outcome in severe acute closed head injury by using discriminant function analysis of normal auditory brainstem response latencies and amplitudes

OBJECT

The auditory brainstem response (ABR) is a useful addition to standard medical measures for predicting outcome in patients with severe acute closed head injury (ACHI). Limiting this success, however, is the poor predictive value of a so-called "normal" ABR. In this study the authors used discriminant function analysis (DFA) of ABR Wave I, III, and V latencies and amplitudes to improve the predictive accuracy of the normal ABR, both as a single measure and in combination with other standard medical measures.

METHODS

The DFAs were conducted using the ABR and medical results in 68 patients with severe ACHI (30 who died [ACHI-died], and 38 who survived [ACHI-lived]) who presented with normal ABR responses in the neurosurgical intensive care unit of the authors' hospital in Johannesburg. All patients had undergone surgery to remove an intracranial hematoma. Correct predictions of outcome by ABR DFA measures were 83% for the ACHI-died group (48% at > or = 90% confidence level) and 87% for the ACHI-lived group (71% at > or = 90% confidence level); by medical DFA measures the correct predictions were 83% for the ACHI-died group (96% at >; or = 90% confidence level) and 95% for the ACHI-lived group (94% at > or = 90% confidence level); and by combined ABR and medical DFA measures correct predictions were 100% for the ACHI-died group (100% at > or = 90% confidence level) and 97% for the ACHI-lived group (100% at > or = 90% confidence level).

CONCLUSIONS

The DFA of ABR Wave I, III, and V latencies and amplitudes improved the predictive ability of normal ABR results to rates similar to those obtained using DFA for the medical measures, although at lower confidence levels. The DFA of the combined ABR and medical measures improved correct predictions to rates significantly higher than for either of the measures on its own.

Representation of somatosensory evoked potentials using discrete wavelet transform

OBJECTIVE

Somatosensory evoked potentials (SEP) have been shown to be a useful tool in monitoring of the central nervous system (CNS) during anaesthesia. SEP analysis is usually performed by an experienced human operator. For automatic analysis, appropriate parameter extraction and signal representation methods are required. The aim of this work is to evaluate the discrete wavelet transform (DWT) as such a method for an SEP representation.

METHODS

Median nerve SEP were derived in 52 female patients, scheduled for elective surgery with SEP monitoring, under clinically proven conditions in the awake state. The discrete wavelet transform implemented as the multiresolution analysis was adopted for evaluating SEP. The suitability of the wavelet coefficients was investigated by calculating the error between the averaged response and the corresponding wavelet reconstructions.

RESULTS

SEP can be represented by a very small number of wavelet coefficients. Although the individual SEP waveform has an influence on the number and selection of wavelet coefficients, in all subjects more than 84% of the SEP waveform energy can be represented by a set 16 wavelet coefficients.

CONCLUSIONS

The discrete wavelet transformation provides an efficient tool for SEP representation and parameterisation. Depending on the specific problem the DWT, can be adjusted to the desired accuracy, which is important for the subsequent development of automatic SEP analysers.

Electrophysiologic monitoring in neurointensive care

Cumulative evidence of potential benefits of electroencephalography (EEG) and evoked potentials in the management of patients with acute cerebral damage has been confirmed. Continuous EEG monitoring is the best method for detecting nonconvulsive seizures and is strongly recommended for the treatment of status epilepticus. Continuously displayed, validated quantitative EEG may facilitate early detection of secondary cerebral insults and may play a decision-making role in the management of patients with head injury, stroke, or subarachnoid hemorrhage. Long-latency auditory evoked potentials and cognitive components constitute a new field of interest for the progress of comatose patients. Motor evoked potentials may become clinically important both in acutely injured and elective postoperative patients. In the neurointensive care units adequate techniques can be selected to answer targeted clinical questions. The efficacy can be improved by implementing educational projects based on ad hoc training of nurses and neurointensive care specialists.

Quantitative analysis of the loss of distinction between gray and white matter in comatose patients after cardiac arrest

BACKGROUND AND PURPOSE

Anecdotal reports suggest that a loss of distinction between gray (GM) and white matter (WM) as adjudged by CT scan predicts poor outcome in comatose patients after cardiac arrest. To address this, we quantitatively assessed GM and WM intensities at various brain levels in comatose patients after cardiac arrest.

METHODS

Patients for whom consultation was requested within 24 hours of a cardiac arrest were identified with the use of a computerized database that tracks neurological consultations at our institution. Twenty-five comatose patients were identified for whom complete medical records and CT scans were available for review. Twenty-five consecutive patients for whom a CT scan was interpreted as normal served as controls. Hounsfield units (HUs) were measured in small defined areas obtained from axial images at the levels of the basal ganglia, centrum semiovale, and high convexity area.

RESULTS

At each level tested, lower GM intensity and higher WM intensity were noted in comatose patients compared with normal controls. The GM/WM ratio was significantly lower among comatose patients compared with controls (P:<0.0001, rank sum test). There was essentially no overlap in GM/WM ratios between control and study patients. The difference was greatest at the basal ganglia level. We also observed a marginally significant difference in the GM/WM ratio at the basal ganglia level between those patients who died and those who survived cardiac arrest (P:=0. 035, 1-tailed t test). Using receiver operating characteristic curve analysis, we determined that a difference in GM/WM ratio of <1.18 at the basal ganglia level was 100% predictive of death. At the basal ganglia level, none of 12 patients below this threshold survived, whereas the survival rate was 46% among patients in whom the ratio was >1.18. The empirical risk of death was 21.67 for comatose patients with a value below threshold.

CONCLUSIONS

The ratio in HUs of GM to WM provides a reproducible measure of the distinction between gray and white matter. A lower GM/WM ratio is observed in comatose patients immediately after cardiac arrest. The basal ganglia level seems to be the most sensitive location on CT for measuring this relationship. Although a GM/WM ratio <1.18 at this level predicted death in this retrospective study, the difference in this study is not robust enough to recommend that management decisions be dictated by CT results. The results, however, do warrant consideration of a prospective study to determine the reliability of CT scanning in predicting outcome for comatose patients after cardiac arrest.

Auditory evoked potentials to spectro-temporal modulation of complex tones in normal subjects and patients with severe brain injury

In order to assess higher auditory processing capabilities, long-latency auditory evoked potentials (AEPs) were recorded to synthesized musical instrument tones in 22 post-comatose patients with severe brain injury causing variably attenuated behavioural responsiveness. On the basis of normative studies, three different types of spectro-temporal modulation were employed. When a continuous 'clarinet' tone changes pitch once every few seconds, N1/P2 potentials are evoked at latencies of approximately 90 and 180 ms, respectively. Their distribution in the fronto-central region is consistent with generators in the supratemporal cortex of both hemispheres. When the pitch is modulated at a much faster rate ( approximately 16 changes/s), responses to each change are virtually abolished but potentials with similar distribution are still elicited by changing the timbre (e.g. 'clarinet' to 'oboe') every few seconds. These responses appear to represent the cortical processes concerned with spectral pattern analysis and the grouping of frequency components to form sound 'objects'. Following a period of 16/s oscillation between two pitches, a more anteriorly distributed negativity is evoked on resumption of a steady pitch. Various lines of evidence suggest that this is probably equivalent to the 'mismatch negativity' (MMN), reflecting a pre-perceptual, memory-based process for detection of change in spectro-temporal sound patterns. This method requires no off-line subtraction of AEPs evoked by the onset of a tone, and the MMN is produced rapidly and robustly with considerably larger amplitude (usually >5 microV) than that to discontinuous pure tones. In the brain-injured patients, the presence of AEPs to two or more complex tone stimuli (in the combined assessment of two authors who were 'blind' to the clinical and behavioural data) was significantly associated with the demonstrable possession of discriminative hearing (the ability to respond differentially to verbal commands, in the assessment of a further author who was blind to the AEP findings). Behavioural and electrophysiological findings were in accordance in 18/22 patients, but no AEPs could be recorded in two patients who had clear behavioural evidence of discriminative hearing. The absence of long-latency AEPs should not, therefore, be considered indicative of complete functional deafness. Conversely, AEPs were substantially preserved in two patients without behavioural evidence of discriminative hearing. Although not necessarily indicative of conscious 'awareness', such AEP preservation might help to identify sentient patients who are prevented by severe motor disability from communicating their perception.