Electrophysiological assessment of the afferent sensory pathway in cardiac arrest survivors

A Comprehensive Review and Practical Guide of the Applications of Evoked Potentials in Neuroprognostication After Cardiac Arrest

Cardiorespiratory arrest is a very common cause of morbidity and mortality nowadays, and many therapeutic strategies, such as induced coma or targeted temperature management, are used to reduce patient sequelae. However, these procedures can alter a patient's neurological status, making it difficult to obtain useful clinical information for the reliable estimation of neurological prognosis. Therefore, complementary investigations are conducted in the early stages after a cardiac arrest to clarify functional prognosis in comatose cardiac arrest survivors in the first few hours or days. Current practice relies on a multimodal approach, which shows its greatest potential in predicting poor functional prognosis, whereas the data and tools to identify patients with good functional prognosis remain relatively limited in comparison. Therefore, there is considerable interest in investigating alternative biological parameters and advanced imaging technique studies. Among these, somatosensory evoked potentials (SSEPs) remain one of the simplest and most reliable tools. In this article, we discuss the technical principles, advantages, limitations, and prognostic implications of SSEPs in detail. We will also review other types of evoked potentials that can provide useful information but are less commonly used in clinical practice (e.g., visual evoked potentials; short-, medium-, and long-latency auditory evoked potentials; and event-related evoked potentials, such as mismatch negativity or P300).

Somatosensory Evoked Potentials and Neuroprognostication After Cardiac Arrest

Improved understanding of post-cardiac arrest syndrome and clinical practices such as targeted temperature management have led to improved mortality in this cohort. Attention has now been placed on development of tools to aid in predicting functional outcome in comatose cardiac arrest survivors. Current practice uses a multimodal approach including physical examination, neuroimaging, and electrophysiologic data, with a primary utility in predicting poor functional outcome. These modalities remain confounded by self-fulfilling prophecy and the withdrawal of life-sustaining therapies. To date, a reliable measure to predict good functional outcome has not been established or validated, but the use of quantitative somatosensory evoked potential (SSEP) shows potential for this use. MEDLINE and EMBASE search using words "Cardiac Arrest" and "SSEP," "Somato sensory evoked potentials," "qSSEP," "quantitative SSEP," "targeted temperature management in cardiac arrest" was conducted. Relevant recent studies on targeted temperature management in cardiac arrest, plus studies on SSEP in cardiac arrest in the setting of hypothermia and without hypothermia, were included. In addition, animal studies evaluating the role of different components of SSEP in cardiac arrest were reviewed. SSEP is a specific indicator of poor outcomes in post-cardiac arrest patients but lacks sensitivity and has not clinically been established to foresee good outcomes. Novel methods of analyzing quantitative SSEP (qSSEP) signals have shown potential to predict good outcomes in animal and human studies. In addition, qSSEP has potential to track cerebral recovery and guide treatment strategy in post-cardiac arrest patients. Lying beyond the current clinical practice of dichotomized absent/present N20 peaks, qSSEP has the potential to emerge as one of the earliest predictors of good outcome in comatose post-cardiac arrest patients. Validation of qSSEP markers in prospective studies to predict good and poor outcomes in the cardiac arrest population in the setting of hypothermia could advance care in cardiac arrest. It has the prospect to guide allocation of health care resources and reduce self-fulfilling prophecy.

Beyond dichotomy: patterns and amplitudes of SSEPs and neurological outcomes after cardiac arrest

Background

We hypothesized that the absence of P25 and the N20–P25 amplitude in somatosensory evoked potentials (SSEPs) have higher sensitivity than the absence of N20 for poor neurological outcomes, and we evaluated the ability of SSEPs to predict long-term outcomes using pattern and amplitude analyses.

Methods

Using prospectively collected therapeutic hypothermia registry data, we evaluated whether cortical SSEPs contained a negative or positive short-latency wave (N20 or P25). The N20–P25 amplitude was defined as the largest difference in amplitude between the N20 and P25 peaks. A good or poor outcome was defined as a Glasgow-Pittsburgh Cerebral Performance Category (CPC) score of 1–2 or 3–5, respectively, 6 months after cardiac arrest.

Results

A total of 192 SSEP recordings were included. In all patients with a good outcome (n = 51), both N20 and P25 were present. Compared to the absence of N20, the absence of N20–P25 component improved the sensitivity for predicting a poor outcome from 30.5% (95% confidence interval [CI], 23.0–38.8%) to 71.6% (95% CI, 63.4–78.9%), while maintaining a specificity of 100% (93.0–100.0%). Using an amplitude < 0.64 μV, i.e., the lowest N20–P25 amplitude in the good outcome group, as the threshold, the sensitivity for predicting a poor neurological outcome was 74.5% (95% CI, 66.5–81.4%). Using the highest N20–P25 amplitude in the CPC 4 group (2.31 μV) as the threshold for predicting a good outcome, the sensitivity and specificity were 52.9% (95% CI, 38.5–67.1%) and 96.5% (95% CI, 91.9–98.8%), respectively. The predictive performance of the N20–P25 amplitude was good, with an area under the receiver operating characteristic curve (AUC) of 0.94 (95% CI, 0.90–0.97). The absence of N20 was statistically inferior regarding outcome prediction (p < 0.05), and amplitude analysis yielded significantly higher AUC values than did the pattern analysis (p < 0.05).

Conclusions

The simple pattern analysis of whether the N20–P25 component was present had a sensitivity comparable to that of the N20–P25 amplitude for predicting a poor outcome. Amplitude analysis was also capable of predicting a good outcome.

Electronic supplementary material

The online version of this article (10.1186/s13054-019-2510-x) contains supplementary material, which is available to authorized users.

Pain-related Somato Sensory Evoked Potentials: a potential new tool to improve the prognostic prediction of coma after cardiac arrest

Introduction

Early prediction of a good outcome in comatose patients after cardiac arrest still remains an unsolved problem. The main aim of the present study was to examine the accuracy of middle-latency SSEP triggered by a painful electrical stimulation on median nerves to predict a favorable outcome.

Methods

No- and low-flow times, pupillary reflex, Glasgow motor score and biochemical data were evaluated at ICU admission. The following were considered within 72 h of cardiac arrest: highest creatinine value, hyperthermia occurrence, EEG, SSEP at low- (10 mA) and high-intensity (50 mA) stimulation, and blood pressure reactivity to 50 mA. Intensive care treatments were also considered. Data were compared to survival, consciousness recovery and 6-month CPC (Cerebral Performance Category).

Results

Pupillary reflex and EEG were statistically significant in predicting survival; the absence of blood pressure reactivity seems to predict brain death within 7 days of cardiac arrest. Middle- and short-latency SSEP were statistically significant in predicting consciousness recovery, and middle-latency SSEP was statistically significant in predicting 6-month CPC outcome. The prognostic capability of 50 mA middle-latency-SSEP was demonstrated to occur earlier than that of EEG reactivity.

Conclusions

Neurophysiological evaluation constitutes the key to early information about the neurological prognostication of postanoxic coma. In particular, the presence of 50 mA middle-latency SSEP seems to be an early and reliable predictor of good neurological outcome, and its absence constitutes a marker of poor prognosis. Moreover, the absence 50 mA blood pressure reactivity seems to identify patients evolving towards the brain death.

Predictors of poor neurological outcome in adult comatose survivors of cardiac arrest: a systematic review and meta-analysis. Part 1: patients not treated with therapeutic hypothermia

AIMS AND METHODS

To systematically review the accuracy of early (≤7 days) predictors of poor outcome defined as death or vegetative state (Cerebral Performance Categories [CPC] 4-5) or death, vegetative state or severe disability (CPC 3-5) in comatose survivors from cardiac arrest not treated using therapeutic hypothermia (TH). PubMed, Scopus and the Cochrane Database of Systematic reviews were searched for eligible studies. Sensitivity, specificity, false positive rates (FPR) for each predictor were calculated and results of predictors with similar time points and outcome definitions were pooled. Quality of evidence (QOE) was evaluated according to the GRADE guidelines.

RESULTS

50 studies (2828 patients) were included in final analysis. Presence of myoclonus at 24-48h, bilateral absence of short-latency somatosensory evoked potential (SSEP) N20 wave at 24-72h, absence of electroencephalographic activity >20-21μV ≤72h and absence of pupillary reflex at 72h predicted CPC 4-5 with 0% FPR and narrow (<10%) 95% confidence intervals. Absence of SSEP N20 wave at 24h predicted CPC 3-5 with 0% [0-8] FPR. Serum thresholds for 0% FPR of biomarkers neuron specific enolase (NSE) and S-100B were highly inconsistent among studies. Most of the studies had a low or very low QOE and did not report blinding of the treating team from the results of the investigated predictor.

CONCLUSIONS

In comatose resuscitated patients not treated with TH presence of myoclonus, absence of pupillary reflex, bilateral absence of N20 SSEP wave and low EEG voltage each predicted poor outcome early and accurately, but with a relevant risk of bias.

Biochemical markers and somatosensory evoked potentials in patients after cardiac arrest: the role of neurological outcome scores

Biochemical markers, e.g. NSE or S100B, and somatosensory evoked potentials (SSEP) are considered promising candidates for neurological prognostic predictors in patients after cardiac arrest (CA). The Utstein Templates recommend the use of the Glasgow-Pittsburgh Cerebral Performance Categories (GP-CPC) to divide patients according to their neurological outcome. However, several studies investigating biochemical markers and SSEP are based on the Glasgow Outcome Score (GOS). We noticed that many studies failed to exclude patients who died without certified brain damage from patients classified as poor outcome, instead including all patients who died into this category. Therefore, we summarized the published NSE cut-off values and the derived sensitivity and specificity to predict poor outcome of those studies which only included patients with certified brain death in GOS-1 or GP-CPC-5 (group A) vs. those studies which did not differentiate between death from any cause or death due to primary brain damage (group B). On average, mean NSE cut-off values and sensitivity were higher (56 ± 35 ng/ml, 56 ± 18%) in group A than in group B (41 ± 17 ng/ml, 44 ± 25%), respectively. The specificity remained equally high in both groups. In analogy, the average sensitivity of SSEP to predict poor outcome was higher in group A (76 ± 11%) than in group B (50 ± 15%), while the specificity was similar in both groups. Conclusively, inclusion of deaths without certified brain damage after CA in neurological outcome studies will lead to underestimation of the prognostic power of biochemical or electrophysiological markers for brain damage. A modified GOS and GP-CPC score might help to avoid this bias.

Assessment of somatosensory evoked potentials during resuscitation of a 15-year-old boy with Duchenne muscular dystrophy

Patients with Duchenne muscular dystrophy (DMD) are likely to suffer from cardiac insufficiency. Subclinical cardiac insufficiency may decompensate intraoperatively. During spinal surgery, recording of somatosensory evoked potentials (SSEP) is the standard method of spinal cord monitoring. Assessment of SSEP has proven to be a highly prognostic measure of neurological outcome after cardiopulmonary resuscitation (CPR). In the case presented, scalp SSEP as response to stimulation of both median and tibial nerves were recorded during spinal surgery in a 15-year-old boy with DMD. The patient developed severe hypotension and circulatory collapse intraoperatively. SSEP were measured before, during and up to 3h after circulatory collapse. He was successfully resuscitated and fully recovered. Latencies of SSEP remained stable from all extremities whereas amplitudes significantly decreased during CPR, but recovered completely within 3h. The amplitudes of SSEP serve as a more sensitive marker for brain ischaemia than latencies. Stability of latencies and full recovery of amplitudes within 3h indicated sufficient CPR and predicted a good neurological recovery.

Clinical neurophysiologic monitoring and brain injury from cardiac arrest

Electrophysiologic testing continues to play an important role in injury stratification and prognostication in patients who are comatose after cardiac arrest. As discussed previously, however, the adage about treating whole patients, not just the numbers, is relevant in this situation. EEG and SSEP can offer high specificity for discerning poor prognosis as long as they are applied to appropriate patient populations. As discussed previously, EEG and SSEP patterns change during the first hours to days after cardiac arrest and negative prognostic information should not be based solely on studies performed during the first 24 hours. Both electrophysiologic techniques also are susceptible to artifacts that may worsen the electrical patterns artificially and suggest a falsely poor prognosis. EEG is suppressed by anesthetic agents and hypothermia, both of which may produce ECS and burst suppression. Patients who experience respiratory arrest from a toxic ingestion of narcotics or barbiturates, in particular, may present with high-grade EEG patterns initially. Many patients also receive anesthetic medications at the time of tracheal intubation, which may linger beyond their normal half-life in patients who have hepatic or renal insufficiency or concurrent use of interacting medications. SSEP is much less susceptible to sedative anesthetic agents, but hypothermia is demonstrated to prolong evoked potential latencies. As therapeutic hypothermia becomes more common after cardiac arrest, the effect of temperature on electrophysiologic testing needs to be taken into account. The publications discussed previously also emphasize the need to adjust the prognostic value of electro-physiologic tests to the pretest probability of meaningful neurologic recovery in individual patients. Clearly, grade I EEG patterns and normal N20 potentials indicate a much better prognosis in patients who have a short du-ration of cardiac arrest, short duration of coma after resuscitation, and when the studies are performed within the first few days. In patients who remain in coma days after resuscitation and lack appropriate brainstem reflexes, however, even the most normal appearing electrophysiologic patterns do little to change the overall prognosis. Aside from prognostication, electrophysiologic testing holds great promise in defining the basic anatomy and physiology of coma emergence after cardiac arrest. In addition, quantitative EEG and automated evoked potentials have the potential to render these tools less subjective and arcane and more applicable for monitoring patients in the period during and immediately after resuscitation. Quantitative EEG also has great potential asa tool to define the time window for neuroprotective intervention and the means to track the response to such therapies in real time.

Somatosensory and brainstem auditory evoked potentials in cardiac arrest patients treated with hypothermia*

OBJECTIVE

To evaluate the prognostic value of short-latency median nerve somatosensory evoked potentials and brainstem auditory evoked potentials in outcome prediction for comatose cardiac arrest patients treated with hypothermia.

DESIGN

Prospective, randomized, controlled trial of mild hypothermia after out-of-hospital cardiac arrest; a substudy of the European Hypothermia After Cardiac Arrest study.

SETTING

Intensive care unit of a tertiary referral hospital (Helsinki University Central Hospital).

PATIENTS

Sixty consecutive patients (aged 18-75 yrs) resuscitated from out-of-hospital ventricular fibrillation and comatose at 24 hrs after cardiac arrest; all patients were randomly assigned either to therapeutic hypothermia of 33 degrees C or normothermia.

INTERVENTIONS

All patients received standard intensive care for at least 2 days. Patients randomized to hypothermia were cooled with an external cooling device for 24 hrs and then allowed to rewarm slowly for 12 hrs. In the normothermia group, the core temperature was kept below 38 degrees C with antipyretics and by physical means. The clinical outcome was assessed 6 months after cardiac arrest.

MEASUREMENTS AND MAIN RESULTS

Somatosensory evoked potentials and brainstem auditory evoked potentials were recorded 24-28 hrs after cardiac arrest. All wave latencies were significantly prolonged in the hypothermia group. Bilaterally absent N20 waves predicted permanent coma with a specificity of 100% in both treatment groups. Brainstem auditory evoked potential recordings did not correlate with the outcome in either treatment group.

CONCLUSIONS

The prognostic ability of median nerve short-latency somatosensory evoked potentials does not seem to be affected by therapeutic hypothermia. Brainstem auditory evoked potentials had no additional value in outcome prediction.

Brain function after resuscitation from cardiac arrest

PURPOSE OF REVIEW

In industrial countries the incidence of cardiac arrest is still increasing. Almost 80% of cardiac arrest survivors remains in coma for varying lengths of time and full cerebral recovery is still a rare event. After successful cardiopulmonary resuscitation, cerebral recirculation disturbances and complex metabolic postreflow derangements lead to death of vulnerable neurons with further deterioration of cerebral outcome. This article discusses recent research efforts on the pathophysiology of brain injury caused by cardiac arrest and reviews the beneficial effect of therapeutic hypothermia on neurologic outcome along with the recent approach to prognosticate long-term outcome by electrophysiologic techniques and molecular markers of brain injury.

RECENT FINDINGS

Recent experimental studies have brought new insights to the pathophysiology of secondary postischemic anoxic encephalopathy demonstrating a time-dependent cerebral oxidative injury, increased neuronal expression, and activation of apoptosis-inducing death receptors and altered gene expression with long-term changes in the molecular phenotype of neurons. Recently, nuclear MR imaging and MR spectroscopic studies assessing cerebral circulatory recovery demonstrated the precise time course of cerebral reperfusion after cardiac arrest. Therapeutic hypothermia has been shown to improve brain function after resuscitation from cardiac arrest and has been introduced recently as beneficial therapy in ventricular fibrillation cardiac arrest.

SUMMARY

Electrophysiologic techniques and molecular markers of brain injury allow the accurate assessment and prognostication of long-term outcome in cardiac arrest survivors. In particular, somatosensory evoked potentials have been identified as the method with the highest prognostic reliability. A recent systematic review of 18 studies analyzed the predictive ability of somatosensory evoked potentials performed early after onset of coma and found that absence of cortical somatosensory evoked potentials identify patients not returning from anoxic coma with a specificity of 100%.