Brain-stem auditory evoked potentials and brain death

Rapid hearing threshold assessment with modified auditory brainstem response protocols in dogs

Introduction

Auditory brainstem response (ABR) is the gold standard for hearing testing in dogs. ABR is commonly used in puppies to diagnose congenital sensorineural deafness. Long test times limit the use for a more comprehensive hearing screening in veterinary practice. This study aimed to establish a super-fast hearing screening protocol in dogs.

Methods

Hearing thresholds were routinely measured with a mobile device designed for newborn hearing screening in 90 dogs. We introduced modifications of the ABR protocol, e. g., a binaural test mode, higher stimulus rates, a broadband chirp stimulus, and an algorithm for automatic peak V detection in a stepwise fashion. Hearing thresholds were then measured with fast protocols utilizing either 30 Hz click or 90 Hz broadband chirp stimuli with 80, 60, 40, 30, 20, 10, 0 and -10 dBnHL stimulation intensities. Interrater reliability, agreement between click and chirp hearing thresholds and correlations with clinical characteristics of the dogs were assessed.

Results

Using all innovations, the test time for hearing threshold assessment in both ears was reduced to 1.11 min (mean). The chirp stimulus accentuated both, peak V and the subsequent trough, which are essential features for judgement of the hearing threshold, but preceding peaks were less conspicuous. Interrater reliability and agreement between click and chirp hearing threshold was excellent. Dogs >10 years of age and dogs with abnormal hearing score or otitis score had significantly higher hearing thresholds than younger dogs (p ≤ 0.001) or dogs without abnormalities (p < 0.001).

Conclusion

The results demonstrate that modifications in ABR protocols speed-up test times significantly while the quality of the recordings for hearing threshold assessment is maintained. Modified ABR protocols enable super-fast hearing threshold assessment in veterinary practice.

Inconsistencies Between the Criterion and Tests for Brain Death

The whole-brain criterion of death provides that a person who has irreversibly lost all clinical functions of the brain is dead. Bedside brain death (BD) tests permit physicians to determine BD by showing that the whole-brain criterion of death has been fulfilled. In a nonsystematic literature review, we identified and analyzed case reports of a mismatch between the whole-brain criterion of death and bedside BD tests. We found examples of patients diagnosed as BD who showed (1) neurologic signs compatible with retained brain functions, (2) neurologic signs of uncertain origin, and (3) an inconsistency between standard BD tests and ancillary tests for BD. Two actions can resolve the mismatch between the whole-brain criterion of death and BD tests: (1) loosen the whole-brain criterion of death by requiring only the irreversible cessation of relevant brain functions and (2) tighten BD tests by requiring an ancillary test proving the cessation of intracranial blood flow. Because no one knows the precise brain functions whose loss is necessary to fulfill the whole-brain criterion of death, we advocate tightening BD tests by requiring the absence of intracranial blood flow.

Technical aids in the diagnosis of brain death: a comparison of SEP, AEP, EEG, TCD and CT angiography

BACKGROUND

The use of technical aids to confirm brain death is a controversial matter. Angiography with the intra-arterial administration of contrast medium is the international gold standard, but it is not allowed in Germany except in cases where it provides a potential mode of treatment. The currently approved tests in Germany are recordings of somatosensory evoked potentials (SSEP), brain perfusion scintigraphy, transcranial Doppler ultrasonography (TCD), and electroencephalography (EEG). CT angiography (CTA), a promising new alternative, is being increasingly used as well.

METHODS

In a prospective, single-center study that was carried out from 2008 to 2011, 71 consecutive patients in whom brain death was diagnosed on clinical grounds underwent recording of auditory evoked potentials (AEP) and SSEP as well as EEG, TCD and CTA.

RESULTS

The validity of CTA for the confirmation of brain death was 94%; the validity of the other tests was: 94% for EEG, 92% for TCD, 82% for SSEP, and 2% for AEP. In 61 of the 71 patients (86%), the EEG, TCD and CTA findings all accorded with the clinical diagnosis. The diagnosis of brain death was established beyond doubt in all patients.

CONCLUSION

In this study, the technical aids yielded discordant results in 14% of cases, necessitating interpretation by an expert examiner. The perfusion tests, in particular, can give false-positive results in patients with large cranial defects, skull fractures, or cerebrospinal fluid drainage. In such cases, electrophysiologic tests or a repeated clinical examination should be performed instead. CTA is a promising, highly reliable new method for demonstrating absent intracranial blood flow. In our view, it should be incorporated into the German guidelines for the diagnosis of brain death.

Brain death diagnosis and apnea test safety

The apnea test is a mandatory examination for determining brain death (BD), because it provides an essential sign of definitive loss of brainstem function. However, several authors have expressed their concern about the safety of this procedure as there are potential complications such as severe hypotension, pneumothorax, excessive hypercarbia, hypoxia, acidosis, and cardiac arrhythmia or asystole. These complications may constrain the examiner to abort the test, thereby compromising BD diagnosis. Nevertheless, when an appropriate oxygen-diffusion procedure is used, this technique is safe. We review here the prerequisites to begin the test, its procedure, potential complications, and the use of alternative ancillary tests. We recommend that the apnea test be retained as a mandatory procedure for the diagnosis of BD. In those situations when the apnea test is terminated by the examiner for some reason or when it is impossible to carry it out in a patient due to the presence of some pathologic condition, alternative ancillary tests should be used to confirm BD.

Diagnosis of brain death

Brain death (BD) should be understood as the ultimate clinical expression of a brain catastrophe characterized by a complete and irreversible neurological stoppage, recognized by irreversible coma, absent brainstem reflexes, and apnea. The most common pattern is manifested by an elevation of intracranial pressure to a point beyond the mean arterial pressure, and hence cerebral perfusion pressure falls and, as a result, no net cerebral blood flow is present, in due course leading to permanent cytotoxic injury of the intracranial neuronal tissue. A second mechanism is an intrinsic injury affecting the nervous tissue at a cellular level which, if extensive and unremitting, can also lead to BD. We review here the methodology of diagnosing death, based on finding any of the signs of death. The irreversible loss of cardio-circulatory and respiratory functions can cause death only when ischemia and anoxia are prolonged enough to produce an irreversible destruction of the brain. The sign of such loss of brain functions, that is to say BD diagnosis, is fully reviewed.

Diagnostic value of neurophysiological tests in the diagnosis of brain death--do we need changes in national guidelines?

For a final diagnosis of brain death one needs particular proof that the brain has fully stopped functioning. In this respect, diagnostic tests need to be used to confirm the clinical findings. Since in different countries there are various national guidelines for the determination of brain death, the aim of this study is to point to the diagnostic value of electroencephalography (EEG) and evoked potentials (EP) in verifying brain death. The need is emphasized for the two methods to be used in combination, and, accordingly, for the current national guidelines to be changed in our country and also in other countries round the world.

INTRACRANIAL SUBARACHNOID HEMORRHAGE FOLLOWING LUMBAR MYELOGRAPHY IN TWO DOGS

Intracranial subarachnoid hemorrhage is a rare but serious complication of lumbar puncture in humans. Possible sequelae include increased intracranial pressure, cerebral vasospasm, or mass effect, which can result in dysfunction or brain herniation. We describe two dogs that developed intracranial subarachnoid hemorrhage following lumbar myelography. In both dogs, myelography was performed by lumbar injection of iohexol (Omnipaque). Both the dogs underwent uneventful ventral decompressive surgery for disk herniation; however, the dogs failed to recover consciousness or spontaneous respiration following anesthesia. Neurologic assessment in both dogs postoperatively suggested loss of brain stem function, and the dogs were euthanized. There was diffuse subarachnoid hemorrhage and leptomeningeal hemorrhage throughout the entire length of the spinal cord, brain stem, and ventrum of brain. No evidence of infectious or inflammatory etiology was identified. The diagnosis for cause of brain death was acute subarachnoid hemorrhage. Our findings suggest that fatal subarachnoid hemorrhage is a potential complication of lumbar myelography in dogs. The cause of subarachnoid hemorrhage is not known, but may be due to traumatic lumbar tap or idiosyncratic response to contrast medium. Subsequent brain death may be a result of mass effect and increased intracranial pressure, cerebral vasospasm, or interaction between subarachnoid hemorrhage and contrast medium.

Brief review: the role of ancillary tests in the neurological determination of death

PURPOSE

The acceptance of brain death by society has allowed for the discontinuation of "life support" and the transplantation of organs. In Canada we accept the clinical criteria for brain death (essentially brain stem death) when they can be legitimately applied. Ancillary tests are needed when these clinical criteria cannot be applied or when there are confounders. Ancillary tests include tests of intracranial blood circulation, electrophysiological tests, metabolic studies and tests for residual vagus nerve function. The ideal confirmatory test is one which, when positive, would be incompatible with recoverable brain function (i.e., has no false positives), is not influenced by drugs or metabolic disturbances and which can be readily applied. A critical review of the various ancillary tests used to support the neurological determination of death (brain death) was undertaken.

METHODS

A literature review based on a MEDLINE search of relevant articles published between January 1966 to January 2005 was undertaken.

RESULTS

Tests of whole brain perfusion/intracranial blood circulation are the only ones that meet stated criteria.

CONCLUSIONS

At present only cerebral angiography and nuclear medicine tests of perfusion are accepted by Canadian standards, but computed tomography and magnetic resonance angiography should prove to be suitable. Transcranial Doppler studies may be suitable for specific cases once appropriate guidelines are established.

Role of short latency evoked potentials in the diagnosis of brain death

OBJECTIVE

The aim of this study is to confirm the effectiveness of auditory brain-stem responses (ABRs) and somatosensory evoked potentials (SEPs) in the diagnosis of brain death (BD).

METHODS

ABRs and SEPs were recorded at the same session in 130 BD patients (age range 8-77 years, 81 male and 49 female). Twenty-four cases were submitted to serial recordings from preterminal conditions through BD.

RESULTS

ABRs were absent in 92 cases (70.8%), only waves I or I-II were present in 32 cases (24.6%), while in the remaining 6 patients (4.6%) waves V and/or III were still present, excluding the death of the brain-stem. In 4 cases (3.1%) SEPs showed the absence of all components following the cervical N9, preventing the diagnosis of BD. Among 126 cases (96.9%) with preserved cervical N9-N13 SEPs confirmed the absence of brain-stem activity in 122 cases (93.7%), in whom no waves following P11 or P13 were recordable. SEPs excluded the diagnosis of BD in the remaining 4 cases (3.2%) showing preserved P14 and/or N18. In all pre terminal patients the far-field P14-N18 were present, and their disappearance was closely related to the onset of BD.

CONCLUSIONS

The combined us of ABRs and SEPs was able to confirm BD in almost all patients, providing an objective confirmation of the diagnosis, and to exclude it in 7 cases, thus improving the reliability of diagnosis.

[Current concepts in diagnosing brain death in Germany]

Diagnosis of brain death requires definite evidence of an acute CNS catastrophe and exclusion of complicating medical conditions that may confound clinical assessment. Acute CNS catastrophe may be due to direct ("primary") brain damage (e.g., intracerebral hemorrhage, severe concussion, brain tumors), or indirect ("secondary") brain damage (e.g., cerebral hypoxia following cardio-pulmonary resuscitation). The cardinal findings in brain death are coma, absence of brainstem reflexes, and apnea. Persistence of these clinical signs determines brain death. In Germany, the intervals of a repeat clinical evaluation are at least 12 hours in patients with primary, and at least 72 hours in those with secondary brain damage. Electroencephalographically documented absence of electrical activity for at least 30 minutes or by means of transcranial Doppler ultrasonography or isotope angiography documented intracranial circulatory arrest also confirm brain death. Under such conditions, a repeat clinical evaluation is unnecessary in patients with clinical brain death signs. First of all, brain death is a clinical diagnosis. Confirmatory tests are not mandatory in most situations. In Germany, confirmatory tests are required in newborns, infants below the age of 2 years, and patients with infratentorial brain damage.

N18 in median somatosensory evoked potentials: a new indicator of medullary function useful for the diagnosis of brain death

OBJECTIVES

To record N18 in median somatosensory evoked potentials (SEPs) for deeply comatose or brain dead patients and to demonstrate the usefulness of N18 for the diagnosis of brain death in comparison with auditory brain stem responses (ABRs) and P13/14 in median SEPs, which have been conventionally used as complementary tests for the diagnosis of brain death.

METHODS

Subjects were 19 deeply comatose or brain dead patients. Thirteen recordings were performed in deeply comatose but not brain dead conditions, and 12 recordings were performed in brain death. N18 was evaluated in the CPi-C2S lead (or other scalp-C2S leads) to obtain a flat baseline.

RESULTS

N18 was preserved in 12 of 13 non-brain dead comatose recordings whereas it was completely lost for all of the 12 brain death recordings. P13/14 in median SEPs was preserved for all the comatose recordings, whereas apparent P13/14-like potentials, usually of low amplitude, were seen in nine of 12 brain death recordings-that is, frequent false positives. The ABRs already showed features which were characteristic for brain death (loss of components other than wave 1 or small wave 2) for four comatose recordings, in three of which N18 was preserved. The last result not only corresponds with the fact that ABRs can evaluate pontine and midbrain functions and not medullary function, but further supports the medullary origin of N18. In the four patients followed up for the course of progression from coma to brain death, N18s preserved in normal size during the comatose state were completely lost after brain death was established.

CONCLUSIONS

The N18 potential is generated by the cuneate nucleus in the medulla oblongata in the preceding studies. N18 is suggested to be a promising tool for the diagnosis of brain death because there were no false positives and rare false negatives in the present series for detecting the remaining brain stem function.

Brain-stem auditory evoked potential monitoring. The increase of the stimulus artifact in the development of brain death: a biological phenomenon?

Brain-stem auditory evoked potentials (BAEPs) were recorded in 12 dead subjects (mean age, 72.6 +/- 14.8 years), 30.6 +/- 19.5 hours (range 9-70) after abolished systemic circulation. Death was due to cardiac failure (n = 10), intracerebral hemorrhage (n = 1) and larynx cancer (n = 1). The presence and amplitude of the stimulus artifact were evaluated. The mean (+/- SD) amplitudes of the stimulus artifact was 0.03 +/- 0.02 microV on the left side and 0.01 +/- 0.02 microV on the right side. These findings in accordance with previous studies on comatose patients and brain dead subjects confirm that the increase of the stimulus artifact in the development of brain death, in spite of stimulation with alternating polarity, seems to reflect a biological phenomenon which is not found in dead subjects after complete cessation of systemic circulation.

Robust moving averages, with Hopfield neural network implementation, for monitoring evoked potential signals

This technical note describes a robust version of moving averages, that enables reliable monitoring of the evoked potential (EP) signals. A cluster analysis (CA) procedure is introduced to robustify the signal averaging (SA). It is implemented via a Hopfield neural network (HNN), which performs selection of the trials forming a cluster around the current state of the EP signal. The core of this cluster serves as an estimate of the instantaneous EP. The effectiveness of the method, indicated by application to real data, and its computation efficiency, due to the use of simple matrix operations, makes it very promising for clinical observations.

Brain-stem auditory evoked potential monitoring. Variations of stimulus artifact in brain death

Brain-stem auditory evoked potentials (BAEPs) were recorded in 20 subjects with brain death (mean age, 33.2 +/- 15.1 years) and 20 healthy volunteers (mean age, 29.8 +/- 6.8 years). Brain death was due to head injury (n = 14), encephalitis (n = 3), brain-stem hemorrhage (n = 1), cerebellar hemorrhage (n = 1) or cerebral infarction (n = 1). The presence, latency and amplitude of the individual BAEP components and variations of the stimulus artifact were evaluated. The mean (+/- S.D.) amplitude of the stimulus artifact was 0.26 +/- 0.12 microV in the brain-dead subjects and 0.09 +/- 0.05 microV in the control group (P < 0.001, t test). The causes of the phenomenon of increasing stimulus artifacts in the evolution of brain death remain unclear.

Multimodality evoked potentials (auditory, somatosensory and motor) in coma

Auditory brainstem responses (ABRs) have proved to be significantly related to outcome, both in severe head injury and brain hemorrhage. Nevertheless, the usefulness of ABR is limited by the anatomic extent of the investigated pathways. The combined use of ABRs and somatosensory evoked potentials (SEPs) improves the outcome prediction in comparison to the use of only one modality. It mainly decreases the rate of false negatives, since patients with severe hemispheric damage sparing the brain stem may have a poor outcome despite normal ABRs. The use of motor evoked potentials (MEPs) from magnetic transcranial stimulation is also significantly related to outcome: it appears to be far superior to the clinical evaluation of motor responses, while the combined use of MEPs and SEPs gives a new opportunity of checking sensorimotor dysfunction. ABRs and SEPs may also be useful tools in the confirmation of brain death, the kernel of which is the assessment of brainstem death: they allow to check lemniscal pathways, which cannot be properly evaluated by clinical examination, and provide an objective confirmation of absence of brain stem activity.

Visual evoked potentials and electroretinography in brain-dead patients

Visual evoked potentials and electroretinograms were elicited by light-emitting diode stimulation and recorded simultaneously, with cephalic and noncephalic references, in 30 normal subjects and in 30 brain-dead patients. A characteristic pattern was found in the group of patients: when a cephalic reference was used for both visual evoked potentials and the electroretinogram, the a- and b-waves of the electroretinogram were recognized in all patients, and visual evoked responses consisted of waves with inverse polarity, similar morphologic characteristics, the same latency, and less amplitude than those of the electroretinogram. When a noncephalic derivation was chosen for the electroretinogram and visual evoked potentials, electroretinogram did not change in either morphologic features or latency, whereas the visual evoked potential channel showed no response. Only in two cases was it possible to record waves in the visual evoked potential lead with a noncephalic reference, showing a spread of the electroretinogram to the occipital area, with a considerably reduced amplitude. These results suggest that, although contamination of visual evoked potential records by the spread of the electroretinogram to the occipital area could occur, it is easy to confirm the absence of a true cortical visual response in brain-dead patients by means of a noncephalic reference. This pattern clearly confirms that in the visual pathways of brain-dead patients, electrical activity is confined to the retina.