Auditory brain-stem (ABRs) and middle latency auditory responses (MLRs) in the prognosis of severely head-injured patients

Brainstem Evoked Potential Indices of Subcortical Auditory Processing After Mild Traumatic Brain Injury

OBJECTIVES

The primary aim of this study was to assess subcortical auditory processing in individuals with chronic symptoms after mild traumatic brain injury (mTBI) by measuring auditory brainstem responses (ABRs) to standard click and complex speech stimuli. Consistent with reports in the literature of auditory problems after mTBI (despite normal-hearing thresholds), it was hypothesized that individuals with mTBI would have evidence of impaired neural encoding in the auditory brainstem compared to noninjured controls, as evidenced by delayed latencies and reduced amplitudes of ABR components. We further hypothesized that the speech-evoked ABR would be more sensitive than the click-evoked ABR to group differences because of its complex nature, particularly when recorded in a background noise condition.

DESIGN

Click- and speech-ABRs were collected in 32 individuals diagnosed with mTBI in the past 3 to 18 months. All mTBI participants were experiencing ongoing injury symptoms for which they were seeking rehabilitation through a brain injury rehabilitation management program. The same data were collected in a group of 32 age- and gender-matched controls with no history of head injury. ABRs were recorded in both left and right ears for all participants in all conditions. Speech-ABRs were collected in both quiet and in a background of continuous 20-talker babble ipsilateral noise. Peak latencies and amplitudes were compared between groups and across subgroups of mTBI participants categorized by their behavioral auditory test performance.

RESULTS

Click-ABR results were not significantly different between the mTBI and control groups. However, when comparing the control group to only those mTBI subjects with measurably decreased performance on auditory behavioral tests, small differences emerged, including delayed latencies for waves I, III, and V. Similarly, few significant group differences were observed for peak amplitudes and latencies of the speech-ABR when comparing at the whole group level but were again observed between controls and those mTBI subjects with abnormal behavioral auditory test performance. These differences were seen for the onset portions of the speech-ABR waveforms in quiet and were close to significant for the onset wave. Across groups, quiet versus noise comparisons were significant for most speech-ABR measures but the noise condition did not reveal more group differences than speech-ABR in quiet, likely because of variability and overall small amplitudes in this condition for both groups.

CONCLUSIONS

The outcomes of this study indicate that subcortical neural encoding of auditory information is affected in a significant portion of individuals with long-term problems after mTBI. These subcortical differences appear to relate to performance on tests of auditory processing and perception, even in the absence of significant hearing loss on the audiogram. While confounds of age and slight differences in audiometric thresholds cannot be ruled out, these preliminary results are consistent with the idea that mTBI can result in neuronal changes within the subcortical auditory pathway that appear to relate to functional auditory outcomes. Although further research is needed, clinical audiological evaluation of individuals with ongoing post-mTBI symptoms is warranted for identification of individuals who may benefit from auditory rehabilitation as part of their overall treatment plan.

The Application of the International Classification of Functioning, Disability and Health to Functional Auditory Consequences of Mild Traumatic Brain Injury

This article reviews the auditory consequences of mild traumatic brain injury (mTBI) within the context of the International Classification of Functioning, Disability and Health (ICF). Because of growing awareness of mTBI as a public health concern and the diverse and heterogeneous nature of the individual consequences, it is important to provide audiologists and other health care providers with a better understanding of potential implications in the assessment of levels of function and disability for individual interdisciplinary remediation planning. In consideration of body structures and function, the mechanisms of injury that may result in peripheral or central auditory dysfunction in mTBI are reviewed, along with a broader scope of effects of injury to the brain. The activity limitations and participation restrictions that may affect assessment and management in the context of an individual's personal factors and their environment are considered. Finally, a review of management strategies for mTBI from an audiological perspective as part of a multidisciplinary team is included.

Koma

BACKGROUND

The prognosis after traumatic coma is often unclear. We investigated the prognostic value of somatosensory (SSEP) and early acoustic (EAEP) evoked potentials on comatose patients in the intensive care unit regarding long-term outcome. Different evaluation systems were investigated.

METHODS

This was a retrospective analysis of 100 patients. SSEP and EAEP were examined at different times and analysed according to the Riffel score. Combinations of the different types of potentials were evaluated regarding possible improvement of outcome prediction.

RESULTS

The positive predictive value of at least one missing peak V of the EAEP regarding a fatal prognosis was 83%. The negative predictive value of the EAEP was 96%. A good outcome (GOS 4+5) could be predicted by bilateral normal SSEP and EAEP with a positive predictive value of 98%.

CONCLUSIONS

Early evaluation of SSEP and EAEP allows reliable prognostic predictions regarding a later outcome in patients with severe traumatic brain injury and should therefore be used more often for intensive care patients.

Evoked potentials in severe brain injury

Three-modality evoked potentials (EPs) have been used for several years in association with the electroencephalogram (EEG) as a diagnostic and prognostic tool in acute traumatic or nontraumatic coma. In 1993 we proposed to combine these in two indices: the index of global cortical function (IGCF) and the index of brain-stem conduction (IBSC). Four EP patterns based on both indices emerge at the acute stage of severe head trauma. These are easily explainable by pathophysiology. Pattern 1 corresponds to alterations in the index of global cortical function without changes in the index of brain-stem conduction. Its prognosis is good (80 to 90% of these patients recover). Pattern 2 is characterized by alterations of somatosensory EPs that are suggestive of midbrain dysfunction. The prognosis depends both on the reversibility of the midbrain dysfunction and on the extent of associated diffuse axonal lesions, whose evaluation requires MRI. Patients who recovered from Pattern 2 sometimes did so after a long interval during which they remained vegetative. Pattern 3 is characterized by alterations of brain-stem auditory EPs that are suggestive of pontine involvement. It usually follows uncontrolled intracranial hypertension and corresponds to evolving transtentorial herniation. All patients with that transient pattern eventually died. Pattern 4 is categorized by the disappearance of all activities of intracranial origin, contrasting with the preservation of all activities of retinal, spinal-cord, and peripheral-nerve origin. This pattern corresponds to brain death. In our experience, three-modality EPs are currently the best bedside brain-death confirmatory tool.

[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.

Medical technology assessment EEG and evoked potentials in the intensive care unit

We review the principal aspects of EEG and evoked potential (EP) neuromonitoring in the intensive care unit. The electrophysiological methods allow functional assessment of comatose patients and can be used (a) as a help to diagnose the origin of coma, (b) as a means to predict outcome, and (c) for monitoring purposes. The combination of the EEG and long-, middle-, and short-latency EPs allows widespread assessment of the cerebral cortex, the brain-stem, and the spinal cord. The EEG and the EP interpretation first requires taking into account non-neurological factors that may interfere with the recorded activities (sensory pathologies, toxic or metabolic problems, body temperature). The sensitivity and the specificity of any neurophysiological technique depend on the etiology of coma. Anoxic comas are associated with a predominantly cortical involvement, while the cortical and brain-stem functions are to be taken into account to interpret the EEG and the EPs in head trauma. The EEG and the EPs can be used to differentiate the comas due to structural lesions from those of metabolic origin, to confirm brain death and help to diagnose psychogenic unresponsiveness or a de-efferented state. While the prognostic value of the EEG is markedly hampered by the widespread use of sedative drugs, it has been possible to design efficient systems based on early- and middle-latency multimodality evoked potentials in anoxic and traumatic comas and, more generally, in all comas associated with an increase of the intracranial pressure. Continuous neuromonitoring techniques are currently under development. They have already been proven useful for the early detection and for the prevention of subclinical seizures, transtentorial herniation, vasospasm, and other causes of brain or spinal-cord ischemia.

Brainstem auditory evoked potential in Japanese encephalitis

Japanese encephalitis (JE) is associated with varying degrees of coma and brainstem involvement is frequent which can be evaluated and monitored by brainstem auditory evoked potential (BAEP). The present study has been undertaken to evaluate the BAEP changes and their role in predicting the outcome. Twelve adult patients with JE were subjected to CT scan, MRI and BAEP studies after detailed neurological evaluation. The severity of coma was assessed by Glasgow coma scale and outcome was defined at the end of 3 months into good and poor recovery on the basis of Barthel Index score (BI). The mean age of the patients was 28.3 years (range 14-50), and four of them were females. Most of the patients were comatose. The mean Glasgow coma scale (GCS) score was 7 (range 4-11). There were no brainstem signs or cranial nerve palsy. Cranial CT scan revealed thalamic hypodensity in four, whitematter oedema in three and left putaminal hypodensity in one patient. Cranial MRI was carried out in eight patients which revealed bilateral thalamic lesions in all, basal ganglia and midbrain lesions in three each and pontine and cerebellar lesions in one patient each. Brainstem auditory evoked potentials were recordable bilaterally. The absolute latency of wave I, II, III, IV and V and interpeak latencies (IPL) of I-V, III-V, and I-III were normal. The V/I amplitude ratio were significantly reduced in five patients. The BAEP abnormalities correlated with brainstem lesions on CT or MRI but not with severity of coma or outcome. The reduced amplitude ratio of wave V/I may be due to raised intracranial tension or brainstem involvement in JE.

Prognosis in anoxic and traumatic coma

This article presents a systematic review of the prognostic factors in coma caused by hypoxia and anoxia, and traumatic head injury. In the case of anoxic coma, poor prognosis can be determined very accurately at the bedside by the lack of brainstem reflexes assessed at day three post-insult. Electroencephalogram (EEG) and evoked potentials (i.e., auditory and somatosensory) detect additional patients with poor prognosis. Clinical examination is not as helpful in the case of traumatic coma. Evoked potentials performed within the first week detect approximately half of patients with a poor prognosis. EEG adds only a small amount of prognostic information.

Middle latency auditory evoked potentials in intensive care patients and normal controls

Evoked potentials have been introduced into intensive care unit to objectively measure parameters of coma. In particular, auditory brainstem evoked potentials have been useful for localizing brainstem dysfunction in comatose patients. The middle latency auditory evoked potentials (MLAEPs) believed to be a response of subcortical auditory radiations and the primary auditory cortex. MLAEPs were measured in 40 adults (mean age 24.9 +/- 2.9 years; range: 19-34 years) with normal hearing and in 102 intensive care patients (mean age: 48.4 +/- 18.9 years; range: 14-86 years) under the influence of biological variables. Latencies (control group, mean +/- SD: V = 5.74 +/- .29 ms, N0 = 9.11 +/- 1.74 ms, P0 = 12.94 +/- 1.87 ms, Na = 17.23 +/- 1.77 ms, and Pa = 29.22 +/- 3.43 ms), amplitudes (control group, mean +/- SE: N0-P0 = 2.00 +/- .34 microV, P0-Na = 3.88 +/- .67 microV, Na-Pa = 2.83 +/- .29 microV) and the amplitude ratio (control group, mean +/- SE: P0-Na/Na-Pa = 1.53 +/- .39) were calculated. In the control group in both females and males, right-sided stimulation produced shorter average MLAEP latencies and higher amplitudes than left-sided stimulation (Pa-right 28.26 +/- 3.53 ms; Pa-left 30.17 +/- 3.33 ms). MLAEPs showed significant differences according to sex but did not depend significantly on age. A temperature dependence was found for the latency of wave V (short latency AEP), which was prolonged at lower temperatures and for the amplitude Na-Pa, which was increased at decreased temperatures between 38.9 and 35.4 degrees C. There was a significant association between the amplitude Na-Pa and PO2 (P = .017). Alterations of PCO2 in the range of 26 to 54 mmHg did not influence the MLAEPs. Also, renal dysfunction or hepatic dysfunction and alterations of mean arterial pressure (range: 50-102 mmHg) did not affect MLAEP latencies and amplitudes significantly. Increases in latencies and decreases in amplitude were seen in sedated patients. These results in intensive care patients suggest that the combination of early AEP (acute phase) and MLAEP (post acute phase) may be useful to monitor comatose patients.

Predictive value of brain-stem auditory evoked potentials in children with post-traumatic coma produced by diffuse brain injury

In the present study, brain-stem auditory evoked potentials (BAEPs) in 31 children in post-traumatic coma with diffuse brain injury were examined. The BAEPs were recorded in the first 72 h after hospital admission and the findings of 29 patients related to the level of neurological recovery at 6 months after head injury. On the basis of the results, children were divided into three groups: the first consisted of children with bilateral and normal BAEPs (n = 19); the second of those with asymmetrical BAEPs (n = 6); and the third of those in whom BAEPs has disappeared or in whom only responses of the seventh cranial nerve and cochlear nucleus were recorded (n = 4). All the patients in the first group presented a good clinical outcome, with excellent recovery in 80%. In the second group three children (42.8%) had a good recovery, two (28.6%) were moderately disabled and one (14.3%) died of extraneurological causes. All the patients in the third group died. Abnormal BAEPs showed a significant correlation with absence of pupillary and/or corneal reflex, but not with the Glasgow Coma Score and anisocoria. Good statistical correlation was observed between normal BAEPs and visualization of basal cisterns on computed tomographic scan. The incidence of increased intracranial pressure was higher in patients with abnormal BAEPs, but the differences were not significant (P = 0.06). Our study confirms the predictive value of BAEPs in children's post-traumatic coma due to diffuse brain injury.

Auditory brainstem responses after out-of-hospital cardiac arrest: Are they useful for outcome prediction?

We evaluated whether we could predict the neurologic outcome in 55 out-of-hospital cardiac arrest patients using auditory brainstem responses (ABR). ABR patterns were classified into one of 3 types by evaluation of 5 components: type 1, with all 5 components; type 2, lack of at least one response between the 2nd and 5th components; type 3, with only the first component or no response. The relation between the ABR patterns on the 3rd day following resuscitation and the neurologic outcome on hospital discharge was evaluated. The specificity that the 5 awake patients had type-1 ABR was 38%. The sensitivity that the 10 brain dead patients had type-3 ABR was 60%. In the type-1 ABR patients, the negative predictive value that the patients were awake was 100%. In the type-3 ABR patients, the negative predictive value that the patients became brain dead was 90.9%. These results suggest that ABR on the 3rd post-resuscitation day may not be useful for predicting if patients are awake or become brain dead, although the loss of components may be a sign of morbidity, and the presence of the 2nd or later components indicates possible future prevention of brain death.

EEG-controlled "overdosage" of anesthetics in a patient with a history of intra-anesthetic awareness

In spite of the ever-growing pharmacologic arsenal available for induction and maintenance of anesthesia, to our knowledge no treatment regimen exists that will provide full protection against intraoperative awareness. To date, no single monitoring technique is able to detect awareness or predict recall. Although the frequency of these complications is rare, the occurrence of any such event can be very distressful for the patient. Based on our clinical experience with a patient with a history of recall and a marked resistance to benzodiazepines, we present electroencephalogram-based anesthetic management as a technique to address this difficult problem.

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.

Audiological findings following head trauma

Traumatic head injury is the principal cause of death and serious physical disability in adolescents and young adult. Hearing loss is a common sequela of head trauma. The results of pure tone and auditory brain-stem response (ABR) assessment of minor head injury revealed that 20 per cent of these cases showed impairment of hearing including both conductive and sensorineural hearing loss mostly in the high frequencies loss. ABR findings showed a significant difference of absolute wave V latency and interwave (III-V) latencies at repetition rate 11.1 click/sec., and a highly significant difference at a repetition rate of 51.1 click/sec., compared with the control group. In severe head injury, conductive hearing loss was found in only one case, in which a longitudinal fracture of the temporal bone was identified radiologically. No ABR could be recorded at a repetition rate of 11.1 click/sec. in either ear of this case; this patient died a few days later. Three cases were found with a mixed sensorineural hearing loss; one of them showed a longitudinal fracture radiologically. ABR findings in two cases showed prolongation of wave V and I-V interwave latencies, and the third case showed absence of ABR recording. In comatose patients, an ABR grading system is a more sensitive index of brainstem dysfunction than the simple division of normal versus abnormal ABR waves and interwave latencies. Presence or prolongation of wave V and interwave latency I-V even in one ear is of good prognostic value in the comatosed patient.

The brain-stem and 40 Hz middle latency auditory evoked potentials in brain death

Bilateral recordings of brain stem auditory evoked responses were compared with the 40 Hz middle latency auditory evoked responses in 38 patients either brain dead or in coma grade IV. In 4 cases the 40 Hz auditory evoked potential was partly preserved, while the brain stem auditory evoked potential was not reproducible beyond wave II. Recording of the 40 Hz auditory evoked potential is found to be more sensitive in some rare instances.

Auditory brain stem responses (ABR) in patients with acute severe closed head injuries. The use of a grading system

ABR were recorded in 68 cases suffering from severe acute closed head injuries. Abnormal recordings were observed in 60 per cent of patients, and the abnormalities ranged from prolongation of the interpeak latency intervals to complete absence of auditory brain stem activity. Based on the results obtained, a grading system was developed and correlated with the neurological outcome. It was concluded that ABR is a useful diagnostic and prognostic tool which can provide valuable information about the function of the brain stem in these patients. Inclusion of ABR studies in the head injury management protocol is strongly recommended.

Serial recordings of auditory brainstem responses in severe head injury: relationship between test timing and prognostic power

ABRs have proved to be very accurate prognostic indicators in severe head injury, even when predictions are based on single ABR recordings. In this study we submitted 30 severely head-injured patients to serial ABR recordings (during the clinical course of posttraumatic coma) in order to verify whether the ABR prognostic power may depend on test timing in relation to the injury. 15 patients (50%) died, 5 (16.7%) remained vegetative, 2 (6.6%) severely disabled and 8 (26.7%) recovered. All recovered patients had normal ABR throughout clinical course, while severely disabled and vegetative patients showed at least transently ABR abnormalities (namely, an interpeak latency of waves V-I greater than 4.48 ms). Among dead patients, 8 showed steady and 3 transient ABR abnormalities during the clinical course, while 2 of 3 patients with normal ABR dead from extracranial complications. ABRs were significantly related to the outcome at any time, but gave more accurate prognostic indications on days 3-6 after the injury. The use of serial ABR recordings appeared to improve the outcome predictions in comparison with single ABR tests. Finally our result confirm the previously reported existance of a breakpoint between reversible brainstem dysfunction and irreversible brainstem damage defined by an IPL V-I of about 4.50 ms.

Brain-stem monitoring. II. Preterminal BAEP changes observed until brain death in deeply comatose patients

Preterminal BAEP changes were studied until brain death in 8 head-injured patients out of a series of 38 comas monitored by means of a system allowing high-rate sequential recording. Two different modalities of BAEP degradation were disclosed: (1) simultaneous latency increase of all components associated with a decrease of cerebral perfusion pressure (CPP), consistent with ongoing ischaemia of the posterior fossa; (2) deterioration of brain-stem components (waves III-V) with preserved or even enhanced wave I. The latter pattern was not consistently associated with any haemodynamic change and might be related to mechanical factors causing rostro-caudal deterioration of brain-stem function. The time course of BAEP degradation ranged from a few minutes to more than 10 h. In the case of slow preterminal evolution definitely pathological trends were identified even when individual BAEPs were still within normal limits. Such trends would have remained unnoticed in single BAEP records. Hypothermia and anaesthetic drugs were found to induce falsely alarming BAEP changes very similar to those seen during preterminal evolution. Our results suggest that continuous brain-stem monitoring could be helpful for management of comatose head-injured patients.

40 Hz--middle latency auditory evoked response in comatose patients

Brain-stem auditory evoked response (BAER) and 40 Hz middle latency auditory evoked response (40 Hz AER) were elicited in 12 comatose patients. The concept of a midbrain generator of 40 Hz AER is being discussed.