Middle latency auditory-evoked potential index monitoring of cerebral function to predict functional outcome after emergency craniotomy in patients with brain damage
To our knowledge, this is the first prospective study to evaluate the changes of MLAEPi
in patients with brain damage in an ICU. Several studies have found that white matter
tracts within the cerebral hemisphere and brain stem are injured following TBI or
acute stroke 10], 11]. MLAEP is usually obtained intermittently and derived from AEPs, which reflects the
morphology of MLAEP curves. Previous studies have failed to demonstrate the usefulness
of single components of the early somatosensory-evoked potentials (SEP) and short
latency AEP for predicting the clinical outcome after TBI or acute ischemic injuries
following stroke or cardiac arrest 12], 13]. Although middle latency activities are considered to be good predictors of prognosis
in comatose patients, the predictive value of MLAEP remains uncertain. Therefore,
we hypothesized that MLAEPi decreases among patients with DOC caused by head injury
or acute stroke and can predict the neurological outcomes after emergency craniotomy.
Most studies have evaluated MLAEPi as an indicator of the state of anesthesia with
100 % specificity using an MLAEPi cut-off value of 37 for unconsciousness during anesthesia
14]. We previously demonstrated that the recommended range of MLAEPi in non-sedated patients
with DOC between 35 and 61 8]. MLAEPi is profoundly affected by the decreasing amplitudes and increasing latencies
induced by hypnotic drugs such as midazolam and propofol. Although the recommended
range of MLAEPi in surgical anesthesia settings is 30 to 45, the adequate range of
MLAEPi for critical care patients in ICUs remains unknown 3]. Our results show that MLAEPi did not differ significantly in patients with brain
damage across the three outcome groups during sedative periods. However, we were able
to demonstrate that MLAEPi increased significantly between patients with favorable
outcome or unfavorable outcome compared with patients with BD immediately after finishing
sedation. Moreover, MLAEPi increased significantly among patients with favorable outcome
compared with patients with unfavorable outcome within 1Â week after the onset of brain
damage.
Several studies found that the GCS score and BIS value were relatively correlated
in critically ill patients, and the processed EEG is a non-invasive method for monitoring
consciousness during anesthesia or critical care sedation 9], 15]. BIS is the numerical value acquired using spontaneous EEG. Although a BIS value
of 0 is useful in severe TBI or ischemic brain injury for the early detection and
confirmation of BD with a GCS score of 3, there has been no satisfactory report on
BIS monitoring of cerebral function to predict functional outcome after brain damage
16], 17]. Middle latency SEP may be valuable for increasing sensitivity without any loss of
specificity for predicting unfavorable outcome in patients after stroke, ischemic
brain injury or TBI 13], 18]. However, the prognostic value of SEPs remains controversial and should never be
considered in isolation but should be integrated with other neurophysiological tools
and clinical examination 19]. It has been reported that aepEX monitoring is a more effective indicator for determining
the state of consciousness than BIS or any other EEG-based monitoring method 4], 5], 14], 20]. The major difference between any other EEG-based monitoring method and the aepEX
method is that MLAEPi is the numerical value acquired using MLAEP. The strong correlation
between GCS scores and MLAEPi in patients with DOC has been previously reported 7], 8]. Furthermore, because of its small size and battery operation facility, aepEX monitoring
can provide a consistent assessment of MLAEP during life-saving procedures while transporting
patients within the hospital and in patients admitted to ICUs. In this study, we demonstrated
that MLAEPi may be a reasonable indicator of neurological outcomes in patients with
brain damage who have underwent emergency craniotomy in an acute care setting.
This study has several limitations, particularly the small number of evaluated patients
sustaining different types of brain damage such as focal TBI, diffuse TBI, and cerebral
stroke. Second, we did not measure initial MLAEPi in ED and did not use other monitors
such as BIS, short latency AEP, early SEP, or MLAEP to evaluate the degree of DOC
in the ICU. Third, the measurement of MLAEPi was performed by a single emergency physician
(Dr. TJ). Thus, the patients with DOC were not enrolled sequentially in this study.
Fourth, we only obtained MLAEPi data for a period of up to two weeks and had no records
from the late phase following discharge from the hospital. The purpose of this study
was to assess MLAEPi monitoring for predicting functional outcomes in the acute phase
in patients admitted to ICUs. Thus, we limited the study endpoint to the initial evaluation
in the acute phase and did not perform long-term MLAEPi follow-up.