Brainstem Auditory-Evoked Potentials as an Objective Tool for Evaluating Hearing Dysfunction in Traumatic Brain Injury

Abstract

Lew HL, Lee EH, Miyoshi Y, Chang DG, Date ES, Jerger JF: Brainstem auditory-evoked potentials (BAEPs) as an objective tool for evaluating hearing dysfunction in traumatic brain injury.

Because of the violent nature of traumatic brain injury, traumatic brain injury patients are susceptible to various types of trauma involving the auditory system. We report a case of a 55-yr-old man who presented with communication problems after traumatic brain injury. Initial results from behavioral audiometry and Weber/Rinne tests were not reliable because of poor cooperation. He was transferred to our service for inpatient rehabilitation, where review of the initial head computed tomographic scan showed only left temporal bone fracture. Brainstem auditory-evoked potential was then performed to evaluate his hearing function. The results showed bilateral absence of auditory-evoked responses, which strongly suggested bilateral deafness. This finding led to a follow-up computed tomographic scan, with focus on bilateral temporal bones. A subtle transverse fracture of the right temporal bone was then detected, in addition to the left temporal bone fracture previously identified. Like children with hearing impairment, traumatic brain injury patients may not be able to verbalize their auditory deficits in a timely manner. If hearing loss is suspected in a patient who is unable to participate in traditional behavioral audiometric testing, brainstem auditory-evoked potential may be an option for evaluating hearing dysfunction.

Hearing loss is intuitively detrimental to daily communication and community reintegration. However, there is a paucity of data on its prevalence, classification, and effects on disability in traumatic brain injury (TBI) patients. Because TBI patients are unable to report their hearing deficits until they reach a certain level of cognitive awareness, diagnosis of this silent handicap may be delayed in many cases. This case report demonstrates the use of brainstem auditory-evoked potential (BAEP) as an objective and noninvasive tool to identify hearing dysfunction in the early stage of recovery for patients with severe TBI.

CASE REPORT

A 55-yr-old Hispanic male sustained severe TBI after a fall from his ladder and was admitted to a local hospital for emergency treatment. Premorbidly, he had a history of alcohol abuse, but no history of cognitive, hearing, or communication impairment. On the day of injury, the computed tomographic scan revealed bifrontal hemorrhagic contusions with subarachnoid hemorrhage, vertical fractures of right occipital/parietal bones, and a transverse left temporal bone fracture (Fig. 1, left). Initially, the patient was very agitated and combative. Because of the temporal bone fracture and suspicion of hearing loss, a pure-tone audiogram was performed at the fourth week of injury. However, results from both behavioral audiometry and bedside evaluation (including Weber and Rinne tests) were inconclusive because of the patient’s poor cooperation. It was unclear whether his problems with compliance were caused by hearing loss or cognitive impairment.

Figure 1:

Left, transverse left temporal bone fracture (white arrow head) on the initial head computed tomographic image; right, subtle right temporal bone fracture (white arrow head) detected on the thin-slice computed tomographic scanning after brainstem auditory-evoked potential testing suggested bilateral hearing loss.

After stabilization, the patient was transferred to our rehabilitation center at the seventh week postinjury. At admission to the rehabilitation unit, he was awake but disoriented, restless, and uncooperative. He made eye contact but could not respond appropriately to verbal commands. The admitting physician noted that he was more responsive to written instructions and gestures than to verbal questions. He also had mild left facial weakness, whereas other cranial nerves were intact. His gait was stable, and he was able to perform basic activities of daily living with supervision.

The admission evaluation performed by a speech-language pathologist revealed hypernasal voice quality, decreased articulatory precision, and inappropriately increased speech volume. Auditory comprehension was severely impaired. Reading comprehension, verbal expression, written expression, and pragmatics were mildly to moderately impaired. Although bilateral hearing loss was suspected, objective evidence was still lacking for a definitive diagnosis. Therefore, at the eighth week postinjury, BAEP was performed on the rehabilitation floor by a physiatrist. BAEP was performed using a Viking IV electrodiagnostic instrument (Nicolet Instrument, Madison, WI). Recording electrodes were placed on bilateral mastoids (M1 and M2), and the reference electrode was placed at the forehead (Fpz). Alternating click stimuli (duration, 0.1 msec; frequency, 11.1 Hz), starting with 35 dB normalized hearing level, were presented monaurally via earphones, with masking white noise to the contralateral side. Recordings were done with 1,000 presentations of auditory stimuli using two channels. The same test was repeated for each ear to verify reproducibility of the response.

The entire test took about 30 mins to perform. For this particular patient, BAEP could not be elicited with auditory presentations up to 85dB normalized hearing level bilaterally (Fig. 2b), and the results strongly suggested severe bilateral hearing loss.

Figure 2:

Brainstem auditory-evoked potentials: (a) normal response from a healthy subject using 65 dB normalized hearing level and (b) no response from the patient using 85 dB normalized hearing level. Traces 1–4, left-side stimulation (1–2, ipsilateral recordings; 3–4, contralateral recordings); traces 5–8, right-side stimulation (5–6, ipsilateral recordings; 7–8, contralateral recordings).

The initial head computed tomographic images, which showed unilateral (left) temporal bone fracture, were reviewed carefully with the radiology department. However, right temporal bone fracture was not evident. Hence, a head computed tomographic scan was repeated with thin slices, focusing on bilateral temporal bones and cochleas. The results showed a subtle transverse fracture of the right temporal bone, in addition to the previously known left temporal bone fracture. Specifically, there was a conspicuous left transverse temporal bone fracture that extended across the cochlea and the superior and inferior semicircular canals (Fig. 1, right). The right temporal bone fracture was subtle and nondisplaced (Fig. 1, right), with less cochlear involvement than the left side. Bilateral ossicles appeared intact.

The Wechsler Adult Intelligence Scale-III was performed at the end of the eighth week postinjury. It showed verbal intelligence quotient of 103, performance intelligence quotient of 84, and full scale intelligence quotient of 95. At the tenth week postinjury, the audiology service responded to our initial consultation because the patient’s cognitive status and cooperation improved to a point at which behavioral audiometry could be performed. Despite a normal otoscopic examination and tympanometry/impedance audiometry, his pure-tone audiogram showed profound sensorineural hearing loss bilaterally (Fig. 3). Conventional hearing aids were tried, but they were unsuccessful. On further consultation with the otolaryngology service, a cochlear implant to the right ear was suggested as a possible treatment option. The patient and his family were still undecided about this suggestion at discharge. On the twelfth week post-TBI, the patient was discharged to his local VA hospital for treatment of his alcohol problems. His admission and discharge FIM™ scores (recorded within 72 hrs after admission and 72 hrs before discharge) from our unit are listed in Table 1. Evidently, he made improvements in several categories of FIM. Considering the current commercial insurance polices in the United States, we realize that this patient probably might not have met the admission criteria for acute TBI rehabilitation in many hospitals. Fortunately for this patient, his health insurance coverage allowed him not only to be admitted as an inpatient, but it also approved for extended therapy.

Figure 3:

Audiogram of the patient showing profound bilateral hearing loss. CNT, cannot test.

TABLE 1.

Admission and discharge FIM™ scores

Category	Admission FIM	Discharge FIM
Self care	30	41
A. Eating	5	7
B. Grooming	5	7
C. Bathing	5	6
D. Dressing, upper body	5	7
E. Dressing, lower body	5	7
F. Toileting	5	7
Sphincter	13	14
G. Bladder management	7	7
H. Bowel management	6	7
Transfer	18	21
I. Bed, chair, wheelchair	6	7
J. Toilet	6	7
K. Tub, shower	6	7
Locomotion	10	13
L. Walk/wheelchair	5	7
M. Stairs	5	6
Communication	7	13
N. Comprehension, auditory/reading	1/3	1/6
O. Expression, verbal/written	4/4	6/7
Social cognition	6	12
P. Social interaction	2	2
Q. Problem solving	2	4
R. Memory	2	6
Total FIM score	84	114
Total FIM improvement	30
FIM efficiency, FIM change/length of stay	30/31 = 0.97

DISCUSSION

Because cognitive issues often interfere with behavioral performance after TBI, hearing deficits in this population are difficult to detect in the early stages of recovery.¹ This may result in delays in audiologic, surgical, and rehabilitative interventions and, ultimately, may adversely affect the functional outcome of patients with TBI.

A review of literature from 1975 to 2003 showed that the prevalence of hearing problems after TBI varies from 7% to 50%.^2–8 To obtain an estimate of hearing impairments in our own inpatient TBI population, we conducted a retrospective chart review of 63 moderate to severe TBI patients admitted to our service from September 2000 to August 2002. The mean age of the patients was 38 (±15) yrs, and 89% of them were men. On the average, they were admitted 6 wks after injury. The causes of their injuries included motor vehicle accidents, assaults, and mechanical falls. Historically, for our unit, behavioral audiometry was requested only when a patient complained of a hearing problem or when the rehabilitation team suspected that the patient had a hearing loss. Our chart review showed that 11 (17%) of the 63 patients were found to have abnormal audiograms. Seven (11%) were found to have sensorineural hearing loss, two (3%) had conductive hearing loss, and the remaining two (3%) had mixed hearing loss. If hearing screenings were performed prospectively on all TBI referrals to our unit, the prevalence would likely be higher than 17%.

BAEP (also known as BAER, brainstem auditory-evoked response) represents the electrophysiologic response of the central nervous system to an auditory click stimulus and is characterized by a pattern of five neurogenic waves (Fig. 2a). The BAEP wave I originates from the cochlear nerve, and wave II arises from the cochlear nucleus. Waves III, IV, and V originate from the superior olivary complex, lateral lemniscus, and inferior colliculus, respectively. In cases of TBI, prolongation of the interval between the BAEP waves I–III, III–V, or I–V have been reported.^9–13 The most ominous finding is complete absence of a discernible waveform, as seen in the current case report (Fig. 2b).

Because of its objective nature and ease of use, the value of BAEP for hearing evaluation has been established in the pediatric population.^14–17 In a study on comatose and medically compromised children and adolescents with TBI, BAEP abnormality was found in about 40% of the 62 patients tested.⁵ In another study on 40 TBI patients,¹⁰ BAEP results were compared with audiologic findings, and the authors concluded that BAEP was more diagnostic than pure-tone audiometry in the evaluation of brainstem dysfunction. However, the study did not specify the chronicity of TBI (time elapsed postinjury) in these patients. Moreover, only patients with mild or severe brain injury were studied, whereas moderate TBI patients (Glasgow Coma Scale scores, 9–12) were not included.

Our case report shows that BAEP can be readily performed in a comprehensive physiatric practice environment, in which an electrodiagnostic laboratory and collaboration with an audiology service are available. On further review of literature and consultation with audiology experts, it is apparent that the three main objective hearing tests that have been proven to be effective are (1) BAEP, (2) impedance audiometry, and (3) otoacoustic emissions test. A normal BAEP indicates integrity of the cochlear nerve and the ascending auditory pathway along the brainstem. The tympanometry portion of impedance audiometry provides good assessment of middle ear function.^18,19 Otoacoustic emissions evaluate hair cell function in the cochlea, and portable otoacoustic emission machines have been widely used in neonatal hearing screening.^20–24 Otoscopic examination gives an estimate of outer ear and tympanic membrane integrity. The above test battery, which can be done at the bedside, may be used to identify the site of lesion along the auditory pathway. As a follow-up study, we plan to submit a more comprehensive protocol to prospectively identify and classify different types of hearing impairments in patients with moderate to severe TBI and to investigate their effects on different aspects of functional outcome. In conclusion, physicians may consider BAEP as part of an objective and noninvasive test battery for evaluating hearing problems in TBI patients.