Neuroaudiological Considerations for the Auditory Brainstem Response and Middle Latency Response Revisited: Back to the Future

Abstract

The auditory brainstem response (ABR) and middle latency response (MLR) are two sets of evoked potentials that have made major contributions to the field of diagnostic audiology. Many of these contributions were guided by clinical research audiologists. Though many of these auditory evoked potentials (AEPs) are still being used diagnostically by audiologists, there has been a steep decline in their popularity both clinically and in the research laboratory. This is indeed most unfortunate because these AEPs could and should be advancing our field and benefitting many patients. In this article, some critical research is overviewed that addresses some of the reasons why these AEPs (ABR and MLR) are not being utilized as frequently as they should be for neuroauditory assessments. Reflecting on our past when ABR and MLR were more commonly used can serve as a model for our future. Multiple applications and the diagnostic value of these AEPs are presented in an effort to convince audiologists that these electrophysiologic procedures should be revisited and reapplied in the clinic and research settings. It is argued that the dwindling use of ABR and MLR (and AEPs in general) in the field of audiology is not only remarkably premature but also lacks good scientific grounding. While on the other hand, if applied clinically, the value of these AEPs is both substantial and promising.

The auditory brainstem response (ABR) and the middle latency response (MLR) have an impressive diagnostic track record. The ABR, in particular, is the most powerful diagnostic procedure to ever grace the audiology landscape. Unfortunately, audiologists have abandoned to a great degree the use of not only the ABR and the MLR procedures but also auditory evoked potentials (AEPs) in general. A review of a recent data report provided by Medicare ¹ covering over 1 million procedural codes filed by audiologists revealed the use of ABR only approximately 5% of the time. Other AEPs were used even far less frequently. ¹ This unfortunate behavior on the part of much of the audiology community has not only compromised audiology but also has weakened its value for patients and other professionals. In addition, there are several disorders for which the ABR and MLR are highly applicable that are often overlooked or not sufficiently utilized that can bolster the role of audiology in the diagnostic community. This overview article will focus on some of the neuroauditory disorders for which the ABR and MLR have been (and now perhaps more than ever are) well suited.

The lack of use of the ABR/MLR and other AEPs for neuroauditory diagnostic purposes is not only unfortunate but largely unfounded. A brief account of pertinent ABR history is germane to shed light on the trends that halted much of the diagnostic use of ABR, and, by extension, the MLR.

In the late 1980s and well into the 1990s, reports surfaced arguing against the use of ABR due to its proclaimed insensitivity to small acoustic tumors (vestibular schwannomas) when compared with the superior sensitivity of magnetic resonance imaging (MRI) for these types of lesions. ^{2 3} While these reports were to some degree true, the trends of audiological practice (and related research) that followed remain rather puzzling and shortsighted to the present authors. The exodus from the use of the ABR and other AEPs seems, in our view, somewhat illogical. There are numerous ways in which ABR/MLR procedures can and should be utilized in audiology and they are simply not. It appears that audiology has subscribed to an “all or none” diagnostic use of ABR favoring the “none” approach based on some debatable reports which appeared some 30 years ago. In this article, we argue for the reinvigoration of the application of the ABR and the MLR. This will be based on revisiting clinical situations in which the early and middle evoked potentials can be used as powerful indicators of neuroauditory system integrity that can significantly enhance neuroaudiological diagnosis and inform patient status. This review, which is not intended to be exhaustive, will focus on two particular topics: (1) the audiologist as a gatekeeper: disorders for which ABR and MLR are complementary to imaging and (2) disorders for which imaging may not be useful. In addition, some related considerations such as the difference between anatomical structure and physiological function and patient acceptance of and access to imaging will be briefly overviewed. By revisiting the historical evidence of the efficacy of the ABR and MLR potentials in the diagnosis of neuroauditory disorders, we hope that we can move “ back to the future ” where the applications of these potentials are an essential component of the diagnostic assessment of patients at risk for neuroauditory system (auditory nerve/brainstem) compromise.

One of the longstanding arguments for the use of electrophysiological and behavioral test procedures versus imaging tests is the slight but significant differences between structure and function. It is well known and accepted that there is a strong relationship between anatomy and physiology. This is an important relationship and one that is of great value to all of us. However, in some instances, this relationship is not 1:1. Biochemical and micro-anatomical changes can affect physiology, but these alterations may not be seen on “structural” imaging procedures. As will be discussed in the ensuing text, there are numerous disorders for which imaging may not identify structural anomalies that do affect auditory function. This differential is enhanced if the resolution for the imaging procedure is less than ideal.

In this article, the ABR and MLR will be highlighted. These procedures can be done separately, but can also be recorded simultaneously saving both time and effort (see Fig. 1 ). The advantage of utilizing both the ABR and the MLR is that these two procedures can provide an integrity check of the auditory nerve and pontine auditory pathways as well as the thalamocortical connections, respectively. ⁴ The combination of ABR and MLR can also provide valuable site-of-lesion information. For example, if the ABR is normal and the MLR is not, the implication is that dysfunction of the higher-order thalamocortical areas of the auditory system exists. However, in most instances, if there is auditory brainstem involvement, both the ABR and MLR will be compromised. Certainly, there are cases where the ABR can be abnormal and the MLR normal, but these are usually cases where the ABR is only minimally affected. ⁵

Figure 1.

An example of a combined auditory brainstem response (ABR) and middle latency response (MLR) waveform and its replication. The figure depicts an ABR and MLR waveform measured in milliseconds and viewed in a 0.3-microvolt amplitude calibration marker. The first major peak of both waves is marked as V at ∼6 ms. Both waves enter a trough at ∼15 ms, marked as Na, and then both waves then peak again at ∼25 ms and the peak is marked with Pa. Then they fall into a trough at ∼40 ms, which is marked with Nb. The final peak is at 50 ms and is labeled Pb.

THE AUDIOLOGIST AS A GATEKEEPER: DISORDERS FOR WHICH ABR/MLR IS COMPLEMENTARY TO INDICATED IMAGING

Recently, there has been much discussion regarding the audiologist acting as the gatekeeper or being the first contacted hearing health care professional for individuals with hearing impairment. Incumbent on the audiologist in these situations is a careful and in-depth evaluation of the patient. After the evaluation of the patient is completed, a determination for further testing and referral(s) must be made. Given that many cases with neuroauditory involvement experience hearing difficulties, the audiologist is often the initial health care professional consulted. It is therefore critical that the audiologist generate a well-grounded and appropriate referral that can contribute to the overall diagnostic workup. Missing a neurologically based problem, making an inappropriate referral, or taking a “hands off” approach to challenging patients is certainly not consistent with responsible audiology practice or good gatekeeping.

The ABR/MLR can be most helpful to the audiologist in determining the presence of neuroauditory involvement. As the potential initial healthcare contact, the audiologist is not going to have direct imaging accessibility for a variety of brain disorders, such as in the case of mass lesions, vascular problems, and certain degenerative disorders (e.g., multiple sclerosis [MS]) for which imaging is the standard diagnostic approach. Therefore, identifying potential neurological involvement related to hearing difficulties is key to providing accurate and appropriate referrals for diagnostic patient follow-up.

Vestibular Schwannomas (Acoustic Tumors)

It is well known that ABR, with the exception of small (<1.0 cm) tumors, has an excellent sensitivity (>90%) and specificity (>80%) for the detection of vestibular schwannomas with the most common waveform abnormality being a wave I–III latency extension. ^{2 3 6} A remarkable illustration of excellent gatekeeping is a recent case report by Ivey et al. ⁷ In this case, a patient in his early 20s went to an audiologist because of hearing difficulties and slight imbalance. The audiogram showed normal hearing sensitivity and excellent speech recognition in the left ear and normal hearing to a mild sensorineural hearing loss with good speech recognition for the right ear. The audiologist choses to perform an ABR to better elucidate the problem. The ABR was normal for the left ear but showed only a wave I for the right ear. A referral was made, and a subsequent and timely diagnosis of neurofibromatosis was rendered. Currently, in many clinics (but certainly not all) direct referral for MRI without ABR is made for patients with unilateral hearing loss. Without rehashing the extensive debate of the 1990s, ² a few comments need to be voiced in this regard. First, only a small percentage of cases (1–7.5%) ⁸ with unilateral hearing loss have acoustic tumors and the risk of over referral for expensive MRI procedures is often a possibility. This is strongly supported by an extensive study published in a radiology journal by radiologists. ⁹ This study analyzed referrals for MRI made by otologists/otolaryngologists for acoustic tumor detection. ABR was not employed by the otologists, but audiograms were. There were 881 consecutive cases referred for MRI for acoustic tumor detection. Of the 881 cases, only 12 (1.4%) had acoustic tumors. This means that 869 patients were examined without a positive result. The current cost of MRIs of the brain ranges from $1,600 to $8,400 depending on the procedural techniques employed. ¹⁰ Using the lowest MRI cost and the numbers in the study of Vandervelde and Connor, ⁹ a total of $1,390,400 would be spent without yielding a positive finding. Use of the ABR at a 94% hit rate would have slashed the over referral numbers dramatically. While it is true that ABR may have missed some small acoustic tumors (85% hit rate), small acoustic tumors would be only a portion of the already rare prevalence of this type of lesion. ^{2 3} In addition, current philosophies in the medical field have trended toward a wait-and-observe approach for small lesions due to the slow growth of these lesions unless such an approach is contraindicated medically (as in the case of patients who present with neurological symptoms). Therefore, it is likely that many of the small tumors not identified with ABR may not be detected or acted upon during a medical examination in the early stages of tumor development and growth. In a large study of 207 patients with small vestibular schwannomas (<2.0 cm), only 15 showed significant tumor growth. ¹¹ Another factor is that when referrals are made based solely on symptoms and audiogram findings, there is no way of knowing if some patients with tumors are being missed. There are data indicating that approximately 5% of all acoustic tumors have normal or near-normal hearing sensitivity. ² However, we would submit that there is a fair possibility that some patients with tumors may be overlooked if ABR testing is not completed. ²

Brainstem Lesions

As with vestibular schwannomas, there are disorders of the brainstem for which imaging is key, but for which the audiologist may be seen either as the initial contact or after imaging has been performed. Often in these cases, determination as to involvement of the auditory system is of value in audiological management and counseling. The ABR can be of value to the audiologist for a variety of brainstem disorders. Intra-axial brainstem lesions (lesions arising from within the brainstem) and extra-axial lesions (arising from outside of, but affecting, the brainstem) are two main categories of brainstem lesions. In general, the intra-axial lesions are more disruptive due to their locus and the ABR is highly sensitive to these. Though study numbers have been small, hit rates for ABR and intra-axial lesions have been reported to be greater than 90%. ⁶ The extra-axial lesion of most interest to the audiologist is the acoustic tumor which has been discussed earlier. However, mass lesions of the facial and trigeminal nerves and the cerebellum also fall into this category. Over the course of one's professional career, a diagnostic audiologist may see a variety of brainstem lesions such as mass lesions, vascular disorders, and degenerative anomalies. ABR sensitivity for this “garden variety” of brainstem involvement is slightly less than for acoustic tumors—generally trending in the 80% range, ^{6 12 13} but clearly this sensitivity rate would support the use of ABR.

The brainstem disorders mentioned earlier obviously can result in hearing problems. If hearing problems exist, patients with these disorders may be seen by audiologists before imaging is completed and therefore the onus for appropriate referral exists. However, these patients are indeed relatively rare in audiology clinics.

Multiple Sclerosis

One disorder often affecting hearing/balance and the brainstem that is relatively common (especially in the northern hemisphere) and one that is frequently overlooked or under-assessed audiologically is MS. Though MRI is indeed valuable in defining MS, the audiologist again may see these patients early on in the diagnostic process before imaging is completed. Moreover, as MS is a progressive disease, often the diagnostic question when the patient with MS is noting hearing problems is “ Are these hearing difficulties due to the MS or to some other auditory (peripheral) problem .” This differential diagnosis would be critical to proper and optimal audiological management for these patients. The opportunity for audiologists to see patients with MS is likely. There are over a million patients with MS in the United States and since its onset is commonly in the early adulthood years and there is no cure, individuals can live for many years with the disease with the average life expectancy of approximately 75 years. ¹⁴ This results in an increasing prevalence of the disorder. Hence, the prevalence of MS is increasing every year. Of interest to audiologists is that the percentage of patients with MS that report auditory difficulties ranges from approximately 6 to 40%. ^{15 16 17} Also, it has been reported that the prevalence of vestibular problems in the MS population is 5 to 78%—many of these individuals would or should be seen by audiologists for both hearing and vestibular exams. ¹⁸

Both the ABR and MLR have been shown to be sensitive to dysfunction of the neuroauditory system in patients with MS. Differing ABR and MLR criteria and varying degrees of involvement in patients with MS have contributed to the variability in sensitivity in reports. However, in reviewing several well-conducted studies with relatively large populations, the ABR and MLR revealed reasonable statistics in defining the disorder and associated auditory dysfunction. ^{19 20 21 22 23 24 25} In the studies where ABR and MLR were combined, sensitivity was increased. Of note was the neurological support for completing both evoked potentials and MRI tests as they can reveal differing features of the central nervous system (CNS; anatomic vs. physiologic), and as such should be viewed as complementary procedures. ²³

Further support for the value of evoked potential testing in patients considered to be at risk for MS is provided by a recent meta-analysis on sudden sensorineural hearing loss (SSHL) as an early indicator of MS. ²⁶ In this study, 92% of the SSHL cases occurred in the early stages of the disease and was the only symptom at the time of presentation for almost one-half of the female patients. In the early presenting SSHL cases, 60 and 40% of the MS lesions were located in the brain and internal auditory meatus, respectively. All ABRs performed on the SSHL patients were abnormal and remained abnormal—even after complete recovery of hearing in many of the patients. This report is critical to alerting clinicians that SSHL may not be cochlear in origin—which is a common misconception. Rather, SSHL should be considered as a potential early sign of MS, and as Di Stadio and colleagues strongly suggest, ABR should be part of the diagnostic protocol (see Fig. 2 ). ²⁶

Figure 2.

Auditory brainstem response (ABR) results from a young adult patient who suffered a sudden sensorineural hearing loss for the right ear accompanied by tinnitus and imbalance. The patient had normal hearing sensitivity in the left ear and a moderate flat sensorineural loss in the right ear and was diagnosed with multiple sclerosis related to a MS plaque in the right auditory brainstem pathway. The figure depicts the patient's ABR as a function of latency in milliseconds and has peaks I to V labeled. Three ABR waveforms were recorded at 80 dB nHL and one was recorded at 90 dB nHL. The right ABR shows extended central conduction latencies (I–III, III–V, I–V) and poor waveform morphology. The left ABR is essentially normal.

ABR/MLR AND AUDITORY DISORDERS FOR WHICH IMAGING MAY NOT BE USEFUL

Of special interest are the auditory disorders for which imaging often may not be of help in diagnosis. In these disorders, ABR and MLR often become the keystone of the auditory diagnosis. In the forefront of these disorders are auditory neuropathy spectrum disorder (ANSD) hyperbilirubinemia (HB; jaundice), neurotoxicity (heavy metals), phenylketonuria (PKU), and concussion/mild head injury.

Auditory Neuropathy Spectrum Disorder

The diagnosis of ANSD is essentially determined by ABR results and other audiological tests. This is in part because imaging is of little help to diagnosing ANSD. ²⁷ The typical ABR findings in ANSD are a cochlear microphonic (CM) that is present and an ABR that is most commonly absent. Otoacoustic emissions (OAEs) can be variable but generally are present. Acoustic reflexes are often absent and if the patient is old enough to test speech recognition, it is often poorer than expected. The pure-tone audiogram can show considerable hearing loss, yet OAEs can be present, indicating neural involvement. It is fair to say, however, that the diagnosis of ANSD cannot be made without employment of the ABR.

Though ANSD is most often associated with newborns, it can occur at any age. The incidence/prevalence of ANSD is difficult to determine, as the breadth of criteria for testing and subject inclusion criteria vary considerably. Estimates of approximately 2% and even much lower have been reported. ^{28 29} The point, however, is that audiologists without ABR testing cannot be involved in the diagnosis in this interesting population of patients. Auditory nerve damage giving rise to ANSD has been associated with anoxia, hypoxia, low birth weight, prematurity, heredity, viral infection, high fever, Friedrich's ataxia, Charcot–Marie–Tooth syndrome, Stevens–Johnson syndrome, and Ehlers–Danlos syndrome. ³⁰

Hyperbilirubinemia (Jaundice)

Another auditory disorder for which the ABR is critical is HB. This disorder, often included under ANSD, is kept separate here as it is primarily a brainstem disorder. ³⁰ HB is related to high levels of bilirubin (>20 mg/dL) in the blood, which is a pathological condition usually noted at birth. There is high variability in the incidence of HB with approximately 2 to 8% of newborns presenting with clinically significant bilirubin levels which are routinely treated with phototherapy. ³¹

This disorder will result in yellowing of the skin (jaundice) and can result in damage to the basal ganglia and auditory brainstem pathways if left untreated. ³⁰ For many years, HB has been associated with hearing loss but in particular with central auditory involvement, as it appears to have a particular affinity for affecting the cochlear nucleus. Therefore, the ABR becomes an important tool in evaluating auditory brainstem integrity in cases of HB, as imaging is usually not employed in this diagnosis as it is of little help in said diagnosis. ^{29 31}

The ABR in HB can show a variety of waveform configurations. If the hearing sensitivity is sufficient, the ABR will often reveal a normal wave I with delays in the I to III and/or the III to V interwave interval(s). Later, ABR waves (III, V) can also be absent or of very poor morphology. ^{32 33} ABRs are often abnormal bilaterally since HB is a systemic disorder. Because of its central site of involvement, screening ABRs is preferred to OAEs, as OAEs are often normal in patients with HB; hence, if ABR testing is not administered, the potential exists that the presence of this disorder may be missed. Numerous studies have shown the ABR to have significant value in detecting newborns with HB and discriminating them from newborn control populations for almost 40 years. ^{32 33 34 35} In addition, ABRs have also been shown to be of value in monitoring the effects of phototherapy and serum levels of bilirubin. ^{33 34}

Neurotoxicity (Heavy Metals)

Neurotoxic agents such as heavy metals can compromise function of the peripheral and central auditory nervous systems. ^{29 36} Two of the most common heavy metals are lead and mercury with humans often exposed to these two neurotoxins. Exposure to these heavy metals can be in manufacturing, industrial, mining, and agricultural activities. Exposure can also be through drinking water (lead pipes) or through lead exposure present in dust, old paint, or food. ^{29 36} It is well accepted that structural imaging procedures play a limited role in identifying heavy metal effects on the central auditory system; hence, functional measures become key in the assessment of patients with histories of heavy metal exposure.

Castellanos and Fuente ³⁶ conducted an extensive review and analysis of the adverse effects of heavy metals on the auditory system and strongly supported ABR as a key auditory procedure for defining auditory involvement. Additional support for the use of the ABR in the assessment of individuals with lead exposure can be found in an earlier study that compared test results for individuals who had occupational lead exposure with those obtained from a well-controlled group of nonexposed workers. ³⁷ These data showed ABR absolute latencies of wave V and the I to V wave interval to be significantly extended in the lead-exposed group. In another study, increased lead blood levels were correlated with an extended ABR wave III for workers exposed to lead. ³⁸ These ABR studies (and others) document dysfunction of the auditory brainstem (central) pathway in individuals with heavy metal exposures.

Perhaps one of the most lead-exposed populations is children aged 1 to 5 years. ³⁹ This is often a result of children ingesting lead paint, playing in lead dust areas created from heavy traffic in cities, and/or engaging in other activities where they could be exposed to potentially pathological levels of lead. The prevalence of lead poisoning in children is difficult to determine, as measurement type and criteria vary across studies. Children living close to lead mining and/or lead-processing industries (such as those that extract lead from batteries) have a higher prevalence of lead poisoning. In these situations, prevalences near 5% have been reported. ³⁶ Other surveys away from these industries yield much lower prevalence numbers. As was the case with adults, studies on children show ABRs with extended latencies for waves III and V that are correlated with high lead blood levels. ^{36 40}

Mercury poisoning often related to employment in mining jobs, factories, and agriculture is less common and less often studied than lead poisoning. In an interesting study, Discalzi et al ⁴¹ compared ABRs from mercury-exposed workers who had normal pure-tone audiograms to workers in a control group. As often noted with lead poisoning, the ABR central conduction latencies (I–V wave interval) were significantly extended for the mercury-exposed group. This is consistent with other reported findings for mercury poisoning. ⁴²

This review of heavy metal effects on ABR gives rise to an interesting observation. Many reports of heavy metal poisoning often reveal pure-tone hearing loss, and, when performed, abnormal central conduction times on the ABR. We would postulate that in many cases, the decreased hearing sensitivity may be secondary to compromise of the auditory nerve and/or auditory brainstem pathway as is reflected by the abnormal central conduction time on the ABR. Evaluation of possible heavy metal poisoning in adults and children is an important responsibility of the audiologist. If ABR is not completed, perhaps the most valuable audiological tool in the assessments of exposed individuals is lost and a less than ideal and complete evaluation ensues (see Fig. 3 ).

Figure 3.

The auditory brainstem response (ABR) from a middle-aged patient who suffered from acute mercury poisoning. The audiogram for this patient showed bilaterally symmetrical normal hearing to a mild hearing loss. The ABR is measured in a span of 10 milliseconds and all traces were recorded at 80 dB nHL. Two replicated traces were recorded at 15 clicks per second, and two replicated traces were recorded at 73 clicks per second. The right ABR showed poor wave morphology, a missing wave III and an extended I–V latency. The left ABR shows extended central conduction latencies (I–III, III–V, I–V) and a severe deterioration (essentially no waves present) at a high rep rate. (Reprinted with permission from Musiek FE, Hanlon DP. Neurological effects in a case of fatal dimethyl mercury poisoning. Ear Hear 1999;20:271–273.)

Phenylketonuria

Another population for which ABR/MLR may play a critical role though one that is seldom considered is PKU. Though PKU testing is a well-known newborn screening procedure as it is done worldwide, auditory aspects of this disease are not as well-known as they should be. PKU is a metabolic genetic disorder that can affect myelination of neural fibers and neurotransmission within the CNS. It has a prevalence rate estimated at 1/15,000 to 1/5,000. ⁴³ This is another disorder for which MRI is of little help and generally not used. ⁴⁴ Since PKU can involve the CNS, it was logical for some early studies to consider possible effects on hearing and to turn to evoked potentials as a possible indicator of this influence. ^{45 46} These initial studies revealed that ABR central conduction times were sensitive to elevated PKU levels. This opened the door to thinking about the relationship between hearing and PKU and the use of evoked potentials to measure possible influences on the auditory system.

More recently, a key study by Mancini et al ⁴⁷ has allowed insight to the advantageous use of ABR/MLR in PKU. These researchers showed that greater than 50% of their PKU population had significantly extended ABR central conduction latencies compared with those of their control group. In addition, the MLR (when applying interhemispheric electrode comparison procedures) yielded significant differences from the control group 87.5% of the time. The authors of this research suggest it is important to monitor children with PKU with these auditory measures.

Though more clinical studies are needed, the known link between PKU's effect on the CNS and the ability to monitor these effects with ABR/MLR creates a compelling role for neuroaudiology.

Concussion, Mild Head Injury

Concussion, mild head injury, and mild traumatic brain injury (all three terms abbreviated as HI) seem to be on the rise in the United States. It has been estimated that the incidence of concussion is approximately 1.2 million people. ⁴⁸ Furthermore, there is evidence that approximately 50% of those with concussion/mild head injury have auditory deficits. ^{49 50} Like MS, auditory involvement in cases of concussion/head injury is dependent on the locus of the injury. Though HI occurrence is somewhat random, there are trends reported that reveal certain anatomical regions that are more often involved than others. Among the most commonly involved areas are in the frontotemporal region, the internal capsule, the upper brainstem, and the corpus callosum. ⁵⁰ Interestingly, auditory tracts course through all of these anatomical areas.

Consistent with this segment of this article, imaging is often of little help in defining HI. For example, Bonow et al ⁵¹ reported that MRI was consistent with traumatic brain injury in only 0.5% of children with documented sports injuries to the head. These findings level more emphasis on other measures to document HI—including audiology test measures.

Several ABR studies dating back to 1980 have demonstrated a hit rate that hovered around 50% for individuals with mild HI. ^{49 52 53 54} This hit rate is considered to be reasonably good given that in HI the auditory system is not always involved. ⁵⁵ The most common ABR finding in these studies was a significant extension of the I–V inter-wave interval, thus implicating involvement of the auditory nerve and/or the brainstem auditory pathway. Additionally, Munjal et al ⁵⁶ showed a relationship between the severity of HI and an increased latency of wave V and a prolonged I–V ABR interval.

A study that does not quite fit into our ABR category, but needs mentioning, is the speech FFR report by Kraus et al. ⁵⁷ The speech FFR is a derivative of the ABR using an abbreviated CV (consonant–vowel stimulus, 40 ms in duration) as the stimulus and a longer time window (60 ms). The speech FFR has shown excellent sensitivity (90%) to HI in a population of children with concussions. Another population, specifically infants, where speech FFR may offer diagnostic value, was discussed by Musacchia et al. ⁵⁸ The results of this study found that speech-evoked FFR may track neuronal function alterations observed in infants affected with high levels of bilirubin with more sensitivity than traditional ABR. Therefore, it is logical to see how this electrophysiologic measure may be appropriate in these and other related areas of auditory neurophysiology.

The MLR has not received as much attention in the evaluation of HI as perhaps it should. Some key studies have demonstrated significant Na–Pa wave amplitude and latency differences between mild HI and control populations. ^{59 60} In one study of 20 patients with mild HI, 6 of the patients revealed an absent MLR waveform. ⁵⁹ In addition to the ABR findings discussed earlier, Munjal et al ⁵⁶ also showed a relationship between the severity of HI and a decreased amplitude of the Pa wave in their patients.

Certainly, more HI studies need to be conducted using the MLR to substantiate its value. However, to reiterate, combining the ABR and MLR in HI can herald important site-of-lesion information. A present and normal ABR with an abnormal or absent MLR may help rule out brainstem involvement and implicate thalamocortical pathway involvement.

ADDITIONAL COMMENTS

Two major areas of consideration for increasing the current use of the ABR/MLR have been overviewed. Though beyond the scope of this article, there are other critical applications of the ABR/MLR. For example, intraoperative monitoring has grown into a large clinical discipline involving a variety of professionals including audiologists. ⁶¹ The ABR/MLR to monitor change in patient status related to various otologic and neurologic conditions is another valuable use of these evoked potentials. Also, the ABR/MLR can be applied for monitoring changes in patients undergoing nonmedical therapeutic procedures such as auditory training. ⁶²

FINAL COMMENTS

The purpose of this overview article was to urge a greater use of evoked potentials, specifically the ABR/MLR, in audiology. It is clear that the use of evoked potentials has decreased rather dramatically in recent history. This is indeed unfortunate for our field as well as for our patients. As conveyed in this article, there are many valuable applications for the ABR/MLR that many audiologists are not involved in and perhaps should be. Currently, there are trends in our profession such as over-the-counter hearing aid sales that are usurping audiological practice activities. ⁶³ Revisiting the use of ABR/MLR could be an ideal way of filling this void currently being noted by many audiologists. These evoked potentials have an impressive track record in their application to a variety of otologic and neurologic disorders. It would be a shame for more audiologists to not take advantage of these valuable tools that can contribute in important ways to enhanced diagnostic and rehabilitative efficacy for our patients.

In this article, evidence has been offered to show that ABR/MLR can and should be used as a complement to imaging procedures as opposed to dropping their use entirely. Key to this view is the differences between structure and function in defining disorders of hearing. It is also important to consider that many patients cannot or will not undergo imaging procedures. A meta-analysis has shown that between 1 and 2 people out of 100 have a claustrophobic event that negates a successful MRI. ⁶⁴ In addition, in the United States, there are over 2 million people with implanted metal devices, which in most instances negates MRIs. ^{65 66} These are considerable numbers. For patients with the neuroauditory disorders discussed earlier who cannot or will not undergo MRI, ABR and MLR can be utilized as an alternative, for example, in cases of pregnancy. Also, there are considerations of the high-intensity sound emitted during MRI and its effects on the patient, as well as the world shortage of helium used for cooling the MRI machine. ⁶⁷ This is another avenue for the use of evoked potentials that audiologists should be ready to contribute to and utilize. In these situations, ABR/MLR can play a critical role in the diagnosis of serious health problems.

In addition, there are a myriad of auditory disorders for which imaging procedures are of little help for which ABR/MLR are well suited. A lesson can be learned from our past when ABR and MLR as well as neuroaudiology in general were employed. Audiology should pursue these areas, as they hold potential for expansion of the clinical profession and better service to our patients. With much current discussion about audiologists as the “gatekeeper” for those with hearing problems, proper patient management and referral is paramount. To fulfill this kind of responsibility, audiologists need all the diagnostic information possible. Without the use of powerful tools such as ABR/MLR, audiologists will be handicapped in attempting to follow through on their “gatekeeper's” obligations. Previous, more popular use of ABR/MLR established an impressive record in our field. We need to embrace that record and move back to the future.