The auditory middle latency response can be a valuable diagnostic tool for central auditory function, especially in combination with an auditory brainstem response because it provides information about the integrity of important auditory structures such as the brainstem, thalamus, thalamo-cortical pathway, and primary auditory cortices.
Multiple sclerosis (MS), a progressive autoimmune demyelinating disease, affects regions of the auditory system confined to areas in the central auditory nervous system (CANS) where there are myelinated axons, including the corpus callosum, thalamo-cortical pathway, and the proximal portion of the auditory nerve. MS does not directly affect any specific nerve tract, but auditory presentations of MS are often the first signs of the disease in many people.
An auditory brainstem response (ABR) and auditory middle latency response (AMLR) can be recorded simultaneously and provide information about the reliability of the auditory system through the thalamo-cortical pathway and auditory cortices. Performing a combined ABR-AMLR increases the sensitivity to detecting MS lesions along the central auditory pathway.
The Effects of MS on the Auditory System
MS destroys and scars myelin on the axons of nerve fibers, and is associated with swelling and, consequently, neuronal dysfunction. Auditory complaints from those with MS are typically mild in quiet listening situations but become more pronounced in noise. One study of MS patients found that 40 percent had auditory complaints, though only 18 percent had abnormal audiograms. (Am J Otol 1989;10:343.)
Continued and prolonged periods of inflammation can cause permanent damage to axons throughout the central nervous system, including the CANS, if affected. Etiology is unknown, but MS is more common in cold climates and in young adult women. The characteristics of MS and its auditory manifestations when present herald the use of diagnostic central auditory processing evaluations with this population, including electrophysiologic techniques. Dichotic listening abilities are typically depressed in individuals with MS and temporal processing skills are also moderately affected because axonal demyelination affects neural conduction speed and synchrony. (Disorders of the Auditory System, San Diego: Plural Publishing, 2012.)
MS does not affect any specific nerve tract so manifestations of MS may appear in other sensory and motor tracts of the CANS, including those involved in vision (diplopia, blurred vision), tactile sensation (paresthesias), motor control, balance (vertigo), and coordination. This is not an exhaustive list of nonauditory manifestations of MS because it can present in a multitude of ways. Combinations of systems may be involved due to the global nature of neural systems that can be affected.
The AMLR is a scalp-recorded evoked potential composed of a series of negative and positive deflections from approximately 10 to 70 milliseconds poststimulus onset. Giesler and colleagues first described AMLR, then referred to it as the “fast response,” after developing and using averaging techniques in 1958, and Picton and colleagues in 1974 identified and labeled the waves (i.e., Na, Pa, Nb, Pb). (Science 1958;128:1210; Electroencephalogr Clin Neurophysiol 1974;36:179.) The Na-Pa wave complex is “the most consistent, most often used, and most researched” part of the MLR, even though there are many components to the waveform. (J Am Acad Audiol 1999;10:124.)
AMLR generation involves anatomical structures of the auditory system rostral to those that generate the ABR, but more caudal to structures generating the later cortical auditory evoked potentials such as N1-P2 and P300. No single anatomical structure appears to generate the AMLR; a combination of synchronous neuronal contributions from multiple structures creates the potential. The major generator of the AMLR is the thalamo-cortical pathway, an extended axonal pathway coursing from the medial geniculate body of the thalamus to the primary auditory cortex in the temporal lobe of the brain. Other contributors to the response include the midbrain structures of the inferior colliculus and the reticular formation. (Handbook of Clinical Audiology, Fourth Ed., Baltimore: Williams and Wilkins, 1994.)
The first major negative component of the potential, Na, is thought to originate from deep subcortical regions, in particular the inferior colliculus and the medial geniculate body of the thalamus. Little effect was found on the generation and configuration of the Na portion of the AMLR in individuals with cortical lesions, supporting the subcortical origin of Na. (J Neurol 1991; 238:427.) Subcortical and cortical generators for the Pa portion of the AMLR have been suggested, and support for these has been demonstrated in studies of patients with cortical lesions. Studies of the AMLR in those with unilateral temporal lobe lesions resulted in reductions in Pa amplitude for the electrode placed over the hemisphere with the lesion. (Electroencephalogr Clin Neurophysiol 1982;54:275; Electroencephalogr Clin Neurophysiol 1987;66:108.)
Some individuals with bilateral temporal lobe lesions still demonstrate Pa waves for both hemispheres, suggesting that there may also be thalamo-cortical contributions to Pa in addition to cortical generators leading to the presence of the response. (Electroencephalogr Clin Neurophysiol 1987;68:132.)
Multiple Sclerosis and AMLR
MS plaques are often found in highly myelinated areas such as the corpus callosum, medial longitudinal fasciculus, and the periventricular trigone region, although the effects of MS on nerve tracts are nonspecific. CANS integrity in MS is typically evaluated through ABR testing, although this response is only sensitive to lesions caudal to the thalamus. An ABR and AMLR can be simultaneously recorded, giving information about the reliability of the auditory system through the thalamo-cortical pathway and auditory cortices. A combined ABR-AMLR increases the chances of detecting lesions along the central auditory pathway.
Variability in results across studies is likely influenced by the inherent variability in MS plaque location, even when the same diagnostic criteria are utilized. Differences in disease progression and time since last flare-up may also contribute to variability. Samples dominated by probable diagnoses and longer recovery times since last flare-up are likely more variable and have poorer hit rates than results obtained from samples dominated by definite diagnoses of MS requiring shorter recovery times. Diagnostic criteria are defined based on age, symptoms, and objective evidence of plaques in the central nervous system as determined through MRI or neurological exam. Electrophysiologic findings in MS are dependent on the extent of CANS involvement, with location and degree of demyelination uniquely affecting individual physiology and behavioral manifestations.
Many researchers have found the AMLR to be clinically sensitive to MS because it primarily tests the myelinated CANS from the inferior colliculus to the primary auditory cortex, while the ABR is only sensitive to a portion of this pathway up to the level of the inferior colliculus. It was found that the ABR alone was abnormal in 79 percent of MS patients with definite brainstem lesions and in 51 percent of those without clinical signs. (Brain 1977;100[Pt 1]:19.) The late potentials were normal in the majority of these cases. These researchers also found that the AMLR showed abnormalities in the Pa, Nb, and Pb components in MS patients with a hit rate of 45 percent, while the ABR was normal in 12 percent of these cases. These findings demonstrate that some individuals with MS will present with normal ABR and abnormal AMLR results. Combining the evaluation procedure so that both potentials are recorded simultaneously allows for the collection of more clinically relevant patient data in about half the time necessary to measure the potentials independently.
Rehabilitation and Counseling
The main form of treatment for those with MS involves pharmaceutical management. Injections aimed at reducing exacerbations have become the cornerstone of medical management of MS, although drugs to adjust the immune system are also beginning to be used. Medications commonly used to treat MS include beta interferons, fingolimod, natalizumab, mitoxantrone, and glatiramer, an immunosuppressant injection administered once daily.
Audiologically, routine evaluation of the peripheral and central auditory systems should be performed to establish baselines for auditory sensitivity and to document any changes in auditory function. Routine evaluation may also help predict the course of the disease. MS could coexist with an independent peripheral auditory dysfunction, so it is important to separate central auditory effects of the disease from peripheral hearing loss, especially when determining hearing aid candidacy. Expectations that loss of hearing function may progress because of the disease or the natural process of aging should be incorporated in honest and sensitive conversations with the patient.
Incorporating the AMLR into routine audiological evaluations and follow-up of those with MS allows for a more complete understanding of an individual's unique effects and progression of the disease on auditory function beyond what may be seen on the audiogram alone. MS is a demyelinating disease that can affect areas rostral to the thalamus, and it is logical to utilize testing procedures that pinpoint those myelinated areas of the auditory system to determine if neural dysfunction is contributing to auditory complaints. Combined use of the AMLR alongside traditional audiometric evaluations and the ABR may arm clinicians not only with information about hearing status as affected by age, noise exposure, or other peripheral auditory factors, but also with information about the extent to which the CANS is affected by the demyelinating process. A personalized course of treatment and management may be pursued to best address each person's unique audiological profile once peripheral and central auditory status has been established.