What is audiology?

Science and Profession

The field of audiology has become an indispensable adjunct in the objective diagnosis of hearing loss and auditory disorders. The scope of audiology practice is rather extensive and includes broad categorical services. According to the US Department of Labor, the audiologist “specializes in diagnostic evaluation of hearing, prevention, habilitative and rehabilitative services for auditory problems and research related to hearing and attendant disorders.” Among other functions, the audiologist
determines range, nature, and degree of hearing function . . . using electroacoustic instrumentation; . . . coordinates audiometric results with other diagnostic data, such as educational, medical, social, and behavioral information; . . . differentiates between organic and nonorganic hearing disabilities through evaluation of total response pattern and use of acoustic tests; . . . [and] plans, directs, conducts, or participates in conservation, habilitative and rehabilitative programs, including hearing aid selection and orientation, counseling guidance, auditory training, speech reading, language rehabilitation, and speech conservation.

Audiologists may have primary affiliations in private practice, clinics and hospitals, military installations, universities and colleges, or public and private school systems.

Because of the significant advances that have been made in providing differential diagnosis of impaired auditory behavior, the number of institutions providing audiology programs increased dramatically in the latter half of the twentieth century. Although the highest degree offered in audiology at many institutions is the PhD, a number of schools offer a professional doctorate of audiology, which stresses clinical diagnosis, auditory prosthetic evaluation, and rehabilitative practice.

A number of professional audiology organizations exist to provide resources and support to those in the field. The membership of the American Speech-Language-Hearing Association (ASHA) consists of speech pathologists and audiologists, with the former having significantly greater numbers. In 1988, James Jerger and other prominent audiologists in the United States formed the American Academy of Audiology. Its members consist exclusively of audiologists holding a master’s or PhD degree in that field.

Diagnostic and Treatment Techniques

One of the most common services associated with the practice of audiology is the basic assessment of the auditory system relative to pure-tone air conduction thresholds. This is a procedure in which the patient’s ability to detect the presence of a tone delivered through earphones or a speaker is determined. Additionally, speech threshold detection is determined by assessing the patient’s ability to identify correctly 50 percent of a list of two-syllable words. Measurements of the acoustic reflex provide information about hearing loss, as do reflex-eliciting auditory tests. The acoustic reflex is the contraction of the stapedial muscle produced by a strong acoustic signal. The strength of the response and the level at which it is elicited are important diagnostic indicators of system malfunction, as is the absence of a reflex response. The degree to which the reflex response deviates in morphology and amplitude from normal is diagnostically significant. Communication handicap inventories also are an essential part of the basic assessment procedure. Such inventories provide useful information as to the degree of social handicap as a concomitant part of hearing impairment. Serial communication inventories serve as indicators of the effectiveness of habilitative or rehabilitative programs designed to enhance communicative skills. The term “basic” is applied to indicate a routine assessment of auditory function. Basic assessment does not provide the preponderance of clinical evidence needed to determine the site of injury or disease or to suggest its medical or surgical management.

Another service associated with audiology is an extended evaluation of the auditory system, which is composed of all anatomical structures that contribute to human hearing. Such an evaluation may include the determination of air conduction, bone conduction, and speech thresholds, as well as the administration of word and sentence recognition tests. Air conduction tests are performed by placing calibrated headphones over the patient’s ears and presenting a broad range of discrete frequencies. In practice, that frequency range extends from 250 to 8,000 hertz. Even though the normal human ear is capable of perceiving a much broader frequency range (from 20 hertz to 20,000 hertz), the range between 250 hertz and 8,000 hertz contains all the essential frequencies needed to understand speech. Bone conduction thresholds are determined by placing a vibrator, or bone oscillator, at the mastoid bone and presenting the same frequency range. Often, differences in the patient’s response to air-conducted and bone-conducted signals provide essential diagnostic information and suggest the site of injury or disease.

Speech threshold and word or sentence evaluations provide the clinician with performance scores that indicate the degree to which speech understanding has been compromised by the hearing disorder. Such measurements also indicate the probability of understanding connected discourse in communicative situations. There are a number of speech tests that provide information about the status of the auditory system. The most commonly used speech stimuli are two-syllable words to determine an individual’s speech reception threshold and one-syllable words to assess the auditory system’s discrimination function.

Another standard audiological practice is a comprehensive behavioral evaluation to determine the sensorineural site of lesion, that is, the place in the auditory system from which the hearing disorder originates. For most hearing disorders affecting auditory performance, it is critical that this site be located. It may be found in the peripheral system (the cochlea), which contains specialized sensory tissue that responds to sound pressure changes. The problem could also lie in the ascending auditory pathway, including its terminal projection in the auditory cortex of the brain. To arrive at an accurate diagnosis, the audiologist employs a number of advanced tests, such as sophisticated acoustic reflex tests, tests of frequency discrimination (the ability to detect differences between two or more signals), tests of intensity discrimination, and tests of auditory adaptation. The latter is a clinical procedure in which one determines whether a continuous sound decays over time to the point of inaudibility; such abnormal decay of the test signal indicates possible malfunction of the neural pathway of the auditory system. The results of these several tests increase significantly the probability that the site of lesion can be found.

One of the most promising clinical advances in audiology has been the development of evoked response audiometry (ERA). ERA is best defined as the measurement of neuroelectrical activity generated in the brain stem or of higher orders of brain function elicited by an acoustic signal. Acoustic signals, clicks, and tone pips are submitted to the external auditory ear. If the signals are detected by the auditory system, there is a change in neuroelectrical activity for each signal presented. A computer stores these minute changes in activity. When a sufficient number of acoustic signals have been processed, the computer prints out a response pattern consistent with the transmission of the electrical response from cochlear and subsequent responses as the signal travels to the brain. Response patterns have been classified as first (from the cochlea, zero to two milliseconds), fast (from the acoustic nerve and auditory brain stem, two to ten milliseconds), slow (from the primary and secondary areas of the cerebral cortex, fifty to three hundred milliseconds), and late (from the primary and associated areas of the cerebral cortex, more than three hundred milliseconds). More recent terminology of these time-related events refers to them as early, middle, and late responses.

Evoked potential measurement is significant because it offers a method of auditory assessment for those patients unwilling or unable to give reliable voluntary responses to acoustic stimuli. For example, evoked response audiometry provides a means of detecting hearing impairment in the neonate and very young. It also provides a clinical method of determining normal or abnormal hearing function for those who are intellectually disabled. Evoked responses to acoustic stimuli aid in the diagnosis of various types of tumors or neuromas that affect the transmission of auditory signals to the brain. If such lesion sites are detected early, it may be possible to remove them surgically and save the patient’s hearing. Certainly, early detection of retrocochlear pathology increases the probability that surgical intervention will preserve auditory system performance.

In 1978, D. T. Kemp published a germinal paper identifying the presence of otoacoustic emissions. Spontaneous emissions are generated within the cochlea and can be measured by a probe microphone assembly inserted into the external ear canal. Not all individuals have spontaneous otoacoustic emissions that can be measured by current probe microphone systems. Evoked otoacoustic emissions can be measured, however, in individuals having normal hearing or hearing loss of no more than forty to forty-five decibels. Such emissions are evoked by presenting a series of clicks or other compatible acoustic stimuli to the patient’s auditory system. The cochleomechanical activity induced by these acoustic signals is “picked up” by a probe microphone and processed by a computer. The graphic information obtained has proven to be of significant benefit in the screening of neonates and the very young. The literature would seem to suggest that otoacoustic emission measurement is fast, reliable, and repeatable. Research is under way to assess the range of losses that can be measured and the most appropriate stimuli to be employed in order to gain specific bits of information about cochlear behavior.

Auditory prosthetic evaluations have become common practice in audiology. When tests for hearing function determine that a hearing impairment exists, medical referral is mandatory for appropriate clinical management. For a sizable number of hearing-impaired individuals, however, medical or surgical intervention will not alter the hearing loss. For cases in which hearing impairment is a permanent sensorineural condition, a hearing aid or assistive listening device (or both) is often the preferred treatment modality in the rehabilitative process. To determine the appropriate electroacoustic characteristics of the hearing aid device to compensate best for the hearing deficit, special tests are conducted. Such tests may measure differences in word and speech understanding with and without the hearing aid. Another important test measures the degree of comfort or discomfort resulting from the sound level that is produced by the hearing aid device; if such a test is not performed, the patient may reject the hearing aid because it is too loud and unpleasant. Other tests designed to help determine the appropriate level of amplified sound involve narrow-band noise thresholds and various environmental sounds to which the patient may be periodically exposed during the activities of daily living. The use of environmental sound recordings provides the audiologist with objective indications of the electroacoustic responses that will yield maximum speech discrimination in the presence of specific background noises.

Audiology has helped many children and adults with hearing impairment through the use of hearing aids and the practice of aural rehabilitation. The selection and fitting of hearing aid devices have become important parts of the professional responsibilities of the clinical audiologist. With the many advances in hearing aid technology that have occurred, the audiologist has been given a much broader array of electroacoustic devices from which to select the one that offers the best correspondence with a patient’s needs. For example, some commercial hearing aid systems can be digitally programmed to meet the specific acoustic requirements of the hearing-impaired individual. In some cases, programmable hearing aids provide more than one acoustic response at the immediate command of the user. Should the device fail to meet the acoustic requirements of the patient, it can be reprogrammed in a short period of time to achieve a better correlation with the patient’s need for amplification.

Auditory (aural) rehabilitation is also of clinical concern to the audiologist, and a major branch of audiological practice is in this field. In this context, rehabilitation refers to the development and conduct of special programs to assist the hearing impaired in using and understanding verbal language (speech) more efficiently. For example, for those children born with a severe auditory deficit, the early introduction of aural rehabilitation programs is of paramount importance to the development of verbal language. Consistent with the development of rehabilitation programs is the early detection of hearing impairment that cannot be ameliorated by surgical or medical intervention. From a rehabilitative point of view, early introduction of hearing aid amplification and supportive auditory and speech-reading training programs have been of inestimable value in speech and language development for the hearing-impaired child. In some of the major school systems throughout the United States, there is an “educational audiologist” whose task it is to develop and maintain special programs intended to assist hearing-impaired children.

Equally as important are auditory rehabilitation programs for the hearing-impaired adult. Hearing impairment is a rather insidious phenomenon, gradually worsening over time. Consequently, adult patients are somewhat unaware of hearing loss until they fail to recognize enough of speech sounds to understand intended messages. When speech understanding has been degraded by hearing impairment, rehabilitative programs stress the use of hearing aid amplification and the value of speech reading. Training programs may assist the adult in learning speech-reading skills or in adapting to a hearing aid device. It is important that the audiologist be aware of attitudes or behaviors that may restrict or limit a patient’s acceptance of and participation in programs designed to assist the hearing impaired.

Perspective and Prospects

Audiology, as a recognized academic discipline, originated during World War II. At that time, thousands of military personnel needed diagnostic and rehabilitative services for ear injuries incurred during active service. It was essential that an organized program be developed to meet the demand. Several military hospitals and selected universities and colleges undertook the task of developing programs to accomplish these diagnostic and rehabilitative tasks. One of the first textbooks dealing exclusively with audiological practice was authored by Hayes Newby in 1958, while he was teaching at Stanford University. Since that memorable introduction, hundreds of special texts have been published relative to various aspects of audiological practice.

Since the early pioneering days during World War II, the field of audiology and the clinical skills of audiologists have expanded appreciably. Significant advances in auditory disorder diagnosis and in prosthetic and rehabilitative care have been made. Although audiology is a relatively new academic and professional discipline, its contributions to the understanding and treatment of auditory system disorders have greatly advanced the understanding of its role in human communication.

Bibliography

Alpiner, Jerome G., and Patricia A. McCarthy, eds. Rehabilitative Audiology: Children and Adults. 3d ed. Philadelphia: Lippincott Williams & Wilkins, 2000.

Gelfand, Stanley A. Essentials of Audiology. 3d ed. New York: Thieme, 2009.

Hall, James W. Introduction to Audiology. Boston: Allyn & Bacon, 2013.

Jerger, James, ed. Pediatric Audiology. San Diego, Calif.: College-Hill Press, 1984.

Katz, Jack, ed. Handbook of Clinical Audiology. 6th ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins, 2009.

Lubinski, Rosemary, and Carol M. Frattali, eds. Professional Issues in Speech-Language Pathology and Audiology. San Diego, Calif.: Thomson/Delmar Learning, 2007.

Madell, Jane R., and Carol Flexer. Pediatric Audiology Casebook. New York: Thieme, 2011.

Mendel, Lisa Lucks, Jeffrey L. Danhauer, and Sadanand Singh. Singular’s Illustrated Dictionary of Audiology. San Diego, Calif.: Singular, 1999.

Newby, Hayes, and Gerald Popelka. Audiology. 6th ed. Englewood Cliffs, N.J.: Prentice Hall, 1992.

Roeser, Ross J. Roeser's Audiology Desk Reference. 2d ed. New York: Thieme, 2013.