Учебники / Otolaryngology - Basic Science and Clinical Review

of a typical communication system. These two reports were instrumental in the awareness that a person’s language was dependent on both intrinsic and extrinsic factors and the beginning of the concept of a critical/ sensitive period in language development.

CRITICAL PERIODS

The need for intrinsic brain mechanisms and external linguistic input and the time frame of critical periods have been further quantified by several controlled and/or prospective human studies performed over the past 3 decades.The acquisition of language appears to be a progression that first involves phonology (the sound of words), followed by semantics (the meaning of words) and syntax (the rules of grammar).

The ability of the person to select from the cacophony of environmental sounds that are salient for language is a developmental process that begins at least by the sixth month of fetal life and approaches maturity by the end of the first postpartum year of life. The fetus develops in a sound-attenuated environment—the womb—with audible frequencies between 30 and 40 dB. The ability of the fetus to react to sound has been demonstrated by observing through ultrasound eye blinks to pulsed tones at 28 weeks gestation, as well as by noting electrocardiogram changes with sound stimuli.This gives rise to the question, Do such physiological responses in any way reflect, or themselves create, a shaping of the fetal central nervous system (CNS) with regard to preferential sound perception for the language to which they are exposed in utero? Simply stated, Do the mother’s voice and native language sounds have any advantage over all other sounds at birth? The answer is yes.The newborn prefers the mother’s voice to other female voices and the mother’s voice to a male voice. It has further been shown that the newborn will recognize sounds and patterns heard in utero. Other studies (Jusczyk et al. [1988] and Mehler et al. [1988]) have demonstrated that infants prefer the unique sounds of their mother’s language to foreign languages. One study was performed on 2-day-old infants and the other at 2 months of age. Collectively, these data indicate that there is intrauterine phonemic learning.

During the first year of life, the infant evolves from a relatively poorly differentiating language receptor to one exquisitely tuned to the unique sounds—phonemes—of his or her native language.This has been shown in several studies demonstrating that infants from birth to 4 months are able to discriminate the sounds of all languages, whereas at the end of the first year they no longer

discriminate the foreign sounds but only those of their native language. This is illustrated by two studies from Werker and colleagues (1981, 1990), who showed this using English and Japanese in one group, and Hindi and Salish in another. All of the younger infants were able to distinguish all of the phonemes, but at 1 year of age the children raised in an English (Canadian) environment had lost their ability to distinguish the Japanese, and those raised in a Japanese environment had lost their ability to discriminate the English phonemes; the same results found for those raised in the Hindi or Salish contexts.The basic foundation of language—phonology—is formed by the end of the first year of life.This process of going from many to few (specialization) parallels the known neuroanatomical process of the reduction of dendrites in the developing CNS.

There are limited data as to the time dimensions of the critical/sensitive period(s) for semantics and syntax from a group of neurophysiological studies conducted by Neville et al (1992, 1997).The evidence is based on the achievement of a fixed (i.e., mature) neurophysiological response pattern. It is assumed that when the neurophysiological pattern becomes fixed, the plasticity is reduced or extinguished, and this decrease in neurophysiological plasticity is considered as the end of the critical/sensitive period. This is a tenuous set of assumptions based on few data and serves only as an approximate time frame for the end of the critical/sensitive period(s) of semantics and syntax.

Neville and colleagues recorded event-related brain potentials (ERPs) to words in hearing children, adolescents, and adults. They used two types of words. The first type is categorized as “open class” words, which consist of nouns, verbs, and adjectives that make reference to specific objects and events.These words are considered as semantic.The second group, “closed class” words, are articles, conjunctions, and auxiliaries. In the English language, these “closed class” words are part of the basis for the grammatical structures and may be thought of as an approximation of syntax.

Open class words (semantic representation) evoke in the normal adult a negative ERP occurring at 350 ms after the stimulus of the presentation of the word. The response is called the N350. The adult N350 is a large ERP found over the posterior brain regions of both hemispheres.The ERP to closed class words (syntax) in the normal adult occurs 280 ms after the stimulus. It is called the N280 and is found in the anterior temporal region of the left hemisphere.

Developmental studies found that, at 4 years of age, the youngest age tested, there were adult-like ERPs (N350) to open class/semantic stimuli. These N350 responses were robust and located at the posterior

hemispheres.The N280 ERP, the response to the closed class/syntactic words, has a different ontogeny. The N280 does not achieve its mature, or final, configuration until 15 to 16 years of age.These data suggest that, based on ERPs, the critical period for semantic organization may occur before the fourth year of life, whereas the critical period of the assessed portion of syntax as evidenced by the closed class words may not occur until age 15 to 16.

Three aspects of aural language structure— phonology, semantics, and syntax—may have different time periods for optimal shaping of the CNS (i.e., the critical/sensitive period). The data suggest that the earliest specialization is phonological, and at the end of the first year the repertoire of phonemes that can be discriminated is limited. Phonological specialization comes before semantic specialization, and it is only suggested the critical period for phonological specialization may occur before 4 years of life.The last portion of the language structure to be set is syntactical, which will not be fixed until the late teens.

Although these time dimensions are approximations and certainly will be supplanted by more accurate and precise measures in the years to come, they do describe several important aspects of critical/sensitive period(s) for language acquisition. The concept/aspect is that different parts of language are governed by different biological constraints. Language does not happen in an instant, and full development is, in part, a time-dependent process.

A second concept is that different portions of the CNS, in a normal person, mediate different aspects of language. Up to now the investigation of language functions of the CNS have been explored based on the behavioral descriptions of language. These behavioral studies should produce information so that language could begin to be conceptualized based on the neurological mechanisms that underlie the process.The classification of language components based on the concepts of phonology, semantics, and syntax may or may not be the model for the neuroanatomical and physiological mechanisms that mediate the complex process called language.

Third, there appears to be a hierarchal sequencing of language development. The basis of language is the earliest restriction, in the case of an aural language, phonology. If there is little or no early phonological input, then the resultant semantic and syntactical portions cannot or do not develop optimally. The maturation of the semantics and syntax can be considered as being dependent upon early sensory input. It is well documented

that either early auditory or visual linguistically organized sensory input will result in the development of sophisticated (i.e., functional) language. Much less is known about the informational characteristics of visual language. Are there any studies of the critical period of visual “phonology”? I know of no study that has looked to see if there would be the same restriction of aspects of the visual signs as in the phonological restrictions found at the end of 12 months in aural language. Are there signing characteristics that are found in American sign language (ASL) and not in Japanese sign language (JSL), and vice versa? Would an ASL child not be able to categorize the JSL at 12 months of age? It is known that if a child is given some sensory bases of language during the first 2 years of life, the semantic and syntactic aspects of that language develop into a useful and sophisticated communication system (effective language).

Neville and colleagues showed that deaf adults had normal N350, the ERP to open class/semantic words. However, these early-deafened adults, on the whole, lacked the N280 ERP to the closed set/syntactical words. It is unclear as to how, where, or even if syntax was being performed in these deaf adults. The few deaf adults who did have good grammatical judgment did have the expected left hemisphere N280s. This may indicate that the syntactical dimension may be an integral aspect of the left hemisphere specialization.

A recent series of retrospective studies byYoshinagaItano (1998) examined the language outcomes in two groups of hearing-impaired children.The first group had a diagnosis and intervention with some form of language intervention before 6 months of age, and the second group had intervention and diagnosis when they were older than 6 months of age. Children who were diagnosed before 6 months of age had superior but not normal expressive and receptive language skills as compared with those who were diagnosed at the older age (i.e., greater than 6 months).These data strongly suggest that the earlier that language is instituted, the better the language outcome is.The researchers noted that the children with interventions before 6 months of age had a variety of interventions, including hearing aids, total communication, and/or sign language. Future types of linguistic input—auditory and/or visual—did not make any difference in the outcome. It has been documented that deaf children whose language is based solely on signs, on the average, will develop their first words somewhat sooner than the average hearing child exposed to normal aural language. Word order and morphology occur at the same age in both the sign-reared deaf and the aural-

reared hearing child. These observations show that linguistic input other than hearing will allow for the development of language.These observations suggest that the early formation of a language base is not dependent upon any particular sensory modality, but if the information is appropriately coded, language can develop from a sensory input that is not auditory.There is further evidence that this may occur from the education of the deaf blind in which the linguistic input is that of touch. Many of these patients, such as Laura Bridgman and Helen Keller, have had 1 or 2 years of aural language before they became deaf and blind. However, in both of these women and many others, the use of touch was sufficient to allow for complex linguistic development.That there are different sensory inputs such as auditory, visual, and tactile that can create bases for semantics and syntax indicates that language as described by semantics and syntax is not dependent upon any particular sensory form of input, but is dependent upon some form of linguistically organized sensory input.

There has been and continues to be controversy that a language initially learned with one sensory modality may not be transferable to one that is based on another sensory modality. More directly, a child learns sign language who is thought by many not to be able to develop optimal aural language. The establishment of language based on one sensory modality does appear to allow for the utilization of other sensory systems for the expression and reception of language, as has been shown.A study by Dee et al. (1982) performed some years ago with a group of, by today’s standards, late-diagnosed deaf children in a total communication program found that 11 of the 12 developed speech and a few, for the time when this was done, excellent aural language. The 12th child had a tracheotomy and a cleft palate and was avocal.

CENTRAL NERVOUS

SYSTEM PLASTICITY

The ability of various sensory inputs to allow for linguistic development suggests that the central nervous system is, to some extent, flexible (plastic) in regard to the formation and accomplishment of language. We have seen that interference with the establishment of the components of language, especially phonology in spoken language, results in substantial and apparently irrevocable linguistic deficits. Has nature created such a rigid structure that there is no redundancy or ability to compensate for deficiency? What is, if any, the ability of the CNS to compensate for defects? How plastic is the CNS for language?

There are numerous studies (Sharma et al. [1982] and van Melcher et al. [2000]) that demonstrate the plasticity of the CNS in response to a loss of a portion of the cochlear.The CNS has the ability to “cannibalize” the unused portion of the nervous system to respond to the remaining cochlear inputs. One of the more dramatic and recent has been a study in immature ferrets in which the visual systems were implanted into the auditory system in the newborn ferret.These ferrets grew up and saw with their auditory cortex.

The studies of Knudsen et al (1990, 2000) of sound localization in the barn owl are more pertinent to our interest in oral language.The researchers placed optical prisms on the eyes of newborn owls, resulting in a visual perversion of space, and found that the auditory CNS would reorganize itself to compensate for the distortion of vision. When they did the same to the mature owl, there was no compensation.The older owls’ CNS could not adjust. These data would indicate that there was a very sharp critical period for the organization of the owls’ auditory system, which was dependent upon visual and acoustic input. The study of the mature owl was carried a little further.The researchers fitted the mature owls with prisms that had very little distortion. The mature owl and its CNS compensated for this lesser perversion of visual space. The owls were then fitted with another prism that changed the visual field a little more and more progressively until the mature owls now had their visual field change so much that neither of the newborn nor the mature owl could compensate in one step.This is evidence of a quantitative change in plasticity in contrast to an all-or-none conceptualization of the process. It would appear that the owls’ CNS maintained an attenuated plasticity.

King and colleagues (2000, 2001) have performed parallel studies, plugging one external auditory canal in a young ferret, causing a perversion of auditory space. They have found that the young ferret and its CNS would compensate for the “misinformation” about auditory space.This was evidenced by anatomical and physiological changes in the auditory CNS of the young ferret.When the same was done to the mature ferret, like the mature owl, they did not compensate. However, when given many trials, the adult ferrets recognized their CNS and were able to complete the task. Both of these studies suggest that the auditory CNS remains plastic throughout life, with a diminution of the amount of change it is able to accomplish as the organism ages.

The limits of plasticity or critical period for language are suggested but not directly addressed by the aforementioned studies. An indication of the human CNS’s

linguistic plasticity is to be found in the studies of young children who have had removal of either their right or left cerebral hemisphere. There are several indications for this procedure of which the hemispherectomies performed. One of these, Rasmussen’s syndrome, is a unilateral progressive inflammatory disease characterized by normal development and late-onset seizures between 6 and 13 years of age. It allows for a more homogeneous population in which the possibility of the utilization of unusual brain areas for language may be studied. Several case studies have shown that righthanded children with left hemispherectomies recover and continue to develop linguistically. Their recovery recapitulates the usual development sequences: first, receptive skills are evident, followed in time by expressive language. Most of these children are able to carry on a conversation despite the loss of their left brain. It is not known whether the right brain was able to be the language center and is not used because of a preference for the left brain, or if the right brain is able to adapt so as to carry out language function.There are other conjectures, but the observations remain that the child’s CNS is able to compensate for the severe injury.There is ability, given the circumstance of age and/or situation, for the CNS to adapt to change so that the person may be linguistically competent.

SUMMARY

The concept of a “critical/sensitive” period has undergone change in the past 2 decades. Today we must consider that the normal ontogeny for optimal language is dependent upon the institution of some form of linguistic input before the first 6 months of life.The CNS has an ability to adapt itself to the sensory changes—dif- ferent or perverted inputs and/or intrinsic loss, such as with a hemispherectomy—that appears to be inversely proportional to age. This plastic tenet of the CNS does not entirely close down.The next frontier, the biology of language, will be to learn how to use this plastic ability to optimize the linguistic ability of each individual.

SUGGESTED READINGS

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SELF-TEST QUESTIONS

For each question select the correct answer from the lettered alternatives that follow.To check your answers, see Answers to Self-Tests on page 716.

1.Language development can progress

A.Only if there is hearing

B.If there is an organized sensory linguistic input

C.If the child is raised in isolation

2.The order of linguistic development is

A.Syntax, phonology, semantics

B.Phonology, syntax, semantics

C.Phonology, semantics, syntax

3.There is evidence that the sensitive/critical period for language acquisition

A.Is over by 18 months of age

B.Does not exist

C.Diminishes with age

Audiometry is concerned with the measurement of hearing and auditory function.Audiology, being broader in scope, encompasses not only the evaluation of hearing and its disorders, but also the myriad skills and techniques involved in hearing health care such as habilitation/rehabilitation of hearing loss, identification of hearing loss in newborn infants, the selection and fitting of hearing aids, evaluation and postsurgical management of patients with cochlear implants, assessment of vestibular function, and prevention of hearing loss through programs of hearing conservation.

Audiometric assessment may be undertaken for a variety of reasons. In many cases it is aimed at quantifying the amount of usable hearing so as to determine whether a person’s communicative abilities are at risk and, if so, whether they would benefit from a sensory device such as a hearing aid or cochlear implant. In other cases, assessing the functionality of the auditory system focuses more specifically on how intact that system is, or how well a particular component of that system is working. For example, the question of interest may be whether surgery has been successful in restoring mobility to the middle ear system. Because audiometry is multifaceted, encompassing procedures aimed at both an assessment of hearing and an assessment of auditory function, a useful framework in which to describe audiometry is to

distinguish between subjective and objective procedures. Subjective procedures are based on behavioral responses from the listener and, in most cases, are the preferred methods to determine that a sound has indeed been heard. Objective procedures, on the other hand, do not rely on behavioral responses but instead use various physiological test procedures to measure the integrity or functionality of various components of the auditory system.The audiologist plays a critical role in the assessment process by selecting and applying test procedures appropriate to a given situation. Often this requires the application of a test battery, permitting a systematic “cross-check” of clinical findings based on a combination of behavioral and objective measures. This chapter will describe a general framework of audiometric indices, beginning with a discussion of behavioral assessment and then describing various objective procedures.

BEHAVIORAL ASSESSMENT

OF HEARING

PURE-TONE AUDIOMETRY

Pure-tone audiometry is designed to measure thresholds of detection for pure-tone signals presented via air or bone conduction. Air conduction audiometry involves

the presentation of test stimuli from an earphone, an insert receiver, or a loudspeaker, whereas bone conduction audiometry refers to the presentation of signals through a bone vibrator, usually placed behind the ear on the mastoid process.Testing is generally performed at octave intervals from 250 to 8000 Hz for air conduction and 250 to 4000 Hz for bone conduction. Detection thresholds are graphically displayed as an audiogram that plots threshold levels in decibels hearing level (dB HL) as a function of frequency using standard symbols (see Fig. 30-1). Degree of hearing loss can be summarized

as the pure-tone average, based on the average of the air conduction thresholds at 500 Hz, 1000 Hz, and 2000 Hz. Descriptive terms commonly used to categorize hearing levels include normal (0–15 dB HL), borderline normal (16–25 dB HL), mild loss (26–45 dB HL), moderate loss (46–75 dB HL), severe loss (76–90 dB HL), and profound loss ( 90 dB HL). Borderline categories of hearing impairment are often described with a combination of terms, such as mild to moderate, as are sloping configurations due to differences in hearing sensitivity across the frequency range.

Methods used to obtain the behavioral pure-tone audiogram vary depending on the patient’s age and developmental status. Adults and children at a developmental age of 5 years raise their hand or press a response button when the tone is heard; toddlers require a play audiometry task, and infants from 6 to30 months require an operant conditioning procedure known as visual reinforcement audiometry (VRA). For infants at a developmental age less than 6 months, behavioral test procedures are not considered valid; thus hearing status must be inferred from objective measures described later in this chapter.

When pure-tone air conduction thresholds are abnormally elevated, bone conduction thresholds are obtained to differentiate between sound transmission problems lateral to the inner ear and dysfunctions of the auditory system in or medial to the inner ear. Conductive hearing loss is characterized by normal or near normal bone conduction thresholds in the presence of elevated air conduction thresholds (Fig. 30-1A). When air and bone conduction thresholds are equally elevated, the loss is described as sensorineural (Fig. 30-1B).A mixed hearing loss (Fig. 30-1C) is characterized by abnormal responses to both air conduction and bone conduction signals, with air conduction thresholds being poorer than bone conduction thresholds. It should be noted that some middle ear pathologies, in addition to resulting in conductive hearing losses, may show elevated bone conduction responses at or around 2000 Hz due to loss of the normal middle ear participation in the bone conduction response. This pattern of results, described as Carhart’s notch, can be diagnostically relevant in the context of reviewing patient candidacy for middle ear surgery. In such cases a more accurate estimate of sensory loss can be achieved using a forehead placement of the bone vibrator, because the effects of middle ear pathology on the bone conduction response are diminished using frontal placement. It should be noted that adjustments in calibration are needed for forehead placement.

Pure-tone audiometry may be complicated by transmission of test signals across the skull to the nontest ear. The audiologist isolates the test ear by applying masking noise to the nontest ear. Because diagnosis and treatment are often influenced by the audiometric findings, accurate use of masking noise is imperative.

PEDIATRIC MODIFICATIONS IN

PURE-TONE AUDIOMETRY

An operant conditioning procedure, based on reinforcement of head-turn responses, is useful for the behavioral

assessment of hearing in infants beginning at a developmental level of 6 months.This technique, called visual reinforcement audiometry, involves placing the infant in a calibrated sound room while test signals (frequency modulated pure tones) are presented from a loudspeaker. An assistant is seated across from the child to maintain attention away from the loudspeaker and a reinforcer (illuminated mechanical toy in a smoked-glass enclosure). Once the infant is conditioned, stimuli are presented at lower levels to determine an approximate threshold. Testing can be performed with stimuli presented from a loudspeaker or via insert receivers.When test signals are delivered from a loudspeaker, the responses are considered an indication of “better ear” sensitivity for each test frequency. Responses obtained via insert receivers permit evaluation of each ear separately. The VRA technique also can be used to obtain bone conduction thresholds.

When the child reaches a developmental level of30 months, it is generally feasible to conduct manual pure-tone “play” audiometry.The response task for older children (5 years) involves raising a hand or pressing a response button when the tone is heard; younger children (30 months–4 years) respond by engaging in some age-appropriate play activity, such as dropping a block in a container or placing a peg in a board. Play audiometry is usually performed under earphones (or with insert receivers) so that each ear can be evaluated separately. Bone conduction thresholds are obtained in the same manner.

SPEECH AUDIOMETRY

Routine speech audiometry consists of speech threshold measures and speech recognition measures. The speech threshold test, also known as the speech reception threshold (SRT), is performed using two-syllable spondee words that have equal emphasis on both syllables (e.g., baseball). SRTs are obtained in a manner similar to pure-tone threshold assessment. Speech recognition measures, more commonly referred to as speech discrimination tests, are typically performed using lists of prerecorded single-syllable words presented at a fixed level above threshold.To illustrate, a patient with a mild hearing loss and 35 dB HL pure-tone average would be expected to have an SRT of 35 dB HL because the SRT is normally in general agreement with the pure-tone average (average of thresholds at 500, 1000, and 2000 Hz). The speech discrimination test would then be administered at a level 40 dB above the SRT (i.e., at a 40 dB “sensation level”), which, in this example, would be at 75 dB HL.The SRT serves as a cross-check on the validity

of the pure-tone responses as well as a reference level for speech discrimination testing.The speech discrimination test provides a qualitative measure of understanding for speech in quiet, delivered at or near a comfortable level above threshold; the score is expressed in percent correct for each ear.Taken together, these measures are useful for diagnostic purposes and in the planning of rehabilitation (e.g., hearing aid use). In addition to single words presented in a quiet background, many other speech recognition measures have been developed for diagnostic and rehabilitative applications.

OBJECTIVE MEASURES OF

AUDITORY FUNCTION

IMMITTANCE AUDIOMETRY

The assessment of middle ear function requires a battery of procedures that measure the ease with which a sound is transmitted through the conductive mechanism. This battery, which includes tympanometry and a series of related measures, provides information concerning the mobility of the tympanic membrane and middle ear system, middle ear pressure, and equivalent volume estimates. Each of these indices may be associated with various middle ear abnormalities.

Tympanometry begins with a sealed probe tip in the ear canal. The probe assembly provides both a sound source and a measuring microphone. As the air pressure in the ear canal is varied by the tympanometer, the associated movements of the eardrum and middle ear system are detected as amplitude and phase changes relative to a constant low-frequency probe tone (200/226 Hz) measured from a microphone in the probe assembly. These changes are due to alterations in the physical impedance to the sound at the plane of the tympanic membrane, as positive and negative pressure causes changes in the stiffness of the tympanic membrane. In essence, they provide a measure of the ease with which the probe tone is admitted through the tympanic membrane and middle

Figure 30-2 Measures derived from the standard (226 Hz) tympanogram.

ear system. When plotted as a function of the relative pressure in the sealed ear canal, the admittance changes result in a graphic representation known as the tympanogram, which may be thought of as an “objective” form of pneumatic otoscopy.A complete acoustic immittance battery includes measures of tympanometric peak pressure, tympanometric shape (width), static admittance, and estimates of equivalent ear canal volume, all of which are derived from the tympanogram (Fig. 30-2).

Tympanometric peak pressure, the pressure in the external canal when static admittance is highest, is measured in decapascals (daPa) and provides an estimate of middle ear pressure. As illustrated in Fig. 30-3A, a normal tympanogram will have a peak that approximates ambient air pressure ( 50 150 daPa).Tympanometry performed on an ear with middle ear effusion typically results in a “flat” tympanogram (immobile or noncompliant; Fig. 30-3B).The tympanogram shown in Fig. 30-3C has a pressure peak at approximately 240 daPa, meaning that middle ear admittance was maximum when the tympanometer created negative pressure ( 240 daPa) in the ear canal (i.e., equal pressure on either side of the tympanic membrane). In this case, it can be assumed that the middle ear is characterized by negative pressure, presumably due to insufficient aeration of the middle ear space. It should be noted, however, that changes in middle ear pressure are caused by a complex interaction of many factors. Consequently, tympanometric peak pressure is of limited value in the detection of middle ear dysfunction.

Static measures of tympanic membrane mobility also can be obtained. Static admittance, which is measured in acoustic millimhos (mmho), is calculated by measuring the height of the tympanometric peak and comparing it to one of its tail values. Patients with otosclerosis or ossicular fixation usually show reduced middle ear mobility, evidenced by low static admittance.The resulting tympanogram often has a shallow peak with normal middle ear pressure. Static admittance measures are useful for detecting middle ear effusion. For children 3 to

Figure 30-3 Tympanometric patterns commonly associated with (A) normal middle ear function, (B) middle ear effusion, (C) negative tympanometric peak (middle ear) pressure, and (D) an abnormally wide configuration that may or may not be associated with middle ear disease.

10 years of age with no history of chronic or recurrent otitis media with effusion (OME), mean values are typically 0.5 mmho with a 90% range of 0.3 to 1.0 mmho. Infants and toddlers, especially those with significant OME histories, usually demonstrate lower peak static admittance, even when middle ear effusion is not present. Because middle ear effusion reduces the mobility of the middle ear mechanism, a tympanogram associated with OME is typically characterized by low static admittance (0–0.2 mmho).The flat tympanogram in Fig. 30-3B is typical of that seen in a child with OME.

The shape of the tympanogram can be altered by middle ear effusion even when a pressure peak is identifiable. For most screening instruments, tympanometric width is reported as the pressure interval (in daPa) corresponding with a 50% reduction in static admittance (see Fig. 30-2).Tympanometric width has proven to be useful in the detection of middle ear effusion.The tympanogram is usually flat when middle ear effusion is present, but wide tympanograms (e.g., 200 daPa) may also be associated with middle ear effusion. The tympanogram in Fig. 30-3D is abnormally wide. Such a tympanogram may occur when middle ear effusion occupies a portion of the middle ear space while still allowing some aeration of the middle ear.

Estimates of equivalent ear canal volume are useful in identifying a perforation of the tympanic membrane or confirming tympanostomy tube patency. In either case the middle ear space is added to the ear canal space, resulting in an abnormally large equivalent volume (e.g., 2.5 cc). Estimates of equivalent ear canal volume are especially useful in the interpretation of “flat” tympanograms. Flat tympanograms may be due to (1) perforation of the tympanic membrane, a condition that would result in an abnormally large equivalent canal volume;(2) OME,which would result in a normal equivalent canal volume; or (3) occlusion of the probe tip (cerumen or canal obstruction), which would show abnormally small equivalent volume.

The acoustic reflex occurs when a high-intensity sound causes contraction of the stapedius muscle, momentarily restricting the transmission of low-frequency sound through the middle ear due to the stiffening in the middle ear.When used clinically, the acoustic reflex is elicited by a pure tone delivered via the tympanometer probe assembly. Instruments designed for middle ear assessment typically deliver ipsilateral or contralateral activating stimuli. The acoustic reflex is generally absent in an ear with middle ear dysfunction because the conductive hearing loss reduces the level of the acoustic stimulus reaching the inner ear and/or because the stiffness caused by the middle ear disorder prevents its measurement. Inclusion of the acoustic reflex in acoustic immittance screening batteries has been shown by some studies to improve the identification of ears with otitis media, but this often occurs at the expense of lower specificity (i.e., a higher number of false-positive referrals).

The acoustic reflex has many clinical applications. In the 1970s and 1980s, the acoustic reflex was used to screen for retrocochlear disease. Since then, it has been replaced by tests with better predictive value. It is still useful, however, as a test of cranial nerve (CN) VII and VIII function. It is also useful in the context of screening for middle ear effusion when the tympanogram is ambiguous. A flat tympanogram is expected when middle ear effusion is present, but as noted earlier, middle ear effusion can be present even when the tympanogram has an identifiable peak, especially in the case of wide tympanograms (Fig. 30-3D). Because the acoustic reflex will almost always be absent when middle ear fluid is present, the acoustic reflex can assist in differentiating wide tympanograms associated with effusion from those without effusion.

OTOACOUSTIC EMISSIONS

Basis of OAEs

The inner ear, or cochlea, is characterized by an active mechanism that has the effect of amplifying low-level