Otologic and Neurotologic Diagnostics and Tests

3 Otologic and Neurotologic Diagnostics and Tests


Physical Examination


General Otologic/Neurotologic Physical Examination


The Neurotologic Cranial Nerve Examination


Evaluation of Hearing


Clinical Evaluation


Behavioral Testing


Objective Audiometry


Assessment of Middle Ear Function


Otoacoustic Emissions


Auditory Brainstem Response


Other Auditory Evoked Potentials: Electrocochleography, Auditory Steady-State Responses, and Cortical Potentials


Evaluation of the Balance System


Clinical Evaluation


Basic Principles


Dix–Hallpike Test


Objective Balance Testing


Electronystagmography


Computerized Platform Posturography


Vestibular Evoked Myogenic Potentials


Evaluation of the Facial Nerve


Clinical Evaluation of the Facial Nerve


Objective Testing of the Facial Nerve


Imaging of the Temporal Bone


Conventional Radiography


Computed Tomography


Magnetic Resonance Imaging


Conventional MRI


Magnetic Resonance Angiography and Venography


Other Imaging Procedures


Digital Subtraction Angiography


Nuclear Medicine




Physical Examination


Examination of the neurotologic patient is a combination of a brief general physical and otolaryngologic examination as well as the formal neurotologic examination.


General Otologic/Neurotologic Physical Examination


Definition

A brief general examination should be performed in every patient with neurotologic pathology. Also, a complete otolaryngologic physical examination should be performed before the cranial nerve status can be assessed during the formal neurotologic examination.


Closely Related Topics

• External ear and eustachian tube—see p. 9


• Physiology of the middle ear—see p. 27


• Clinical evaluation (of hearing)—see p. 53


Introduction

Primarily, attention should be paid to the general appearance and mannerisms of the patient. A good deal of information regarding the patient’s general strength and gait can be obtained by simple observation. Auscultation of the chest and neck may yield valuable information regarding the patient’s general health.


Otolaryngologic Examination

The otolaryngologic examination should be performed once a baseline health status has been established. Inspection of the oral cavity, oral pharynx, and basic anterior rhinoscopy may provide useful clues regarding the etiology of neurotologic disease. Careful palpation of the bony and soft tissue structures should follow, including a lymph node examination, thyroid examination, and palpation of the scalp and temporomandibular joint. Flexible fiberoptic laryngoscopy can be performed when indicated, especially if lower cranial nerve pathology is suspected.


Formal Neurotologic Examination (Fig. 3.1A, B)

This part of the physical examination consists of inspection and palpation of both auricles, a binocular microscopic examination with appropriate speculae of the external auditory canal (EAC) and tympanic membrane (TM) (Fig. 3.1A, B), serial testing of the relevant cranial nerves, and special maneuvers when indicated by history and other physical examination findings described later in this chapter.


Auricle (Fig. 3.1C, D): The external anatomic survey consists of the careful inspection and palpation of the auricle and periauricular areas looking for malformations, indicators of trauma, inflammation, masses, otorrhea, ecchymosis, crepitance, or auricular proptosis.


External auditory canal: EAC and TM inspection can be initially approached with a handheld pneumatic otoscope. However, most neurotologic patients will require a binocular microscopic examination for definitive evaluation. This frees both hands of the examiner and allows for more precise manipulation of the patient’s head, auricle, and speculum as well as the safe removal of foreign bodies and cerumen. Attention should be paid to the color, texture, and size of the EAC looking for cerumen, foreign bodies, swelling of the soft tissues, infection, and masses within or intruding upon the canal. Figure 3.1E provides some common examples of EAC findings in the neuroto-logic patient.


Tympanic membrane: Examination of the TM includes identification of the normal anatomic landmarks as outlined in Chapter 2, pp. 9 and 12 and any abnormal findings. Figure 3.1F, G show examples of normal and abnormal findings seen in the neurotologic patient.


Recommended Reading

Adad B, Rasgon BM, Ackerson L. Relationship of the facial nerve to the tympanic annulus: a direct anatomic examination. Laryngoscope 1999;109(8):1189–1192


McDivitt K. The pediatric tympanic membrane: see it, describe it, treat it. ORL Head Neck Nurs 2003;21(3):14–17




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Fig. 3.1 A–D




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Fig. 3.1 E–G


The Neurotologic Cranial Nerve Examination


Definition

A thorough clinical cranial nerve examination is essential in the assessment of the neurotologic patient. Cranial nerve function provides important information necessary for proper diagnosis and management.


Closely Related Topics

• Anatomy of the IAC, jugular foramen, and petrous apex—see p. 19


• Peripheral nerves, CNS, and CSF—see p. 42


• Behavioral testing (of hearing)—see p. 55


• Clinical evaluation (of the balance system)—see p. 70


• Rehabilitation of cranial nerve paralyses—see p. 386


Introduction (Fig. 3.2A–C)

Figure 3.2A lists each cranial nerve (CN), its innervated structures, and basic testing principles. Sensory distributions, reflexes, and muscle innervations provided by each nerve should be tested as outlined in Fig. 3.2B. The basic reflexes are also outlined in Fig. 3.2C.


Cranial Nerves I, II, III, IV, VI (Fig. 3.2D–G)

Olfaction is a product of sensory input from cranial nerves I and V. Formal testing is generally not indicated. Isopropyl alcohol 70% may be wafted by the nose to test for basic detection. No detection at <1.5 meters (5 feet) defines anosmia. CN II examination includes inspection of the sclera, conjunctivae, visual fields, visual acuity, pupillary reflexes, and a funduscopic examination. Visual fields can be tested by having the patient and physician cover opposing eyes while presenting stimuli (wiggling a finger) into each of the main visual field quadrants (Fig. 3.2D). Pupillary reflexes are tested bilaterally with a light source for both the direct and consensual responses. Funduscopic examination of the retina is best accomplished by an ophthalmologist. Testing of CN III, IV, and VI consists of movement of each eye through the six cardinal motions outlined in Fig 3.2E with special attention to the presence and direction of any nystagmus. The forced duction test can help distinguish between true nerve palsy and mechanical interference of the normal range of motion. This test consists of grabbing the sclera of the anesthetized eye with a forceps and passively moving the eye through the six cardinal movements (Fig. 3.2E). Normal ROM indicates intrinsic muscular problems (e.g., atrophy) or nervous problems (e.g., nerve palsy or myasthenia gravis). CN III palsy leaves the eye deviated laterally and inferiorly (down and out) along with mydriasis and ptosis. CN VI palsy results in inability to laterally deviate the globe on the affected side. CN IV palsy results in extorsion of the eye and difficulty depressing the eye during medial gaze. The Bielschowsky test can help confirm a CN IV lesion: Tilting the head toward the side of the lesion worsens diplopia because of the loss of the inward torsional pull on the affected side (Fig. 3.2G).


Cranial Nerve V (Fig. 3.2A, B)

Cranial nerve V provides both afferent and efferent innervation via three main branches: the ophthalmic (V1), the maxillary (V2), and the mandibular (V3) branches (Fig. 3.2B). These combine to provide sensory input from the face and anterior neck. V3 also provides motor innervation to the muscles of mastication (the masseter, the temporalis, and the medial and lateral pterygoid muscles), the tensor veli palatini, the tensor tympani, the anterior belly of the digastric, and the mylohyoid muscle. Corneal reflexes can be tested as follows: The eye should be deviated medially and a cotton swab should be used to stimulate the reflex from the lateral to medial edges. Blinking should normally only occur with direct corneal stimulation (Fig. 3.2C).


Cranial Nerves VII and VIII (Fig. 3.2H)

Cranial nerve VII provides somatic efferent innervation to the muscles of facial expression, the stapedius muscle, and taste sensation from the anterior two-thirds of the tongue, and carries para-sympathetic fibers to the lacrimal, submandibular, and submental glands. Special consideration can be found on p. 386. CN VIII provides hearing and balance perceptions via its cochlear and vestibular branches. Clinical and objective testing is reviewed on p. 87. Figure 3.2H provides an anatomic overview over facial nerve function.


Lower Cranial Nerves IX, X, XI, and XII

Testing of lower cranial nerve function is fundamental for neurotologic diseases. This is especially important in lesions of the jugular foramen such as temporal bone paragangliomas. Specifically, lower cranial nerve function provides useful clinical information to predict postoperative swallowing and voice function. Also, monitoring of lower cranial nerves is generally performed during auditory brainstem implantation.




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Fig. 3.2 A–B




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Fig. 3.2 C–F




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Fig. 3.2 G–H


The lower cranial nerves are easily tested by observation and simple movements. Normal phonation and articulation as well as articulation of speech require the use of CN V, VII, X, and XII. Hoarseness may indicate a CN X lesion (recurrent laryngeal branch). The gag reflex assesses CN IX (afferent limb) and X (efferent limb). It can be tested by touching the posterior pharyngeal wall with a tongue blade. Symmetric elevation of the palate requires proper CN X function. CN XI is tested by having the patient shrug their shoulders (trapezius muscle) and turning their head from side to side against resistance. The tongue should be observed for atrophy and fasciculations upon protrusion and for paresis upon bilateral deflection to assess CN XII integrity. Flexible fiberoptic pharyngolaryngoscopy should be performed to assess CN X.


Recommended Reading

Bloustine S, Langston L, Miller T. Ear-cough (Arnold’s) reflex. Ann Otol Rhinol Laryngol 1976;85(3 pt 1): 406–407


Dickins JR. Approach to diagnosis of skull base lesions. Am J Otol 1981;3(1):35–45



Evaluation of Hearing


Definition

The purpose of audiologic testing is to provide a quantitative assessment of hearing. The resulting audiometric profile indicates the pattern and degree of hearing loss, and helps to characterize the nature of auditory dysfunction.


Closely Related Topics

• Physiology of the middle ear—see p. 27


• Auditory physiology of the inner ear—see p. 30


• Sensorineural hearing loss—see p. 158


Introduction (Fig. 3.3A)

The perception of sound requires that the acoustic signal be conducted to the sensory organ of hearing (the cochlea) and then successfully transduced into neural impulses that travel to the brain. Disruptions at any stage in this sequence will result in a loss of hearing. Hearing loss can therefore be broadly differentiated into two categories: conductive hearing loss and sensorineural hearing loss (Fig. 3.3A).


• Conductive hearing loss results from deficits in the transmission of sound through the outer and middle ears due to such problems as impacted cerumen, perforated tympanic membrane, fluid in the middle ear space, or adhesions of the ossicular chain. A reduction in sound transmission means that the sound reaches the cochlea at a reduced intensity. Conductive hearing loss therefore results in a simple attenuation of sound in most cases.


• Sensorineural hearing loss results from dys-functions of the transduction mechanism (the sensory apparatus) or pathophysiology affecting the neural elements of the auditory pathway. Sensorineural hearing loss can therefore be further subdivided into sensory loss and neural loss (or retrocochlear loss). Sensory loss can result from such disruptions as hair cell degeneration or electrochemical imbalances in the cochlear fluids. Sensory loss associated with hair cell dysfunction depends on the population of hair cells affected. The electromotile outer hair cells amplify low-amplitude basilar membrane vibrations and their loss results in a decrease in sensitivity and an absence of normal nonlinear cochlear function such as compression. The inner hair cells are the primary input to the cochlear nerve and their loss results in a much more profound hearing loss. Neural loss can result from such problems as acoustic neuromas or demyelinating diseases. Because damage to the sensory and neural substrates affects the encoding and internal representation of sound, sensorineural hearing loss usually results in both reduced audibility and reduced fidelity of sound.


Clinical Evaluation


Classification (Fig. 3.3B)

The clinical evaluation of hearing is directed toward two goals: (1) measuring the auditory capabilities of the patient; and (2) assessing the site of the lesion or underlying nature of the hearing loss. The first goal necessitates measuring the degree and pattern of hearing loss. The degree of hearing loss categorizes the magnitude of loss on the basis of the amount of gain required to make sound audible. The intensity of sound is expressed in logarithmic units using a standardized scale known as decibels hearing level (dB HL), where 0 dB HL is the threshold of audibility for a young normal-hearing adult. Using this scale, the degree of hearing loss is categorized below. The degree of hearing loss may vary with the frequency of sound (Fig. 3.3B):


• Normal/near-normal hearing (≤25 dB HL)


• Mild hearing loss (26–40 dB HL)


• Moderate hearing loss (41–55 dB HL)


• Moderately severe hearing loss (56–70 dB HL)


• Severe hearing loss (71–90 dB HL)


• Profound hearing loss (>90 dB HL)


A variety of testing tools are available to assess the nature and degree of hearing loss. These tools fall into two broad classes: (1) those that rely on voluntary responses from the patient; (2) those that measure responses elicited without the express cooperation of the patient. The former are referred to as behavioral (or subjective) tests; the latter are referred to as objective tests.


Recommended Reading

Gelfand SA. Essentials of Audiology. 3rd ed. New York: Thieme Medical; 2008


Katz J, ed. Handbook of Clinical Audiology. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2001


Roush J, Grose J. Principles of audiometry. In: Van de Water T, Staecker H, eds. Otolaryngology: Basic Science and Clinical Review. New York: Thieme Medical; 2001




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Fig. 3.3 A–B


Behavioral Testing


Definition

Behavioral testing refers to the assessment of auditory function using metrics that depend on voluntary responses from the listener. These responses may include, for example, raising a hand to indicate the perception of a tone (pure-tone audiometry) or repeating a presented word (speech audiometry).


Closely Related Topics

• Physiology of the auditory system—see p. 25


• Congenital sensorineural hearing impairment—see p. 158


• Occupational hearing loss and ototoxicity—see p. 165


• Otosclerosis—see p. 154


Tuning Fork Tests (Fig. 3.4A–C)

Although tuning fork tests are qualitative tests that serve, at best, a screening role, they provide a simple introduction to some concepts of auditory testing. Tuning forks resonate at specific frequencies and can therefore act as sound sources for air-conducted sounds or bone-conducted sounds (Fig. 3.4A). Air conduction implies that sound reaches the ear through the medium of air, and therefore relies on the coupling of the middle ear mechanism to reach the inner ear. Bone conduction indicates that the sound source is coupled directly to the bones of the skull, thereby providing a direct vibratory pathway to the cochlea. To a large extent, the bone conduction pathway bypasses the outer and middle ears.


Because a conductive loss impedes the transmission of sound through the outer and middle ears, a tuning fork vibrating in the air near the entrance to the ear canal will have reduced audibility in an ear with a conductive loss. However, the bone conduction pathway will be minimally affected by the conductive loss. Therefore, a vibrating tuning fork placed directly on the mastoid will not have reduced audibility. In contrast, a sensorineural loss will result in reduced audibility of the tuning fork regardless of whether it is vibrating in the air near the ear or placed on the mastoid bone. The Rinne test is a simple qualitative test to gauge whether hearing loss is conductive or sensorineural. The vibrating tuning fork is alternated between the proximity of the concha (air conduction) and placement on the mastoid (bone conduction), and the patient subjectively compares the loudness of the tone at the two locations (Fig. 3.4B). If the tuning fork is louder when placed on the mastoid than when near the entrance of the ear canal, the nature of the hearing loss is likely to be conductive.


Tuning forks can also be used to determine the nature of a one-sided (or unilateral) hearing loss. Occlusion of the air conduction pathway to the cochlea often leads to a slight gain in low-frequency sounds reaching the cochlea through the bone conduction pathway because of resonant effects. A vibrating low-frequency tuning fork (e.g., 500 Hz) placed directly on the midline of the forehead will therefore lateralize to the ear with conductive loss relative to the normal ear. On the other hand, if the unilateral loss is sensorineural in nature, a tuning fork placed on the midline will lateralize to the normal ear. This simple manipulation is known as the Weber test (Fig. 3.4C).


Pure-Tone Audiometry (Fig. 3.4D–G)

Because the cochlea maps frequencies to certain locations along the basilar membrane (see p. 30), the configuration of hearing loss may vary as a function of frequency. To quantitatively measure sensitivity to sound as a function of frequency, pure-tone audiometry is used. This test generates a graph called an audiogram that plots detection thresholds for pure tones in dB HL, as a function of frequency (Fig. 3.4E). Note that the ordinate scale of the audiogram is inverted, such that the increasing sound level is plotted in the downward direction. This sometimes leads to the ordinate scale being colloquially referred to as “dB Hearing Loss.” Typically, octave frequencies between 250 Hz and 8000 Hz are tested, with half-octave points included if thresholds change markedly over an octave. For the purposes of ototoxic monitoring, higher frequencies up to roughly 16 000 Hz can also be tested. Thresholds are plotted separately for each ear, and for each mode of stimulus delivery (air conduction and bone conduction). Standard symbols exist to denote which ear is stimulated and the stimulation mode on the audiogram. For sensorineural hearing loss, thresholds measured by air conduction and bone conduction will be similar (Fig. 3.4E, panel B). For conductive hearing loss, thresholds measured by bone conduction will be lower (better) than air conduction, resulting in an air–bone gap (Fig. 3.4E, panel A). In rare conditions, an air–bone gap can exist because of a dehiscence in the bony labyrinth (e.g., in the wall of the superior semicircular canal), which results in the bone conduction thresholds being abnormally low without affecting the air conduction thresholds. This air–bone gap is not indicative of a true conductive loss since the site of lesion is not in the conductive pathway.




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Fig. 3.4 A–G


It is possible for sound presented to one ear to be conveyed through the bones of the skull to the opposite ear. If the sound is presented via air conduction, this transcranial transmission involves some attenuation of the sound reaching the contralateral ear (typically, ~40 dB for headphones and 70 dB for insert phones). If the sound is presented via bone conduction, usually no attenuation is involved in transcranial transmission. To control which cochlea is responding to the test stimulus, irrespective of which phone (left or right) is presenting the sound, contralateral masking noise is used in conjunction with the ipsilateral signal (Fig. 3.4D). The masking noise ensures that the signal tone is inaudible in the ear not being tested. For cases in which headphones or insert phones cannot be used to deliver air-conducted signals to the patient, loudspeakers can be used. Presentation of stimuli in the sound field, however, introduces two limitations: (1) it is difficult to isolate the response ear since both ears receive the sound; (2) standing waves within the enclosed testing environment can invalidate pure-tone threshold measurements, so frequency-modulated tones or narrow bands of noise are the preferred frequency-specific stimuli for sound-field testing.


To obtain pure-tone thresholds, a bracketing procedure is used wherein the tone level is reduced by 10 dB following a correct response, and raised by 5 dB following an incorrect response. For older children and adults, listeners indicate their response by raising a hand or pressing a button. For babies and toddlers between ~6 and 24 months of age, responses are gauged by means of a conditioned response—a physical head turn toward a visual target (Fig. 3.4F). This technique is known as visual reinforcement audiometry (VRA). In this technique, correct detection of the presented stimulus is rewarded with a presentation of the reinforcer. The visual reinforcer consists of an illuminated mechan ical toy or an image on a video screen, and is located to the side of the baby. For children older than 24 months, threshold is measured using some variation of conditioned play audiometry (CPA). In this technique, the child responds to detection of the signal by placing a game piece in a receptacle(Fig. 3.4G). The game or puzzle is varied as needed to maintain the child’s interest and participation.


Speech Audiometry

Pure-tone thresholds in quiet conditions are informative about the configuration of hearing loss, but are not informative about the ability to perceive complex sounds when multiple frequency components are present simultaneously. The complex sound of most interest to human communication is speech, and so speech audiometry forms part of the basic behavioral test battery. Two speech measures are most common: (1) threshold for correctly recognizing speech—the speech reception threshold (SRT); (2) the percent correct recognition of comfortably audible speech—the word recognition score (WRS). In both tests, the patient repeats aloud the speech sound heard. The speech stimuli for SRT measurements are usually spondees, which are two-syllable words with equal stress on each syllable (e.g., “sailboat”). There is typically a close association between the SRT and the pure-tone average (PTA) threshold at 500, 1000, and 2000 Hz. The stimuli for WRS measurements are usually prerecorded single-syllable words. In special applications, such as testing for cochlear implant candidacy, measurements of speech recognition with and without visual cues (lip reading) may be required. For young children, speech testing is modified to fit the child’s capability. For example, toddlers may be asked to point to body parts rather than repeat words (“show me your nose”), and young children may be asked to point to a picture from a closed set of possibilities that corresponds to the word they perceived. Although speech testing in quiet conditions is informative, it is not representative of everyday listening conditions where background noises are typically present. Speech-in-noise tests exist but are not routinely administered in most basic hearing assessments.


Recommended Reading

British Society of Audiology. Recommended procedure for Rinne and Weber tuning-fork tests. Br J Audiol 1987;21(3):229–230


Guidelines for the audiologic assessment of children from birth to 5 years of age. Rockville, MD: American Speech Language and Hearing Association; 2004. Available at: www.asha.org/docs/pdf/GL2004-0002.pdf. Accessed April 6, 2010


Madell JR, Flexer C, eds. Pediatric Audiology: Diagnosis, Technology and Management. Stuttgart: Thieme Medical Publishers; 2008



Objective Audiometry


Objective audiometry refers to the assessment of auditory function using tests that do not depend on voluntary responses from the listener. These tests are useful in patients who will not, or cannot, reliably respond to sound because of their age and/or condition.


Assessment of Middle Ear Function

Definition

Objective assessment of middle ear function refers to the physical characterization of the ease with which a sound is transmitted through the outer and middle ears to the cochlea. This transmission depends on the function and integrity of the tympanic membrane, middle ear system, and eustachian tube.


Closely Related Topics

• Physiology of the middle ear—see p. 27


• Chronic suppurative otitis media (CSOM)—see p. 130


• Otosclerosis—see p. 154


Introduction

The middle ear acts as an impedance transformer to better match the impedance of air with the input resistance of the cochlea (see p. 27). This transformation is frequency dependent because the impedance of the middle ear depends on the reactance of its stiffness and mass components. In particular, the middle ear is relatively poor at passing low frequencies because the overall impedance of the middle ear tends to be dominated by stiffness. The status of the middle ear can be objectively assessed by measuring how well sound is admitted through the tympanic membrane. Admittance is the inverse of impedance. (Because middle ear function can be described in terms of both impedance and admittance, sometimes the generic term immittance is used.) Pathologies that affect the stiffness of the middle ear will consequently affect how well a low-frequency sound is admitted into the ear. There are two main tests of middle ear function based on admittance measurements: (1) tympanometry; (2) acoustic reflex testing.


Tympanometry (Fig. 3.5A)

The tympanogram is a graph that plots the admittance of a probe tone through the tympanic membrane into the middle ear system as a function of the relative pressure in the sealed ear canal (Fig. 3.5A). The abscissa of the tympanogram is pressure relative to ambient pressure (in decapascals [daPa]) and the ordinate is admittance (Y) measured in millimhos (mmhos). When a low-frequency probe tone is used (e.g., 256 Hz) the admittance term is almost a pure compliance and therefore the ordinate is sometimes labeled “Compliance.” When the middle ear and eustachian tube are working normally, the pressure of the air trapped in the middle ear space is the same as the ambient pressure existing in the ear canal. The tympanic membrane is therefore most flexible at ambient pressure and will maximally admit the low-frequency probe tone at this pressure. When the pressure in the sealed ear canal is increased above ambient pressure and then systematically reduced below ambient pressure, the stiffness of the tympanic membrane progressively changes from a state of increased tautness (at positive pressure), through maximal compliance (at ambient pressure), and returns to increased tautness (at negative pressure). The resulting tympanogram evidences peak admittance at ambient pressure (0 daPa), with relatively symmetric declines in admittance at both higher and lower pressures (Fig. 3.5A 1—Type A tympanogram). Various metrics can be derived from the tympanogram such as static admittance, tympanometric slopes and widths, pressure associated with the tympanometric peak, and equivalent ear canal volume (the volume of air with equivalent compliance). Departure from normative values for these metrics is of diagnostic significance.


If a slight vacuum exists in the middle ear due to resorption of gases secondary to eustachian tube dysfunction, the tympanic membrane will be chronically retracted. The peak in the tympanogram will now occur when pressure in the sealed ear canal is reduced below ambient pressure and is made equal to the negative pressure existing in middle ear space (Fig. 3.5A 3—Type C tympanogram). Many middle ear pathologies, including effusions or cholesteatomas, will increase the stiffness of the middle ear to the point that admittance will remain low irrespective of the pressure change in the sealed ear canal. The resulting tympanogram will be flat (Fig. 3.5A 2—Type B tympanogram). A Type B tympanogram can also result from a perforation in the tympanic membrane (e.g., patent tympanostomy tube) since any potential pressure differential across the tympanic membrane is ablated even with pressure variations in the sealed ear canal. A Type B tympanogram due to a perforation can be distinguished from middle ear dysfunction in the presence of an intact tympanic membrane on the basis of the equivalent ear canal volume. In cases of perforation, this metric will be abnormally large since it includes the middle ear space. A tympanogram may have its peak at 0 daPa but can still be associated with middle ear dysfunction: A shallow peak can result from otosclerosis (Fig. 3.5A 4—Type A Stympanogram), and a markedly increased peak can result from a disarticulated ossicular chain (Fig. 3.5A 5—Type A Dtympanogram).




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Fig. 3.5 A–C


Because the impedance characteristics of middle ears of infants are very different from those of more mature ears, tympanometry using 256-Hz probe tones is generally unreliable in children less than 3 months of age. The use of higher-frequency probe tones (e.g., 1000 Hz) is therefore appropriate for testing the admittance of infant ears.


Acoustic Reflex Thresholds (Fig. 3.5B, C)

Two middle ear muscles are attached to the ossicular chain: the stapedius muscle and the tensor tympani muscle (see p. 12). The stapedius muscle contracts in response to loud sound (in either ear) and this is known as the acoustic reflex. Because the attachment of the stapedius muscle to the stapes is orthogonal to the axis of rotation of the ossicular chain, its contraction stiffens the ossicular chain. By increasing the stiffness of the middle ear system, the admittance of (low-frequency) sounds will be reduced. The same principles underlying tympanometry can therefore be applied to detecting the acoustic reflex. A low-frequency probe tone is continuously presented to the ear while a second sound—the eliciting sound—is presented in brief durations to the ear on the same side as the probe tone (ipsilateral) or the opposite side (contralateral). The level of this eliciting sound is increased incrementally until a reduction in admittance of the probe tone is measured. This level of the eliciting sound is referred to as the acoustic reflex threshold and represents the minimum sound level that results in a measurable contraction of the stapedius muscle (Fig. 3.5B 1). Normally, further increases in sound level above the acoustic reflex threshold will result in a growth of the reflex magnitude; i.e., an increased admittance change. The acoustic reflex threshold can be measured for a variety of eliciting stimuli including broad band noise and pure tones. Abnormally elevated acoustic reflex thresholds are of diagnostic significance. In addition, comparison of acoustic reflex thresholds measured separately for ipsilateral and contralateral presentation of the eliciting tone can be informative as to the integrity of the neuronal reflex arc across the brainstem (Fig. 3.5C). Acoustic reflexes can only be measured in the presence of a functioning middle ear system (e.g., Type A tympanogram), and therefore this test is precluded by middle ear pathology. Acoustic reflexes can also only be measured in ears with sufficient sensory reserve at the eliciting tone frequency to respond to the loud eliciting sounds.


The stability of the acoustic reflex for longer presentations of the eliciting sound can be quantified as acoustic reflex decay (Fig. 3.5B 2). In this test, the reflex is elicited by a sound presented at 5 or 10 dB above its acoustic reflex threshold. The sound is presented continuously for 10 seconds, and a decline in the reflex (return of admittance toward baseline) of 50% or greater during this period is considered abnormal. Abnormal reflex decay may be indicative of retrocochlear dysfunction.


Recommended Reading

Alaerts J, Luts H, Wouters J. Evaluation of middle ear function in young children: clinical guidelines for the use of 226- and 1,000-Hz tympanometry. Otol Neurotol 2007;28(6):727–732


American Academy of Family Physicians; American Academy of Otolaryngology-Head and Neck Surgery; American Academy of Pediatrics Subcommittee on Otitis Media With Effusion. Otitis media with effusion. Pediatrics 2004;113(5):1412–1429


Margolis RH, Hunter LL. Acoustic immittance measurements. In: Roeser RJ, Valente M, Hosford-Dunn H, eds. Audiology: Diagnosis. New York: Thieme Medical Publishers; 2000: 381–342



Otoacoustic Emissions

Definition

Otoacoustic emissions (OAEs) are sounds recorded in the ear canal that are generated as a result of the electromotility of outer hair cells. They are used as an objective test of normal cochlear function.


Closely Related Topics

• Auditory physiology of the inner ear—see p. 30


• Early acquired hearing loss and auditory neuropathy—see p. 162


• Occupational hearing loss and ototoxicity—see p. 165


Introduction (Fig. 3.6A)

The electromotility of the outer hair cells in the cochlea allows them to function as an active feedback mechanism that amplifies the basilar membrane response in a frequency-selective manner (see p. 30). An epiphenomenon of this active feedback mechanism is that the driven oscillations can be transmitted in the reverse direction out of the cochlea. With this reverse transmission, the tympanic membrane acts like the cone of a loudspeaker rather than the diaphragm of a microphone. If the volume of air in front of the tympanic membrane is minimized by sealing the ear canal with a probe assembly, the small force of the vibrating membrane can generate sufficient sound pressure to be detected by a sensitive microphone (Fig. 3.6A). The pressure waves recorded in the ear canal are termed otoacoustic emissions (OAEs). A basic tenet of OAEs is that normal outer hair cell function is required for their generation, and therefore the detection of OAEs is indicative of normal cochlear function. Because OAEs must also be transmitted in retrograde fashion through the middle ear, detection of OAEs also indicates a functional middle ear system. In current clinical applications, OAEs constitute a binary test where present OAEs are indicative of normal outer hair cell function and absent OAEs are indicative of cochlear dysfunction. The absence of OAEs is associated with sensory or conductive hearing loss, but does not indicate degree of hearing loss. However, current research is focused on deriving estimates of behavioral thresholds based on OAE input–output functions. In this section, only the two most clinically useful classes of OAEs will be described: transient-evoked OAEs and distortion product OAEs.


Transient-Evoked Otoacoustic Emissions (Fig. 3.6B)

Transient-evoked OAEs (TEOAEs) are elicited by a brief stimulus such as a click or tone pip. Following the stimulus, a TEOAE emerges after a latency of a few milliseconds. This latency reflects a combination of the forward and reverse travel times into and out of the cochlea, including travel time of the basilar membrane. When a click is utilized, the stimulus contains a wide range of frequencies and therefore stimulates an extensive portion of the basilar membrane. This broad stimulation allows for the emission energy from the basal, high-frequency region of the cochlea to emerge before that from the apical, low-frequency region. A click-evoked OAE therefore typically exhibits a dispersion of frequencies in its time waveform (Fig. 3.6B). A tone pip is more frequency-specific than a click, and therefore the OAE evoked by a tone pip is more limited in its spectral content. Because a TEOAE is analogous to an echo (i.e., it emerges after a slight delay following the stimulus and its frequency content reflects that of the stimulus), and because OAEs were first brought to public attention by a scientist named David Kemp, TEOAEs were initially referred to as “Kemp echoes.” Both clicks and tone pips allow for the probing of the functionality of the outer hair cells along the cochlear partition. For frequency regions where hearing loss in excess of ~30 dB HL exists, TEOAEs will likely be absent.


Distortion Product Otoacoustic Emissions (Fig. 3.6C)

Distortion product OAEs (DPOAEs) are elicited by two continuously presented tones (or primaries). The frequencies and levels of these two tones are selected to optimally generate an intermodulation distortion product in the region where the basilar membrane responses overlap. When the frequency ratio of the two tones is roughly 1.2, and the higher-frequency tone (f2) is 10 dB lower than the lower-frequency tone (f1), the distortion product with frequency 2f1 − f2 will be maximally generated. This distortion product propagates back in the middle ear system and can be detected in the ear canal as a DPOAE. The frequency of the DPOAE is precisely controlled by the selection of the two primary tones. The presence of the DPOAE can be confirmed by an analysis of the component amplitude at this frequency relative to amplitudes at neighboring frequencies that constitute the noise floor (i.e., the signal-to-noise ratio meets some criterion such as +6 dB). A plot of DPOAE levels as a function of the frequency of the generation site (usually near f2) is known as the DPGram (Fig. 3.6C). The DPGram can indicate the frequency regions where outer hair cell function is normal. Absent DPOAEs typically indicate a hearing loss of at least 30–40 dB. Because the DPOAE test and the analysis of the results can be fully automated, DPOAEs are a popular screening test for normal cochlear function.




image


Fig. 3.6 A–C


Recommended Reading

Hall JW. Handbook of Otoacoustic Emissions. San Diego: Singular Publishing Group; 2000


Robinette MP, Glattke TJ, eds. Otoacoustic Emissions: Clinical Applications. New York: Thieme; 2002



Auditory Brainstem Response

Definition

The auditory brainstem response (ABR) is an auditory evoked potential that assesses the function of the peripheral auditory system including the eighth cranial nerve and brainstem. Its clinical applications include screening for hearing loss (in the context of newborn infant hearing screening), objectively estimating behavioral thresholds, and identifying sites of lesion in retrocochlear pathology—although modern imaging methods have mostly replaced ABR for this latter application.


Closely Related Topics

• Central auditory and vestibular pathways—see p. 22


• Central auditory pathways—see p. 33


• Electrocochleography and auditory steady-state potentials—see p. 67


• Congenital sensorineural hearing impairment—see p. 158


Introduction

All neural activity is essentially electrochemical in nature, and electrical events within a neuron can be reflected as extracellular potential changes. If the summed extracellular potentials across a population of neurons are of sufficient magnitude, these potentials can be detected in the far field. Such conditions are most likely to be met if the population of neurons is synchronously active, such as when the neurons are responding in unison to the onset of an eliciting stimulus. The ABR refers to the recording from the scalp (i.e., far field) of the electrical potentials generated by synchronous neural activity in the eighth cranial nerve and auditory brainstem pathway. Because it is generated below the cortex, the ABR is not affected by sleep or anesthesia.


ABR Waveforms (Fig. 3.7A, B)

The ABR consists of a series of five vertex-positive peaks occurring in a time window lasting several milliseconds after the onset of the stimulus (Fig. 3.7A). The peaks are traditionally labeled with Roman numerals I–V. The latency of individual peaks relative to the stimulus onset and the interval between successive peaks depend on the stimulus parameters, the recording system, and the status of the patient’s auditory system. However, for a given set of parameters, the latencies are quite consistent across listeners with normal hearing. Of the five peaks, the final peak (wave V) is usually the most robust in that it can be detected at low stimulus levels. Figure 3.7B shows a series of ABR traces recorded as the stimulus intensity is gradually reduced. Wave V develops a prolonged latency as the intensity is reduced. For this stimulus, there is a strong association between the ABR threshold and the behavioral threshold.


ABRs and Frequency Specificity (Fig. 3.7C)

Although the ABR requires an abrupt stimulus to generate sufficient neural synchrony for far field recording, some frequency specificity can be obtained by using tone pip stimuli. Figure 3.7C shows ABR traces recorded in a young child using four different frequency tone pips. For each tone pip, wave V latency increases with decreasing level. Across tone pip frequencies, wave V latency is inversely proportional to frequency because of the travel time along the basilar membrane. The morphology of an ABR elicited with a tone pip tends to be less distinct because of the lack of neural synchrony.


Clinical Utilization (Fig. 3.7D)

Because there is a strong association between the ABR threshold (the lowest stimulus intensity at which wave V is reliably identified) and behavioral threshold, the ABR is often used to estimate hearing sensitivity in babies and other special populations who are unable to provide behavioral responses. By controlling the frequency content of the stimulus, it is possible to estimate the configuration of auditory sensitivity across frequencies. Because the ABR requires an abrupt stimulus to elicit the synchronous neural response that is recorded, the frequency specificity of the test is limited. It is important to remember that the ABR is a test of synchronous neural response and not of hearing per se.


The ABR is also used as a screening test for neonatal hearing screening. A single low-level stimulus is presented (typically 35 dB nHL) and automated detection algorithms are applied to determine whether a valid response is present. Because these statistical detection algorithms do not rely on subjective human decision making, the test result is quite objective. In this sense, the automated ABR can be compared with OAEs (see p. 61) as an objective screening tool for auditory function. However, whereas OAEs can provide better frequency specificity than the automated ABR, they only probe auditory function to the level of the outer hair cells. The automated ABR, on the other hand, provides information to the level of the brainstem. Infants with auditory neuropathy will therefore likely pass their newborn hearing screening if it is performed with OAEs, but will fail an automated ABR.




image


Fig. 3.7 A–D


The ABR can also be used as a functional test for locating retrocochlear lesions. A vestibular schwannoma pushing on the eighth nerve can result in interaural asymmetry in the latencies of the ABR peaks (Fig. 3.7D). A technique known as stacked ABRs can be helpful in detecting small tumors. In this technique, frequency-specific ABRs are derived and time-shifted to align the waves V, thus eliminating differences in cochlear travel time with frequency. The aligned (“stacked”) frequency-specific ABRs are summed to give an enhanced wave V. This summed response is sensitive to the absence of frequency-specific contributions from tumor-affected neurons.


ABRs can be elicited in cochlear implant recipients by direct electrical stimulation. These responses differ from the normal acoustic-elicited ABR in two main respects: (1) the early waves are usually obscured by the large stimulus artifact; (2) the latency of the ABR waves is shorter because of the absence of basilar membrane travel time.


Recommended Reading

American Academy of Pediatrics, Joint Committee on Infant Hearing. Year 2007 position statement: principles and guidelines for early hearing detection and intervention programs. Pediatrics 2007;120(4): 898–921


Burkard RF, Don M, Eggermont JJ, eds. Auditory Evoked Potentials. Basic Principles and Clinical Application. Philadelphia: Lippincott, Williams & Wilkins; 2007


Don M, Kwong B, Tanaka C, Brackmann D, Nelson R. The stacked ABR: a sensitive and specific screening tool for detecting small acoustic tumors. Audiol Neurootol 2005;10(5):274–290


Thompson DC, McPhillips H, Davis RL, Lieu TL, Homer CJ, Helfand M. Universal newborn hearing screening: summary of evidence. JAMA 2001;286(16): 2000–2010



Other Auditory Evoked Potentials: Electrocochleography, Auditory Steady-State Responses, and Cortical Potentials

Definition

Electrocochleography measures gross potentials emanating from the vicinity of the cochlea. The auditory steady-state response measures the synchronization of the central auditory system to the modulation rate of the stimulus. Cortical auditory evoked potentials record the response of the cortex to auditory stimuli.


Closely Related Topics

• Auditory, physiology of the inner ear—see p. 30


• Sensorineural hearing loss—see p. 158


• Ménière disease—see p. 188


Electrocochleography (Fig. 3.8A)

Electrocochleography (ECochG) refers to the recording of summed potentials generated within the cochlea. These gross potentials include the whole nerve action potential (WNAP) or compound action potential(CAP), which corresponds to wave I of the ABR, the cochlear microphonic (CM), and the summating potential (SP). The CM is an alternating current (AC) potential that represents the summed extracellular potentials of the hair cells. The CM is typically dominated by the more numerous outer hair cells and follows the cycle-by-cycle waveform of the stimulus. In clinical ECochG recordings, the CM reflects the activity of the hair cells in the base of the cochlea, irrespective of the frequency of the stimulus, since the phase of the hair cell activity tends to be more uniform at the base. The SP is more of a direct current (DC) potential and is generally insensitive to the phase of the stimulus. Although scalp electrodes can be used to record the ECochG, electrode placement closer to the cochlea, such as on the tympanic membrane or promontory, results in more robust recordings.


The CM and SP may be recorded in response to both clicks and tone bursts. Because the CM is phase-locked to the stimulus, alternating the phase of the stimulus will cancel the CM and leave only the SP remaining. Figure 3.8A shows the ECochG response to a 1000-Hz toneburst in rarefaction phase (top traces), condensation phase (lower traces), and alternating phase (middle trace). The slowly drifting “baseline” is the SP. A present CM in the absence of a measurable ABR may be indicative of auditory neuropathy. It has been suggested that an abnormally large SP relative to the WNAP can be associated with Ménière disease. This change may possibly be due to a shift in the operating point of the basilar membrane secondary to endolymphatic hydrops. In cochlear implant patients, recording the WNAP through the implanted electrodes delivering the current pulses is a routine test of the integrity of the electrode/neural interface.


Auditory Steady-State Response

The transduction of sound from a vibratory format into a neural code results in an “internal representation” of the sound that may contain frequencies not present in the original stimulus. The representation of these frequencies is due to the nonlinearity of the transduction process (e.g., half-wave rectification). For example, a sinusoidally amplitude modulated (SAM) tone contains three components in its spectrum (the carrier frequency and two side-bands), none of which corresponds to the modulation frequency. However, the spectrum of the internally represented SAM tone contains an additional component at the frequency of the modulation rate (Fig. 3.8B). The ASSR test is designed to detect this modulation rate component in an electroencephalogram (EEG) when a SAM tone is continuously presented. Successful detection of the modulation rate component signifies that the frequency region of the cochlea corresponding to the carrier frequency of the SAM tone is functional. Because the stimulus is essentially continuous (i.e., in a “steady state”), its frequency content is circumscribed, allowing for good frequency specificity of the test.


The threshold of the auditory steady-state response (ASSR) is measured by lowering the level of the SAM tone until the modulation rate component can no longer be statistically distinguished. Behavioral threshold predictions can then be calculated based on the ASSR threshold. When the ASSR stimulus does not need to be particularly intense, multiple frequency regions can be tested simultaneously using a family of SAM tone stimuli with different carrier frequencies and modulation rates. It is also possible to test both ears simultaneously using an appropriate electrode montage. One limitation of the ASSR test is that at lower modulation rates, the response reflects some contributions from the cortex. Therefore, at these lower rates, the test result can be affected by factors such as sleep, anesthesia, and arousal state. Although the ASSR has not replaced the ABR in general clinical practice, it has one advantage in that it allows stimulation at higher levels than the ABR. The ASSR is therefore particularly helpful in cases of severe-to-profound hearing loss, where ABRs cannot be recorded.


Apr 14, 2018 | Posted by in NEUROSURGERY | Comments Off on Otologic and Neurotologic Diagnostics and Tests
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