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Types of Hearing Loss
Peripheral hearing loss comes in two broad types, conductive and sensorineural. Conductive hearing loss results from deficits in the sound-conducting apparatus of the outer and middle ear. Problems such as fluid in the middle ear and immobility of the middle ear bones are the main cause of a conductive hearing loss. Sensorineural hearing losses (SNHLs) that result from damage to the hair cells and the stria vascularis can be considered “sensory” hearing loss. Neural loss occurs when the auditory nerve fibers (ANFs) are involved, as might occur in a tumor of the eighth nerve (a vestibular schwannoma) or in a recently recognized type of hearing loss known (most commonly) as auditory neuropathy (ANP). Types of hearing loss that cause dominantly “central changes” have been described in Chapter 4 , Hearing Problems.
5.1
Site of Lesion Testing
5.1.1
Air/Bone Conduction Audiograms
Conductive hearing losses are defined by an audiometric difference for air- and bone-conducted sound, the air–bone gap. Bone conducted sound, produced by a vibrator on the scalp (see chapter: Implantable Hearing Aids ), bypasses the external and middle ear in stimulation of the cochlea, but is less effective than air conduction in normal hearing people.
5.1.2
Speech Discrimination Testing
Speech audiometry is a fundamental tool in hearing loss assessment. Together with pure-tone audiometry, it can aid in determining the degree and type of hearing loss. Speech audiometry provides information on word recognition and about discomfort or tolerance to speech stimuli. Speech audiometry outcomes help also in setting the gain and maximum output of hearing aids for patients with moderate to severe hearing losses. An adaptation of speech audiometry is the Hearing-in-Noise Test, in which the stimuli are presented to both ears together and the patient is required to repeat sentences both in a quiet environment and with competing noise being presented from different directions.
5.1.3
Acoustic Immittance
Acoustic immittance testing evaluates the patency of the eardrum and the middle ear space behind the eardrum, as well as a middle ear muscle reflex (MEMR). The primary purpose of impedance audiometry is to determine the status of the tympanic membrane (TM) and middle ear via tympanometry. The secondary purpose of this test is to evaluate the MEMR pathways, which include the facial nerve, the audio-vestibular nerve, and the auditory brainstem. This test cannot be used to directly assess auditory sensitivity, although results are interpreted in conjunction with other threshold measures.
5.1.3.1
Tympanometry
Tympanometry is an examination used to test the condition of the middle ear, the mobility of the eardrum, and the conduction bones by creating variations of air pressure in the ear canal. A tone of 226 Hz is generated by the tympanometer into the ear canal, where the sound strikes the TM. Some of this sound is reflected back and picked up by the instrument. Middle ear problems often result in stiffening of the middle ear, which causes more of the sound to be reflected back. Tympanometry is an objective test of middle ear function. In evaluating hearing loss, tympanometry assists in diagnosing between sensorineural and conductive hearing loss.
5.1.3.2
Middle Ear Muscle Reflex
A standard clinical immittance test battery also includes the MEMR. The MEMR is the contraction of the stapedius muscle in response to high-level acoustic stimulation. The MEMR is a bilateral response, which means that presenting the loud tone or noise to one ear will elicit the response in both ears. MEMR measurements are used as a cross-check with the behavioral audiogram and as a way to separate cochlear from retrocochlear pathologies ( ).
5.1.4
Oto-Acoustic Emission Testing
Otoacoustic emissions (OAEs) can be measured with a sensitive microphone in the ear canal and provide a noninvasive measure of cochlear amplification (see chapter: Hearing Basics ). There are two main types of OAEs in clinical use. Transient-evoked OAEs (TEOAEs) are evoked using a click stimulus. The evoked response from a click covers the frequency range up to around 4 kHz. Distortion product OAEs (DPOAEs) are evoked using a pair of primary tones f1 and f2 (f1<f2) and with a frequency ratio f2/f1<1.4. The most commonly measured DPOAE is at the frequency 2f1–f2. Recording of OAEs has become the main method for newborn and infant hearing screening (see chapter: Early Diagnosis and Prevention of Hearing Loss ).
5.1.5
Electrocochleography
Electrocochleography (ECochG) is the recording of stimulus-related potentials generated in the human cochlea, including the first-order neurons forming the auditory nerve (see chapter: Hearing Basics ; Appendix). These potentials are the cochlear microphonic (CM), the summating potential (SP), and the compound action potential (CAP). CM, SP, and CAP can be recorded from the promontory of the human cochlea by inserting (under local anesthesia) an electrode through the eardrum or from the eardrum itself. CM and SP are generated by the hair cells, CAP by the auditory nerve and is also represented by wave I of the auditory brainstem response (ABR). SP and CAP comparisons are used in the diagnosis of Ménière’s disease. The presence of the CM and the absence of a CAP is a defining characteristic of ANP.
5.1.6
Auditory Brainstem Response Testing
5.1.6.1
The Auditory Brainstem Response
The ABR is an auditory evoked potential obtained by signal averaging from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The resulting recording is a series of vertex positive waves of which I through V are evaluated. These waves occur in the first 10 ms after onset of an auditory stimulus. Wave I is generated in the proximal part of the auditory nerve (as is the CAP) and wave III in the lower brainstem. Wave IV and V are generated in the upper brainstem ( ). The ABR represents initiated activity beginning at the base of the cochlea and moving toward the apex over a 4 ms period of time. For click stimuli, the peaks largely reflect activity from the most basal regions on the cochlea because the disturbance hits the basal end first and by the time it gets to the apex, a significant amount of phase cancellation occurs. Amplitude and latency of waves I, III, and V are the basic measures for quantifying the ABR. Amplitude is dependent on the number of neurons firing and above all their synchrony, latency depends on hearing loss and again neural synchrony, interpeak latency (the time between peaks) depends on conduction velocity along the brainstem, and interaural latency (the difference in wave V latency between ears) is sometimes used in acoustic neuroma diagnosis. The ABR is used for newborn hearing screening, auditory threshold estimation using tone-pip stimuli or high-pass mashed clicks, intraoperative monitoring, determining hearing loss type and degree, and auditory nerve and brainstem lesion detection.
5.1.6.2
The Stacked ABR
The stacked ABR is the sum of the synchronous neural activity generated from five frequency regions across the cochlea in response to click stimulation and high-pass noise masking. The technique is an application of high-pass noise masking of the click-evoked ABR resulting in derived ABRs ( ).
The derived waveforms representing activity from more apical regions along the basilar membrane have wave V latencies that are prolonged because of the nature of the traveling wave (see chapter: Hearing Basics ). In order to compensate for these latency shifts, the wave V component for each derived waveform is stacked (aligned), added together, and then the resulting amplitude is measured. showed that in a normal ear, the sum of the stacked derived ABRs has the same amplitude as the unmasked click-evoked ABR. However, the presence of even a small vestibular schwannoma results in a reduction in the amplitude of the stacked ABR in comparison with the unmasked click ABR.
5.1.6.3
The Cochlear Hydrops Analysis Masking Procedure
The Cochlear Hydrops Analysis Masking Procedure (CHAMP) test is a basically a high-pass noise masking ABR test that is used to screen for the presence of cochlear hydrops. It is hypothesized that cochlear hydrops alters the response properties of the basilar membrane which results in reduced masking effectiveness of high-pass noise on the ABR to click stimuli. It is the degree of this undermasking that is used as the diagnostic.
5.1.7
The Auditory Steady-State Response
The auditory steady-state response is an auditory evoked potential, elicited with modulated tones that is used to predict hearing sensitivity in patients of all ages. It is a cortical EEG or MEG response to rapid (or amplitude modulated) auditory stimuli and creates a statistically validated audiogram.
5.1.8
Tone Decay
The tone decay test is used in audiology to detect and measure auditory “fatigue” or abnormal adaptation. In people with normal hearing, a tone whose intensity is only slightly above their hearing threshold can be heard continuously for 60 s. The tone decay test produces a measure of the “decibels of decay,” i.e., the number of decibels above the patient’s absolute threshold of hearing that are required for the tone to be heard for 60 s. A decay of between 15 and 20 dB is indicative of cochlear hearing loss. A decay of more than 25 dB is indicative of damage to the auditory nerve. The outcome of the tone decay test is used as a diagnostic criterion for retrocochlear pathology (damage to the auditory nerve).
5.2
Conductive Hearing Loss
Here I will first describe some forms of conductive hearing loss caused by the following conditions: ossicular chain interruption with intact TM, missing TM and ossicles, otosclerosis, TM collapse, and TM perforations. I base this overview on the work of .
5.2.1
Ossicular Interruption With Intact Tympanometry
Interruption of the ossicular chain in the presence of an intact TM produces an air–bone gap of 50–60 dB. When the ossicles are interrupted, there is no ossicle-coupled sound, and acoustic coupling provides the sole sound input to the cochlea (see chapter: Hearing Basics, Fig. 1.2 ). demonstrated that 50–60 dB air–bone gaps could be explained by the remaining acoustic coupling with the stapes.
5.2.2
Loss of Tympanometry, Malleus, and Incus
When the TM, malleus, and incus are missing, the resulting air–bone gap may be explained in terms of remaining acoustic coupling ( ). With the TM and ossicles missing, ossicular coupling is abolished and acoustic coupling is approximately 10–20 dB larger than in the normal ear. Therefore, the air–bone gap for these conditions is 40–50 dB. Similar gaps should also occur when there is a large perforation of the TM in conjunction with ossicular disruption ( ).
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