Cochlear dead zones: What are they and how do you detect them?

Abstract

Definition: A dead zone is a portion of the cochlea in which the inner hair cells have been destroyed. Everyday we see the Xs and Os—indicating thresholds—on audiograms. We assume the Xs and Os have meaning—that they represent the integrity of the part of the ear we are testing. We need to be careful, however, in interpreting audiograms. Advances in our understanding of how the cochlea works are showing us how to spot false responses that come from dead zones in the cochlea. Knowing this can help us improve our hearing tests and our hearing aid fittings. Cochlear dead zones are areas where the inner hair cells have been destroyed. The outer hair cells “tune” the inner hair cells, but we “hear” with the input from the inner hair cells. Outer hair cells mechanically help the inner hair cells sense sounds below 50 to 60 dB SPL; they also serve to sharpen the traveling wave, thus enabling us to distinguish among frequencies close together. Outer hair cell death eradicates otoacoustic emissions (OAEs). Inner hair cell death makes the acoustic reflex (AR) absent. OAEs and the AR can be seen as non-behavioral tests of the outer and inner hair cells, respectively. Both are also obliterated by middle ear pathology; as such they are excellent cross tests. In this two-part series we will discuss dead zones and their warning signs. We will also examine a test strategy to identify cochlear dead zones that was developed by Moore and discussed in the March 2009 Page Ten. To understand this test's rationale, one must understand the unique traveling wave of the cochlea. Look at the 65-dB threshold at 1000 Hz in Figure 1. When a 65-dB, 1000-Hz pure tone was presented, the patient responded. Does this mean the patient can hear at 1000 Hz? Not necessarily. This apparent threshold might be an artificial response from a dead zone. It could very possibly be the patient “hearing” the tone via hair cells representing lower frequencies. If there are no functioning inner hair cells in this frequency region (the gray area on the audiogram in Figure 1), it would be pointless to put amplification here.Figure 1: If the hair cells in the high-frequency zone are dead, you may see an audiogram showing a pronounced high-frequency SNHL. The steep “front” of the traveling wave (intense, high-frequency stimulation) may “excite” the healthy low-frequency hair cell region. The person reported hearing the tone, but the response did not truly arise from the high-frequency hair cells regions. Note: All the figures in this article are teaching illustrations designed to help explain these concepts. They do not replicate the exact proportions or shape of the waves.Moore has identified three hearing loss configurations in which “dead zones” tend to occur: severely sloping, high-frequency hearing loss reverse curves “cookie bite” audiograms Before talking about audiometric configurations, let's review a little hearing theory. THE TRAVELING WAVE The traveling wave passes instantly through the fluid-filled inner ear and moves the basilar membrane. The wave grows in amplitude and slows as it goes along the basilar membrane until reaching a peak at some particular hair cell region. To understand how sound passes through the inner ear and causes the basilar membrane to move, hold the ends of a pen between your index fingers. Pretend the right finger is the stapes of the middle ear and the left finger is a cilium of a hair cell on the basilar membrane. Vibrate your right finger. Notice how the opposite finger immediately moves in response. Like the pen between your fingers, the endolymphatic fluid in the inner ear is not compressible. That's important. We would be unable to hear in real time if the sound wave traveling through the inner ear were delayed by more than a few milliseconds. When the stapes vibrates, waves are transmitted almost instantly through the ear, moving the basilar membrane and stimulating the hair cells. Notice in Figure 2 that the cochlea has been unrolled and we have assigned high and low frequencies to different zones along the basilar membrane. The basilar membrane, the floor upon which the hair cells are situated, has a tapered shape, which grows wider from the stapes to the apex. That's the opposite of the cochlea, which is also tapered but is widest at its apex and narrows toward its base.Figure 2: The cochlea has been unrolled and we have assigned different frequencies to different zones along the basilar membrane. The basilar membrane has a tapered shape: narrow at the base where the stapes attaches to the inner ear and much wider at the apex, the opposite end of the cochlea.The basilar membrane is narrow and stiff at the wide base of the cochlea near where the stapes is attached to the oval window or cochlear entrance. It is much wider and less stiff at the narrow apex of the cochlea (see Figure 2). The inner hair cells convert high-pitched tones from hydraulic waves into electrical signals near the base of the cochlea. Low-pitched tones are transformed by the inner hair cells near the apex. The reason that highs stimulate hair cells at the base of the cochlea is that the basilar membrane is narrow and stiff in that region (highs resonate stiffness); lows stimulate the hair cells at the apex because lows resonate with mass. To understand testing for cochlear dead zones one must appreciate the asymmetrical shape of the “wave” envelope. It has a long, sloping tail toward the high-frequency base of the cochlea and a relatively steep leading edge facing the low-frequency apex of the cochlea. Figure 3 shows the basilar membrane moved by a wave created by a low-frequency pure tone. Note the shape of the wave and how the leading edge drops off quickly, while the tail of the wave is much longer.Figure 3: A low-frequency pure tone moves through the inner ear and creates maximum movement of the basilar membrane in the apex, the “far end” of the inner ear.Figure 4 illustrates a wave created by a high-frequency pure tone. The action all occurs near the base. The basilar membrane is not deflected at all in the middle and higher regions.Figure 4: A high-frequency pure tone affects only the area closest to the base.Study the similarities and differences between Figures 3 and 4. Both figures show an asymmetrical wave. A low-frequency pure tone creates maximum movement of the basilar membrane in an area close to the apex and also vibrates the basilar membrane to a lesser extent toward the high-frequency base of the cochlea. However, a high-frequency pure tone affects only the area closest to the base. When you present a moderate-to-high-intensity test signal, the traveling wave is taller and longer. You stimulate the hair cells at the peak of the wave, but if all the inner hair cells at this peak are dead a response might arise from a neighboring frequency region of living hair cells. An illustration of the envelope of a traveling wave created by a 65-dB, 1000-Hz pure tone is shown in Figure 5.Figure 5: The traveling wave created by a 65-dB pure-tone at 1000 Hz. The point of maximum basilar membrane movement is seen at the expected location (1000 Hz), but the hair cells in this zone do not respond because this is the dead zone. The leading edge of this wave also stimulates living hair cells along the basilar membrane at 500 Hz (viable zone).Note the viable zone, the darker shaded area on the left of the figure. This is an area of living hair cells. The point of maximum basilar membrane movement is seen at 1000 Hz, but the hair cells in this zone are dead! The leading edge of this wave also stimulates 500 Hz, a viable zone. The threshold was marked on the audiogram because the patient heard something and raised his hand. In fact, it was the functioning hair cells in the 500-Hz zone that detected the test signal. When you use a 2000-Hz tone to test someone you have to increase the intensity of the tone dramatically to enable the steep front of the “wave” to extend into the regions with viable living inner hair cells (the darker shaded area in Figure 5). This is precisely why high-frequency sensorineural hearing losses (SNHL) associated with cochlear dead regions must be: (1) steeply sloping or precipitous and (2) pronounced in degree. When the slope of the hearing loss takes a drastic drop (over 40 dB per octave), consider the likelihood of dead zones. When we encounter steeply sloping, severe-to-profound high-frequency hearing loss, we need to ask ourselves, how much of the high-frequency zone is really dead? Sometimes, most of the zone is dead, which makes amplification in this zone pointless. Or, we may determine that most of the high-frequency zone is functional, so the hair cells can detect and respond to the signal. In such a case, we conclude that substantial amounts of amplification are needed. Moore has produced a CD designed to evaluate cochlear dead regions.* * To order the CD go to www.hearing.psychol.cam.ac.uk It uses pure-tone test signals and masking noise called Threshold Equalizing Noise (TEN). TEN is broadband noise very different from the masking noise used in an audiometer. It is always presented ipsilaterally to the test ear, not contralaterally, as most masking procedures are. The TEN is unique in that when it is presented it raises all of one's audiometric thresholds to the level of the noise. For example, TEN presented at 30 dB HL to patients with normal hearing should raise their thresholds to 30 dB HL. The pure-tone and TEN stimuli are played on a two-channel audiometer: one channel for tones, the other for TEN. Intensity can be adjusted separately by the audiometer controls. First, test for a patient's thresholds using the pure tones from the CD; next, retest the same ear for thresholds while the TEN masking noise is presented ipsilaterally. The hearing loss shown in Figure 1 has been redrawn in Figures 6A and 6B. Mild masking (50 dB of TEN) shown as shaded bars has been introduced in both cases. In Figure 6A the “high-frequency” hair cells are alive. In Figure 6B they are dead.Figure 6: In panel A, the 1000-Hz threshold did not move because this zone is viable. In Panel B, mild ipsilateral masking lowered all the thresholds by 20 dB. This shows the presence of a dead zone. The original threshold at 1000 Hz is 65 dB: 45-50 dB of masking will produce no effect if this zone is alive.A way to use the CD clinically for testing cochlear dead zones is to present the TEN ipsilaterally with enough intensity to easily mask the better hearing thresholds. Theoretically, the poorer thresholds should not be changed at all by this masking because the intensity level is too soft to mask them. This will be the case if the patient has SNHL without any dead regions. This is what's shown in Figure 6A; the TEN masking noise at 50 dB HL shifted only the 30-dB-HL low-frequency thresholds, while the high-frequency thresholds remained unchanged. Here, the high-frequency inner hair cells are damaged but not dead. The TEN masking was not audible to the patient at these frequencies. In Figure 6B the TEN shifted both the low- and high-frequency thresholds. The low-frequency thresholds shifted because they were masked; the high-frequency thresholds shifted because they did not truly arise from the high-frequency hair cells. This finding shows that the high frequency thresholds actually come from the viable hair cells in the low-frequency zone. It helps to ask the patient to describe the quality of the sound he or she hears. In a viable zone the pure-tone sounds should be clear. The test signal can take on a buzzing or rattling quality when presented to a dead zone. A steeply sloping, severe, high-frequency SNHL should make a clinician suspect a possible cochlear dead region. The patient's report of a non-tonal, scratching type of sound quality might add more fuel to that suspicion. We all see many patients with severely sloping hearing losses. When we fit them with hearing aids we need to know what part of the ear is functional and what part is dead. We want to provide amplification to the working portion while avoiding the dead zone. Often this means providing amplification to the transition frequencies, those between the good areas and the dead zones, rather than the frequencies where the hearing loss is worst. One can help the dying, but not the dead. We will continue this discussion next month when we look at reverse audiometric configurations.