Auditory Disabilities

Hearing Impairment Classification

The ICIDH distinguishes six degrees of hearing impairment (Table 1):

Designation According to ICIDH	Hearing Loss [in dB HL]
Mild hearing loss	$26 - 40 d B H L$
Moderate hearing loss	$41 - 55 d B H L$
Moderatly severe hearing loss	$56 - 70 d B H L$
Severe hearing loss	$71 - 91 d B H L$
Profound hearing loss (bordering on deafness)^[1]	$> 91 d B H L$
Complete loss of hearing	$-$

Table 1: Categories of hearing impairment according to ICIDH [1].

The term deaf should be used only for persons whose hearing impairment is so severe that they are incapable of deriving any benefit from amplification. The usefulness of drawing a valid line of demarcation between hard of hearing and deaf by means of an indication in dB HL seems to be small. It is much more important to evaluate the functionality of the auditory system in relation to the most socially significant task, the hearing and understanding of spoken language. With this in mind, practitioners suggest drawing the line where auditory communication fails despite amplifying aids [4].

In classifying hearing impairment, the primary distinction is whether the reduction in auditory performance is due to reduced conduction of sound to the receptors (hair cells) of the inner ear or whether there is damage to the cochlea or downstream neural pathways (auditory pathway). After this general distinction, some more typical forms of hearing loss will be discussed

Conductive hearing loss (general)
Conductive hearing loss is caused by a disorder in the external auditory canal (e.g. a plug), in the eardrum or in the middle ear (e.g. stiffening of the ossicles). Hearing is diminished, but never completely lost.
In the bone-threshold audiogram (see Determination of Hearing Ability - Audiometry for details), a conductive hearing loss is expressed by a drop in the air-conduction curve (AC curve), while the bone-conduction curve (BC curve) remains unchanged in that range that is the norm for persons with normal hearing (Figure 1, for the symbols used see Table: Symbols used in the audiogram). This is referred to as the occurrence of a “AC-BC difference” or “air-bone gap”.
Figure 1: Tone threshold audiogram in conductive hearing loss – the air conduction curve (x) is below the bone conduction curve (]) = “air-bone-gap” [3].
Closing the ears with the fingers results in an attenuation of approximately $20 d B H L$ , thus simulating a conductive hearing loss that is even lower than a “mild hearing loss” according to ICIDH (see Table 1).
Sensorineural hearing loss (general).
The causes of sensorineural hearing loss are damage to the inner ear (hearing loss, ototoxicosis, acoustic trauma), the auditory nerve or the central nervous system. In the tone threshold audiogram, sensorineural hearing loss is manifested by a joint drop in the air conduction curve and the bone conduction curve (Figure 2).
Figure 2: Tone threshold audiogram in sensorineural hearing loss – air conduction curve (x) and bone conduction curve (]) in congruence [3].
Sensorineural hearing loss primarily affects the high frequencies, which is noticeable in poor perceptibility of the phonemes “s”, “f” and “sh”. Thus, not only hearing itself is affected, but especially understanding. A (complete) deafness is always caused by a disturbance of the inner ear.
Combined conductive and sensorineural hearing loss (general)
As a third possible general hearing impairment, the combined conductive and sensorineural hearing loss in the tone threshold audiogram shall be presented. Because of the sensorineural hearing loss, the AC and BC curves decrease together. The additional conductive hearing loss causes a further drop of the AC curve (Figure 2).
Figure 3: Tone threshold audiogram in combined conductive and sensorineural hearing loss – both curves lower, but the air conduction curve (x) is even further below the bone conduction curve (]) [3].
Noise-induced hearing loss
Any exposure of the auditory system to high sound pressure levels leads to an upward shift of the hearing threshold. If the exposure is short or if it does not exceed sound pressure levels of $90 d B S P L$ , adaptation, which occurs in all sensory organs, occurs, but the hearing threshold drops back to the values before the sound exposure after a recovery period.
Prolonged exposure to sound, especially when it exceeds $90 d B S P L$ , poses a serious threat to the ear. Three mechanisms may come into play. First, sound exposure (e.g., $15$ minutes at $95 d B S P L$ or $30$ seconds at $115 d B S P L$ ) can mechanically damage hair cells (kinking or fusing of stereocilia). Second, hair cells have an increased energy demand during excitation that cannot be met during prolonged stimulation. If the cell is not allowed to recover in time, the sensory cell may die (starve). Thirdly, noise leads to a stress load of the entire organism, through which the blood circulation and thus the oxygen supply of the ear is reduced and a damaging effect on the inner ear cannot be excluded [3].
Hearing loss triggered by noise exposure particularly affects the frequencies around $4 k H z$ . Apparently, this region of the cochlea enters a supply bottleneck earliest when exposed to noise. Because of the musical note $c^{5}$ (five-dashed C, top key of a piano with $4186 H z$ ) located at $4 k H z$ , it is also called the $c^{5}$ sink^[2] [3]. An audiogram typical of noise-induced hearing loss is shown in Figure 4.
Figure 4: Tone threshold audiogram in noise-induced hearing loss – typical drop at $4000; H z$ , $c^{5}$ dip.
All impulsive sounds are particularly dangerous, because in this case the regulating mechanism of the middle ear (attenuation by contraction of the middle ear muscles) does not take effect in time and the high levels can reach the inner ear unhindered.
While the harmful long-term effects of occupational noise are not only sufficiently well known today (or at least should be) and effective hearing protection measures are available, “recreational noise” poses the far greater threat to public health. The effects of over-loud music (discotheques, open-air concerts^[3], “car sound”) and, in particular, music from portable devices consumed through headphones and earphones pose an enormous risk. All lovers of such “sound sources” must be warned at this point that damage to the hair cells is absolutely irreversible.
Age-related hearing loss – presbycusis (presbyacusis)
Age-related hearing loss (presbycusis) refers to a hearing threshold at higher frequencies that increases with age (the typical course of the audiogram is shown in Figure 5).
Figure 5: Tone threshold audiogram in age-related hearing loss – typical loss in the range of high tones.
Strictly speaking, one can only speak of “true” presbycusis if the hearing loss occurs symmetrically in both ears and if it is also ensured that other factors have not also led to a loss of high frequencies. In most cases, one will have to deal with a mixture of age effects and long-term consequences of noise exposure and drug damage (ototoxic effects of drugs), which cannot be separated from each other^[4].

Tinnitus – Ringing in the Ears

Tinnitus refers to sounds and noises that are perceived by the affected person but are not caused by any external sound source. Approximately $32 %$ of the adult population is affected by tinnitus, with $20 %$ reporting that it affects them so severely that the ringing in the ears is bothersome. In most cases, tinnitus is a concomitant of hearing loss. Most patients with sensorineural hearing loss complain of tinnitus in the high frequency range, while sensorineural hearing loss usually leads to tinnitus in the low frequency range. Otosclerosis, like Meniere's disease, can lead to tinnitus in the low frequency range [7].

Pulse-synchronous ear noises have their cause in circulatory disturbances or are caused by hypertension (increased blood pressure). Tinnitus is generally understood to be the non-pulsating ear noises that manifest themselves as buzzing, humming or noise in the case of diseases of the middle ear, and as hissing or whistling in the case of damage to the inner ear [8].

Triggering cause for tinnitus can be spasms (cramps) of the middle ear muscles, degenerations of the cervical spine or functional disorders of the cochlea. In the latter case, the active movements of the outer hair cells may be affected, making it possible to objectively measure the resulting sounds in the auditory canal. In most cases, however, it is subjective tinnitus, the cause of which is suspected to be a disturbance in the amplification effect of the outer hair cells. This results in increased stimulation of the outer hair cells, which in turn acts on the inner hair cells to produce an excitation perceivable as a tone or noise [3]. Other sources suggest that the cause is an increase in the spontaneous discharge of the nerve fibers of the auditory nerve, since the inhibitory effect of a direct current conducted through the cochlea leads to a reduction in ear noise [7].

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Author: Dr. Wolfgang L. Zagler
Title: Rehabilitationstechnik
Date: March 1, 2008
Location: Vienna, Austria
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Footnotes

The ICIDH uses the term “deafness”, which is frowned upon by those affected. It should therefore be avoided at all costs and always replaced with deafness. The reason for the negative connotation of the words “deafness” and “deaf” is historical. The terms “stupid” or “tumb” (ahd.) and “deaf” or “toub” used to have the same meaning, namely “dull” or also “dull-witted” (Duden) and “befuddled”, “confused” and “stunned” [2]. In ancient times, the ear was considered the seat of memory. For Paracelsus, large ears were signs not only of good hearing, but also of a good memory and a keen mind [3]. ↩︎
Acousticians and musicians use different designations of pitches. In acoustics, the notes belonging to each octave of the piano are given subscripts, so $C_{1}$ denotes the lowest C (first complete octave) on the piano. The highest note of the piano is $C_{8}$ . In music, the pitched octave begins with middle C on the piano. Superscript indices are used here. The designation is therefore $c^{1}$ . The concert pitch lying in the same octave with $440 H z$ has the designation $a^{1}$ . The musical $c^{1}$ thus corresponds to the acoustic $C^{4}$ [5]. ↩︎
The sound pressure level produced by an “average” rock band is given in the literature as $120$ to $130 d B S P L$ . For the group “The Who” values up to $160 d B S P L$ (!) are found. The consequences for the hair cells are easy to imagine. Only the launch of a space shuttle (measured at a distance of $50 m$ ) clearly exceeds this value with $180 d B S P L$ [5, 6]. ↩︎
In this context, it is noteworthy that studies on primitive peoples (Africa and Asia) have shown no signs of presbycusis. It is therefore reasonable to suspect that presbycusis is nothing else than the effect of socially conditioned damage to hearing (socioacusis) integrated over a lifetime [3]. ↩︎

List of Abbreviations

AC: Air conduction
BC: Bone conduction

List of Figures

Figure 1: Tone threshold audiogram in conductive hearing loss – the air conduction curve (x) is below the bone conduction curve (]) = “air-bone-gap” [3].
Figure 2: Tone threshold audiogram in sensorineural hearing loss – air conduction curve (x) and bone conduction curve (]) in congruence [3].
Figure 3: Tone threshold audiogram in combined conductive and sensorineural hearing loss – both curves lower, but the air conduction curve (x) is even further below the bone conduction curve (]) [3].
Figure 4: Tone threshold audiogram in noise-induced hearing loss – typical drop at $4000; H z$ , $c^{5}$ dip.
Figure 5: Tone threshold audiogram in age-related hearing loss – typical loss in the range of high tones.

List of Tables

Table 1: Categories of hearing impairment according to ICIDH [1].

List of Sources

[1]: ICIDH : International classification of impairments, disabilities, and handicapsDie Vorlage enth. insgesamt 2 Werke
[2]: The Biomedical Engineering Handbook (ISBN: 9780849383465)
[3]: Hören: Physiologie, Psychologie und Pathologie (Jürgen Hellbrück, ISBN: 9783801704919)
[4]: Computer for the deaf (and hearing-impaired): towards an integrated solution from a linguistic standpoint (Franz Dotter)
[5]: Einführung in die Psychologie - Informationsaufnahme und -verarbeitung beim Menschen (Human Information Processing) (Peter H. Lindsay, Donald A. Norman)
[6]: Psychology of Perception (Donald F. Kendrik)
[7]: Diagnostic Principals in Neuro-otology: The Auditory System (B. Todd Troost, Melissa A. Waller)
[8]: Pschyrembel Klinisches Wörterbuch (Willibald Pschyrembel, ISBN: 9783110148244, DOI: 10.1515/9783112328545)

Auditory Disabilities ​

Hearing Impairment Classification ​

Tinnitus – Ringing in the Ears ​

Attribution/Citation ​

Attribution ​

Citation ​

Footnotes ​

List of Abbreviations ​

List of Figures ​

List of Tables ​

List of Sources ​