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3.4.Hearing impairment
3.4.1.Definitions and evaluation
Hearing impairment may be defined to include a reduction in hearing acuity or sensitivity, or presence of tinnitus. It relates primarily to the inability of the affected individual to hear sounds at certain levels. This is tested by presenting of pure tones at frequencies of 250 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 6 kHz and 8 kHz and is shown in steps of 5 dBHL on a chart known as an audiogram. The threshold of hearing is defined as 0 dBHL on the basis of testing a number of young people. It is generally accepted that hearing thresholds lying between 0 dBHL and 20 dBHL across the frequency range tested may be deemed within “normal” limits.* Thus any threshold levels at any of the audiometric frequencies listed above may constitute a hearing loss at that frequency of a given amount raised above normal.
There are two types of hearing impairment, defined according to where the problem occurs:
- Conductive hearing impairment, which is a problem in the outer or middle ear. This type of hearing problem is often medically or surgically treatable, if there is access to the necessary services; childhood middle ear infection is the most common example;
- Sensorineural hearing impairment, which is usually due to a problem with the inner ear, and occasionally with the auditory nerve going from there to the brain.
This type of hearing problem is usually permanent and requires rehabilitation, such as with a hearing aid. Common causes are ageing, excessive noise and ototoxic drugs etc.World Health Organisation defines hearing impairment (www.who.int/pbd/deafness/en/
) as below: Hearing impairment is a broad term used to describe the loss of hearing in one or both ears. There are different levels of hearing impairment:
- Hearing impairment refers to complete or partial loss of the ability to hear from one or both ears. The level of impairment can often be usefully graded as mild, moderate, severe or profound;
- Deafness refers to the complete loss of ability to hear from one or both ears.
Table 4: WHO Grades of hearing impairment
The WHO table (table 4) relates to the remediation after the acquisition of hearing loss and not to the purposes of protection to prevent noise damage. For the purposes of prevention it is important to consider any significant audiometric threshold shift as a sign of impairment. In order to prevent hearing impairment from occurring it is important to assess the sensitivity of hearing change as soon as possible. Consequently, changes in hearing sensitivity between 0 and 20 dB maybe important, especially in children and young people. Another area of concern is the lack of differentiation between hearing acuity or sensitivity which may be deemed “normal” for children, and young people and older adults as hearing is considered to be “normal” below 20 dB HL for all groups.
The above definitions of hearing impairment reflect one aspect of hearing that of an inability to hear sounds of certain level of intensity. Another factor of importance in hearing and understanding speech is the spectral and timing information. This is crucial for clarity of speech hearing. Timing information is also necessary for another hearing function which allows localisation of sounds in space. These aspects are not considered by the above definition of hearing impairment which considers only intensity information.
Hearing impairment may therefore arise despite a “normal audiometric “threshold which may be due to loss of timing information. In these instances a speech test may show a relatively poor score relative to what may be expected from the audiometric evaluation. This is normally seen in neural or central nervous system lesions.
Hearing impairment may also result from loss of frequency selectivity resulting in poor tuning or selective listening for sounds of interest relative to background sounds which may distract from the information of interest. This may be assessed using frequency tuning curves or the relative strength of the efferent auditory system in modulating incoming signals.
Another aspect of hearing impairment may result from a long-lasting buzzing or ringing known as tinnitus. This may result from over exposure to sounds of high intensity and be short lived or may remain a constant irritation for the listener. Tinnitus may also occur as a consequence of developing a hearing loss from any other cause.
* However, WHO proposed to set a limit of hearing impairment to 25 dB(A) (see Table 4)
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3.4.2.Speech communication difficulties
The ability to understand speech can be described by mathematical models like the Speech Intelligibility Index SII (ANSI S3.5-1997, 1997) and Speech Transmission Index STI (Steeneken and Houtgast, 1980), as a function of the hearing loss and speech level. The STI approach shows that a normal-hearing person can understand speech (normal sentences, no contextual information) if about 30% of the information is present (STI of 0.33). Information can be inaudible due to either a masking noise, or speech being below the audibility threshold as determined in the pure-tone audiogram.
Verschuure and van Benthem (1992), using unmodulated speech noise (i..e, a noise with the same long-term spectrum as the speech), showed that the speech-in-noise threshold depends primarily on the high-frequency hearing loss. A 3 dB poorer threshold is considered clinically relevant and means that the communication distance has to be reduced by a factor 2. If we assume that a normal-hearing person can communicate at a party at a distance of about 1 m a high-frequency hearing loss of about 40 dB makes it impossible to do so; we have to come closer to a speaker and reduce the distance to 50 cm, close to the minimum distance socially acceptable. They also showed that hearing aids can only partly (about 2/3) compensate for that loss. This effect can be well described by the STI approach (Plomp et al. 1978; George et al. 2006). The poorer auditory discrimination causes an additional loss of auditory processing. High-frequency hearing loss, whether aided or unaided by hearing aids, will cause poorer speech understanding in a noisy environment.
Everyday environments consist of many different situations, usually with modulated noises in the background. Only when we are at a noisy party with many participants the noise becomes almost unmodulated. The effect on the speech-in-noise threshold of a six speaker babble is still somewhat better than the effect of an unmodulated noise. Tests as described above have been done using modulated speech noise by Smoorenburg et al. (Smoorenburg et al. 1982). They found that the higher the pure-tone hearing loss the poorer was the ability of a person to listen in the gaps of the modulated noise. This resulted in an extra effect of hearing loss on speech intelligibility in noise. For normal-hearing persons the beneficial effect of modulating noise was about 7 dB, for people with a hearing loss of about 60 dB, this advantage over unmodulated was almost nullified.
As to reverberation, Plomp and Duquesnoy (1980) showed that it has the same effect as a background noise. In a population of healthy elderly people he studied the effect of reverberation on the speech-in-noise threshold. He stated that for normal-hearing persons the maximum reverberation time can be quite long. A person with a relative small high-frequency hearing loss of about 40 dB will on average have a S/N ratio of about 0 dB, resulting in a maximum acceptable reverberation time of about 1.7 s. It means that this person can no longer understand speech in a big church or large meeting hall even if it is completely quiet. In case the loss is about 60 dB the person can only communicate well in well-furnished offices, but not in a large living with a modern design interior.
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3.4.3.Tinnitus
Tinnitus may be defined as “a phantom perception of sound“, which a person perceives as spontaneous auditory sensations, such as ringing, buzzing, or hissing in the absence of an external signal. The source of the tinnitus sound lies within, rather than outside, the auditory system. In almost all cases, persons with tinnitus have peripheral or central auditory nervous system involvement. Subjective tinnitus, tinnitus originating within the auditory system, is far more common than objective tinnitus.
Tinnitus is often associated with hearing impairment, ageing and noise (Davis, 1989). Estimates for the prevalence of tinnitus in the population need careful attention to detail concerning the wording used and how the response is obtained (either by postal survey, response to questionnaire in clinic, verbal response to clinician). It is useful to determine the prevalence of tinnitus that last for more that five minutes and is not only after loud sounds (Prolonged Spontaneous Tinnitus, PST), which was determined to be 10% by postal questionnaire and 16% by clinical interview (Davis, 1995). There was evidence that clinicians were not as rigorous as the patients in excluding tinnitus just after loud sounds. Davis et al 2007 have shown that whilst 17.7% of people said that they had ever experience such tinnitus about 4% have tinnitus most of the time and 0.4% have their quality of life substantially affected by tinnitus. An other study by Job et al. (2000) showed that in a representative sample of French youth of 18-24 years old, tinnitus was frequently experienced in 8% of the subjects. In young people Smith et al. (2000) and Lovell et al (1998) showed that 9.2% of young people aged 18-25 years reported PST with only 6.8% in those who had not had substantial social noise exposure equivalent to 80 dB(A) Lequ 40 for 50 years. For those who had a greater social noise exposure then the prevalence of PST was 20%, a substantial increase over those who have less than this significant level of noise exposure.
Many theories exist and have been published about the underlying physiological mechanisms that cause subjective tinnitus (Baguley 2002).
Generally, theories involve hyperactive hair cells or nerve fibers activated by a chemical imbalance across cell membranes or decoupling hair cell stereocilia. An alternate theory proposes that injury to cochlear integrity from any cause reduces the suppressive influence of the central nervous system, allowing increased neuronal activity higher in the auditory system.
Whatever the cellular mechanism is it appears that tinnitus can be caused by abnormal conditions in the cochlea, the cochlear nerve, the ascending auditory pathway, or the auditory cortex.
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3.4.4.Age-associated hearing loss
Hearing ability deteriorates with increasing age in virtually all members of human populations. Numerous studies have quantified this phenomenon, to the extent that it is characterised in international standard ISO 7029 (ISO 7029, 2000). The standard models the distribution of hearing threshold levels in males and females separately in terms of deviations from a baseline set at the age of 18 years. The distributions are semi-normal, defined by mean values and standard deviations representing the upper and lower parts of the distribution. The mean (equal to median) values rise gradually with age at first then accelerate for older people. The standard deviations also increase with age, giving a wide spread for older people. Hearing deteriorates more with age in men than in women. The standard only specifies the distributions up to the age of 80 years, due to limitations on the source data.
For the purposes of the present study, which involves only young adults, ISO 7029 shows little change over the age range from 18-25 years. For example, at the median there is an increase of less than 1 dB at any frequency from age 18 to 25 years in either males or females.
There has been substantial debate about the baseline value used to represent hearing threshold level for 18-year-olds. The original standard implied that a value of 0 dB should be assumed based on numerous studies of highly screened populations of young adults. However, those studies involved participants who were not representative of the population at large and had thresholds that are slightly better than the whole population. More recent studies in the UK have shown that a baseline in the range 2-7 dB is more representative of the otologically normal UK population (Lutman and Davis 1994).
3.4.5. Conclusions
Hearing impairment may be defined as a reduction in hearing acuity or sensitivity, or tinnitus. Hearing loss is the inability of the affected individual to hear sounds at certain levels that can be measured with pure-tone audiometry. According to the WHO Grades of hearing loss no or very slight hearing problems exist when hearing threshold in the better ear is at or below 25 dB.
The reduction in hearing acuity results in the impairment of speech understanding. High-frequency hearing loss, which is typical for age and noise-related hearing impairments may result in worsening speech-in-noise comprehension. It is on average by 1.2 dB per 10 dB. The ability to understand speech may be also impaired despite a normal audiometric threshold due to loss of timing information.
Tinnitus defined as “a phantom perception of sound" is a common problem. It is usually originating within the auditory system and is caused by abnormal conditions in the cochlea, the cochlear nerve, the ascending auditory pathway, or the auditory cortex.
Hearing ability deteriorates with increasing age in virtually all people and this deterioration accelerates for older people. In young adults, up to the age of 40, this process is slow and leads to negligible levels of hearing impairment.
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