Pathology and Mechanisms of Hearing Loss
The human auditory system is composed of several systems. The ear is comprised of three parts, the external ear, the middle ear and the inner ear. The ear takes up sound from the environment and transduces it into electrical signals, which are carried on by the auditory nerve to the brain.
The external ear (see Video) collects the sound from the outer world and transmits it through the external ear canal to the ear drumor tympanic membrane. In the middle ear the vibrations induced by the sound waves are taken up by the three auditory ossicles, malleus, incus and stapes and are transferred to vibrations of the stapes footplate placed in the oval window of the inner ear. Thus, the middle ear acts as an impedance matching device efficiently transferring sound energy from air into the fluid medium in the cochlea. The oscillation of the stapes footplate in the oval window induces fluid waves in the inner ear watery liquids called perilymph and endolymph. As the fluids move, the cochlear partition (basilar membrane and organ of Corti) moves; the sensory hair cells sense the motion via their cilia, and convert that mechanical motion to electrical signals that are transmitted via neurotransmitters to of the auditory nerve cells of the spiral ganglion in the cochlea. These primary auditory neurons transform the signals into electrochemical impulses known as action potentials, which travel along the auditory nerve to structures in the brainstem for further processing.
Hearing loss can be caused by a variety of reasons, noise, ototoxic drugs or age but may also occur as sudden idiopathic hearing loss especially in men of middle age. Currently noise-induced hearing loss counts for about half of all cases and since this is life-style and environment-related, it is expected that noise trauma will remain the most important reason for hearing loss. However, due to the demographic development the numbers of people with prebyacousis will significantly increase over the next decades.
Hearing Loss induced by ototoxic compounds
Ototoxicity is drug-induced damage to the inner ear and/or vestibular systems that may lead to hearing loss and tinnitus. A wide range of drugs (e.g. platinum based anti-cancer drugs cisplatin and carboplatin, aminoglycoside antibiotics like gentamicin and streptomycin and the loop diuretic furosemide) have been implicated in causing ototoxicity. Even though there is a lot of variability in the reported incidence of hearing loss caused by ototoxic drugs, recent studies report hearing loss in 61% of the 67 paediatric cancer patients undergoing either monotherapy (with cisplatin or carboplatin) or combination therapy. Other sources estimate the prevalence of hearing loss induced by ototoxic drugs to be 600,000 in the US alone.
Noise Induced Hearing Loss (NIHL)
Noise-induced hearing loss can occur when the ear is exposed to noise above ’safe limits‘ and can follow acoustic trauma or prolonged exposure to high levels of sound. Exposure to excessive noise could be in the workplace, military, or by sudden acoustic trauma. There is currently great concern worldwide over the rise in use of personal music listening devices, which can produce sound above the ’safe‘ sound threshold. Prolonged exposure to noise in this way may in the future lead to a rise in cases of noise-induced hearing loss. Around 10 mio people in top 7 markets experience disabling NIHL. The number of individuals suffering any form of NIHL is expected to be far greater. The exact scale of NIHL is not simple to assess. The major hearing loss charities and research groups report many conflicting figures. To assess the number of NIHL, we used data published by RNID biomedical research in 2009 (Noise-induced hearing loss market report, 2009). An overview over the top 7 markets is shown in the following Table.
Prevalence of disabling NIHL in selected WHO countries
|Country||Population 2009||Estimated prevalence of NIHL|
Source: Noise-Induced Hearing Loss Market report, 2009
Similar figures for the US were recently published by the US Department of Health and Human Services (Centers for Disease Control and Prevention), who reports an adjusted severe hearing impairment prevalence ratio of 1.1% among the US population (Morbidity and Mortality weekly report, 2011).
Sudden sensorioneuronal hearing loss (SSHL)
Acute idiopathic sensorineural hearing loss (for review see Schreiber et al. 2010) is defined as a hearing loss of 30 dB or more, over at least three contiguous audiometric frequencies, that develops over 72 hours or less. Typically, adults are affected between the ages of 43 and 53. This is a medical emergency and various possible causal factors have been postulated. National surveys in UK as well as in US have estimated the incidence of SSHL at between five and 30 cases per 100 000 per year. The number of new cases each year could be much higher, since sudden deafness often goes undiagnosed because many people recover quickly and never seek medical help or they think their hearing loss is due to congestion or earwax blockage. A study from Germany has shown an incidence as high as 160 cases per 100 000 per year (Klemm et al., 2009). As all other subtypes of hearing loss, SSHL has been shown to increase over the last years and is expected that the numbers of affected people will to continue to grow. A Japanese study reported a constant increase by comparing epidemiology data over the last 30 years (Theranishi et al., 2007).
Age-related hearing Loss
Hearing loss is the most widespread sensory impairment in aging people. Hearing acuity declines with age – physiologically beginning by the third decade, predominantly in the high frequencies. It begins to affect the frequencies of the speech spectrum within the fifth decade. In a recently published review article the prevalence of age-related hearing loss have been studied across Europe (Roth et al., 2011). As described earlier due to various definitions and inclusion criteria it is difficult to compare prevalence of age-related hearing loss across countries. However data suggest that as many as up to 50% of people older than 65 years in Europe suffer from at least moderate age-related hearing loss. Similar numbers will apply to all western countries and due to the demographic development we can expect these numbers to increase continuously over the next decades.
- Klemm E, Deutscher A, Mosges R (2009) [A present investigation of the epidemiology in idiopathic sudden sensorineural hearing loss]. Laryngo- rhino- otologie 88:524-527.
- Roth TN, Hanebuth D, Probst R (2011) Prevalence of age-related hearing loss in Europe: a review. European archives of oto-rhino-laryngology : official journal of the European Federation of Oto-Rhino-Laryngological Societies 268:1101-1107.
- Schreiber BE, Agrup C, Haskard DO, Luxon LM (2010) Sudden sensorineural hearing loss. Lancet 375:1203-1211.
- Teranishi M, Katayama N, Uchida Y, Tominaga M, Nakashima T (2007) Thirty-year trends in sudden deafness from four nationwide epidemiological surveys in Japan. Acta oto-laryngologica 127:1259-1265.