Deafness is a general term that describes the complete loss of the ability to hear. Deafness may be congenital or acquired, and may be provoked by anomalies affecting any part of the auditory system, e.g., outer, middle or inner ear as well as the vestibulocochlear nerve and higher brain centers.
While older children and adults generally report their inability to hear, deafness may not be easily recognized in neonates and infants. Here, a progressive decline of vocalization and a considerable delay of language and speech acquisition may not be noted by parents before their children are aged six months and older. In such cases of pre-lingual deafness, communication skills cannot be developed appropriately unless the child's condition is recognized and treated at an earlier point in time. In order to assure the correct diagnosis of congenital deafness, newborn hearing screens have been implemented in many countries. However, such programs are often lacking in developing countries . Owing to the inability to hear and thus the inability to interact with their surroundings, many deaf children are initially diagnosed with behavioral disorders like autism or attention deficit. Parents may describe their child to be stubborn, not willing to listen to their advice. Consequently, hearing evaluations may be indicated in case of a suspected behavioral disorder.
Depending on the underlying disease, deaf patients may present a wide variety of additional symptoms that may or may not predominate the clinical picture. Individuals suffering from Down syndrome, for instance, are generally diagnosed with this genetic disorder before their inability to hear becomes apparent . This also applies to other syndrome-related forms of deafness and developmental defects. In contrast, a progressive, bilateral hearing loss is the main and often the only symptom of presbycusis.
Distinct diagnostic measures are available to assess development and function of anatomical structures forming part of the auditory system. The above explained physiological conduction and processing of acoustic waves may serve as a reference for a possible upstream workup. Of course, anamnestic data and clinical presentation of the individual patient may allow for the direct selection of a specific diagnostic approach.
In general, treatment should be initiated as early as possible to avoid irreversible damage to the cochlea or neural structures. In case of neonates and infants, early therapy of deafness is a prerequisite for an undisturbed development of communication skills. Causal therapy of deafness may comprise administration of antimicrobial and anti-inflammatory drugs, myringotomy, tumor resection, reconstructive surgery and a wide variety of other measures depending on the underlying disease. In contrast, hearing aids and/or cochlea implants are required if tissue damage is not reversible by means of the aforementioned measures. Patients suffering from bilateral sensorineural deafness may benefit from brainstem implants. Even if the ability to hear cannot be regained completely, day-to-day communication is largely facilitated if patients hear how words are intonated. It is also very helpful if they can hear environmental sounds like honking cars.
Deaf patients should be offered the possibility to attend linguistic courses guided by speech therapists, and possibly to learn lipreading. Their families should be trained regarding an appropriate dealing with affected individuals. Ideally, such educational programs would be available to any interested person.
Prognosis largely depends on the underlying disease. There is no causative treatment for genetic disorders and congenital malformations, but hearing aids and cochlea implants may significantly improve the quality of life of an affected individual. With regards to infectious diseases like otitis media, morbidity has decreased considerably during the last decades, particularly because more effective drugs have become widely available. In contrast, neural damage is still largely irreversible. Thus, the outcome in cases of sensorineural hearing loss is often worse than that of conductive deafness.
Moreover, pre-lingual deafness may severely interfere with the acquisition of language and speech. Here, early diagnosis and treatment are of utmost importance to improve the infant's ability to hear in order for them to develop appropriate communication skills . With respect to the latter, further unfavorable prognostic factors are inner ear anomalies, a narrow bony cochlear nerve canal as well as additional perinatal issues .
As has been indicated above, functional impairment of a wide variety of anatomical structures may provoke deafness. From an etiological point of view, the inability to hear most commonly results from genetic disorders, pre- or postnatal infections, neoplasms or trauma.
Due to its clinical importance, sudden hearing loss shall be mentioned as another entity that may lead to deafness. It is generally considered idiopathic and can therefore not be assigned to any of the aforementioned groups.
This list is by far not complete and the interested reader is referred to excellent reviews available elsewhere .
Deafness may affect people of both genders, any race and any age. However, there are significant differences regarding the etiology and incidence of deafness in distinct patient groups.
There is no unique chain of pathophysiological events leading to deafness. In order to understand which anatomic structures may be implicated in hearing loss, it is important to comprehend how acoustic waves are recognized.
Acoustic waves propagate in form of otherwise unperceivable alterations in air pressure. These are captured by the auricles, which function as funnels and guide those waves into the external auditory canal. Eventually, they reach the tympanic membrane and cause it to vibrate. These vibrations are transmitted to the auditory ossicles malleus, incus, and stapes. They are suspended in the tympanic cavity and connect to the so-called oval window, which is the point of entry to the inner ear. If the ear is exposed to loud noises, a reflectory contraction of the stapedius muscle attenuates the transmission of vibrations from the stapes to the oval window and thereby protects the inner ear. The inner part of the ear contains the bony labyrinth and within that structure, the membranous labyrinth can be found. It is filled with endolymph and perilymph and does not only perceive acoustic stimuli - which occurs in the cochlea -, but does also play a pivotal role in maintaining balance. The latter is achieved by the vestibular system.
Any disturbance affecting the conduction of acoustic waves into the cochlea may cause conductive hearing loss. Accumulation of fluid in the middle ear and otosclerosis are common causes of this form of hearing impairment.
Pressure waves propagating inside the cochlea stimulate the organ of Corti, the sensory epithelium lining its ducts. Here, hair cells register pressure fluctuations. Depending on the frequency of the acoustic wave, sensory cells located in different parts of the cochlea are stimulated. They are all connected to the auditory nerve, i.e., the cochlear nerve, which is, in turn, part of the eighth cranial nerve, the vestibulocochlear nerve. Activation of ion channels and release of excitatory neurotransmitters allows for the conversion of mechanical stimuli into electrical impulses that may be conducted to higher centers in the brain. After passing medulla, pons and brainstem, the auditory cortex is reached.
If an effective conduction of electrical impulses to the brain or processing of those signals is not possible, the affected individual suffers from sensorineural hearing loss. This condition may be provoked by cerebral ischemia, hemorrhages or inflammatory processes as well as presbycusis. Ototoxic drugs may also destroy cochlear hair cells and cause this type of deafness.
For many forms of deafness, no specific preventive measures can be recommended. However, the risk of developmental defects may be reduced by complying with vaccination plans, by maintaining a healthy lifestyle during pregnancy, by avoiding possible sources of infection. The latter also applies to postnatal care of the ear. Exposure to loud noises should be avoided as far as possible throughout life and, if this is not possible, ear protectors should be worn.
Deafness, sometimes also referred to as hearing loss, is a very broad term that describes a symptom rather than a particular entity. Hearing is a complex cognitive process that depends on the correct functioning of all organs comprising the auditory system. In brief, any severe disturbance of acoustic wave perception, mechanic conduction, conversion into electrical impulses, conduction of nerve signals and processing in the central nervous system may cause hearing impairment and eventually deafness. Accordingly, the following forms of deafness may be distinguished:
Deafness may affect both ears or occur unilaterally, whereby unilateral deafness may or may not be associated with contralateral hearing impairment. Complete bilateral hearing loss severely compromises life quality and this applies particularly to pre-lingual forms of deafness . Here, language and speech acquisition is impaired by the inability to hear. In contrast, acquired hearing loss does typically not affect the patient's ability to speak.
Moreover, deafness is not necessarily an irreversible condition. If the underlying disease can be treated before permanent damage to involved anatomic structures occurs, the ability to hear may be regained partially or completely.
Deafness describes the complete loss of the ability to hear. This condition is severely disabling and thus compromises life quality. In infants, hearing impairment additionally interferes with language and speech acquisition. Fortunately, some forms of deafness can be treated or, even better, avoided. For instance, congenital deafness may result from prenatal infections with pathogens like Treponema pallidum (syphilis), rubella virus (congenital rubella syndrome), or Toxoplasma gondii (toxoplasmosis). This also applies to consumption of alcohol and drugs. With regards to acquired deafness, constant exposure to loud noises, trauma and neoplasms may cause damage to anatomical structures forming part of the auditory system. The former can be reduced by taking preventive measures.
Therapy needs to be adjusted to the cause of deafness. If causal treatment is not available, hearing aids, cochlea implants or brainstem implants may improve the patient's ability to distinguish words and environmental sounds. Speech therapy and educational programs may be attended by the patient and their family in order to learn how to deal with each other.