How we hear
Hearing is one of human beings’ five senses. It is a complex process of collecting sounds and assigning meaning to them.
The human ear is fully developed at the time of birth and is capable of responding to very soft and very loud sounds. In the uterus the foetus already reacts to sounds.
But how does this mechanism work and what are the structures involved?
The human hearing system may be divided into two parts: the peripheral auditory system and the central auditory system (consisting of the auditory cortex and the brainstem nuclei).
The peripheral auditory system is composed of the outer ear, the middle ear and the inner ear.
The outer ear consists of the auricle (the visible part of the ear) and the auditory canal, which is totallyenclosed in the tympanic membrane (the eardrum).
The ear/auricle is a cartilage and mostly collects surrounding sound vibrations, which travel to the external auditory canal. It is important for determining the direction and source of sound.
The external auditory canal extends from the auricle to the tympanic membrane. The external 2/3 of the canal appear as folds of cartilage covered by skin with hair and cerumen-secreting glands. The hair and the cerumen help prevent the entry of foreign bodies, such as insects or dust.
The middle ear includes a cavity with ossicles: the hammer, the anvil and the stirrup are attached to each other and enable communication from the outer ear (tympanic membrane) to the inner ear (oval window). The hammer, which is attached to the eardrum, is in turn attached to the anvil, which is the bridge between the hammer and the stirrup. The mechanical energy produced by this chain of ossicles causes an oscillating movement at the base of the stirrup that travels to the inner ear.
Besides the fundamental part of transmitting sound waves, the middle ear is also very important for maintaining a constant pressure gradient inside the ear and out. This function is only possible through the Eustachian tube that links the nasopharynx to the middle ear.
The inner ear includes the sensory organs for hearing and balance. It includes the cochlea for hearing, and the vestibule (utricle and saccule and semicircular canals) for balance.
The loss of hearing is one of the most common sensory disabilities, affecting 28 million North Americans. Approximately 1 to 3 of every 1,000 children are born deaf. Elderly men and women are also among those most affected, as 40-50% of people over 75 experience hearing loss. In Europe, about 71 million adults between the ages of 18 and 80 have a hearing loss greater than 25 dB, the definition of hearing impairment recognised by the World Health Organisation, WHO. In the EU alone, over 55 million people suffer from hearing loss.
Hearing impairment refers to any loss of the ability to hear. It is generally described according to 3 parameters: type, degree and configuration.
There are many causes linked to hearing loss.
The aetiology of hearing loss in children
Congenital hearing impairment means hearing loss present at birth. It can include hereditary hearing loss or hearing loss due to other factors present either in uterus (prenatal) or at the time of birth.
Hereditary causes are thought to cause more than 50% of all incidents of congenital hearing loss. Genetic transmission may be autosomal dominant (when one parent, usually hearing impaired, carries the dominant gene for hearing loss and passes it on to the child in 50% of the cases); in autosomal recessive hearing loss (when both parents who are typically normal, i.e., who do not manifest any hearing impairment, carry a recessive gene), the probability of the child having a hearing loss is 25%; in X-linked hearing loss the mother carries the recessive trait for hearing loss on the sex chromosome and passes on the trait to her children, but usually only male children are affected).
There are some genetic syndromes, in which hearing loss is one of the known characteristics. Some examples are Down syndrome (abnormality on a gene), Usher syndrome (autosomal recessive), Treacher Collins syndrome (autosomal dominant), Crouzon syndrome (autosomal dominant), and Alport syndrome (X-linked).
Other causes of congenital hearing loss that are not hereditary in nature include prenatal infections, illnesses, toxins consumed by the mother during pregnancy or other conditions occurring at the time of birth or shortly thereafter. These conditions typically cause sensorineural hearing loss. They include:
- Intrauterine infections such as rubella, cytomegalovirus and herpes simplex infection.
- Complications associated with the Rh factor
- Premature birth
- Maternal diabetes
- Toxaemia of pregnancy
Acquired hearing loss that occurs after birth, at any stage of life, is caused by different clinical conditions, including:
- Middle ear Infections
- Ototoxic drugs (damaging the hearing system)
- Chicken pox
- Head trauma
- Exposure to noise
The aetiology of hearing loss in adults
Hearing impairment in adults may have multiple causes, its characteristics thus varying according to the underlying aetiology.
Examples of the causes of hearing loss in early adulthood:
- Otosclerosis – condition involving the otic capsule, affecting stirrup movement, in particular.
- Meniere’s disease – involves the membranous duct of the inner ear and is characterised by hearing loss, vertigo and tinnitus.
- Drugs used that harm the hearing system (called cytotoxic) and cause hearing loss. Some of these pharmaceuticals are well known: Aminoglycoside antibiotics (streptomycin, kanamycin and neomycin), large doses of salicylate (most common being aspirin), loop diuretics (furosemide – LasikÒand ethacrynic acid) and drugs used in chemotherapy medication (cisplatin, carboplatin and nitrogen mustard).
- Exposure to high levels of noise – long-term exposure to noise damages the cochlea hair cells, resulting in permanent hearing loss. Such injury may also result from trauma to the ear – single or infrequent exposure to very high noise levels.
- Acoustic neuroma – example of a tumour that causes hearing impairment. It originates in the auditory nerve. Primary symptom is the loss of hearing in one ear, accompanied by the feeling of full ear.
- Head or ear trauma – may be related to temporal bone fracture, perforated eardrum caused by a foreign body or rapid air pressure changes.
- Ageing – presbycusis – particularly involves degeneration of the inner ear structures, and possibly other parts of the hearing system. Hearing loss is progressive, mostly affecting high-pitched sounds.
Forms of treatment and recovery
Once the characteristics and causes of the hearing impairment have been identified, a strategy to try to solve such impairment may be established or, where this is not an option, action can be taken to minimise the impact on daily life.
There are various treatment options for hearing impairment, which are understandably related to the underlying cause and the severity/degree of impairment.
In the case of transmission hearing loss, treatment aims at removing the barrier to the propagation of sound waves. This may include removing cerumen, antibiotic therapy for infection treatment, eliminating fluid inside the middle ear via medical therapy or surgery, optimising the functioning of the Eustachian tubes, surgical repair of ruptured eardrum and even removing and replacing an ossicle in the middle ear with a prosthesis.
Sensorineural hearing loss is slightly different, as there is no physical barrier to sound propagation. The hair cells and/or auditory nerve fibres are damaged, in which case some strategies may be adopted to try to reduce the impairment.
A variety of simple procedures may help to significantly increase the capacity to perceive speech – the main complaint of patients with neurosensorial hearing loss. The first critical step consists of eliminating or reducing unnecessary sound (for example, radio or television) to enhance the signal-to-noise ratio. Comprehension is also reinforced through lip reading, which is why listener and speaker must stand face to face. The speaker must speak slowly, although speaking too slowly may cause distraction and loss of context.
Hearing-impaired people may benefit from amplification, achieved through the use of a hearing aid. Although in the past they were synonymous with social stigma, today hearing aids come in all sorts of shapes and sizes and tend to go unnoticed. The choice depends on the characteristics of the hearing impairment. Most devices are digital, thus being programmed for each individual. Additionally, the existence of multiple and multidirectional microphones helps manage the surrounding noise.
In the case of more severe hearing loss or when the amplification offered by hearing aids is insufficient, there are other options, in particular, implantable devices or osteointegration – such as middle-ear implants – like Vibrant Soundbridge and osteointegrated implants – like BAHA.
Cochlear implant is an implantable hearing aid that converts sound energy into electric signals and can be used to stimulate the auditory nerve. In such cases the hair cells are not functioning, thus being necessary to bypass this section of the auditory pathway and stimulate the auditory nerve directly.
When the auditory nerve is also injured or damaged, other options must be weighed. The brainstem implant is available in such cases. This is a device that is also implantable, but unlike the cochlear implant, which is placed in the cochlea as the name suggests and stimulates the auditory nerve, in the brainstem implant an excitatory electrode is placed within the brainstem to stimulate the central auditory pathway through which information travels to the brain cortex.
In short, there are several options available for treating hearing impairment. The choices are personal and are made by the hearing-impaired individual.