The ear is the organ of hearing and balance. One of the very advanced sensitive organs of the human body, the ear detects, transmits, and transduces sound to the brain and maintains a sense of balance. Just as ripples are spread out in circles from the point where a stone is dropped in water, the sound source creates pressure variations in the air, which are perceived by the ear as sound.
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Examination of the ear.
The outer ear consists of the pinna or auricle, the part that we see on the side of the head, and the tube or external auditory canal that connects the outer ear to the middle ear through which sound travels.
The tympanic membrane or the eardrum lies between the outer and middle ear.
The middle ear is composed of ossicles and the eustachian tube. Ossicles consist of three small bones–malleus, incus, and stapes—that are coupled to transmit sound waves to the inner ear. The eustachian tube, lined with mucous, is a canal that helps to equalize pressure in the middle ear so that the sound waves are transferred properly. Sound enters the outer ear and makes the eardrums vibrate, and the vibrations are passed along the ossicles.
The inner ear consists of the cochlea, vestibule, and semicircular canals. The cochlea is the hearing part, comprising a coiled spiral tube containing nerves for hearing. The cochlea, a snail-shaped and fluid-filled structure, is in the inner ear. An elastic partition, which is the basilar membrane (serves as a base on which the important hearing structures resides), runs from the start to the end of the cochlea and splits it into an upper and a lower part.
Vestibule and semicircular canals are part of the balance system. The semicircular canals provide information to the brain about the direction of the head movement.
The process of hearing
Many stages are involved in the process of transduction i.e. the complex process by which sound waves are transformed into electrical signals, which are then conveyed by the auditory nerve to the brain.
When we hear a sound, it is transmitted as a wave and reaches the outer ear. The sound waves pass through the ear canal, a slender passage, leading to the eardrum. When the eardrum is struck, the vibrations are sent to the ossicles in the middle ear. These small bones are responsible for amplifying the vibrations and sending them to the cochlea. A rippling effect is caused by the vibrations in the cochlea, and this results in the formation of a traveling wave along the basilar membrane. The sensory cells present on the top of the basilar membrane, called hair cells, recognize the sound waves.
Detection of a sound is dependent on its pitch – while high-pitched sounds are detected by the hair cells near the wide end of the cochlea, low-pitched sounds are identified by the hair cells closer to the center of the cochlea.
Stereocilia are vital for the process of electro-mechanical transduction. They are the microscopic hair-like projections that rest on and protrude from the top of the hair cells. These knock against a covering structure and are deflected when the hair cells move up and down. This process opens up the stretch-sensitive ion channels present at the tips of the stereocilia and allows the influx of specific cations, namely K+ and Ca2+, which generates an electrical signal. This signal travels through the auditory nerve to the brain, which is ultimately responsible for the processing of sounds, and recognition of inter-sound variability.
Pitch and intensity
It is important to understand the terms pitch and loudness, as together they are used to describe characteristics of a sound. The pitch, or sound frequency, is measured in hertz (Hz) and the loudness, or the intensity, of the sound is measured in decibels (dB).
Typically, the most important sounds that humans hear daily are in the range of 250–6,000 Hz where the normal ranges are 20-20,000 Hz. However, hearing becomes most sensitive in the 2000–5000 Hz frequency range.
The audible range for humans is 0-140 dB. While 0 decibels is the quietest, a whisper is around 25–30 dB and conversations are usually 45-60 dB, as speech is a combination of low- and high-frequency sounds.
Hearing loss
If there are obstacles that stop the sound passing through the outer or inner ear, there could be conductive hearing loss. In sensorineural hearing loss, the problem could be with the inner ear or the hearing nerve. A combination of both may result in mixed hearing loss.
The sound information is passed from the ear to the brain via the auditory nerve, with the information processed by the auditory pathway as it travels along the auditory hearing system. This indicated the great role of brain pathways in the hearing process. Although sound can enter the ears, if there are damages to the inner ear or the auditory nerve, the brain cannot ‘understand’ the sound, resulting in Auditory Neuropathy Spectrum Disorder.
Exposure to more than 85 dB for long periods is dangerous to the hearing and may lead to temporary or permanent hearing loss.
Other aspect of hearing loss
Hearing loss is most commonly caused due to two major causes – age related hearing loss and hearing loss due to exposure to loud noises of acoustic trauma. There are numerous other causes that include disease and drugs that may damage the functioning of the ears and lead to hearing loss.
Causes of hearing loss include age related hearing loss, acoustic trauma and so forth. 1-5
Age related hearing loss
This also called presbyacusis and is the largest single cause of hearing impairment. Hearing loss begins normally between ages of 30 and 40. This increases with age until by 80 most people have significant hearing impairment.
Presbyacusis occurs when the sensitive hair cells inside the cochlea gradually become damaged or die.
Common manifestations initially are inability to hear high-frequency sounds, such as female or children’s voices and inability to hear consonants, such as the letters s, t, k, p and f. This makes understanding conversations difficult especially with a background noise.
Hearing loss due to acoustic trauma
Ear damage may be caused due to exposure to loud noises over long periods of time. This repeated exposure to noise causes damage to the delicate inner structure of the ear. The hair cells within the cochlea may be inflamed.
Acoustic trauma and the extent of damage depend on the length of time and the loudness of the noise the person is exposed to. People who work in noisy environment and those who listen to music at a high volume through headphones are at a greater risk.
Hearing loss due to a perforated ear drum
This occurs due to injury or infection of the ear. In children an acute ear infection (acute otitis media) may lead to formation of pus within the middle ear. This may bulge out through the ear drum and the membrane may be perforated. Injury with sharp objects may also cause ear drum to perforate. This leads to a conductive hearing loss.
Hearing loss due to ear wax
Ear wax may get impacted within the ear canal and cause blockage of the ear canal leading to hearing loss. An obstruction of the ear canal with a foreign body like a bead or a small object may also act similarly. This leads to a conductive hearing loss.
Hearing loss due to Otosclerosis
This occurs when there is an abnormal growth of bone in the middle ear. This leads to lack of movement of the ossicles and the sound waves cannot be passed into the inner ear. This leads to a conductive hearing loss.
Genetic hearing loss
Sometimes hearing loss may run in families. This is more commonly of the sensorineural type.
Hearing loss due to viral infections
Viral infections like Mumps, measles and Rubella (German measles) may also cause hearing loss especially in children. This is more commonly of the sensorineural type. Brain infections like meningitis or encephalitis may also cause hearing loss.
Hearing loss due to Meniere’s disease
Meniere’s disease leads to a sensorineural hearing loss along with vertigo and tinnitus or ringing of the ears and feeling of pressure within the ears.
Hearing loss due to acoustic neuroma
A tumor of the auditory nerve that is non-cancerous may also lead to a sensorineural hearing loss.
Other causes of hearing loss
Multiple sclerosis and stroke are other causes of neurological hearing loss.
Head injury may also lead to hearing loss.
Children born with hearing impairments
Children with congenital conditions like Down syndrome, Usher syndrome, Treacher Collins syndrome, Crouzon syndrome, and Alport syndrome may be born with hearing impairment.
Hearing loss as a consequence of some drugs
Some drugs like those used in cancer and antibiotics like Aminoglycoside (gentamicin, Amikacin, Streptomycin etc.) may lead to nerve damage within the ear and hearing loss.
Treatment of hearing loss
- Removal of wax or foreign object that is leading to a conductive hearing loss
- An acute or long term infection of the middle ear may also lead to hearing loss. These need to be treated with appropriate antibiotics.
- Ear drum perforation after infection or injury can be corrected with a surgery called tympanoplasty. A flap of tissue is taken and the membrane is repaired to restore hearing.
Hearing aids
These are devices that serve to increase the volume of the sounds entering the ear and help the person to hear more clearly. These include parts like a microphone, amplifier, loudspeaker and a battery.
Earlier hearing aids were large and often visible. These days the aids are very small and discreet and can be worn inside the ear. The aid works simply with the microphone picking up the sound, and the amplifier increasing its volume.
These can also distinguish between background noise and conversation and selectively help hear conversation.
Behind-the-ear (BTE) hearing aids are available these days that sit within the ear and have a potion that goes behind the ear. These may have two microphones that help the patient to listen to sounds in the general vicinity or to focus on a conversation or sound from a particular direction.
Other types are In-the-ear (ITE) hearing aids and Completely in-the-canal (CIC) hearing aids.
Hearing aids may also be Body-worn (BW) hearing aids with a small box containing the microphone clipped to the clothes or placed within the pocket.
These may be used with best benefits in mild to moderate hearing loss and are useless in profound and sensorineural deafness.
Some hearing aids are useful in bone conduction defects. These vibrate in response to the sounds going into the microphone. Another variety is the bone anchored hearing aid called the Bone Anchored Hearing Aids (BAHA).
Another variety is the CROS hearing aid that is useful in people who only have hearing in one ear. The aid picks up sounds to the bad ear and transmits it to the good ear.
BiCROS hearing aids may be used similarly in people who do not have any hearing in one ear and have limited hearing in the other ear. Some people may also use Disposable hearing aids.
Other treatments for hearing loss
- Some patients may need middle ear implants as well. These are inserted within the ear with the help of a short surgical procedure. These help the ossicles to conduct the sounds within the ear.
- Cochlear implants – These are inner ear implants that can be placed within the ear with a surgery. These may be used in patients with sensorineural hearing loss in both ears.
- Patients with moderate to profound hearing loss need to learn a sign language or lip reading. These include the British Sign Language (BSL) or Signed English and Paget Gorman Signed Speech.
Methods to cope with hearing loss
Patients with hearing loss may adopt simple tips to cope. These include:
- Patients need to own up about their hearing impairment so that speakers speak louder and clearer and do not resort to short forms and whispers
- Looking at the lips of the speaker when they speak helps
- Conversations can be held at quieter places for easy understandability
- Family and friend support for the patients with hearing loss is vital for coping with hearing loss
Prevention of hearing loss
Prevention of hearing loss involves reducing the risks. These include:
- Ear infections need to be treated in childhood so that they do not lead to long term hearing loss
- Ear wax accumulation should be prevented
- Nothing should be inserted within the ear canal. This includes toys, cotton buds, cotton wool and tissue.
- Loud noises from music systems, work environment can damage hearing. These should be avoided or special ear protection gear should be adopted.
- Since certain viral infections may lead to hearing loss all children need vaccination against measles, mumps and rubella infections
Symptoms and Diagnosis of Hearing Loss
Hearing impairment may be present in some conditions since birth. The symptoms of this condition include:
- the baby not being startled with a loud noise
- not turning his or her head to the source of a sound while under four months old
- not say any word even by the age of one
- sees the mother but does not respond to her calling him or her
- seems to hear some sounds but not all sounds
In general, children with delay in learning to talk, unclear speech, talking loudly or asked the speaker to repeat themselves and turning up the volume of the TV so that it is very loud indicates that there may be hearing impairment.
Symptoms of hearing loss
Common complaints include:
- difficulty in hearing and understanding conversations
- difficulty in following conversations over the telephone
- listening to music or television at a volumes higher than other persons
- missing door bells or phone rings
- difficulty placing the direction of the oncoming sound
Patients often ask the speaker to repeat and have more difficulty in understanding women and children. They may also have a history of working in a noisy environment.
Symptoms of mild hearing impairment
This means that the patient can hear the quietest sounds between 25 to 39dB. This type of hearing loss means difficulty in understanding whispered speech.
Symptoms of moderate hearing impairment
These persons can hear the quietest sound between 40 to 69dB. A normal conversation becomes difficult for these persons to follow.
Symptoms of severe hearing impairment
The quietest sound that these patients can hear is between 70 to 89dB. These patients cannot hear shouted words and may need sign language for communication.
Symptoms of profound hearing impairment
These patients cannot hear very loud sounds that normally hurt the ears of normal hearing persons.
Other symptoms of hearing loss
Other symptoms include:
- discharge of clear fluid or pus from the ears
- pain in the affected ear
- dizziness
- tinnitus or ringing of the ear
- feeling of fullness of the affected ear
Diagnosis of hearing loss
Diagnosis of hearing loss involves ear examination, hearing tests and so forth.
Ear examination
The external ear canal is examined using an otoscope or an auriscope. This helps in detection of a blockage in the ear canal with ear wax or with a foreign object (e.g. a bead), ear canal infection, bulging eardrum indicating an acute middle ear infection, perforation of the ear drum and collection within the ear.
Hearing tests
These are performed using a tuning fork test. A tuning fork is a Y-shaped, metallic object. It is tapped and this releases sound waves at a fixed pitch. The tuning fork is then held close to either ear to detect hearing impairment in the ear.
The physician may also whisper near the ear to detect loss of hearing in the ear. This tests for air conduction of the sound.
Tuning fork tests
A hearing test is followed by further tests called tuning fork tests. Once the tapped tuning fork begins to vibrate, it is held against the bone behind the ear called the mastoid bone.
The sound waves move to the ear and the person hears the sound in the normal ear. This tests for the bone conduction of the sound.
Audiometry test
This is also called Pure Tone Audiometry. Both air and bone conduction can be tested using audiometry. For air conduction, sounds are transmitted to the ear using earphones and the patient is asked to indicate when he or she hears the sound.
Bone conduction is tested to see how well the sound is transmitted through the bone rather than the air. These check for the nerves that transmit sound to the brain as well.
