Learning Center: HEARING

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HEARING IS BRAIN HEALTH

Cognitive decline is well known to occur with age—mild for some, and more severe for others. A growing body of evidence now shows that hearing impairment accelerates cognitive decline. Fortunately, this process is modifiable in that improvement in hearing, such as with use of hearing aids, can significantly slow the rate of cognitive decline.

ANATOMY OF THE EAR

Anatomically, the ear is divided into three main components: the outer ear, the middle ear, and the inner ear.

The outer ear or external ear is the external part of the ear, which consists of the visible auricle (also pinna) and the ear canal. It gathers sound energy and focuses it on the eardrum (tympanic membrane).

The visible part is called the auricle or pinna. It is composed of a thin plate of cartilage, covered with skin, and connected to the surrounding parts by ligaments and muscles.

From the pinna, sound waves move into the ear canal (also known as the external acoustic meatus) a simple tube running through to the middle ear. This tube leads inward from the bottom of the auricula and conducts the vibrations to the tympanic cavity and amplifies frequencies in the range 3 kHz to 12 kHz.

The middle ear is the portion of the ear internal to the eardrum, and external to the inner ear. The mammalian middle ear contains three small bones (the ossicles—maleus, incus, and stapes), which transfer the vibrations of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear is also known as the tympanic cavity and is surrounded by the tympanic part of the temporal bone. The Eustachian tube joins the tympanic cavity with the nasal cavity (more specifically, the nasopharynx), allowing pressure to equalize between the middle ear and throat.

The primary function of the middle ear is to efficiently transfer acoustic energy from compression waves in air to fluid–membrane waves within the cochlea.

The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:

  • The cochlea, dedicated to hearing; converting sound pressure patterns from the outer ear into electrochemical impulses which are passed on to the brain via the auditory nerve.

  • The vestibular system, dedicated to balance

The inner ear conducts information to the brain via the eighth cranial nerve, which is really two nerves together, the acoustic nerve and the vestibular nerve, sometimes called the vestibuloacoustic nerve.

TYPES OF HEARING LOSS

The hearing mechanism converts sound waves into electrical signals that are input into the brain. Here is an excellent video from the National Institutes of Health briefly explaining the process. Any type of disruption of this process, anywhere from the outer ear to the brain leads to hearing loss. Conductive hearing loss is a category of hearing loss resulting from any impairment of the mechanical energy of sound traveling through the ear canal to the eardrum, through the three middle ear bones (ossicles). On the other hand, sensorineural hearing loss refers to any impairment of the cochlea (where the mechanical energy is converted to nerve signals) and the auditory nerve.

AN EMERGENCY: SUDDEN SENSORINEURAL HEARING LOSS

It is not usually clear to a patient (or their general physician) whether a sudden hearing loss is sensorineural or conductive. When in doubt, an otolaryngologist’s evaluation can differentiate the two. If the hearing loss is sensorineural, time is of the essence, and prompt diagnosis and treatment may prevent permanent hearing loss.

• Link to frequently asked questions and evidence-based answers on Sudden Sensorineural Hearing Loss on ENTHealth website.

HEARING AIDS

A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers such as Personal Sound Amplification Products (PSAPs) or other plain sound reinforcing systems are not designated as "hearing aids". PSAPs are mostly off-the-shelf amplifiers for people with normal hearing who need a little boost in volume in certain settings. Hearing aids contain a much higher level of technology prescribed to treat a diagnosed hearing loss. PSAPs are not regulated under the Food, Drug and Cosmetic Act because they are not intended to treat, diagnose or cure hearing impairment and do not alter the structure or function of the body. Modern hearing aid devices are computerized electroacoustic systems that transform environmental sound to make it audible, according to audiometric and cognitive rules. Modern devices also utilize sophisticated digital signal processing to try and improve speech intelligibility and comfort for the user. Such signal processing includes feedback management, wide dynamic range compression, directionality, frequency lowering, and noise reduction.

Modern hearing aids require configuration to match the hearing loss, physical features, and lifestyle of the wearer. The hearing aid is fitted to the most recent audiogram (hearing test) and is programmed by frequency. This process is called "fitting" and is performed by a Doctor of Audiology, also called an audiologist (AuD), or by a Hearing Instrument Specialist (HIS). The amount of benefit a hearing aid delivers depends in large part on the quality of its fitting

COCHLEAR IMPLANT

A cochlear implant (CI) is a surgically implanted neuroprosthetic device to provide a person with moderate to profound sensorineural hearing loss a modified sense of sound. CI bypasses the normal acoustic hearing process to replace it with electric signals which directly stimulate the auditory nerve. A person with a cochlear implant receiving intensive auditory training may learn to interpret those signals as sound and speech.

The implant has two main components. The outside component is generally worn behind the ear, but could also be attached to clothing, for example, in young children. This component, the sound processor, contains microphones, electronics that include Digital Signal Processor chips, battery, and a coil which transmits a signal to the implant across the skin. The inside component, the actual implant, has a coil to receive signals, electronics, and an array of electrodes which is placed into the cochlea, which stimulate the cochlear nerve.

The surgical procedure is performed under general anesthesia. Surgical risks are minimal but can include tinnitus, facial nerve injury (temporary or permanent) and dizziness.

From the early days of implants in the 1970s and the 1980s, speech perception via an implant has steadily increased. Many users of modern implants gain reasonable to good hearing and speech perception skills post-implantation, especially when combined with lipreading. One of the challenges that remain with these implants is that hearing and speech understanding skills after implantation show a wide range of variation across individual implant users. Factors such as duration and cause of hearing loss, how the implant is situated in the cochlea, the overall health of the cochlear nerve, but also individual capabilities of re-learning are considered to contribute to this variation, yet no certain predictive factors are known.

Ear fluid

Tinnitus

Tinnitus is the perception of sound when no corresponding external sound is present. While often described as a ringing, it may also sound like a clicking, buzzing, hiss, or roaring. Tinnitus is usually, but not always associated with hearing loss, and may be considered as a low signal-to-noise ratio from the ears.

Tinnitus may have no cure, but there are several treatments for this vexing problem. In brief, distracting noise (such as “white noise"), use of hearing aids, getting good sleep, getting daily exercise, and avoiding known dietary triggers (caffeine, ibuprofen and similar medications, apple cider vinegar, among others), alcohol, avoiding topical causes (sunscreen), and even cognitive behavioral therapy may help. Of course, prevention of tinnitus or its progression is important, and this is accomplished by avoiding excessive noise exposure and avoiding medications that hurt the inner ear (such as certain malaria medications and intravenous antibiotics called aminoglycosides).

• “How Can I Lessen the Impact of Tinnitus” at ENTHealth.org.

• Tinnitus information and resources from the American Speech and Language Association

Auditory Processing Disorder

Auditory processing disorder (APD), rarely known as King-Kopetzky syndrome or auditory disability with normal hearing (ADN), is an umbrella term for a variety of disorders that affect the way the brain processes auditory information. Individuals with APD usually have normal structure and function of the outer, middle, and inner ear (peripheral hearing). However, they cannot process the information they hear in the same way as others do, which leads to difficulties in recognizing and interpreting sounds, especially the sounds composing speech. It is thought that these difficulties arise from dysfunction in the brain.

The American Academy of Audiology notes that APD is diagnosed by difficulties in one or more auditory processes known to reflect the function of the brain. It can affect both children and adults. Its prevalence is currently estimated to be 2–7% in children in US and UK populations. APD can continue into adulthood. Cooper and Gates (1991) estimated the prevalence of adult APD to be 10 to 20%. It has been reported that males are twice as likely to be affected by the disorder as females, and that prevalence is higher in the elderly and increases with age.

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