Human beings can only hear certain sounds. This is because their hearing apparatus, localized in the ears, respond to a certain spectrum of sound waves. Any sound wave outside of that spectrum is not audible for humans. Sound waves are differentiated based upon their frequency, which is usually measured in Hertz, or Hz. Human beings can perceive sound frequencies from 20 to 20,000 Hz, ideally from 1,000 to 5,000 Hz. Most animals can hear sounds of much higher frequency. Sound waves are mechanical or pressure waves, not electromagnetic, which implies that they require a medium to travel like air, water, or solids, and they do not travel in a vacuum. There is no sound transmission in a vacuum.

 

How is a sound perceived by human beings?

Here is an outline of this concept: There is a complex set of apparatus in each ear for perception of sound waves. Sound waves travel through the air inside the external ear, and due to its mechanical energy, create a rippling effect on the tympanic membrane, or what doctors call the external tympanic membrane. Attached to the inside of this membrane are three tiniest bones in our body (malleus [hammer], anvil [incus], and stirrup [stapes]), which help to transmit this mechanical energy to a narrow window on the other side. This window or the opening on the other side is also covered by a membrane, the so-called internal tympanic membrane. The purpose of this whole system of two membranes, bones and their joints in the middle ear, is to amplify and control the amount of mechanical force transmitted to the internal membrane. On the other side of the internal tympanic membrane is the internal ear, which is a set of two convoluted tubes, one inside the other (a soft tube inside a bony hard shell), shaped like a snail.

The soft inner tube is further divided in two parts, like two tubes molded together. The inner ear is filled with fluid and the nature of fluid in each of the inner two tubes is different. On the floor of one of the inner soft tubes, all along its length, are tiny hair-like structures called cilia. Attached to the cilia are nerve endings that come together and make a nerve, which gets attached to the brain.  Any movement of the internal tympanic membrane creates a ripple in the fluid of the soft tube that creates a movement in the cilia. The location of the maximum movement of cilia in the tube depends upon the wavelength of the sound wave. Sound waves with lower frequency move cilia in a different location compared to the ones with higher frequency. Movement of cilia is noted by the nerve ending underneath and this is where the mechanical signal is converted to an electrochemical signal, which travels through the nerve to the brain. In the brain, there are specialized areas to perceive the sound waves and figure out its significance and meaning.

 

What are different causes of hearing impairment?

Causes of hearing impairment or deafness are easier to understand based upon their type and location of the problem.

Type A: Congenital (deafness starting at birth)

–          Any type of lack of development or abnormal development of hearing apparatus described above

–          Any type of brain disorder resulting in lack of or abnormal sound perception

 

Type B: Acquired (deafness developing during life)

External ear:

-Wax

-Fungus

-Any other foreign body

-Damage to the tympanic membrane, e.g., physical trauma, barotrauma, infection or loud noise

-Abnormal development of ear canal

-Tumor, rare

 

Middle ear:

-Effusion, fluid in the middle ear

-Inflammation

-Infection

-Stiffness of the joints between ear bones, such as in otosclerosis

-Blockage of eustachian tube that creates a vacuum in the middle ear cavity

-Barotrauma

-Tumor, rare

 

Internal ear or the cochlea:

-Loud noise causing damage to the membrane between two inner tubes, or cilia

-Damage to the nerve endings, e.g., from chemical exposure or aging

-High pressure in the system, e.g., in Meniere’s disease.

-Inflammation, auto-immune conditions, or infection

-Mechanical trauma

-Stroke

-Erosion of the bony inner tube

-Genetic causes

-Intracranial hypotension

 

The Nerve (cochlear):

-Tumor of the myelin sheath around the nerve

-Any other tumor or mass pressing the nerve

-Small stroke affecting the nerve

-Inflammation or infection, such as in meningitis

 

The Brain:

-Stroke

-Inflammation, e.g., in Multiple Sclerosis

-Tumor

-Infection

-Trauma

 

What are different types of hearing loss?

Hearing loss or deafness is usually divided in the conductive and the sensorineural types. Conductive hearing loss implies malfunction in the external or the middle ear, whose primary purpose is to conduct sound wave energy to the internal ear. Sensorineural hearing loss is a malfunction beyond the internal tympanic membrane where electrochemical signals are generated, transmitted to the brain, and analyzed. A patient may have either or both types of hearing loss.

 

What should I do if my hearing is affected?

Start with your primary care doctor. There may be something simple or straightforward such as ear wax or middle ear infection, which can be easily treated. An ENT consultation may be needed if ear exam is normal, or if the ear condition does not get better with treatment.

 

How is it determined if a patient has conductive or sensorineural hearing loss?

After physical examination, especially of ears, a tuning fork hearing test is done. Using this instrument, it is not difficult to find out if the hearing loss, especially one-sided hearing loss, is conductive or sensorineural type. An audiogram is especially helpful and provides precise information about the nature and extent of hearing loss.

 

Do I need a CT scan or MRI if I have hearing loss?

Most cases of hearing impairment do not require imaging studies. A CT scan of brain is usually not helpful to investigate hearing problems and is avoided as a screening tool. In some conditions, CT scan of temporal bones may be helpful. Even an MRI is not needed in all cases. It is helpful only in certain conditions, especially if a stroke or a tumor is suspected. The ordering physician should be aware of what exactly is required from imaging, when ordering a CT scan or MRI, as hearing loss is investigated with special sequences, which may not be done if the test is not ordered properly. The decision of testing is made after taking a careful history, physical examination, an audiogram in most cases, and reaching a conclusion for the likely cause.

 

How is deafness treated?

It depends upon the cause. Some causes are fully treatable with hearing back to normal or the baseline, but numerous other causes have no definitive treatment. This is the case when a part of hearing apparatus is either not developed or damaged, especially anything beyond the internal tympanic membrane. In such situations the option is either a hearing aid or a cochlear implant. A hearing aid is basically a device to amplify sound waves reaching the internal ear. The cochlear implant is a sophisticated device which may replace the function of internal ear by catching sound waves, changing it in electrical signals, and transmitting it to the cochlear nerve to be taken to the brain. In this manner, a cochlear implant can create a sound signal for patients who otherwise may not hear it at all, while a hearing aid amplifies sound for someone with decreased hearing.

 

Where can I get more information about hearing impairment?

American Academy of Neurology

American Society of Otolaryngology

American Academy of Audiology