Different Types Of Hearing Loss There are different types of hearing loss, depending on which part of the hearing pathway is affected. A specialist will always try to localize where in the hearing pathway the problem lays, so as to be able to classify the hearing loss as belonging to one of the following groups. This is most important in determining the appropriate treatment. They are: Conductive hearing loss Sensorineural hearing loss Central hearing loss Functional hearing loss Mixed hearing loss Conductive Hearing Loss Conductive hearing loss is due to any condition that interferes with the transmission of sound through the outer and middle ear to the inner ear. This type of hearing loss can be successfully treated in most cases. In cases of conductive hearing loss, sound waves are not transmitted effectively to the inner ear because of some interference in: The external ear canal The mobility of the eardrum (problems with the mobility of the eardrum are often caused by accumulation of fluid in the eustachian tube, the tube that connects the middle ear to the back of the throat) The three tiny bones inside the middle ear The middle-ear cavity The openings into the inner ear The eustachian tube Modern techniques make it possible to cure or at least improve the vast majority of cases involving problems with the outer or middle ear. Even if people with conductive hearing loss are not improved medically or surgically, they stand to benefit greatly from a hearing aid, because what they need most is amplification. Sensorineural Hearing Loss In sensorineural hearing loss, the damage lies in the inner ear, the acoustic nerve, or both. Most physicians call this condition "nerve deafness." The cochlea has approximately 30,000 hearing nerve endings (hair cells). The hair cells in the large end of the cochlea respond to very high-pitched sounds, and those in the small end (and throughout much of the rest of the cochlea) respond to low-pitched sounds. These hair cells, and the nerve that connects them to the brain, are susceptible to damage from a variety of causes. The term "sensory" hearing loss is applied when the damage is in the inner ear. Common synonyms are "cochlear" or "inner-ear" hearing loss. "Neural" hearing loss is the correct term to use when the damage is in the acoustic nerve, anywhere between its fibers at the base of the hair cells and the relay stations in the brain (the auditory nuclei). Other common names for this type of loss are "nerve deafness" and "retrocochlear" hearing loss. Nice To Know: Sensorineural hearing loss is one of the most challenging problems in medicine. A large variety of hearing impairments fall under this category. Although the chances for restoring a sensorineural hearing loss are slim, a small number of cases can be treated, and some people experience dramatic improvements as a result. However, a great need for further research in this area still exists. Central Hearing Loss In central hearing loss, the problem lies in the central nervous system, at some point within the brain. Interpreting speech is a complex task. Some people can hear perfectly well but have trouble interpreting or understanding what is being said. Although information about central hearing loss is accumulating, it remains somewhat a mystery in otology (the medical specialty of ear medicine and surgery). A condition called central auditory processing disorder frequently leads people to think they have hearing loss when their hearing is actually normal. Despite the fact that this problem is extremely common and present in many highly successful people, it is actually classified as a learning disability. Basically, the problem involves a person's inability to filter out competing auditory signals. People with central auditory processing disorders have difficulties that include: Problems "hearing" when there are several conversations going on Inability to read or study with the radio or television on Problems reading if someone turns on a vacuum cleaner or air conditioner near them Generally missing the first sentence from people talking to them if they are involved in an auditory attention task (such as watching television) Although such people (and their families and friends) frequently suspect that they have a hearing loss, the function of the ears is usually normal, and routine hearing tests are normal. Naturally, people with this condition may also develop hearing loss from other causes, and this can make it even more difficult for them to function under everyday circumstances. There is no good treatment for central auditory processing disorders other than educating the person, family, and friends, and trying to control the environment. This is especially important for children, whose grades may go from F to A if they are provided with a silent place in which to do their homework. Nice To Know: The test necessary to diagnose central hearing impairment must be designed to assess a person's ability to handle complex information. Most of the tests now available were not created specifically for this purpose. It requires a very experienced and almost intuitive judgment on the physician's part to make an accurate diagnosis. Functional Hearing Loss Functional hearing loss involves a psychological or emotional problem, rather than physical damage to the hearing pathway. Individuals with this type of hearing loss do not seem to hear or respond; yet, in reality, they have normal hearing. The most important challenge for physicians is to classify this condition properly. It may be difficult to determine the specific emotional cause, but if the classification is made accurately, the proper therapy can begin. Too often, a functional hearing loss is not recognized, and individuals receive useless treatments for prolonged periods. In turn, this process may aggravate the emotional element and cause the condition to become more resistant to treatment. Mixed Hearing Loss Frequently, a person experiences two or more types of hearing impairment, and this is called mixed hearing loss. This term is used only when both conductive and sensorineural hearing losses are present in the same ear. However, the emphasis is on the conductive hearing loss, because available therapy is so much more effective for this disorder.

The Ear: Auditory and Vestibular Systems

Contents:

• Anatomy of the Ear
• How We Hear
• How We Maintain Balance
• Developmental Aspects:
Infant Screening
• To learn more

Introduction
The ears are paired sensory organs comprising the auditory system, involved in the detection of sound, and the vestibular system, involved with maintaining body balance/ equilibrium. The ear divides anatomically and functionally into three regions: the external ear, the middle ear, and the inner ear. All three regions are involved in hearing. Only the inner ear functions in the vestibular system. 
Anatomy of the Ear
The external ear (or pinna, the part you can see) serves to protect the tympanic membrane (eardrum), as well to collect and direct sound waves through the ear canal to the eardrum. About 1¼ inches long, the canal contains modified sweat glands that secrete cerumen, or earwax. Too much cerumen can block sound transmission. 

Click on the image below for more information about each region.

The middle ear, separated from the external ear by the eardrum, is an air-filled cavity (tympanic cavity) carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube. This ear-throat connection makes the ear susceptible to infection (otitis media). The eustachian tube functions to equalize air pressure on both sides of the eardrum. Normally the walls of the tube are collapsed. Swallowing and chewing actions open the tube to allow air in or out, as needed for equalization. Equalizing air pressure ensures that the eardrum vibrates maximally when struck by sound waves.

Adjoining the eardrum are three linked, movable bones called "ossicles," which convert the sound waves striking the eardrum into mechanical vibrations. The smallest bones in the human body, the ossicles are named for their shape. The hammer (malleus) joins the inside of the eardrum. The anvil (incus), the middle bone, connects to the hammer and to the stirrup (stapes). The base of the stirrup, the footplate, fills the oval window which leads to the inner ear.

The inner ear consists of a maze of fluid-filled tubes, running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph.  Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth, filled with the fluid called endolymph. This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti. There are three major sections of the bony labyrinth: 

1. The front portion is the snail-shaped cochlea, which functions in hearing. 
2. The rear part, the semicircular canals, helps maintain balance. 
3. Interconnecting the cochlea and the semicircular canals is the vestibule, containing the sense organs responsible for balance, the utricle and saccule. 

The inner ear has two membrane-covered outlets into the air-filled middle ear - the oval window and the round window. The oval window sits immediately behind the stapes, the third middle ear bone, and begins vibrating when "struck" by the stapes. This sets the fluid of the inner ear sloshing back and forth. The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. Nerve impulses generated in the inner ear travel along the vestibulocochlear nerve (cranial nerve VIII), which leads to the brain. This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium.  How We Hear - The Auditory System
Animation:  How the ear hears
All sounds (music, voice, a mouse-click, etc.) send out vibrations, or sound waves. Sound waves do not travel in a vacuum, but rather require a medium for sound transmission, e.g. air or fluid. What actually travels are alternating successions of increased pressure in the medium, followed by decreased pressure. These vibrations occur at various frequencies, not all of which the human ear can hear. Only those frequencies ranging from 20 to 20,000 Hz  (Hz = hertz = cycles/sec) can be perceived. 
In hearing, air-borne sound waves funnel down through the ear canal and strike the eardrum, causing it to vibrate. The vibrations are passed to the small bones of the middle ear (ossicles), which form a system of interlinked mechanical levers: First, vibrations pass to the malleus (hammer), which pushes the incus (anvil), which pushes the stapes (stirrup). The base of the stapes rocks in and out against the oval window - this is the entrance for the vibrations. The stapes agitates the perilymph of the bony labyrinth. At this point, the vibrations become fluid-borne. The perilymph, in turn, transmits the vibrations to the endolymph of the membranous labyrinth and, thence, to the hair cells of the organ of Corti. It is the movement of these hair cells which convert the vibrations into nerve impulses. The round window dissipates the pressure generated by the fluid vibrations, thus serves as the release valve: It can push out or expand as needed.  The nerve impulses travel over the cochlear nerve to the auditory cortex of the brain, which interprets the impulses as sound. 
How We Balance - The Vestibular System
The semicircular canals and vestibule function to sense movement (acceleration and deceleration) and static position. The three semicircular canals lie perpendicular to each other, one to sense movement in each of the 3 spatial planes. At the base of the canals are movement hair cells, collectively called the crista ampullaris. Depending on the plane of movement, the endolymph flowing within the semicircular canals stimulates the appropriate movement hair cells. Static head position is sensed by the vestibule, specifically, its utricle and saccule, which contain the position hair cells. Different head positions produce different gravity effects on these hair cells. Small calcium carbonate particles (otoliths) are the ultimate stimulants for the position hair cells.
The hair cells for both position and movement create nerve impulses. These impulses travel over the vestibular nerve to synapse in the brain stem, cerebellum, and spinal cord. No definite connections to the cerebral cortex exist. Instead, the impulses produce reflex actions to produce the corrective response. For example, a sudden loss of balance creates endolymph movement in the semicircular canals that triggers leg or arm reflex movements to restore balance.
To Learn More

Web resources: NIDCD Glossary (National Institute on Deafness and Other Communication Disorders)  PATTS AT Links - Hearing/ Deafness PATTS - Nervous System Brain-Sensory Interaction Virtual Tour of the Ear: Hearing Mechanisms The InnerBody: Anatomy Tutorials - Nervous System: Ear Promenade 'round the Cochlea Visible Human Project

Acknowledgments:  Joel DeLisa and Walter C. Stolov, "Significant Body Systems," in: Handbook of Severe Disability, edited by Walter C. Stolov and Michael R. Clowers. US Department of Education, Rehabilitation Services Administration, 1981, pages 51-54. Catherine Parker Anthony and Gary A. Thibodeau, Textbook of Anatomy & Physiology. St. Louis: Mosby, 1983, pages 338-344. Anatomy Clipart (Designs4Free) Promenade 'round the Cochlea (permission)

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