Part I Anatomy, Physiology, Classification of Hearing Loss

The human ear is amazing. Although it is one of the smallest and most complex organs in the body, it is capable of using the tiniest disturbances in air molecules, inducing them into a form the brain can understand, and doing so instantaneously over an enormous range of pitch and loudness. Considering the ear’s delicacy, it is remarkably resilient. Nevertheless, problems can occur which impair our ability to hear properly.  

In recent years, substantial advances have made it possible to determine the cause of hearing impairment in nearly all cases and to treat hearing loss in many ears. It is helpful to understand how the ear works and to be familiar with many of the reasons why it may cease to work properly. Such knowledge allows us to recognize and understand ear problems when they occur, and to judge whether our patients are really receiving the latest and best in medical care.

The ear is divided into three major anatomical divisions: (a) the outer ear, (b) the middle ear, and (c) the inner ear. The outer ear has two parts: (a) the “trumpet-shaped” apparatus on the side of the head called the auricle or pinna, and (b) the tube leading from the auricle into the temporal bone called the external auditory canal. This opening is called the meatus.

The tympanic membrane, or “eardrum”, stretches across the inner end of the external ear to the inner ear. A bony bridge is held in place by muscles and ligaments. The middle-ear chamber is filled with air and opens into the throat through the eustachian tube. The eustachian tube helps to equalize pressure on both sides of the eardrum.

The inner ear is a fluid-filled chamber divided into two parts: (a) the vestibular labyrinth, which functions as part of the body’s balance mechanism, and (b) the cochlea, which contains the hearing-sensing nerve. Within the cochlea is the organ of the Corti, which contains thousands of minute, sensory, hair-like cells. The organ of Corti functions as the switchboard of the auditory system. The eighth cranial or acoustic nerve travels from the inner ear to the brain, serving as the pathway for the impulses the brain will interpret as sound.

Sound creates vibrations in the air somewhat similar to the waves created when a stone is thrown into a pond. The outer-ear “trumpet” collects these sound waves, and they are funneled down the external ear canal to the eardrum. As the sound waves strike the eardrum, they cause it to vibrate. The vibrations are transmitted by mechanical action through the middle ear over the bony bridge formed by the malleus, incus, and stapes. These vibrations, in turn, cause the membranes over the openings to the inner ear to vibrate, causing the fluid in the inner ear to be set in motion. The motion of the fluid in the inner ear excites the nerve cells in the organ of Corti, producing electrochemical impulses that are gathered together and transmitted to the brain along the acoustic nerve.

As the impulses reach the brain, we experience the sensation of hearing. The sensitivity of the hearing mechanism is most extraordinary. Near threshold (the softest detectable sound), the eardrum only moves approximately 1/1,000,000^th of an inch. Our intensity range spans extremes from the softest sounds to sounds of jet engine intensity, covering an intensity range of approximately 100,000,000 to 1. Over this range, we are able to detect tiny changes in intensity, and in frequency. Many young, healthy humans (through teens and early twenties) can hear frequencies from about 20 Hz to 20,000 Hz and can detect frequency differences as small as 0.2%. That is, we can tell the difference between a sound of 1000 Hz, and one of 1002 Hz. Consequently, it is no surprise that such a remarkably complex system can be damaged by various illnesses and injuries.

When a hearing loss is classified, the point at which the auditory pathway is broken down is localized, and a determination is made whether the patient’s hearing loss is conductive, sensorineural, central, functional, or a combination of these.

Conductive hearing loss is due to any condition that interferes with the transmission of sound through the external ear and middle ear to the inner ear. If it is in the middle ear, the damage may involve the footplate of the stapes (bottom of the stirrup bone), as in otosclerosis, or the mobility of the drum and ossicle caused by fluid. Conductive hearing losses are generally correctable. In cases of conductive hearing loss, sound waves are not transmitted effectively to the inner ear because of some interference in the external canal, the eardrum, the ossicular chain, the middle-ear cavity, the oval window, and the round window, or the eustachian tube. For example, damage to either the middle ear, which transmits sound energy efficiently or the eustachian tube, which maintains equal air pressure between the middle ear cavity and the external canal, could result in a mechanical defect in sound transmission. In pure conductive hearing loss, there is no damage to the inner ear or the neural pathway.

Patients diagnosed as having conductive hearing loss receive a much better prognosis than those with sensorineural loss because modern techniques make it possible to cure or at least improve the vast majority of cases in which the damage occurs with the outer or middle ear. Even if they are not improved medically or surgically, these patients stand to benefit greatly from a hearing aid, because what they need most is amplification. They are not bothered by distortion and other hearing abnormalities that may occur in sensorineural loss.

In sensorineural hearing loss, the damage lies in the inner ear, the acoustic nerve, or both. Most physicians call this condition “nerve deafness.” In the majority of cases, it is not curable. The cochlea has approximately 30,000 hearing nerve endings (hair cells). Those 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 word “sensorineural” was introduced to replace the ambiguous terms “perceptive deafness” and “nerve deafness.” It is a more descriptive and more accurate anatomical term. Its dual character suggests that two separate areas may be affected, and, actually, this is the case. The term “sensory” hearing loss is applied when the damage is localized 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 auditory nerve proper, anywhere between its fibers at the base of the hair cells and the auditory nuclei (relay stations in the brain). This range includes the bipolar ganglion of the eighth cranial nerve. Other common names for this type of loss are “nerve deafness” and “retrocochlear hearing loss.” These names are useful if applied appropriately and meaningfully, but too often they are used improperly.

Although at present, it is common practice to group together both sensory and neural components, it has become possible in many cases to attribute a predominant part of the damage, if not all of it, to either the inner ear or the nerve. Because of some success in this area and the likelihood that ongoing research will allow us to differentiate between even more cases of sensory and neural hearing loss, we shall divide the terms and describe the distinctive features of each type. This separation is advisable because the prognosis and the treatment of the two kinds of hearing impairment differ. For example, in all cases of unilateral sensorineural hearing loss, it is important to distinguish between a sensory and neural hearing impairment, because the neural type may be due to an acoustic neuroma (benign tumor in the ear-brain interface) which could become serious. Those cases in which we cannot identify as either sensory or neural and those cases in which there is damage in both regions we shall classify as sensorineural.

There are various and complex causes of sensorineural hearing loss, but certain features are characteristic and basic to all of them. Because the histories obtained from patients are so diverse, they contribute more insight to the etiology (cause) than into the classification of a hearing loss.

Sensorineural hearing loss is one of the most challenging problems in medicine. A large variety of hearing impairments fall under this category. The prognosis for restoring a sensorineural hearing loss with presently available therapy is poor. However, in a minority of cases, specific treatable causes are found, and dramatic improvements are occasionally achieved. Although some spontaneous remissions and hearing improvements with therapy have occurred, particularly in cases involving sensory loss, a great need for further research still exists.

In central hearing loss, the damage is situated in the central nervous system at some point between the auditory nuclei (in the medulla oblongata) and the cortex of the brain. Formerly, central hearing loss was described as a type of “perceptive deafness,” a term now obsolete. Knowledge about the subject still is limited.

Although information about central hearing loss is accumulating, it remains somewhat a mystery in otology (the medical specialty of ear and medicine surgery); a sub-specialty of otolaryngology. Physicians know that some patients cannot interpret or understand what is being said and that the cause of the difficulty is not in the peripheral mechanism but somewhere in the central nervous system. In central hearing loss, the problem is not a lowered pure-tone threshold but in the patient’s ability to interpret what he or she hears. Obviously, it is a more complex task to interpret speech than to respond to a pure-tone threshold; consequently, the test necessary to diagnose central hearing impairment must be designed to access a patient’s ability to handle complex information. Most of the tests now available were not created specifically for this purpose, and, so, it still requires a very experienced and almost intuitive judgment on the physician’s part to make an accurate diagnosis.

One common central condition frequently leads people to think they have hearing loss when their hearing is actually normal. It is called a central auditory processing disorder. 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 difficulty “hearing” when there are several conversations going on, can’t study with the radio or television on, have difficulty reading if someone turns on a vacuum cleaner or an air conditioner near them, and generally miss 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 which sometimes makes 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 patient, 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!

In functional hearing loss, there is no detectable organic damage to the auditory pathways, but some underlying psychological or emotional problem is at fault. Functional hearing loss occurs in clinical practice more frequently than many physicians realize. This is the type of condition in which the patient does not seem to hear or to respond: yet the handicap may not be caused by any organic pathology in the peripheral or the central auditory pathways.

The hearing difficulty may have an entirely emotional or psychological etiology, or it may be superimposed on some mild organic hearing loss, in which case it is called a functional or a psychogenic overlay. Often, the patient really has normal hearing underlying the functional hearing loss. A carefully recorded history usually will reveal some hearing impairment in the patient’s family or some reference to deafness which served as the nucleus for the patient’s functional hearing loss.

The most important challenge in such a case is to classify the condition properly. It may be quite difficult to determine the specific emotional cause, but if the classification is made accurately, the proper therapy can be instituted. Too often, the emotional origin of a functional hearing loss is not recognized, and patients receive useless otologic treatments for prolonged periods. In turn, this process may aggravate the emotional element and cause the condition to become more resistant to treatment. Therefore, early and accurate classification is imperative.

Frequently, a patient experiences two or more types of hearing impairment, a problem called mixed hearing loss. For practical purposes, this term is used only when both conductive and sensorineural hearing losses are present in the same ear. However, the emphasis is on conductive hearing loss because available therapy is so much more effective for this group. Consequently, the otologic surgeon has a special interest in cases of mixed hearing loss in which there is primarily a conductive loss complicated by some sensorineural damage. Each type of hearing loss has specific distinctive characteristics which make it possible to classify in the vast majority of cases. These features are identified through a comprehensive history, physical examination, hearing tests, and sometimes other tests.