Conductive Hearing Loss
Anything that interferes with the transmission of the vibrating sound wave through the eardrum and/or ossicular chain will decrease the intensity of the sound wave that finally reaches the inner ear. This causes a conductive hearing loss. If there is hole (perforation) of the eardrum, vibration will not be transmitted—creating a hearing loss.

Another possible source of conductive hearing loss is a disruption in the connection between the bones of the middle ear (the ossicular chain). The space between the eardrum and the inner ear is called the middle ear space. It measures approximately 1.5 cm3. The ossicles are within this space. The eustachian tube connects the middle ear space to an area of our nasal passage called the nasopharynx. In a eustachian tube that functions normally, the tube opens intermittently as we chew and swallow. This allows the pressure in the middle ear space to equalize.

If that equalization does not occur—as occasionally happens with a cold or sinus infection—It can cause fluid to fill the middle ear space. If that space is filled with fluid, which happens frequently in children and less often in adults, it causes a mild to moderate low frequency hearing loss. Ninety percent of the time, the fluid will disappear on its own within 3 months. The fluid is reabsorbed and/or drains out through the eustachian tube. If the fluid does not clear out, it can be removed by making a small incision in the eardrum, evacuating the fluid.

When the environment in the middle ear is chronically out of balance, it can cause conductive hearing loss. If the middle ear space does not ventilate through the eustachian tube, the body can reabsorb the air in the space. This causes the eardrum to retract—to be “sucked in” towards the inner ear. The middle ear space communicates with the air cells of the mastoid. The mastoid is the bone we feel right behind our ear. The mastoid bone is composed of multiple air cells, like a sponge. As the eardrum retracts it can create a deep pocket—called a “retraction pocket.”

The problem with that is that the skin that lines our ear canal is the same type of skin that covers the rest of our body’s exterior. That skin is constantly being sloughed off. In a normal ear, the skin that is sloughed off migrates out of the ear canal. When that type of skin gets deeply retracted, it is not able to clear itself. Instead it builds up into something called a cholesteatoma. Once a cholesteatoma forms, it continually gets larger and can erode into surrounding structures. Frequently it erodes the ossicles—causing a conductive hearing loss.

Even if no cholesteatoma forms, the pressure of the retracted eardrum on the ear bones, especially the incus (anvil), can cause the bone to erode creating a discontinuity of the ossicular chain—again causing a conductive hearing loss. Correcting this problem often requires two surgeries. The first step is to clean out all the trapped skin (cholesteatoma) and to reconstruct the eardrum. This step is critical in order to eliminate the disease and make the ear safe. The next step is to rehabilitate the hearing with an ossicular prosthesis which will be discussed in a later section.

Otosclerosis
Otosclerosis is a disease that causes new bone to form around the base of the third ossicle in the chain, the stapes (stirrup). As the disease progresses the stapes becomes fixed. Once it is fixed, it will not allow the vibration to be passed along the chain and into the inner ear. Rarely, the new bone can form over the cochlea. When that happens, it may cause sensorineural, or nerve, hearing loss.

However, the distinguishing characteristic of nerve hearing loss from otosclerosis is that the ear maintains its ability to discriminate sound despite significant sensorineural hearing loss. Whereas, most of the time, sensorineural hearing loss is accompanied by a progressive loss of discrimination. Otosclerosis often runs in families. In addition, 20 to 30% of the time, it can affect both ears.

If left untreated, it will progress until the stapes is complete fixed and no vibration is passed on. Oral fluoride has been shown to stabilize the progression of the disease. It will not, however, improve any hearing loss that has already developed.

In the case of otosclerosis there are two options for hearing rehabilitation: hearing aids and surgery on the stapes. Hearing aids are able to improve hearing. In the case of nerve hearing loss from cochlear otosclerosis, hearing aids are the only options. In typical otosclerosis without nerve hearing loss, hearing aids are effective, but may be avoided.

The hearing loss may be corrected with surgery. Incisions are made in the ear canal, and a flap is created elevating the eardrum. Once the eardrum is elevated, the ossicles are inspected. The joint between the incus and the stapes is separated. A laser, or a drill, is used to remove the arch from the stapes, leaving only its oval base, or footplate. The laser is then used to create a 0.7 mm hole in the footplate. A piston made of Teflon, which measures 0.6 mm in diameter, is placed into the hole. There is a shepherd’s crook-shaped wire running from the top of the Teflon. The shepherd’s crook is looped over the second bone, the incus, and the wire is crimped so that it fits snuggly against the incus. The eardrum is returned to its normal position. Once it heals, the chain, with the piston replacing the stapes, is able to move freely and send the vibration into the inner ear.

Rehabilitation for Conductive Hearing Loss
Hearing aids work well for conductive hearing loss because there is typically little or no loss of discrimination. They will be discussed in detail in a later section. However, most conductive hearing loss can be corrected with surgery. Sometimes, the surgery can be as simple as the myringotomy discussed earlier to evacuate fluid from the middle ear.

Surgery to correct conductive hearing loss is directed at restoration of the ear’s normal conduction pathway: through the eardrum, into the ossicular chain (the three ear bones) and then into the inner ear. Tympanoplasty is the term used for surgery to repair or reconstruct an abnormal eardrum. Ossicular chain reconstruction is a blanket term used for surgery to restore the connection between an intact eardrum and the inner ear.

There are several different types of ossicular prostheses that are used. They vary in shape, size, and material. The idea with all ossicular prosthetics is to restore the connection between the eardrum and the inner ear. Currently, most ossicular prostheses are made of titanium or a combination of titanium and a material called plastipore. Reconstruction with a prosthesis will work approximately 85% of the time.

When ossicular reconstruction fails, or if the disease process prevents an attempt at reconstruction, hearing may be rehabilitated with a bone-anchored hearing aid (BAHA). The BAHA involves placing an osteointegrated temporal bone implant into the skull. It is based on the implants that are used in dentistry. A minor surgery is required to place a 3 or 4 mm titanium screw into the skull behind the ear, and at the same time prepare an area on the scalp to accommodate a processor.

After a healing period, typically 3 months, to allow the screw to become integrated into the skull, a processor is clipped onto the screw. The processor is similar to a hearing aid. It picks up the sound waves, amplifies them, and transmits the amplified vibration through the screw into the bone of the skull. In conductive hearing loss this vibration is conducted through the skull and causes a sound wave to be propagated in the inner ear. As a result, the conductive hearing loss is overcome.

Sensorineural Hearing Loss

Congenital Hearing Loss
The inner ear has two distinct parts: one part for hearing, the other part for balance. The entire inner ear is contained within the densest bone in the human body – called the otic capsule. Within this dense bone are delicate membranes that form the hearing organ. These membranes also separate the fluids of the inner ear. The inner ear fluids within each compartment are different. The maintenance of these separate compartments is crucial for hearing to function.

When someone is born with hearing loss it is called congenital hearing loss. There are many different causes of congenital hearing loss. Some are inherited from the parents, but most are not inherited.

Prenatal or infant infection can cause hearing loss. Cytomegalovirus (CMV) infection and meningitis are the two most common infections that cause hearing loss. If a newborn is known to have been exposed to or infected with CMV, aggressive treatment with antiviral medication may reduce the subsequent hearing loss.

Children who have meningitis will often end up with severe sensorineural hearing loss. This same group of children is at risk for having the normally fluid-filled portion of their inner ear be replaced with bone. Children with meningitis need to be evaluated to be sure that their cochlea is not ossifying (filling with bone). Once the cochlea is filled with bone, it becomes more difficult to rehabilitate their hearing with a cochlear implant.

An error in the development of either the membranes of the inner ear, the bone of the otic capsule, or both can cause congenital hearing loss. We are able to visualize the bony part of the inner ear using a CT scan. There is no way to visualize the membranes of the inner ear in a living person. The majority of children born with hearing loss will have a normal structure of the bony part of the inner ear. This means that abnormalities in the development of the membranes of the inner ear are responsible for the majority of congenital hearing loss.

Cadaver studies of people who had been born with hearing loss confirm this. The two most common bony abnormalities are an enlarged vestibular aqueduct and a Mondini malformation. The vestibular aqueduct is a bony canal that plays a role in the regulation of the inner ear fluid. If that duct is too wide or patent, it can cause hearing loss. A Mondini malformation occurs when the snail-like shape of the cochlea has one and one half turns instead of the two and one half turns it normally has. There are many unique difficulties for children with hearing loss.

Sudden Sensorineural Hearing Loss (SSNHL)
Sudden sensorineural hearing loss is defined as a nerve-type hearing loss that develops in less than 3 days and causes a 30 dB or greater hearing loss in at least 3 consecutive frequencies. Almost without exception, the hearing loss is truly sudden. Patients will be able to tell exactly when the hearing loss started, or that they woke up with the hearing loss.

No one has been able to determine the exact cause of SSNHL, and there may be several different causes. Most clinicians believe the initial insult is a viral infection. Another suspected cause is “auto-immune” disease where the body recognizes something in the inner ear as foreign and attacks it. Another theory is that somehow the blood vessels that supply the inner ear are blocked, causing part of the inner ear to die. Regardless of the actual initiating insult—the end result is the same. The degree of hearing loss varies, however.

In addition, in about 45% of people who suffer SSNHL the balance part of the inner ear is affected as well. This causes dizziness. The severity of the hearing loss predicts the chance of recovery. More severe hearing loss, obviously, has a poorer prognosis. The addition of dizziness also bodes poorly for recovery.

Traditional treatment is oral steroids with or without antiviral medication. No well-designed study has ever confirmed that steroids actually help. It is believed that the sooner the steroid therapy is started, the better the odds that it will be effective. Since a significant number of SSNHL do improve on their own, it is possible that the few studies showing steroids to be beneficial have actually just documented natural spontaneous recovery.

Recently, some physicians have advocated a steroid injection through the eardrum as a “last ditch” effort in people who do not respond to oral steroids. The theory behind steroids is that they reduce inflammation. It may be that the inflammation causes the occlusion of the blood vessels to the inner ear and leads to hearing loss, and steroids help reduce the inflammation. Steroids are not entirely without risk. They will make diabetes control difficult in someone with the disease. They may cause severe stomach upset and should be taken with anti-ulcer medication.

Most people, however, are able to tolerate them without too much trouble, but they need to taken under a physician’s supervision. Despite the clear lack of evidence that steroids help, most people are offered a trial because the actual risk from a course of steroids is small. For the same reason, physicians continue to use antiviral medications in SSNHL. There has never been any evidence to show benefit from antiviral therapy, but they are very easy to take and have almost no side effects.

In general, over time the brain will “retrain” itself and adjust for the loss of balance input from the affected side.
Recovery of hearing, if it is going to recover at all, generally occurs within 4-8 weeks. Tinnitus often accompanies SSNHL. Recovery is often incomplete. No study has ever been done to look at how often the opposite ear (the remaining “good” ear) is affected after one ear has had a SSNHL. Anecdotally, it is extremely rare. Anyone who has an SSNHL needs an MRI of the internal auditory canal.

One to three out of 100 people with SSNHL will have a benign tumor growing on the nerve for hearing. This is called a vestibular schwannoma. Although these tumors are not cancerous, they cause significant problems as they grow because of their location near the brainstem.

Unilateral Hearing Loss
SSNHL is probably the most common cause of a unilateral hearing loss (hearing loss in only one ear). Other causes include: prior surgery, cholesteatoma, trauma, brain tumors, and congenital hearing loss.

The challenge with unilateral hearing loss is understanding speech in noisy environments and localizing sound. Having ears on both sides of our head is useful for picking up speech in large, open rooms and when there is background noise. The dual input allows your brain to filter out extraneous noise and background chatter and to pick out specific conversation. The “head shadow” effect causes a slight lag in the time it takes for sound to reach the other ear. The brain uses the time lag to localize which direction a sound came from.

There are two ways to rehabilitate unilateral hearing loss: BAHA or CROS hearing aid. The BAHA was discussed in the section on conductive hearing loss. In unilateral hearing loss, the BAHA uses the skull to transmit the sound signal received from the processor on the non-hearing side to the hearing ear. This helps improve the wearer’s understanding in noisy environments and improves sound localization to a lesser degree.

CROS stands for “Contralateral Routing of Sound.” The CROS hearing aid worn in the non-hearing ear acts as a receiver. It picks up sound for the bad side and sends it (either through a wire in an eyeglass frame or wirelessly) to the hearing aid in the hearing ear. The problem with the CROS system is that the good ear has to wear a hearing aid.

When a hearing aid is put into an ear that works, it causes an “occlusal effect.” The mold of the hearing aid acts essentially as an earplug in that ear. The hearing then has to amplify sound for that ear which never sounds quite as well as the normally functioning ear. When the “better” or “working” ear has some hearing loss as well, the CROS hearing aid works very well. In this case, the hearing aid in the good ear is able to amplify sounds for that ear. In this case it is called a “BiCROS” hearing aid.

One extremely important thing to mention in unilateral hearing loss is the essential need to protect the hearing in the remaining ear. This means wearing hearing protection in noisy environments and being careful when participating in activities that may put the ear at risk.

Noise-Induced Hearing Loss
Long-term exposure to loud noise, or even short-term exposure to extremely loud noise, will cause sensorineural hearing loss. Noise comes in different forms. Impulse noise is the kind of noise you get when a hammer hits a piece of metal. Ambient noise, or environmental noise, is the ongoing noise that surrounds us—for example, the sound of a lawn mower. Acoustic trauma is an extreme form of impulse noise.

Noise exposure can cause an immediate hearing loss called a “threshold shift.” Our hearing “threshold” (how loud a sound has to be before we hear it) is “shifted” (it requires a louder sound). Sometimes it is temporary. When severe, the shift may be permanent. Temporary threshold shifts may respond to treatment. Permanent shifts do not. Most often it takes several years for the adverse hearing effects of prolonged noise exposure to show up. Prolonged exposure to sound intensity 90 dB is enough to cause noise-induced hearing loss. 90 dB is approximately the sound level of a lawn mower.

Most of us do a poor job of protecting our hearing from noise. Because we don’t pay the price in hearing loss until several years down the road, we are creating a truly “silent” disability. The common foam earplugs that are available will reduce noise by at least 10 dB. Properly fit earplugs can reduce noise by up to 30 dB. The difficulty is that our ear canals are not straight, and not everyone gets a good fit with earplugs. “Muff” type hearing protection gives a more reliable fitting, but is a little more cumbersome.

In general, however, muffs are recommended when they can be used because of their superior protection and reliable fitting. Insert earphones, like those used for iPods and cell phones, have the potential to deliver a lot of sound directly to our ears. If you are standing next to someone wearing insert earphones, and you can hear music or sound coming from them, do them a favor and tell them they should turn down their music. At that level they are certainly causing damage.

The mechanism of injury in noise-induced hearing loss is free radical formation in the cochlea. These free radicals in turn kill off the hair cells. It is not completely understood why but the greatest amount of hair cell death occurs at the 4000 Hz region of the cochlea.

Some work has been done using free radical scavenger medication to prevent or reduce the hearing loss associated with noise. The medications have shown some benefit, but the medicine must be given shortly after the exposure—which has made its widespread use difficult. It has also been shown to offer some protective benefit if given prior to noise exposure. However, current formulations of the medicine require large and frequent doses, which again have made its widespread use limited.

Acoustic trauma is a different type of injury. Acoustic trauma, as would be experienced by someone exposed to a blast, causes mechanical disruption of the membranes of the inner ear. The injury and associated hearing loss are more severe with this type of injury. Victims of acoustic trauma will frequently have some injury to the balance portion of their inner ear in addition to the hearing loss. As a result, they will have varying degrees of dizziness.

Age-Related Hearing Loss
As we get older, everyone loses some of the hair cells in our inner ears. The amount of hair cell loss is determined by several different factors: genetics, noise exposure, and medical history (medications, etc.). In general, it begins as a high frequency hearing loss. The hearing loss progresses over time until it eventually begins to interfere with hearing. Age-related hearing loss is nearly universal, but not everyone progresses to the point that it interferes with conversation and communication.

As we age and/or as the high frequency hearing loss progresses, the hearing loss eventually makes its way into the speech range. The first part of hearing speech and language that is affected are the consonant sounds. The upper end of that speech frequency range, between 2,000 Hz and 3,000 Hz, is where most of the consonant sounds are heard.

As we begin to lose the ability to hear consonants, words begin to get confusing. “Heat” sounds like “sheet,” “food” like “mood,” “see” like “she,” “feel” like “peel,” etc. We begin to misunderstand people. Background noise exacerbates this high frequency hearing loss. Television, in particular, becomes difficult to understand and the volume begins to creep up. Certain environments, such as restaurants or meetings, become a challenge. Telephones, at least older ones, do not use frequencies above 3,000 Hz so telephone use is OK. High-pitched voices, particularly women and children, are difficult to understand.