Hearing loss is a huge public health problem, both because of its major negative impact on quality of life and because it can occur at any age, being present at birth or arising later during childhood or adulthood. There are many possible causes of hearing loss – genetic, traumatic, infectious, toxic, inflammatory or neoplastic (tumor) – with these causes being subdivided into two types: conductive and sensorineural. Conductive hearing loss refers to situations where the inner ear and hearing nerves are intact but sound vibrations are blocked from reaching them. Common examples include earwax impaction, torn eardrum, middle ear fluid buildup (serous otitis media), middle ear infection (suppurative otitis media) or damage to middle ear bones (ossicular damage).
Sensorineural hearing loss (SNHL), also called nerve deafness, refers to the situation where sound vibrations reach the cochlea normally but are not properly converted to nerve impulses and transmitted to the brain. This might be due to a chemical change in one of the inner ear fl uids; damage to the sensory hair cells that convert sound vibrations to nerve impulses; a problem with the supporting cells that maintain the inner ear environment; or some disease or damage to the auditory nerve, to name only a few possible explanations.Most people think that SNHL is irreversible but this is not the case at all. It is true that the most common forms of SNHL – congenital SNHL, age-related SNHL, many forms of ototoxicity (hearing loss caused by drugs) and chronic noise injury (acoustic trauma) – are permanent. However, there are forms of SNHL that are reversible. Although not the most common forms of hearing loss, they are extremely important. Some forms of reversible SNHL include acute acoustic trauma, Meniere’s disease, acoustic neuroma, sudden deafness and autoimmune inner ear disease.
ACUTE ACOUSTIC TRAUMA
Loud sound damages hearing, and the louder the sound, the less time it takes to do the damage. Some sounds, like a gunshot or two steel plates banging together, can be loud enough to do instantaneous permanent damage, called permanent threshold shift (PTS). Some other sounds, such as loud music in a dance club, can cause a temporary threshold shift (TTS) that recovers in a matter of hours or days. Repeated TTS exposures will eventually cause PTS. It now also appears that some sounds that are not loud enough to cause TTS may still gradually cause a progressive noise injury over time. The latest research even indicates that, like a snowball rolling down a hill, noise injury that starts as a PTS may gradually get worse over the years even if there is no further acoustic trauma.
Research studies have shown that acoustic trauma causes physical damage to inner ear hair cells. The tufts that stick up from the top of hair cells are delicate. Normally they fl ex back and forth in response to sound vibrations. In reversible TTS, the loud sound causes the hair cells to soften and wilt, becoming unresponsive to sound. When they regain their normal shape, hearing recovers. However, if the sound injury is great enough, they do not recover and the hair cells die, resulting in PTS. Currently, it is best to use hearing conservation equipment to prevent exposure to loud sound. There are many studies underway to fi nd drugs that may someday help prevent or reverse acoustic trauma but none have proven to be effective yet.
MENIERE’S DISEASE
Meniere’s disease is characterized by a degenerating inner ear in which both the hearing and balance functions have become unstable. This condition affects about one of every 5,000 individuals yearly. Patients with this disorder experience fluctuating and progressive SNHL in the affected ear, tinnitus (ringing/roaring/buzzing sounds) and episodes of whirling vertigo lasting 20 minutes to 24 hours. In living organisms, the regulation or balance of complex functions is called “homeostasis.” In a normal inner ear there are a host of internal homeostatic systems that regulate production, chemical composition and recycling of inner ear fluids; regulate incoming and outgoing nerve signals and blood flow; regulate intercellular communication, energy metabolism and more. These systems are so robust that a normal ear is impervious to changes in the rest of the body or in the external environment.
However, if an ear is damaged and degenerating it may lose its homeostatic controls and become vulnerable to changes inside or outside the body. You can liken this to having an old car. When you bought it, it ran well. Someday it will not run at all. But as it deteriorates, it may become unreliable – sometimes it runs, sometimes it stalls, sometimes it runs rough. You can wrack your brain for a pattern but fail to fi nd one. You can take it to the mechanic, who pops the hood, jiggles the wires, puts it on the computer and even swaps a few parts, but cannot make it into a new car. There is a lot going on under the hood. Likewise, an inner ear has “a lot going on under the hood,” but performance depends upon normal homeostasis. It now appears that Meniere’s “disease” should more properly be called Meniere’s “syndrome” because it is really a constellation of clinical signs and symptoms that can arise from a host of different disturbances of inner ear homeostasis. As researchers learn more about these homeostatic systems they may be able to offer new treatments to some Meniere’s patients.
At present, treatment consists of modifying diet and lifestyle to avoid stressing the fragile Meniere’s ear. If these measures fail, diuretic or steroid medications may help restore some homeostatic functions. If these measures also fail, destruction of the vestibular hair cells or removal of the inner ear balance organs will eliminate vertigo attacks. However, once hearing loss has gone from the fluctuating stage to completely broken down, there are no treatments known to reverse the loss.
ACOUSTIC NEUROMA
Acoustic neuroma, more properly called vestibular schwannoma, is a tumor of the vestibular (balance) nerves. It occurs in approximately one of every 100,000 individuals yearly. Schwann cells are supporting cells that wrap around nerve fibers to insulate them, just like the plastic insulation on an electric cord, so they do not short-circuit and fail to carry signals normally. The two vestibular nerves from each ear run in a bundle with the auditory (hearing) nerve and the facial nerve from the inner ear, through the half-inch long internal auditory canal, across the spinal fluid space and into the base of the brain. Schwann cells that undergo certain genetic mutations may begin to grow uncontrollably and form tumors. Although acoustic neuromas are almost always benign and slow-growing, they occur in a location that cannot accommodate their size. As they expand into the bony internal auditory canal, they begin to compress the nerves. If there is a little bleeding into the tumor or if it gets inflamed and swells quickly, it can cause a sudden or rapid SNHL. In such cases, a short course of steroid medication can sometimes reduce the swelling and bring back the hearing. Over the long term, however, many acoustic neuroma patients gradually go deaf in the affected ear – not from acute compression, but from other chemical or physical changes in the nerve or the inner ear environment.
Often the recommendation for small and slow-growing tumors is periodic observation by MRI brain scanning. If the tumor is larger, or if it is growing more quickly, it can be treated by radiation therapy (so-called gamma knife therapy) or surgical removal. While these treatments can sometimes preserve some residual hearing, they cannot restore hearing that is already lost.
AUTOIMMUNE INNER EAR DISEASE (AIED)
AIED is an extremely rare condition of rapidly progressive SNHL that affects both ears, with hearing in at least one side declining rapidly. Serial hearing tests administered monthly show hearing loss progressing each month – too slow to be sudden deafness but much too fast to be age-related or genetically determined progressive loss. About 10 percent of AIED patients have other organ system involvement or other autoimmune diagnoses, such as inflammatory bowel disease, lupus, polymyalgia rheumatica or rheumatoid arthritis. Although the name of this condition suggests that the person’s immune system is attacking their own ears, this has never actually been proven. What is certain is that about 50 percent of persons with rapidly progressive SNHL have a reversible condition treatable with high-dose steroid therapy. Patients are given prednisone at a dose of 60mg per day for one month to see if their hearing loss is steroid-responsive. If so, the drug dose is tapered slowly over the next six to 12 months. If they get no benefit from the first month of high-dose therapy, they are tapered rapidly in about 10 days and treatment is discontinued. Nonresponders are presumed not to be suffering from AIED but rather from some other unknown cause.
Persons with AIED are prone to relapse. Even if they have a good response to a long course of steroid therapy, they may lose hearing again in the future. Typically each relapse is progressively less responsive to treatment. AIED patients usually end up with hearing aids, and occasionally with cochlear implants, to remedy hearing loss when it becomes irreversible.
SUDDEN DEAFNESS
Finally, sudden deafness of unknown cause is surprisingly common. Every year approximately one person in 5,000 suddenly loses hearing in one ear. The deafness may be partial or total in the affected ear and progresses within 72 hours or less. Half the patients experience some transient dizziness or imbalance that clears in hours or a few days. All persons with sudden SNHL are advised to have an MRI brain scan because approximately one percent of cases turn out to have acoustic neuroma. In the remaining 99 percent, the cause is unknown. In fact, there may be several different causes, including viral infection, blocked circulation or immunemediated changes or inflammation. If left untreated, 25 percent to 50 percent of persons with sudden SNHL may recover some or all of their hearing. If caught promptly and treated with a tapering course of oral steroid medication, at least 50 to 60 percent of persons will regain some or all of their hearing. There is a critical window of two to four weeks to get the steroid treatment. If treatment is delayed beyond that window, it does not help.
In recent years some doctors have been treating their patients with steroid injections into the ear in hopes of improving the recovery rate and lowering the risk of steroid side effects. Although a number of early studies have hinted that steroid injections may be as good as or better than steroid pills, these studies have been small and have had design flaws that leave their conclusions somewhat questionable. The National Institutes of Health is currently sponsoring a multicenter trial to compare oral and injected steroid treatment for sudden SNHL (www.suddendeafness.org) but the results of this study will not be available for about another year.
Living with SNHL produces hardship for millions of Americans. For many, this loss is progressive. For the vast majority, it is irreversible. However, Mother Nature has provided us with a number of exciting opportunities in the form of reversible SNHL conditions. These instances of reversible SNHL are not the most common forms of hearing loss but they are disproportionately important because they teach us so much about the workings of the inner ear, about the way it fails and about possible pathways to effective treatments for hearing loss and deafness.
Steven D. Rauch, M.D., is a professor of otology and laryngology at Harvard Medical School, a researcher at the Massachusetts Eye and Ear Infirmary, president of the Association for Research in Otolaryngology and a member of the Deafness Research Foundation Council of Scientific Trustees.
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