Deafness | Hearing loss | classification and causes

Sensory Hearing Loss (SNHL)

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Sensory Hearing Loss (SNHL)

Sensorineural hearing loss (SNHL) is due to problems of the inner ear, also known as nerve-related hearing loss. Having sensorineural hearing loss means there is damage either to the tiny hair cells in your inner ear (known as stereocilia), or to the nerve pathways that lead from your inner ear to the brain. It normally affects both ears. Once you develop sensorineural hearing loss, you have it for the rest of your life. It can be mild, moderate, severe or profound.

Diseases of the cochlea result in sensory hearing loss, a condition that is usually irreversible. Most cochlear diseases result in bilateral symmetric hearing loss. The presence of unilateral or asymmetric sensorineural hearing loss suggests a lesion proximal to the cochlea. The primary goals in the management of sensory hearing loss are prevention of further losses and functional improvement with amplification and auditory rehabilitation.

Causes of Sensory Hearing Loss

Presbyacusis

Presbyacusis, or age-related hearing loss, is the most frequent cause of sensory hearing loss and is progressive, predominantly high-frequency, and symmetrical. Various etiologic factors (eg, prior noise trauma, drug exposure, genetic predisposition) may contribute to presbyacusis. Most patients notice a loss of speech discrimination that is especially pronounced in noisy environments. About 25% of people between the ages of 65 and 75 years and almost 50% of those over 75 experience hearing difficulties.

Noise Trauma

Noise trauma is the second most common cause of sensory hearing loss. Sounds exceeding 85 dB are potentially injurious to the cochlea, especially with prolonged exposures. The loss typically begins in the high frequencies (especially 4000 Hz) and, with continuing exposure, progresses to involve the speech frequencies.

Among the more common sources of injurious noise are industrial machinery, weapons, and excessively loud music. Personal music devices (eg, MP3 and CD players) used at excessive loudness levels may also be injurious. Monitoring noise levels in the workplace by regulatory agencies has led to preventive programs that have reduced the frequency of occupational losses. Individuals of all ages, especially those with existing hearing losses, should wear earplugs when exposed to moderately loud noises and specially designed earmuffs when exposed to explosive noises.

Physical Trauma

Head trauma (eg, deployment of air bags during an automobile accident) has effects on the inner ear similar to those of severe acoustic trauma. Some degree of sensory hearing loss may occur following simple concussion and is frequent after skull fracture.

Ototoxicity

Ototoxic substances may affect both the auditory and vestibular systems. The most commonly used ototoxic medications are aminoglycosides; loop diuretics; and several antineoplastic agents, notably cisplatin.

These medications may cause irreversible hearing loss even when administered in therapeutic doses. When using these medications, it is important to identify high-risk patients, such as those with preexisting hearing losses or kidney disease.

Patients simultaneously receiving multiple ototoxic agents are at particular risk owing to ototoxic synergy. Useful measures to reduce the risk of ototoxic injury include serial audiometry, monitoring of serum peak and trough levels, and substitution of equivalent nonototoxic drugs whenever possible.

It is possible for topical agents that enter the middle ear to be absorbed into the inner ear via the round window. When the tympanic membrane is perforated, use of potentially ototoxic ear drops (eg, neomycin, gentamicin) is best avoided.

Sudden Sensory Hearing Loss

Idiopathic sudden loss of hearing in one ear may occur at any age, but typically it occurs in persons over age 20 years. The cause is unknown; however, one hypothesis is that it results from a viral infection or a sudden vascular occlusion of the internal auditory artery.

Prognosis is mixed, with many patients suffering permanent deafness in the involved ear, while others have complete recovery. Prompt treatment with corticosteroids has been shown to improve the odds of recovery. A common regimen is oral prednisone, 1 mg/kg/day, followed by a tapering dose over a 10-day period.

Intratympanic administration of corticosteroids alone or in association with oral corticosteroids has been associated with an equal or more favorable prognosis. Because treatment appears to be most effective as close to the onset of the loss as possible, and appears not to be effective after 6 weeks, a prompt audiogram should be obtained in all patients who present with sudden hearing loss without obvious middle ear pathology.

Hereditary Hearing Loss

Sensory hearing loss with onset during adult life is often familial. The mode of inheritance may be either autosomal dominant or recessive. The age at onset, the rate of progression of hearing loss, and the audiometric pattern (highfrequency, low-frequency, or flat) can often be predicted by studying family members. The connexin-26 mutation, the most common cause of genetic deafness, may be tested clinically, as can most other single gene mutations known to cause hearing loss.

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Autoimmune Hearing Loss

Sensory hearing loss may be associated with a wide array of systemic autoimmune disorders, such as systemic lupus erythematosus, granulomatosis with polyangiitis (formerly Wegener granulomatosis), and Cogan syndrome (hearing loss, keratitis, aortitis). The loss is most often bilateral and progressive. The hearing level often fluctuates, with periods of deterioration alternating with partial or even complete remission. Usually, there is the gradual evolution of permanent hearing loss, which often stabilizes with some remaining auditory function but occasionally proceeds to complete deafness. Vestibular dysfunction, particularly dysequilibrium and postural instability, may accompany the auditory symptoms. A syndrome resembling Ménière disease may also occur with intermittent attacks of severe vertigo.

In many cases, the autoimmune pattern of audiovestibular dysfunction presents in the absence of recognized systemic autoimmune disease. Responsiveness to oral corticosteroid treatment is helpful in making the diagnosis and constitutes first-line therapy. If stabilization of hearing becomes dependent on long-term corticosteroid use, steroid-sparing immunosuppressive regimens may become necessary.

How does it affect how you hear?

Sensorineural hearing loss affects both the loudness and the clarity of the sounds you hear. You may also experience a reduced range of sounds you find comfortable. Meaning, soft and normal sounds are too soft, but loud sounds very quickly get too loud and may really bother you. (In audiological terms, this is known as “recruitment.”)

Sensorineural hearing loss can affect all ranges of hearing. For people with age-related hearing loss, however, it’s typical to experience what’s known as high-frequency hearing loss, which results in the reduced ability to hear high-pitched sounds.

Many people with sensorineural hearing loss report that they can hear but struggle to understand speech. This is especially true in the presence of background noise, and it can be frustrating and exhausting to deal with.

Sensorineural hearing loss symptoms

SNHL can occur in one ear or both ears depending on the cause. If your SNHL onsets gradually, your symptoms might not be obvious without a hearing test. If you experience sudden SNHL, your symptoms will come on within several days. Many people first notice sudden SNHL upon waking.

Sensorineural hearing loss can lead to:

  • trouble hearing sounds when there’s background noise
  • particular difficulty understanding children’s and female voices
  • dizziness or balance problems
  • trouble hearing high-pitched sounds
  • sounds and voices seem muffled
  • feeling like you can hear voices but can’t understand them
  • tinnitus (ringing in your ears)

SNHL treatment

Right now, there’s no surgical option to treat SNHL. The most common options are hearing aids and cochlear implants to help you compensate for hearing loss. Gene therapy for hearing loss is an expanding field of research. However, at this time it’s not clinically used for SNHL.

Hearing aids

Modern hearing aids can match specific hearing loss symptoms. For example, if you have problems hearing high-frequency sounds, a hearing aid can help dial in these sounds without affecting other frequencies.

Cochlear implants

A cochlear implant is a device that can be surgically implemented to help with severe SNHL. A cochlear implant has two parts, a microphone you wear behind your ear and a receiver inside your ear that sends electrical information to your auditory nerve.

Sensorineural hearing loss prognosis

The outlook for people with SNHL is highly variable depending on the extent and cause of hearing loss. SNHL is the most common type of permanent hearing loss.

In cases of sudden SSHL, the Hearing Loss Association of America says that 85 percent of people will experience at least a partial recovery if they’re treated by an ear, nose, and throat doctor. About 32 to 65 percent of people regain their hearing spontaneously within 2 weeks.

Does sensorineural hearing loss get worse?

SNHL often progresses over time if it’s caused by age-related or genetic factors. If it’s caused by a sudden loud noise or environmental factors, symptoms will likely plateau if you avoid the cause of hearing damage.

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