| Sensorineural hearing loss | |
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| Cross section of the cochlea. | |
| Specialty | Otorhinolaryngology |
Sensorineural hearing loss (SNHL) is a type of hearing loss in which the root cause lies in the inner ear, sensory organ (cochlea and associated structures), or the vestibulocochlear nerve (cranial nerve VIII). SNHL accounts for about 90% of reported hearing loss.[citation needed] SNHL is usually permanent and can be mild, moderate, severe, profound, or total. Various other descriptors can be used depending on the shape of the audiogram, such as high frequency, low frequency, U-shaped, notched, peaked, or flat.
Sensory hearing loss often occurs as a consequence of damaged or deficient cochlear hair cells.[disputed – discuss] Hair cells may be abnormal at birth or damaged during the lifetime of an individual. There are both external causes of damage, including infection, and ototoxic drugs, as well as intrinsic causes, including genetic mutations. A common cause or exacerbating factor in SNHL is prolonged exposure to environmental noise, or noise-induced hearing loss. Exposure to a single very loud noise such as a gun shot or bomb blast can cause noise-induced hearing loss. Using headphones at high volume over time, or being in loud environments regularly, such as a loud workplace, sporting events, concerts, and using noisy machines can also be a risk for noise-induced hearing loss.
Neural, or "retrocochlear", hearing loss occurs because of damage to the cochlear nerve (CVIII). This damage may affect the initiation of the nerve impulse in the cochlear nerve or the transmission of the nerve impulse along the nerve into the brainstem.
Most cases of SNHL present with a gradual deterioration of hearing thresholds occurring over years to decades. In some, the loss may eventually affect large portions of the frequency range. It may be accompanied by other symptoms such as ringing in the ears (tinnitus) and dizziness or lightheadedness (vertigo). The most common kind of sensorineural hearing loss is age-related (presbycusis), followed by noise-induced hearing loss (NIHL).
Frequent symptoms of SNHL are loss of acuity in distinguishing foreground voices against noisy backgrounds, difficulty understanding on the telephone, some kinds of sounds seeming excessively loud or shrill, difficulty understanding some parts of speech (fricatives and sibilants), loss of directionality of sound (especially with high frequency sounds), perception that people mumble when speaking, and difficulty understanding speech. Similar symptoms are also associated with other kinds of hearing loss; audiometry or other diagnostic tests are necessary to distinguish sensorineural hearing loss.
Identification of sensorineural hearing loss is usually made by performing a pure tone audiometry (an audiogram) in which bone conduction thresholds are measured. Tympanometry and speech audiometry may be helpful. Testing is performed by an audiologist.
There is no proven or recommended treatment or cure for SNHL; management of hearing loss is usually by hearing strategies and hearing aids. In cases of profound or total deafness, a cochlear implant is a specialised device that may restore a functional level of hearing. SNHL is at least partially preventable by avoiding environmental noise, ototoxic chemicals and drugs, and head trauma, and treating or inoculating against certain triggering diseases and conditions like meningitis.