Sensorineural hearing loss

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. SNHL is usually permanent and can be mild, moderate, severe, profound, or total. However, if the loss happened suddenly, immediate treatment is needed, Prednisone and other treatments may reverse the loss. 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. 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 gunshot 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 includes hearing strategies and hearing aids. In cases of profound or total deafness, a cochlear implant is a specialised device that may be a useful tool in aiding access to sound, ranging from mild to a functional level of hearing when the device is worn and working well. 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. Unlike other types of hearing loss, Sudden Sensorineural Hearing Loss (SSHL or SSNHL) is often treatable if addressed immediately. Many people notice that they have SSHL when they wake up in the morning. Others first notice it when they try to use the deafened ear, such as when they use a phone. Still others notice a loud, alarming "pop" just before their hearing disappears. People with sudden deafness often feel dizzy, have ringing in their ears (tinnitus), or both. Immediate treatment from an otolaryngologist (sometimes called an ENT surgeon) is needed. The primary treatment for SSHL is oral Prednisone, and two recent formal studies both found that nearly half the patients fully recovered their hearing.

Signs and symptoms

Since the inner ear is not directly accessible to instruments, identification is by patient report of the symptoms and audiometric testing. Of those who present to their doctor with sensorineural hearing loss, 90% report having diminished hearing, 57% report having a plugged feeling in the ear, and 49% report having ringing in the ear (tinnitus). About half report vestibular (vertigo) problems. For a detailed exposition of symptoms useful for screening, the American Academy of Otolaryngology developed a self-assessment questionnaire called the Hearing Handicap Inventory for Adults (HHIA). It is a 25-question survey of subjective symptoms.

Causes Sensorineural hearing loss may be genetic or acquired (i.e., as a consequence of disease, noise, trauma, etc.). People may have a hearing loss from birth (congenital), or the hearing loss may develop later. Many cases are related to old age (age-related).

Genetic Hearing loss can be inherited. More than 40 genes have been implicated in deafness. There are 300 syndromes with related hearing loss, and each syndrome may have causative genes. Recessive, dominant, X-linked, or mitochondrial genetic mutations can affect the structure or metabolism of the inner ear. Some may be single point mutations, whereas others are due to chromosomal abnormalities. Some genetic causes lead to a late-onset hearing loss. Mitochondrial mutations can cause SNHL, i.e., m.1555A>G, which makes the individual sensitive to the ototoxic effects of aminoglycoside antibiotics.

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