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. The most common cause of recessive genetic congenital hearing impairment in developed countries is DFNB1, also known as Connexin 26 deafness or GJB2-related deafness. The most common syndromic forms of hearing impairment include (dominant) Stickler syndrome and Waardenburg syndrome, and (recessive) Pendred syndrome and Usher syndrome. Mitochondrial mutations causing deafness are rare: MT-TL1 mutations cause MIDD (Maternally inherited deafness and diabetes) and other conditions, which may include deafness as part of the picture. TMPRSS3 gene was identified by its association with both congenital and childhood-onset autosomal recessive deafness. This gene is expressed in fetal cochleae and many other tissues, and is thought to be involved in the development and maintenance of the inner ear or the contents of the perilymph and endolymph. It was also identified as a tumor-associated gene that is overexpressed in ovarian tumors. Charcot–Marie–Tooth disease an inherited neurological disorder with delayed onset that can affect the ears as well as other organs. The hearing loss in this condition is often ANSD (auditory neuropathy spectrum disorder), a neural cause of hearing loss. Muckle–Wells syndrome, a rare inherited autoinflammatory disorder, can lead to hearing loss. Autoimmune disease: Although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs. Granulomatosis with polyangiitis, an autoimmune condition, may precipitate hearing loss.
Congenital Infections: Congenital rubella syndrome, CRS, results from transplacental transmission of the rubella virus during pregnancy. CRS has been controlled by universal vaccination (MMR or MMRV vaccine). Cytomegalovirus (CMV) infection is the most common cause of progressive sensorineural hearing loss in children. It is a common viral infection contracted by contact with infected bodily fluids such as saliva or urine and easily transmitted in nurseries, and thus from toddlers to expectant mothers. CMV infection during pregnancy can affect the developing foetus and lead to learning difficulties as well as hearing loss. Toxoplasmosis, a parasitic disease affecting 23% of the population in the U.S., can cause sensorineural deafness to the fetus in utero. Hypoplastic auditory nerves or abnormalities of the cochlea. Abnormal development of the inner ear can occur in some genetic syndromes such as LAMM syndrome (labyrinthine aplasia, microtia, and microdontia), Pendred syndrome, branchio-oto-renal syndrome, CHARGE syndrome GATA2 deficiency, a grouping of several disorders caused by a common defect, viz., familial or sporadic inactivating mutations in one of the two parental GATA2 genes. These autosomal dominant mutations cause a reduction, i.e., a haploinsufficiency, in the cellular levels of the gene's product, GATA2. The GATA2 protein is a transcription factor critical for the embryonic development, maintenance, and functionality of blood-forming, lympathic-forming, and other tissue-forming stem cells. As a result of these mutations, cellular GATA2 levels are low, and individuals develop hematological, immunological, lymphatic, and/or other disorders over time. GATA2 deficiency-induced abnormalities in the lymphatic system are proposed to be responsible for a failure in generating the perilymphatic space around the inner ear's semicircular canals, which in turn underlies the development of sensorineural hearing loss.
Presbycusis
Progressive age-related loss of hearing acuity or sensitivity can start as early as age 18, primarily affecting the high frequencies, and men more than women. Such losses may not become apparent until much later in life. Presbycusis is by far the dominant cause of sensorineural hearing loss in industrialized societies. A study conducted in Sudan, with a population free from loud noise exposures, found significantly fewer cases of hearing loss when compared with age-matched cases from an industrialized country. Similar findings were reported by a study conducted of a population from Easter island, which reported worse hearing among those that spent time in industrialized countries when compared with those that never left the island. Researchers have argued that factors other than differences in noise exposure, such as genetic makeup, might also have contributed to the findings. Hearing loss that worsens with age but is caused by factors other than normal aging, such as noise-induced hearing loss, is not presbycusis, although differentiating the individual effects of multiple causes of hearing loss can be difficult. One in three people has significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is neither preventable nor reversible.
Noise
Most people living in modern society have some degree of progressive sensorineural (i.e., permanent) noise-induced hearing loss (NIHL) resulting from overloading and damaging the sensory or neural apparatus of hearing in the inner ear. NIHL is typically a drop-out or notch centered at 4000 Hz. Both intensity (SPL) and duration of exposure, as well as repetitive exposure to unsafe noise levels, contribute to cochlear damage resulting in hearing loss. The louder the noise, the shorter the safe amount of exposure is. NIHL can be either permanent or temporary, known as a threshold shift. Unsafe levels of noise can be as little as 70 dB (about twice as loud as normal conversation) if there is prolonged (24-hour) or continuous exposure. 125 dB (a loud rock concert is ~120 dB) is the pain level; sounds above this level cause instant and permanent ear damage.
Noise and ageing are the primary causes of presbycusis, or age-related hearing loss, the most common kind of hearing loss in industrial society.