Sensorineural hearing loss

Overview

Hearing loss is a very prevalent medical problem that increases in frequency and severity with age. The afflicted population is likewise large, ranging from newborns through old people, and is practically universal in the 70+ age range. The diagnosis and management of this condition necessitate the collaboration of an interprofessional team that comprises the general practitioner, otolaryngologist, speech therapist, audiologist, and social worker.

The most prevalent kind of hearing loss is sensorineural hearing loss (SNHL), which accounts for the vast majority of all hearing loss.

Understanding the type of hearing loss and the technology required to improve auditory reception is critical for properly addressing hearing loss. Pediatricians must be included into the care of children with hearing loss to guarantee appropriate hearing and language development.

Sensorineural hearing loss meaning

Sensorineural hearing loss, or SNHL, occurs as a result of inner ear injury. SNHL can also be caused by issues with the neural pathways that connect your inner ear to your brain. Soft noises might be difficult to hear. Even louder sounds may be distorted or muted. This is the most frequent kind of long-term hearing loss.

Sound waves arriving at the auricle, travelling through the external auditory canal (EAC), and creating a vibration of the tympanic membrane are all part of normal hearing function.

Vibration is then conveyed to the cochlea via the ossicles (malleus, incus, and stapes). Following that, hair cells within the cochlea activate the eighth cranial nerve, which transmits the sensations to the brain. The processing of crude sounds happens in the upper cortices of the brain, as does language comprehension.

Patients with new-onset hearing loss should be explored and evaluated by a multidisciplinary team that includes an otolaryngologist, audiologist, radiologist, and speech/language therapist.

Epidemiology

Hearing loss affects between 0.5 to 1% of children in the United States and can be severe, mild, or moderate. This figure indicates newborn and baby hearing loss based on neonatal screenings, although the problem may impact school-aged children when they are exposed to infections.

The prevalence of sensorineural hearing loss varies by country. In the United States, abrupt SNHL affects between 5-27 persons out of every 100,000 people each year, with roughly 66,000 new cases reported each year. There is limited agreement in the literature about the epidemiology of age-related hearing loss, known as presbycusis, due to different studies employing varied criteria for defining hearing loss. Hearing loss prevalence doubles every decade of life in presbycusis, from the second to the seventh.

While the majority of newborn hearing loss is inherited, the majority of school-age and teenage hearing loss is acquired. In adults, the frequency appears to be approximately two-thirds of persons over the age of 70 in the United States, while this figure might be substantially higher. The illness is linked to age, noise exposure and length, as well as other co-morbidities such as hypertension, high blood sugar, and smoking history, all of which are considered high-risk factors.

Hearing loss affects roughly two-thirds of persons over the age of 70 in the United States. Noise-induced hearing loss is another major cause of hearing loss in adults (NIHL). It is estimated that workplace noise causes 16% of adult worldwide debilitating hearing loss. Despite regulations in most industrialized nations to prevent NIHL, this is nevertheless a frequent occupational ailment.

Causes sensorineural hearing loss

Hearing loss can be conductive, sensorineural, or a combination of the two. Conductive hearing loss occurs when the transmission of sound waves to the cochlea is disrupted. The most prevalent causes include auricle or helix aberrant development, cerumen impaction, ear canal foreign bodies, otitis externa, ossicular chain malfunction or fixation, and middle ear effusion. Cholesteatoma, a benign but locally harmful accumulation of squamous debris emerging from the tympanic membrane, as well as other benign or malignant tumors, can cause conductive hearing loss.

Sensorineural hearing loss (SNHL) is generally caused by a difficulty with the transmission of stimuli at or after the cochlea. This loss might be the result of hair cell malfunction or a problem with the eighth nerve itself. Aside from the pathophysiological aspects, the fundamental distinction between the two types of hearing loss is that patients with conductive hearing loss experience sounds as lessened, whereas SNHL patients may perceive sounds as diminished and distorted.

The most common causes of sensorineural hearing loss are:

Congenital - syndromic and nonsyndromic
Presbycusis
Noise-induced hearing loss
Head injury
Meniere's disease
Ototoxicity - aminoglycosides, loop diuretics
Systemic conditions - meningitis, diabetes
Vestibular schwannoma
Others - autoimmune, barotrauma, perilymphatic fistula

Pathophysiology

Sensorineural hearing loss is caused by injury to the inner ear's hair cells, the vestibulocochlear nerve, or the brain's central processing centers. This is distinct from conductive hearing loss, which occurs when sound waves are unable to reach the inner ear.

The ear consists of

External ear – pinna, external auditory meatus, and canal
Middle ear – tympanic membrane, ossicles, Eustachian tube opening, oval and round windows
Inner ear – cochlea and part of the auditory nerve

Each of the foregoing components is necessary for sound wave conduction, but in SNHL, we are concerned with disease in the inner ear that results in hearing loss. The interaction between the stapes and the oval window transmits sound to the cochlea. Sound that enters the cochlea is amplified by the outer hair cells and then electrochemically transduced by the inner hair cells.

The cochlea receives an auditory input and generates a traveling wave that travels across the basilar membrane of the cochlea, triggering outer hair cells (OHCs), which serve as a biological amplifier/compressor and modify the signal. The cochlear basilar membrane is very frequency selective. The energy of the traveling wave is converted to an electric action potential by the inner hair cells (IHCs) in the cochlea, which synapse at the spiral ganglion to produce the auditory nerve.

There are several pathophysiological mechanisms by which damage to the inner ear results in SNHL.

Structural abnormality of cochlear components: e.g., trauma or congenital conditions.
Aberrant metabolic activity: The transfer of ions determines cochlear function. Conditions that interfere with this transport, whether genetic or acquired, can cause alterations in the endolymph and impact hearing.
Vascular: Noise damage, ototoxicity, and systemic vascular events can all interfere with the circulatory supply to the cochlea, affecting the function of the stria vascularis.
Noise trauma: Noise trauma causes an increase in the vibrational shift between the tectorial and basilar membranes, which can damage the stereocilia of the OHCs. As a result, the stiffness of the Corti organ decreases. Gentamicin and other aminoglycoside antibiotics act as potassium channel blockers, preventing hair cells from depolarizing. They can also alter the perilymph ion concentration, causing damage to the hair cell bundle and resulting in irreversible hearing loss.

Symptoms of sensorineural hearing loss

When evaluating a patient with SNHL, it is critical to conduct a detailed history. Age of start, laterality of symptoms, velocity of decline, variable symptoms, and accompanying symptoms such as tinnitus, auditory fullness, disorientation, and vertigo are all important points to gather.

Identifying the premorbid hearing level is critical for directing therapy and determining if the hearing loss is new or a worsening of an existing image. Past ear surgery, a history of noise exposure, previous head trauma, barotrauma, or ototoxic aminoglycoside exposure are all questions.

Patients with presbycusis will have a history of progressive hearing loss. They are cranking up the volume on the television and encouraging others to speak up. Often, family members are the first to discover this.

In situations of noise-induced hearing loss, there will be a history of personal or occupational noise exposure. Social history is frequently used to assist management and to gain insight into how the patient's illnesses influence their life and those of their family.

Many individuals who suffer from hearing loss find it extremely solitary. Activities they used to like, such as going to the movies, eating out, and meeting family and friends, become stressful for them, and they retreat from them.

Aside from a focused otological examination, it is vital to undertake a comprehensive head and neck exam, including all the cranial nerves, when meeting patients in the clinic with a new hearing loss, albeit it is normally normal.

Diagnosis

A full audiometric examination is the gold standard for assessing hearing loss and should be conducted on anybody with sensorineural hearing loss. Tuning fork tests, a rapid and straightforward bedside inquiry, are often conducted first, followed by a pure tone audiogram (PTA) and tympanometry.

Rinne and Weber's Test

This is a bedside inquiry that uses a 512Hz tuning fork to help a doctor differentiate between conductive and SNHL.

Weber's test is striking a tuning fork against your knee and then placing it in the center of the patient's forehead. The patient is then asked to determine which ear the sound is loudest in. In a unilateral SNHL, the patient hears the sound loudest in their normal or "good" ear, but in a conductive hearing loss, the sound lateralizes to their deaf or "worse" ear. If the SNHL is bilateral, the sound will not lateralize to one ear.

Rinne's test is conducted by striking the tuning fork and then placing it in two locations, first on the patient's mastoid process until it can no longer be heard, and then roughly 1cm away from the patient's external auditory meatus. The former is measuring bone conduction, whereas the later is testing air conduction. A normal test, or Rinne positive result, will have the patient state that the sound was still heard when the fork was held in front of their ear, suggesting that air conduction is better than bone conduction and that there is no conductive loss.

Pure Tone Audiogram

Patients are frequently sent to an outpatient clinic for an audiogram to assess their hearing. This effectively evaluates both air and bone conduction routes, and the thresholds for both are represented on a graph as a curve at increasing frequencies of sound up to 8000Hz.

In SNHL, both the air and bone conduction curves deteriorate, and there is no air-bone gap. The curve's form will vary depending on the underlying disease. Presbycusis, for example, is characterized by a downward sloping high-frequency loss. With contrast, in conductive hearing loss, the air conduction curve worsens and shifts downward, but the bone conduction curve remains constant. The air-bone gap is the difference between the two curves.

Other Tests

Tympanometry: It is used to evaluate middle ear function and the mobility of the tympanic membrane. This test is frequently used in clinical practice to determine if a patient has otitis media with effusion and eustachian tube dysfunction. It is also possible to evaluate the auditory stapedial reflex. The acoustic reflex threshold is the lowest level of sound that initiates the reflex.
Electrophysiological tests: Auditory brainstem testing examines nervous system activity and can be influenced by cerebellopontine angle tumors that compress the cochlear nerve and neural demyelination. It is also used to predict newborns' hearing thresholds.
Speech audiometry: This test is critical in determining the effect of hearing loss on communication.
Head computed tomographic scans, including thin temporal bone window and brain magnetic resonance imaging (MRI),
Laboratory tests are typically not necessary. There are certain exceptions, such as when an autoimmune etiology of SNHL is suspected, in which case tests such as erythrocyte sedimentation rate, antinuclear antibody, rheumatoid factor, and anti-microsomal antibodies are sought.

Sensorineural hearing loss treatments

Sensorineural hearing loss is treated based on its underlying cause. A regular brain MRI with attention to the internal auditory meatus should be requested in acute instances when no etiology is detected and the condition is thought to be of idiopathic origin. Typically, these patients will be started on oral corticosteroids for seven days, with a prednisone dosage of 1 mg/kg/day (max 60 mg/day) reduced over the next week.

As with any drug, a doctor must assess the risks and advantages of treatment for an individual patient due to insufficient evidence of efficacy. Having said that, spontaneous hearing improvement is most likely to occur during the first two weeks. A repeat audiogram should be performed within 10 to 4 days to evaluate progress, and if there is no improvement, the patient should be considered for salvage intratympanic steroids.

Some physicians operate differently and may provide intratympanic steroids sooner in a patient's therapy cycle. They are a possibility, particularly in refractory instances or in those for whom systemic steroids may be ineffective. If the follow-up PTA improves to within 10 dB of the pre-sudden hearing loss, the patient has fully recovered.

Hearing aids, of which there are several sorts, are the basis of treatment in chronic situations. Hearing aids are beneficial to the majority of individuals with presbycusis, even in mild or severe instances. There is no way to restore former hearing thresholds, and owing to psychological comorbidities, audiological rehabilitation support is especially necessary in these individuals. As a result, the sooner the diagnosis is discovered, the more we can reduce the burden on these people's life.

Typical behind-the-ear air conduction Hearing aids are the most frequent treatment for unilateral or bilateral hearing loss. A microphone detects sound and converts acoustic energy into electrical energy, which is amplified. The signal is then converted back into an acoustic signal by the receiver, which is subsequently replayed by a speaker in the patient's ear canal. They are widely accessible, reasonably priced, and simple to replace. They are not appropriate for external ear abnormalities of the pinna or external auditory canal, as well as infections. Hearing aids can help patients whose hearing thresholds are lower than 30 decibels.

Contralateral routing of signal (CROS) hearing aids detect sound using a microphone in the worse ear, which is wirelessly sent to the opposite hearing aid and played into the better hearing ear through air conduction. Hearing aids with bilateral microphones and contralateral signal routing (BiCROS) are similar, but a microphone also supports the better hearing ear on the same side.

In cases of conductive and mixed hearing loss when there is a considerable air-bone gap, bone conduction hearing devices are employed. It is also beneficial in cases of single-sided deafness. These devices typically include the insertion of a titanium fixture into the bone. Vibration from the associated sound processor is conveyed into the skull via osseointegration and to the cochlea via bone conduction. The external auditory canal, tympanic membrane, and ossicles are bypassed, as is the conventional air conduction transmission pathway.

Personal frequency modulation systems or Roger pens, which are wireless microphones used in conjunction with hearing aids, are other devices that patients may find beneficial. It helps the person wearing it to concentrate on the speech they wish to hear while also reducing background noise in busy surroundings.

The usage of cochlear implants varies from country to country. In the United Kingdom, the National Institute for Health and Care Excellence advises cochlear implants for persons who have thresholds of more than 80 decibels in both ears at two frequencies and obtain insufficient benefit from their hearing aids. Adults in the United States have a threshold of more than 70 decibels.

Differential Diagnosis

Congenital

Hearing loss is the most prevalent congenital sensory condition, and it frequently necessitates the knowledge of pediatricians in collaboration with medical geneticists and pediatric otolaryngologists. There are two types of congenital hearing loss: hereditary and environmental. Genetic reasons are further classified as non-syndromic (70 percent) and syndromic (30 %) (30 % ).

Genetic syndromic:

In children with syndrome-related hearing loss, understanding the underlying etiology is frequently more critical because the accompanying clinical symptoms might be severe. More than 400 syndromes with hearing loss as a characteristic have been discovered; however, just a small number of them account for the majority of SNHL occurrences. The major characteristics of the most prevalent syndromes encountered in children are outlined here.

Waardenburg syndrome is the most frequent, with SNHL being present in more than two-thirds of individuals. Another distinguishing trait is pigmentation anomalies in the eyes, skin, and cochlea.
One of the most frequent autosomal recessive causes of syndromic hearing loss is Usher syndrome. This syndrome is distinguished by increasing SNHL and retinitis pigmentosa, as well as hearing and visual loss.
Pendred syndrome is characterized by variable degrees of SNHL, vestibular impairment, and a thyroid goiter. It is one of the most frequent autosomal recessive forms of syndromic hearing loss, along with Usher syndrome. In almost half of the afflicted individuals, a particular mutation in SLC26A4 is found.
Jervell and Lange-Nielsen syndrome is inherited in an autosomal recessive pattern as well. The main characteristic, in addition to SNHL, is a prolonged QT interval on the ECG. These people may experience syncope, sudden death, or long QT syndrome, or they may have a family history of these conditions.
Alport syndrome is inherited in an X-linked way and is caused by a type IV collagen deficiency. It is typically characterized by glomerulonephritis, end-stage renal disease, and bilateral SNHL. The hearing loss begins in high frequencies, but as it develops, lower frequencies get impacted. As the illness worsens, hematuria and proteinuria are important warning indicators.

Environmental:

Intrauterine infection (toxoplasmosis, cytomegalovirus, herpes, rubella)
Alcohol, smoking
Ototoxic drugs
Premature births, hypoxia, neonatal jaundice

Presbycusis

Presbycusis, also known as age-related hearing loss, is a progressive bilateral SNHL that develops in the mid to late adult years. Presbycusis is an exclusion diagnosis, and main causes such as otosclerosis, Meniere's disease, and cytotoxicity, among many others, must be ruled out first. It is frequently associated with degeneration of cochlear hair cells, primarily OHCs in the cochlea's basal region, and the alterations begin in the cochlea's basal end and progress towards the apex as the disease progresses. Patients usually appear with a gradually worsening hearing loss, especially when there is background noise. The patient frequently describes a lack of clarity rather than a loss of loudness. Tinnitus is a common accompanying symptom that might be the most difficult for the sufferer.

Noise-induced Hearing Loss

This syndrome develops when a patient suffers from hearing loss as a result of excessive loud exposure, whether recreational or industrial. Occupational noise exposure is one of the most common and possibly avoidable health problems. It has a little male preponderance and mainly affects people in their forties and fifties. The symptoms are similar to those of most SNHL diseases, with an insidious trend of increasing hearing loss over time, sometimes accompanied by tinnitus.

A hyperacusis questionnaire may be used to measure the degree of hyperacusis, which is reported in 40% of tinnitus sufferers. The bedside otological examination is usually normal, and the diagnosis is mainly predicated on the history paired with the characteristic finding of a notched appearance at 4kHz that seems to start recovering at 8kHz on a pure tone audiogram.

This is referred to be the Carharts notch, however it is not always present. It is not diagnostic of NIHL in the absence of a prior history of noise exposure. Once a diagnosis is made, it is critical to use ear protection to limit additional loud exposure as much as possible. The Control of Noise at Work Regulations of 2005 establish a framework for companies to use in ensuring the safety of their employees based on their average occupational sound exposure.

SNHL can be caused by ototoxic substances in a variety of ways.

Gentamicin and other aminoglycosides induce hair cell death, resulting in irreversible hearing loss and balance disorders. This can happen after several rounds of systemic treatment. Hearing loss begins in the higher frequencies and progresses to the lower frequencies when more hair cells are lost. Loop diuretics are hypothesized to have an immediate but entirely reversible impact on the stria vascularis.

Sensorineural hearing loss vs conductive

Sensorineural hearing loss is caused by a malfunction in either the inner ear or the auditory nerve, which transmits sound to the brain. Conductive hearing loss occurs when sound does not reach the inner ear, generally as a result of a blockage or trauma.

Prognosis

Sensorineural hearing loss often progresses slowly and, for the great majority of patients, may be controlled with conservative treatments and hearing aids, as well as regular follow-up examinations and audiograms. Even substantial hearing loss can be repaired with a cochlear implant if the patient is suitable.

With sudden SNHL, four factors have been shown to help predict the outcome.

Time since onset - the earlier it presents, the better the prognosis
Age - worst prognosis in older patients
Vertigo - poor prognostic indicator
Degree of hearing loss - if profound and with a downward sloping audiogram indicates a poorer prognosis.

Patients who seek medical help early and participate in their therapy are more likely to have positive outcomes. In SNHL, 32% to 66% of patients are resolved without intervention. In 80-90 percent of instances, the reason is unclear at the time of presentation. Only one-third of patients may be identified as having a cause after a comprehensive evaluation. Improvement during the first two weeks has been found to indicate a great long-term result.

Complications

Physical symptoms as well as the psychological burden of living with hearing loss might be complications. Many people with SNHL experience concomitant symptoms like as tinnitus and dizziness, which are sometimes more difficult to bear than hearing loss. There have been studies conducted to assess the impact of tinnitus on the lives of sufferers. Inability to focus, continual alertness, the influence on sleep, intrusiveness, and total loss of control over their life were common responses.

Fear was another category that received high marks when a questionnaire was administered to a group of patients. Patients indicated dread of the tinnitus itself, fear of living with tinnitus for an extended period of time, and fear of engaging in activities that would aggravate their symptoms.

Many patients struggle in social circumstances and engage in activities they normally love, resulting in increased isolation from friends and family. Although there are community support groups, they may not be easily accessible or available, and some patients sense a social shame associated with hearing loss. Hearing loss will grow more common as the population ages, and it is already a major public health concern in the industrialized world.

Conclusion

Sensorineural hearing loss (SNHL) is a form of hearing loss caused by a problem in the inner ear or sensory organ (cochlea and accompanying tissues) or the vestibulocochlear nerve (cranial nerve VIII).

Hearing loss is a major cause of impairment in adults, linked with substantial communication and behavioral issues as well as high health-care expenses, with society and individual economic ramifications.

Patient education is essential for both children and adults. Hearing loss in children is extremely difficult to manage since parents must learn how to interact with their kid while still ensuring their child's appropriate language and social development. It is critical for parents to be educated on how to evaluate their child's hearing and when to seek help.

Adult and elderly patients must be carefully trained on how to use their hearing aids and when they need a fresh appointment with their otologist. Frequent hearing tests and hearing aid cleanliness are critical aspects in ensuring long-term hearing quality.