Sensorineural Hearing Loss

Etiology

The most common causes of sensorineural hearing loss are:[2][3][4][5]

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

Epidemiology

The incidence of sensorineural hearing loss varies in different countries. In the United States, sudden SNHL affects between 5-27 per 100,000 people each year, with approximately 66,000 new annual cases.[6] Due to different studies using varying thresholds when classifying hearing loss, there is little consensus in the literature regarding the epidemiology of age-related hearing known as presbycusis. In presbycusis, hearing loss prevalence doubles every decade of life from the second through to the seventh decade, and is nearly universal past the eighth decade of life.[7] Another important cause of hearing loss in the adult population is noise-induced hearing loss (NIHL). It has been estimated that 16% of adults worldwide disabling hearing loss is occupational noise related.[8] This remains a common occupational disease despite legislation in place in most developed countries to prevent NIHL. 

Congenital hearing loss is nearly always sensorineural in nature, and can have various etiologies. In patients with robust prenatal care, congenital infectious causes such as cytomegalovirus are rare and the most common causes are genetic. There are many genetic syndromes with hearing loss as a component, and SNHL developing in childhood warrants a thorough workup.[9]

Pathophysiology

Sensorineural hearing loss results from damage to the hair cells within the inner ear, the vestibulocochlear nerve, or the brain's central processing centers. This differs from a conductive hearing loss, which results from the inability of sound waves 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 above components is important for the conduction of sound waves, but in SNHL, we are concerned with pathology in the inner ear that leads to hearing loss. The interface between the stapes and the oval window delivers sound transmission to the cochlea. Sound which reaches the cochlea undergoes first amplification by the outer hair cells and then electrochemical transduction by the inner hair cells. The cochlea receives an acoustic signal, and a traveling wave is generated, which traverses the basilar membrane of the cochlea stimulating outer hair cells (OHCs), which act as a biological amplifier/compressor and modifies the signal. The basilar membrane of the cochlea is highly frequency-specific and tonotopically organized. The base of the basilar membrane responds to higher-frequency sounds, while the apex responds to lo frequencies. [10]The inner hair cells (IHCs) in the cochlea transduce the energy of the traveling wave to an electric action potential and synapse at the spiral ganglion to form the auditory nerve.[11]

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

1. Structural abnormality of cochlear components: e.g., trauma or congenital conditions.
2. Aberrant metabolic activity: Cochlear function is determined by the transport of ions. Genetic or acquired conditions that interfere with this transport can lead to changes in the endolymph and affect hearing.
3. Vascular: Interference with the vascular supply to the cochlea can occur in conditions such as noise trauma, ototoxicity, and systemic vascular events, which will affect the function of the stria vascularis.
4. Overcrowding of the basilar membrane preventing OHCs motility and IHCs transduction capabilities: Prevalent in conditions such as diabetes and autoimmune pathology.
5. Noise trauma: With noise trauma, the vibrational shift between the tectorial and basilar membranes increases, and this shift can damage the stereocilia of the OHCs. In turn, the stiffness of the organ of Corti decreases.[12] Aminoglycoside antibiotics, such as gentamicin, are potassium channel blockers and stop the hair cells from depolarizing. They can also change the perilymph ion concentration leading to damage of the hair cell bundle causing permanent hearing loss.

According to Schuknecht's classification in presbycusis, three major cochlear structures can independently degenerate and influence the degree of hearing loss; afferent neurons, the organ of Corti, and stria vascularis.[13][14]

• Sensory - steep high-frequency hearing loss with preserved speech perception - degeneration of the organ of Corti
• Neural - downsloping high-frequency hearing loss with a disproportionate loss of speech perception - degeneration of spiral ganglion cells
• Strial/metabolic - a flat SNHL with preserved speech perception - degeneration of stria vascularis
• Cochlear conductive - progressive downsloping high-frequency SNHL - increased stiffness of the basilar membrane

Histopathology

The sensory epithelium of the cochlea is the organ of Corti. It is contained within the scala media of the cochlea and is composed of sensory hair cells and non-sensory supporting cells. The sensory cells are the OHCs, and the IHCs are arranged uniquely. There is a single row of IHCs and three rows of OHCs separated by supporting cells.[15] The hair bundle is composed of rows of stereocilia that increase in height in one direction across the apical surface of the hair cell. Movements between the tectorial and basilar membrane cause deflection of the stereocilia, which activate or deactivate receptors on the surface of hair cells. Potassium ions flow into the hair cell, which depolarizes it, causing calcium channels to open, and the auditory nerve then sends information about the sound wave to the brain.

History and Physical

It is important to take a thorough history when assessing a patient with SNHL. Essential points to obtain include age of onset, laterality of symptoms, rapidity of decline, fluctuating symptoms, and associated symptoms such as tinnitus, aural fullness, disequilibrium, and vertigo. Establishing the premorbid hearing level is crucial to direct rehabilitation and to assess if the hearing loss is new or deterioration of an existing picture. Previous ear surgery, history of noise exposure, previous head trauma, barotrauma, or ototoxic exposure to aminoglycosides are asked.

Patients who present with presbycusis will give a history of progressive decline in hearing. They are turning up the television louder than usual and asking other people to speak up. It is often family members who notice this first. There will be a history of personal noise exposure or occupational exposure in noise-induced hearing loss cases. Social history often helps guide management and provides insight into how the patient's symptoms affect their lives and their families. Many patients with hearing loss find it incredibly isolating. Activities they previously enjoyed, such as going to the cinema, going out to eat at a restaurant, and meeting family and friends, become stressful for them, and they withdraw from them.

When seeing patients in the clinic with a new hearing loss, aside from a focused otological examination, it is essential to do a full head and neck exam, including all the cranial nerves, although it is typically normal.

Evaluation

A complete audiometric evaluation is the gold standard for evaluating a hearing loss and should be performed to evaluate someone with sensorineural hearing loss. In clinical practice, tuning fork tests, a quick and easy bedside investigation, are usually performed first alongside a pure tone audiogram (PTA) and tympanometry. 

Rinne and Weber Tests

This is a bedside investigation performed using a 512Hz tuning fork and is useful when a clinician is trying to distinguish between a conductive and SNHL, though there must be at least a 20 dB difference between ears or between conduction and sensorineural thresholds for these tests to detect it.[16][17]

The Weber test involves striking the tuning fork against your knee and then placing the tuning fork in the middle of the patient's forehead. The patient is then asked to identify which ear they can hear the sound loudest in. In a unilateral SNHL, the patient will hear the sound loudest in their normal or "good" ear, whereas if they had a conductive hearing loss, the sound would lateralize to their deaf or "worse" ear. If the SNHL is bilateral, then the sound will not lateralize to a particular ear.[3]

The Rinne test is performed by first striking the tuning fork and then placing the tuning fork in two positions, firstly on the patient's mastoid process until it is no longer heard and then approximately 1cm away from the patient's external auditory meatus. The former is testing bone conduction and the latter air conduction. For a normal test, or Rinne positive result, the patient will report that the sound was still heard when the fork was held in front of their ear, i.e., air conduction is better than bone conduction, implying no conductive loss. If the test yields a Rinne negative result, the patient will report that the sound was not heard when the fork is held in front of their ear, i.e., bone conduction is better than air conduction, which implies a conductive hearing loss. In SNHL, the Rinne test should be positive when the affected ear is tested as there is no conduction loss.[3]

Pure Tone Audiogram

Patients are often sent for an audiogram to evaluate their hearing in an outpatient clinic. This essentially tests both air and bone conduction pathways, and simplistically both air and bone conduction thresholds are plotted on a graph as a curve at increasing frequencies of sound up to 8000Hz. In SNHL, both air and bone conduction curves worsen, with no air-bone gap. The shape of the curve will differ depending on the underlying pathology. For example, in presbycusis, you will see a downward sloping high-frequency loss. Conversely, in a conductive hearing loss, the air conduction curve will worsen and shift downward, while the bone conduction curve remains the same. This difference between the two curves is the air-bone gap. 

Other Tests

• Tympanometry: It is used to assess middle ear function and the tympanic membrane's mobility. This test is often used in clinical practice to evaluate if there is otitis media with effusion and eustachian tube dysfunction. The acoustic stapedial reflex can also be assessed. The lowest intensity of sound that triggers the reflex is the acoustic reflex threshold.
• Otoacoustic emissions are sounds recorded in the external auditory meatus and reflect the proper functioning of the OHCs. When OHCs are damaged, these sounds are absent.
• Electrophysiological tests: Auditory brainstem testing measures nervous system activity and can be affected by cerebellopontine angle tumors compressing the cochlear nerve and neural demyelination. It is also used to predict hearing thresholds in babies.
• Speech audiometry: This test is essential in assessing the impact of hearing loss on communication.
• Head computed tomographic scans, including thin temporal bone window and brain magnetic resonance imaging (MRI), are also carried out to evaluate for cochlear ossification, the presence of a cerebellopontine angle tumor, or active mastoiditis.
• Laboratory tests are typically not necessary. Still, there are exceptions, such as when considering an autoimmune cause of SNHL, in which tests such as erythrocyte sedimentation rate, antinuclear antibody, rheumatoid factor, and anti-microsomal antibodies are requested.

Treatment / Management

Sensorineural hearing loss is managed according to the underlying cause. In acute cases where no cause is found and suspected to be of idiopathic origin, a routine brain MRI with detail to the internal auditory meatus should be requested. Usually, these patients will be commenced on oral corticosteroids with a prednisone dose of 1 mg/kg/day (max 60 mg/day) for seven days tapered over the next week.[3] (A1)

As with all medication and due to limited evidence regarding efficacy, a clinician must weigh up the risks and benefits of treatment for an individual patient. This being said, spontaneous improvement in hearing is most likely to occur during the first two weeks. A repeat audiogram should be done within 10 to 4 days to assess improvement, and if no improvement, the patient should be considered for salvage intratympanic steroids. Some clinicians differ in their practice and may use intratympanic steroids earlier in a patient’s treatment course. They are an option, especially in refractory cases or in those patients where systemic steroids may not be suitable. Fully recovered hearing loss is defined if the follow-up PTA has improved to within 10 dB of the pre-hearing loss level.[18](A1)

Hearing aids are the mainstay of treatment in chronic cases, and there are various types. In most patients with presbycusis, hearing aids are of benefit even in mild or moderate cases.[19] There is no way to regain previous hearing thresholds, and audiological rehabilitation support is particularly important in these patients due to psychosocial comorbidities. Hence the earlier the diagnosis is made, the more we can minimize the impact on these individuals' lives.[20] (A1)

Conventional behind the ear air conduction hearing aids are the most common devices for treating unilateral or bilateral hearing loss. Sound is detected by a microphone where acoustic energy is converted into electrical energy, which is amplified. The receiver then transforms the signal back into an acoustic signal, reproduced by a speaker in the patient's ear canal. They are readily available, inexpensive, and easy to replace. They are not suitable for external ear malformations of the pinna or external auditory canal and in the presence of infection. Generally, if hearing thresholds are worse than 30 dB, a patient can benefit from hearing aids. 

Contralateral routing of signal (CROS) hearing aids uses a microphone in the worse ear to detect sound, transmitted wirelessly to the other hearing aid, and played into the better hearing ear by air conduction. Bilateral microphones with the contralateral routing of signal (BiCROS) hearing aids are similar, but a microphone also aids the better hearing ear on the same side.

Bone conduction hearing systems are used in conductive and mixed hearing loss where there is a large air-bone gap. It is also useful in single-sided deafness. These devices most often involve a titanium fixture into the bone. Through osseointegration, vibration from the attached sound processor is transmitted into the skull and through bone conduction to the cochlea. This bypasses the usual air conduction transmission route via the external auditory canal, tympanic membrane, and ossicles.

Other aids that patients may find useful include personal frequency modulation systems or Roger pens, which are wireless microphones used in combination with hearing aids. It allows the person holding it to focus on the speech they want to hear and diminishing background noise in noisy environments. 

Cochlear implants and their use vary worldwide. In the UK, the National Institute for Health and Care Excellence recommends considering cochlear implants for people with thresholds >80 dB in both ears at two frequencies who receive inadequate benefit from their hearing aids. In the US, the threshold for adults is >70 dB. The thresholds for implantation will be different depending on the socio-economic environment. A cochlear implant is a surgically fitted device that provides speech perception to patients with whom hearing aids do not benefit.[21] Usually, the sound is converted from mechanical to electrical energy via the hair cells in the cochlea. Cochlear implants bypass the damaged hair cells and provide direct stimulation to auditory neurons. When assessing patients for an implant, full audiometric evaluation is undertaken with appropriately fitted hearing aids.

Differential Diagnosis

Congenital

Hearing loss is the most common congenital sensory disorder, often requiring pediatricians' expertise in conjunction with medical geneticists and pediatric otolaryngologists.[22] Congenital causes of hearing loss can be divided into genetic and environmental. Genetic causes can be further divided into non-syndromic (70%) and syndromic (30%).

Genetic Non-syndromic

Of the nonsyndromic causes, the most common inheritance pattern is autosomal recessive, followed by autosomal dominant. More than 60 autosomal recessive genes have been identified, the most common being the gap junction beta 2 (GJB2) gene, which accounts for half of nonsyndromic hearing loss cases.[23] GJB2 encodes connexin 26, a protein important for the proper functioning of the cochlea's potassium ion channels. In these patients, due to an absence of other physical findings or relevant history, diagnostic workup remains a challenge but primarily consists of genetic testing, electrocardiogram (ECG), and testing for cytomegalovirus infection.

Genetic Syndromic

In children who have hearing loss due to syndrome, determining the underlying cause is often more important as the other clinical features can be severe.[24] More than 400 syndromes have been identified with hearing loss as a feature; however, only a small number of these account for most cases of SNHL.[25] Outlined below are the key features of the most common syndromes seen in children. 

1. Waardenburg syndrome is the most common, with SNHL being a significant feature found in over two-thirds of patients.[26] The other key feature is pigmentation abnormalities of the eyes, skin, and cochlea.
2. Usher syndrome is one of the most common autosomal recessive causes of syndromic hearing loss. This condition is characterized by hearing loss and visual loss due to a progressive SNHL and retinitis pigmentosa.
3. Pendred syndrome classically presents with a varying degree of SNHL, vestibular dysfunction, and a thyroid goiter. Along with Usher syndrome, it is another one of the most common autosomal recessive causes of syndromic hearing loss. A specific mutation in SLC26A4 occurs in around half of the affected patients.[27]
4. Jervell and Lange-Nielsen syndrome is also inherited with an autosomal recessive pattern. The key feature, along with SNHL, is a prolonged QT interval seen on the ECG. These patients can suffer from or have a family history of syncope, sudden death, or long QT syndrome.
5. Alport syndrome is inherited in an X-linked manner and occurs due to a defect in type IV collagen. It classically presents with glomerulonephritis, end-stage kidney disease, eye abnormalities,  and a bilateral SNHL. The hearing loss is initially in high frequency, but the lower frequencies begin to get affected as it worsens. Hematuria and proteinuria are key signs as the condition progresses.  

Environmental

1. Intrauterine infection (toxoplasmosis, cytomegalovirus, herpes, rubella)
2. Alcohol, smoking
3. Ototoxic drugs
4. Premature births, hypoxia, neonatal jaundice

Sensorineural Hearing Loss

Acute sensorineural hearing loss is defined as a hearing loss greater than 30 dB in at least three consecutive audiometric frequencies over 72 hours.[18] This is usually classified as an otolaryngologic emergency condition, which requires prompt management. There are several possible causes of SNHL, including trauma, infection, malignancy, and Meniere's disease; however, there is no identifiable cause of their hearing loss in most patients and will be classed as idiopathic.[28] As part of the work-up, patients should have a pure tone audiogram as soon as is possible. Often routine blood tests and an autoimmune screen are sent off, although practice varies between departments.

Presbycusis

Presbycusis or age-related hearing loss can be defined as a progressive bilateral SNHL of mid to late adult-onset. The diagnosis of presbycusis is one of exclusion, and primary causes such as otosclerosis, Meniere's disease, and cytotoxicity, amongst many others, must be excluded first. It is commonly associated with degeneration of cochlear hair cells, mainly OHCs in the cochlea's basal portion, and the changes begin in the basal end of the cochlea and spread towards the apex as the condition worsens. Patients typically present with a slowly deteriorating hearing loss, especially in the presence of background noise. It is often a lack of clarity rather than a loss of volume that the patient describes. Tinnitus is frequently an accompanying symptom and can be the most challenging aspect for the patient.

A typical PTA will show a gradual downsloping hearing loss towards higher frequencies. It has been shown that once a certain amount of hearing loss has occurred (roughly 70-80 dB), further progression is slow, especially in the higher frequencies.[29] In terms of management options, hearing aids often benefit patients and prevent social isolation and depression.

Noise-induced Hearing Loss

This condition occurs when a patient experiences hearing loss due to excessive noise exposure, either recreational or occupational. Occupational noise exposure is one of the most prevalent, potentially preventable health conditions. It has a slight male predominance and usually affects the middle-aged population. The symptoms present similar to most SNHL conditions, with the insidious progression of worsening hearing loss over many years, often accompanied by tinnitus. Hyperacusis is found in 40% of tinnitus sufferers, and its severity can be determined using a hyperacusis questionnaire.[30] Bedside otological examination is usually normal, and the diagnosis is generally based on the history combined with the classical finding of a notched appearance at 4kHz, which appears to start recovering at 8kHz on a pure tone audiogram. This is known as the Carharts notch; however, it is not always present. Without a previous noise exposure history, it is not indicative of NIHL. Once a diagnosis is reached, it is essential to reduce further noise exposure as much as possible using ear protection. The Control of Noise at Work Regulations of 2005 sets out a framework for employers to ensure their employees' safety based on their average occupational sound exposure.  

Others 

Meniere's disease is characterized by a triad of spontaneous episodic vertigo, hearing loss, and tinnitus. Patients may also experience aural fullness. Their PTA usually shows an up sloping curve indicating a low-frequency moderate SNHL. PTA remains the most useful investigation, but a brain MRI is done to rule out lesions such as a vestibular schwannoma. Treatment ranges from conservative such as alcohol, coffee, and salt restriction, to treatments targeted at the symptoms most affecting the patients.[31] Vestibular suppressants such as prochlorperazine can be helpful. Hearing aids and tinnitus retraining therapy can also be used. Psychological support can be key in those patients who suffer from the psychological complications of the condition.

Autoimmune SNHL was first described in 1979 by McCabe as a rapidly progressive bilateral SNHL that responded to steroid therapy.[32] Several antigens have been implicated in the etiology.[33] Many systemic autoimmune disorders have been reported, such as Wegener's granulomatosis, rheumatoid arthritis, and systemic lupus erythematosus. Patients usually present in their early twenties. The condition has a female predominance. Symptoms often start in one ear before becoming bilateral in the majority of patients. Aside from audiometric evaluation, blood tests (erythrocyte sedimentation rate, antinuclear antibody) looking specifically at autoimmune conditions are sent off. Treatment is directed by rheumatologists and includes steroid therapy, long-term intratympanic steroid injections, cyclophosphamide, and IL-1 receptor antagonists.[34]

A head injury that results in a temporal bone fracture can lead to conductive hearing loss or mixed SNHL. Otic capsule fractures cause a severe SNHL through various mechanisms, including disruption of the membranous labyrinth, hemorrhage into the cochlea, perilymph fistula, and avulsion or trauma of the cochlear nerve. In those cases that result in bilateral deafness, a cochlear implant is a treatment option.

Ototoxic agents can cause SNHL in many ways. Aminoglycosides such as gentamicin cause hair cell death resulting in permanent hearing loss and balance dysfunction. This can occur after repeated administration of systemic therapy. The hearing loss initially affects the higher frequencies but continues progressively to the lower frequencies as more hair cells are damaged.[35] Loop diuretics are thought to affect the stria vascularis and cause acute but completely reversible effects.[36]

Prognosis

Sensorineural hearing loss tends to have a typical slow progression and can be managed with conservative measures and hearing aids for the vast majority of patients with regular follow-up appointments and audiograms. If patients are eligible, even profound hearing loss can be rehabilitated with a cochlear implant. 

With sudden SNHL, four factors have been shown to help predict the outcome.[37]

• 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 early medical attention and engage with their treatment will likely have better outcomes. In SNHL, 32% to 65% of the cases resolve without intervention. The cause is unknown at the presentation in 80-90% of the cases. Even after a thorough investigation, it is possible to identify a cause in only one-third of the patients. It has been shown that improvement within the first two weeks may predict an excellent long-term outcome.[38]

Complications

Complications can include physical symptoms and the psychological impact of living with hearing loss. Many patients with SNHL suffer from other associated symptoms such as tinnitus and dizziness, which they often find more challenging to tolerate than hearing loss. Studies have been done to evaluate the impact tinnitus can have on patients' lives. Prevalent responses included inability to concentrate, constant awareness, the effect on sleep, intrusiveness, and overall loss of control over their lives.[39] 

Fear was another domain that scored highly when a questionnaire was carried on a cohort of patients. Patients reported fear of the tinnitus itself, fear of living with tinnitus long term, and a fear of doing activities that may exacerbate their symptoms.

Various studies have shown a correlation between uncorrected hearing loss and cognitive decline, strengthening the argument for early auditory rehabilitation with hearing aids.[40] A prospective 25-year longitudinal study confirms this correlation and suggests that social isolation and depression facilitates this decline.[41] 

Many patients struggle in social situations and partake in activities they usually enjoy, leading to increasing isolation from friends and families. Although there are support groups in the community, they may not be easily accessible or available, and some patients perceive a social stigma regarding hearing loss. With an aging population, hearing loss will become increasingly prevalent, and it is already a significant public health issue in the developed world.