Hearing Loss

Etiology

Normal hearing function involves sound waves arriving at the auricle, passing through the external auditory canal (EAC), causing a vibration of the tympanic membrane.[4] Vibration is then transmitted via the ossicles (malleus, incus, stapes) to the cochlea. Subsequently, hair cells inside the cochlea stimulate the eighth cranial nerve that transfers the stimuli to the brain. Processing of crude sounds occurs in the higher cortices of the brain, and this includes the comprehension of language.

Hearing loss can be conductive, sensorineural, or mixed.[3] Conductive hearing loss takes place with disruption of the transmission of the sound waves to the cochlea. The most common causes include abnormal formation of the auricle or helix, cerumen impaction, ear canal foreign bodies, otitis externa, dysfunction or fixation of the ossicular chain, and middle ear effusion. Cholesteatoma, a benign though locally destructive trapping of squamous debris arising from the tympanic membrane, as well as other benign or malignant tumors, can result in conductive hearing loss. 

Sensorineural hearing loss (SNHL) usually results from problematic transmission of the stimuli at or after the cochlea.[5] This loss could be related to hair cell dysfunction or a disorder of the eighth nerve itself. The main difference between the two kinds of hearing loss, apart from the pathophysiological features, is that patients with conductive hearing loss perceive the sounds diminished, while SNHL patients may perceive the sounds diminished and distorted.

Hearing loss that involves problematic transmission before and after the cochlea is called mixed hearing loss.

There are multiple reasons for hearing impairment. In the pediatric population, genetic causes are the most common, accounting for more than 50% of hearing loss. Genetic causes involve various syndromes that have hearing loss as one of their features; however, there is an entire entity of non-syndromic genetic hearing loss, wherein patients suffer hearing loss while the rest of their function is normal.[6] Mutations, autosomal differences, as well as unknown genetic diversity relate to this type of hearing loss.[7] Prenatal causes can also relate to hearing loss in infants. These include exposure to various bacterial or viral infections as well as different teratogens. Perinatal causes are less common, predominantly related to prematurity, low APGAR score, neonatal jaundice, and sepsis.[8] Postnatal causes such as meningococcal infections and mumps can also cause hearing loss as a late complication, as well as head injuries or chronic or recurrent otitis media.[9]

Age-related hearing loss involves a gradual reduction of the hearing capacity of the individual and poor speech discrimination scores, most noticeable initially in noisy environments, which is likely related to age-related degeneration of the cochlea in various sites, particularly the hair cells.[10] Otosclerosis and cholesteatomas are leading causes of conductive hearing loss.[5] Another entity that can occur in the adult population is sudden sensorineural hearing loss.[11] This condition is very specific, with a sudden or rapid onset of hearing loss in one ear. This is often preceded by a viral upper respiratory infection, and virally-mediated inflammatory markers are the presumed cause.[12]

Epidemiology

Hearing loss affects approximately 0.5 to 1% of children in the United States and can be profound, mild, or moderate.[13] This number reflects the neonatal and infant hearing loss based on neonatal screenings, while the condition may increasingly affect school-aged children as they are exposed to pathogens. While most neonatal hearing loss is genetic, most school-aged and adolescent hearing loss is acquired. In adults, the prevalence appears to be nearly two-thirds of adults over 70 years of age in the United States, though it may be much higher.[10] The condition is associated with aging, contact with noise, and the duration of that contact, as well as other co-morbidities such as hypertension, high blood sugar, and smoking history, which are all recognized as high-risk factors.[14]

Pathophysiology

Hearing loss is a condition that occurs when the sound transmission from the outer ear to the brain suffers a disruption. The disruption can happen at any stage, either before or after the cochlea, and the hearing loss is conductive or sensorineural, respectively. If both sites, pre and post the cochlea, are affected, then the hearing loss is characterized as mixed.

History and Physical

The history in pediatric cases is critical for early diagnosis of hearing loss. This history involves questions regarding the prenatal history of the child, their delivery, and the first days of life, as well as the postnatal history up until the moment of the symptom presentation, as well as a family history of hearing loss. A child with hearing loss may present with non-reaction to sounds, behavioral problems, speech issues, language delay, or even school failure, as well as mispronouncing words. Family history, especially if there is a member with early hearing loss, also has great value in suspecting hearing loss.[15]

Adult history acquisition is more straightforward and involves questions regarding the onset of symptoms, the severity, the presence of vertigo, neurological symptoms, infections, and other conditions that could be related to hearing loss. Past medical history, as well as family history, along with work and noise exposure, are also important. In this way, there is a differentiation between the causes of hearing loss, and the clinician can proceed to the appropriate investigations.[2]

Physical examination involves a full otolaryngology examination, with otoscopy bilaterally (including pneumatic otoscopy) to rule out any obvious conductive hearing loss.[16] Foreign bodies, cerumen, infections, tympanic membrane perforations, as well as middle ear effusion need to be ruled out in the first instance. Subsequently, identification of dysmorphic and other physical findings is essential, especially in young children and infants. These can include facial abnormalities or asymmetry, ear, neck, and skin anomalies, other organ dysfunction, or even balance irregularities. As a result, a comprehensive inspection, otoscopy, and neurological examination are crucial to reach the correct diagnosis. Weber and Rinne tests are easy, fast, and globally useful to differentiate between SNHL and conductive hearing loss and may aid in interpreting the formal audiogram. 

Evaluation

An accurate hearing evaluation is possible for the population of all ages, though there are specific limitations that are age- or cognition-related regarding formal audiometric evaluation that may complicate the picture. According to the American Academy of Pediatrics as well as the Joint Committee on Infant Hearing, all infants should undergo a hearing evaluation to rule out any hearing impairment at birth or by the age of one month.[17] Additionally, all newborns and infants with hearing loss need to get a comprehensive evaluation that focuses on medical and birth history as well as a family history for the previous three generations, according to the American College of Medical Genetics and Genomics.[18]

Hearing loss evaluation can differ according to the age of the child. BAER (brainstem audio-evoked response) test is the method for early diagnosis of hearing loss in newborns and infants. Otoacoustic emissions are also an option in newborns, and it is an easy, inexpensive technique, but they are less reliable than BAER tests. Finally, audiometry works with older children, aged 4 to 5 and older, who can respond to sound stimuli according to instructions. There are age-specific audiometric tests that can be performed. Young, pre-lingual children can be conditioned to respond to play stimuli that can assess whether they can hear. This method of testing is not ear-specific and can only confirm they have at least one ear that can hear at a given test level. This is important because as long as there is one ear that hears at a normal level, normal language development can be expected. Tympanograms and audiograms are of value in adults and children but provide information only regarding the mobility of the tympanic membrane.[19]

Treatment / Management

Management of conductive hearing loss focuses on the treatment of the underlying disease. Conservative methods such as removal of the foreign body, microsuction of the cerumen, or discharge in the ear canal are necessary if the ear canal is blocked [5]. With regards to otitis media, myringotomy to release the middle ear fluid will allow the sound wave to reach the cochlea, while ventilation tubes are useful if the otitis media is persistent, causing hearing loss. However, evidence shows that hearing loss can represent a postoperative complication due to tympanosclerosis, though this is very rare.[20] Finally, if the hearing loss is due to cholesteatoma, this requires surgical removal with results in hearing restoration dependent upon the degree of destruction of the middle ear structures. Chronic or refractory, inoperable, conductive hearing loss can be treated with bone-conduction hearing aids or via a BAHA, a bone-implanted conduction aid, with excellent results.[21](B2)

Conservative treatment of sensorineural hearing involves the use of assistive listening devices and amplification. Hearing aids are devices designed to improve audition up to 40 to 60 dB with good results. They require individualized fittings and venting plans and can be very expensive.  The overall results are very good.[22] Surgical treatment is provided to infants diagnosed with SNHL as they undergo cochlear implantation under the age of 6 months.[23] The intervention requires an ear, nose, and throat specialist, and long-term monitoring is essential to ensure normal linguistic and social development of the child. Similarly, refractory, severe SNHL in adults can be treated with cochlear implantation. This will destroy any remnant native hearing, replacing it with the device, so this must be done judiciously. Excellent outcomes have been achieved routinely with this procedure in severe SNHL.(A1)

Differential Diagnosis

The differential diagnosis for hearing loss is very broad as the condition has various causes. More than 300 congenital syndromes are related to hearing loss. Also, conductive hearing loss, as well as SNHL, has multiple triggering factors mentioned in the Etiology section. Therefore, a thorough history and clinical examination are key for correct diagnosis and management. The most important step is the physical examination and comprehensive audiogram to rule out easily correctible causes such as cerumen or Eustachian tube dysfunction. This must be performed before a more advanced workup is entertained.

Staging

According to the American National Standards Institute, hearing loss ranking is as follows:

• Slight hearing loss: 16 to 25 dB
• Mild hearing loss: 26 to 40 dB
• Moderate hearing loss: 41 to 55 dB
• Severe hearing loss: 71 to 90 dB
• Profound: over 90 dB

Prognosis

The prognosis of hearing loss varies between the multiple types. Sudden SNHL prognosis is very dependent on several factors such as the severity of the disease, age, co-morbidities, vertigo, and the shape of the audiogram. This is discussed in a separate StatPearls article.[24] In terms of childhood congenital hearing loss, the mainstay of outcomes depend on the overall neurologic status. Hearing can be effectively restored via cochlear implantation, but global neurologic status, in addition to family care and speech therapy, are paramount to maximize the benefits of the surgery.[25]

Complications

Complications of hearing loss in children involve speech delay and failure to thrive in school. Any child with speech delay requires a formal hearing evaluation, as this is the most common cause.

Adult patients, who are frequently elderly, can easily become isolated and depressed if their hearing loss is not addressed.[26]