Hearing loss : children’s brains develop in response to exposure to sounds

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Deafness in early childhood is known to lead to lasting changes in how sounds are processed in the brain, but new research published today in eLife shows that even mild-to-moderate levels of hearing loss in young children can lead to similar changes.

Researchers say that the findings may have implications for how babies are screened for hearing loss and how mild-to-moderate hearing loss in children is managed by healthcare providers.

The structure and function of the auditory system, which processes sounds in the brain, develops throughout childhood in response to exposure to sounds.

In profoundly deaf children, the auditory system undergoes a functional reorganisation, repurposing itself to respond more to visual stimuli, for example.

However, until now relatively little was known about the effects of mild-to-moderate hearing loss during childhood.

A research team led by Dr Lorna Halliday, now at the MRC Cognition and Brain Sciences Unit, University of Cambridge, used an electroencephalogram (EEG) technique to measure the brain responses of 46 children who had been diagnosed with permanent mild-to-moderate hearing loss while they were listening to sounds.

Dividing the children into two groups – younger children (8-12 years) and older children (12-16 years) – the team found that the younger children with hearing loss showed relatively typical brain responses – in other words, similar to those of children with normal hearing.

However, the brain responses of older children with hearing loss were smaller than those of their normally hearing peers.

To confirm these findings, the researchers re-tested a subset of the group of younger children from the original study, six years later.

In the follow-up study, the researchers confirmed that as the children with hearing loss grew older, their brain responses changed.

Responses that were present when the children were younger had either disappeared or grown smaller by the time the children were older.

There was no evidence that the children’s hearing loss had worsened over this time, suggesting instead that a functional reorganisation was occurring.

“We know that children’s brains develop in response to exposure to sounds, so it should not be too surprising that even mild-to-moderate levels of hearing loss can lead to changes in the brain,” says Dr Axelle Calcus, lead author of the paper, from PSL University, Paris. “However, this does suggest that we need to identify these problems at an earlier stage than is currently the case.”

This shows a little girl with a hearing aid

There was no evidence that the children’s hearing loss had worsened over this time, suggesting instead that a functional reorganisation was occurring. The image is credited to Kind, Hinter-Ohr-Gerät-Anpassung.

“Current screening programmes for newborn babies are good at picking up moderate-to-profound levels of hearing loss, but not at detecting mild hearing loss.

This means that children with mild hearing impairment might not be detected until later in childhood, if at all,” says Dr Lorna Halliday from the University of Cambridge.

“Children with hearing problems tend to do less well than their peers in terms of language development and academic performance.

Detecting even mild degrees of hearing impairment earlier could lead to earlier intervention that would limit these brain changes, and improve children’s chances of developing normal language.”

Funding: The research was funded by the Economic and Social Research Council and the European Union Horizon 2020 Programme. The research was carried out at University College London (UCL).


Hearing loss is an extremely common medical condition, more prevalent than many other pathologies.[1][2] The affected population is also vast, varying between neonates to elderly patients.[3] 

The diagnosis and management require a multi-disciplinary team that includes the surgeon, speech therapists, and social workers.

To correctly address hearing loss, understanding the nature of hearing loss, and the equipment that is needed to improve auditory reception is crucial. In terms of children’s hearing loss, pediatricians need to be integrated into their care to ensure thriving and normal development of the child.

Etiology

The normal hearing function involves sound waves arriving at the auricle, causing a vibration of the tympanic membrane [4]. Vibration then gets 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.

Hearing loss can be conductive or sensorineural.[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 or dysfunction or fixation of the ossicular chain.

Also, cholesteatoma, a benign growth of 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 after the cochlea.[5] 

This loss could be related to air cells dysfunction or 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 perceive the sounds diminished and distorted.

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

Regarding the etiology of hearing loss, there are multiple reasons for hearing impairment. In children population, genetic causes are the most common, accounting for more than 50% of hearing loss cases. Genetic causes involve various syndromes that have hearing loss as one of their features; however, there is an entire entity of non-syndromic genetic related hearing loss involving children who only suffer from hearing loss while the rest of their function is normal.[6] 

Mutations, autosomal differences, as well as unknown genetic diversity, relates to this type of hearing loss.[7] Prenatal causes are also responsible for hearing loss in infants.

These include exposure to various bacterial or viral infections as well as different teratogens. Perinatal causes are less common in literature, predominantly resulting from prematurity, low APGAR score, jaundice over the exchange level 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 severe otitis media.[9]

Age-related hearing loss involves a gradual reduction of hearing capacity of the individual and poor speech understanding in noisy environments, which is probably related to histological changes and degeneration of the cochlea in various sites.[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 common in the audiology cases, and it is mostly idiopathic as the majority of patients present on an emergency basis without any identifiable cause.[12]

Epidemiology

Hearing loss affects approximately 0.5 to 1% of children in the United States, and it is either profound or mild to moderate. Existing data are very heterogeneous and studies are being conducted to determine that exact number of people with hearing loss.[13] 

This number reflects the neonatal and infant hearing loss, while the condition increasingly affects adolescents. In terms of adults, the prevalence appears to be nearly two-thirds of adults over 70 years of age in the United States.[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 stimuli transmission from the outer part of the ear to the brain suffers 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

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 first days of life as well as the post-natal history up until the moment of the symptom presentation.

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 causes, and the clinician can proceed to the appropriate investigations.[2]

Physical examination involves a full otolaryngology examination, with otoscopy bilaterally to rule out any reasons for 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 could be, 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. Special tests such as Weber and Rinne are useful to differentiate between SNHL and conductive hearing loss. 

Evaluation

An accurate hearing evaluation is possible for the population of all ages at this time. 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 by the age of one month.[17] 

Additionally, all newborns and infants with hearing loss need to get a comprehensive evaluation which focuses on medical and birth history as well as a family history for the previous three generations according to American College of Medical Genetics and Genomics.[18]

Hearing loss evaluation should take place with various tests that differ according to the age of the child. BAER (brainstem audio-evoked response) test is the method for early diagnosis of hearing the 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. Tympanograms and audiograms are of value in adults, with the same principles and the equal diagnostic value.[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, micro-suction 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 as a post-operative complication due to tympanosclerosis.[20] 

Finally, if the hearing loss is due to cholesteatoma, this requires surgical removal with good results in hearing restore.[21]

Conservative treatment of sensorineural hearing involves the use of assistive listening devices. Hearing aids are devices designed to improve audition up until 40 to 60 dB with good results.[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.

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.

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.[24] In terms of childhood congenital hearing loss, the literature has weak information regarding prognostic factors with cochlear implantation; the age of implantation, the inner ear malformations, and the co-existence of ear infections appear as the most dominant factors.[25]

Complications

Complications of hearing loss in children involve linguistic and developmental problems.

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

Deterrence and Patient Education

Patient education is critical both in children and adults. Hearing loss in children is very demanding, as parents need to understand how to communicate with their child as well as ensure their child’s normal linguistic and social development. Parent education on how to evaluate their child’s hearing as well as when to seek help is crucial. 

Adult and elderly patients need to be educated carefully regarding how to use their hearing aids properly as well as when they require a new consultation with their otologist. Frequent hearing checks and hygiene of the hearing aids are essential factors to ensure the best hearing quality in the long term.

Enhancing Healthcare Team Outcomes

Multidisciplinary teams are crucial in hearing loss diagnosis and management. The team responsible for the patients suffering from the condition includes the otolaryngologist, the speech and language therapist, the clinical geneticists, genetic counselors, audiologists as well as pediatricians, early hearing intervention, specialty-trained nurses, and family support specialists if the patient is a child.

The various specialties cooperate to reach the correct diagnosis and ensure the best quality of management takes place, and the patient has the most optimal hearing result possible.


Source:
University of Cambridge
Media Contacts:
Lorna Halliday – University of Cambridge
Image Source:
The image is credited to Kind, Hinter-Ohr-Gerät-Anpassung.

Original Research: Closed access
“Functional brain alterations following mild-to-moderate sensorineural hearing loss in children”. Axelle Calcus, Outi Tuomainen, Ana Campos, Stuart Rosen, Lorna F Halliday.
eLife doi:10.7554/eLife.46965.

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