Sensorineural Hearing Loss And Genetics

Introduction

Hearing loss is a common condition that affects people of all ages, with approximately 466 million people worldwide experiencing some form of hearing loss. In fact, by 2050, it is estimated that over 900 million people will have hearing loss.¹ Age-related hearing loss is the most common type, experienced by around one-third of people over 65. Additionally, approximately 1.1 billion young people between the ages of 12-35 are at risk of hearing loss due to exposure to loud sounds, including music.¹ 

Definition 

Sensorineural hearing loss (SNHL) is a type of hearing loss that occurs due to damage or dysfunction of the inner ear or auditory nerve. This type of hearing loss can be caused by genetic predisposition, noise exposure, ototoxic medications, infections, and ageing. SNHL is a common condition that affects millions of people worldwide and can have a profound impact on a person's quality of life.

The inner ear contains hair cells responsible for converting sound waves into electrical signals, which are sent to the brain for processing. Damage to these hair cells can result in SNHL, ranging from mild to profound. Mild SNHL may cause difficulty hearing in noisy environments, while profound SNHL may result in complete deafness.

Importance

Awareness of sensorineural hearing loss (SNHL) is crucial in promoting prevention, early detection, and timely management of the condition. Studies have shown that awareness of SNHL is low among the general population, and many people delay seeking medical attention for hearing problems. For example, a survey conducted in the United States found that only 25% of adults aged 20-69 who reported difficulty hearing had sought medical treatment for their hearing loss.² 

Early detection and treatment of SNHL are vital in preventing further deterioration of hearing and improving outcomes. Treatment options for SNHL include hearing aids, cochlear implants, and other assistive listening devices.¹ However, delays in seeking treatment can result in irreversible hearing loss and a reduced ability to communicate with others.

Impact of SNHL on children

Sensorineural hearing loss (SNHL) can have a significant impact on children's speech, language, and cognitive development. Children with SNHL may experience delayed language development, difficulty understanding speech, and problems with social and emotional development.³ Early identification and management of SNHL in children is crucial in minimising the negative impact on their development and improving their outcomes.

Delayed language development is a common consequence of SNHL in children. Children with hearing loss may have difficulty discriminating speech sounds, making it harder to understand and learn language.³ This can lead to speech and language development delays, affecting their ability to communicate effectively with others. Studies have shown that children with SNHL are at a higher risk of developing reading difficulties and other learning problems.³

Causes of genetic hearing loss

SNHL in children can have various causes, and genetic mutations account for more than half of SNHL cases. There are two primary forms of genetic hearing loss: syndromic and nonsyndromic.

• Syndromic Hearing Loss: About 30% of children with SNHL have syndromic hearing loss, which is associated with other medical problems or syndromes. Diagnosis can be made by evaluating the child and family and considering additional signs and symptoms.⁴
• Nonsyndromic Hearing Loss: Nonsyndromic hearing loss, accounting for around 70% of SNHL cases, occurs without associated medical problems.⁴ Genetic testing is an important tool for diagnosing nonsyndromic SNHL

Genetic factors are often central to the development of SNHL. Inherited genetic mutations or abnormalities can disrupt the structure and function of the inner ear, leading to hearing loss. These mutations can result in hearing loss at birth or it may manifest later in life. Several genetic conditions are associated with SNHL including:

• Connexin 26 (GJB2) gene mutations: GJB2 gene mutations are the most common cause of genetic hearing loss. These mutations affect the production of a protein called connexin 26, necessary for normal functioning of the inner ear.⁴
• Pendred syndrome: This syndrome is characterized by hearing loss and an enlarged thyroid gland. It is caused by mutations in a gene which affects the transportation of ions in the inner ear.⁵
• Usher syndrome: Usher syndrome is a genetic disorder that causes hearing loss and vision impairment. It is caused by mutations in several genes, affecting the development and function of sensory cells in the inner ear and retina.⁵

Non-genetic causes of SNHL

Other than genetic predispositions, various other factors can lead to the development of SNHL:

• Prenatal infections: Infections such as rubella, cytomegalovirus (CMV), and herpes simplex virus (HSV) can cause hearing loss in infants if contracted during pregnancy^1,6
• Ototoxic medications: Certain medications, such as certain antibiotics, chemotherapy drugs, and high-dose nonsteroidal anti-inflammatory drugs (NSAIDs), can damage the sensory cells in the inner ear, resulting in hearing loss^1,6
• Noise exposure: Prolonged exposure to loud noises, such as loud music or occupational noise, can lead to SNHL.¹ While noise-induced hearing loss is not purely genetic, individual susceptibility to noise-induced damage can have a genetic component

Types of genetic hearing loss

Autosomal Dominant Hearing Loss: Autosomal dominant hearing loss occurs when a single copy of a mutated gene from one parent is sufficient to cause hearing loss even if the other copy is functional. This type of hearing loss can manifest at any age and varies in severity within affected families.⁵

Autosomal Recessive Hearing Loss: Autosomal recessive hearing loss requires two copies of a mutated gene, one from each parent, to cause hearing loss. Individuals with one copy of the mutated gene are carriers but do not typically experience hearing loss themselves. Autosomal recessive hearing loss can range from mild to profound.⁵

X-Linked Hearing Loss: X-linked hearing loss is caused by mutations in genes located on the X (female) chromosome.⁵ Since people assigned male at birth have only one X chromosome, they are more likely to be affected by X-linked hearing loss. This is because they only inherit one copy of the X chromosome, so if it is mutated, there is no correct copy to compensate. People assigned female at birth can also be carriers of the condition.

Mitochondrial Hearing Loss: Mitochondrial hearing loss results from mutations in the mitochondrial DNA, inherited exclusively from the mother.⁵ This type of hearing loss can affect individuals at any age and may be associated with other mitochondrial disorders.

Diagnosis

The first step in diagnosing SNHL usually involves an audiological evaluation. Audiological evaluations are essential to assess the severity and type of hearing loss. This evaluation includes a comprehensive assessment of the individual's hearing abilities, such as pure-tone audiometry, speech audiometry, and tympanometry.^7,8 It helps determine if the hearing loss is sensorineural and provides valuable information for treatment planning.

Genetic testing plays a crucial role in diagnosing genetic SNHL and involves analysing an individual's DNA to identify specific gene mutations associated with hearing loss.⁷ Genetic testing can help determine the underlying cause of SNHL, assess the risk of future hearing loss in individuals and their family members, and guide treatment decisions. Genetic counselling provides information, support, and guidance to individuals and families regarding the inheritance patterns, implications, and management options associated with genetic SNHL.

Treatment of SNHL

There are several treatment options available to improve hearing and communication skills in those who have SNHL, such as:

• Cochlear implants: Cochlear implants are surgically implanted electronic devices to bypass the damaged parts of the inner ear.^1,5 They work by directly stimulating the auditory nerve, allowing individuals with severe to profound SNHL to perceive sound. Cochlear implants are particularly beneficial for those who do not experience significant improvement with conventional hearing aids. By allowing the user to hear sound, cochlear implants can enhance speech perception and improve communication abilities.
• Hearing aids: Hearing aids are amplification devices suitable for individuals with mild to moderate SNHL. They amplify sounds, making them clearer and louder for the wearer. Hearing aids help improve speech intelligibility and allow individuals to communicate better in different listening environments. They are non-invasive and can be customised to address specific hearing needs.
• Communication strategies and speech therapy: Developing effective communication strategies and undergoing speech therapy can greatly enhance the quality of life for individuals with genetic SNHL.¹ Communication strategies may involve lip-reading, using sign language, or utilising assistive listening devices. Speech therapy focuses on improving speech and language skills, enabling individuals to communicate more effectively. These interventions can help individuals with genetic SNHL overcome communication barriers and participate fully in social and professional activities.

Future directions

In terms of future directions, ongoing research and advancements in genetic testing and treatment offer promising prospects for individuals with genetic SNHL. Next-generation sequencing and other genetic testing technologies can improve the accuracy of diagnosing and identifying gene mutations associated with SNHL.⁵ These advancements allow for more precise genetic profiling, which can aid in developing personalised treatment approaches for individuals with genetic SNHL.⁵ By better understanding the underlying genetic causes, healthcare professionals can tailor interventions to target specific genetic abnormalities.

Gene therapy is an area of active research for SNHL. This approach involves delivering functional genes or using gene-editing techniques to correct genetic abnormalities responsible for hearing loss.⁵ Scientists are investigating the potential of gene therapy to restore or enhance hearing function by targeting specific gene mutations associated with SNHL. While still in the experimental stage, gene therapy holds promise for future treatment options for individuals with genetic SNHL.

Summary

Sensorineural hearing loss (SNHL) is often linked to genetic factors, and understanding its diagnosis and treatment is crucial. Genetic testing and counselling play a significant role in identifying specific gene mutations associated with SNHL and guiding treatment decisions. Audiological evaluations help assess the severity and type of hearing loss. Treatment options for genetic SNHL include cochlear implants, which bypass damaged parts of the inner ear, and hearing aids, which amplify sound. Communication strategies and speech therapy can also improve communication abilities. Future directions in the field include advancements in genetic testing and treatment, offering personalised interventions and potential gene therapies. Continued research is essential for improving the lives of individuals affected by genetic SNHL.