Understanding Otoacoustic Emissions and Their Importance for Hearing Health
September 19, 2024
Hearing loss affects millions of people worldwide. While many cases are caused by aging or environmental noise exposure, a significant portion of hearing loss is due to genetic factors. Recent advances in genetic research have transformed our understanding of hereditary hearing loss, enabling healthcare providers, especially Audiologists, to offer more personalised care and treatment options for individuals with hearing impairments.
This article explores how genetics influences hearing loss, the importance of genetic testing, some common genetic syndromes related to hearing loss, and how modern treatments like hearing aids can help manage the condition.
The Genetic Basis of Hearing Loss
Hearing loss can be classified into syndromic and non-syndromic categories:
Non-syndromic hearing loss: This is hearing loss that occurs in isolation, with no other symptoms or health issues involved. It accounts for around 70% of hearing loss cases.
Syndromic hearing loss: In these cases, hearing loss is part of a broader condition that can also affect other body parts, such as vision or balance. Approximately 30% of hearing loss cases are syndromic.
Genetic mutations are often the root cause of both types. For instance, mutations in the GJB2 gene, which encodes the protein connexin 26, account for many non-syndromic hearing loss worldwide. However, hearing loss is highly heterogeneous, meaning that different genetic mutations can lead to varying degrees of hearing impairment. Some may result in mild hearing loss, while others lead to profound deafness. Genetic hearing loss can also be autosomal recessive (requiring two copies of the mutated gene) or autosomal dominant (requiring only one copy).
Common Genetic Syndromes Associated with Hearing Loss
Many syndromes that cause hearing loss also affect other aspects of health. Here are some of the most common syndromes where genetic mutations result in hearing impairment:
Usher Syndrome
Usher syndrome is one of the most well-known syndromes associated with hearing loss. It is characterised by hearing loss, vision loss (due to a condition called retinitis pigmentosa), and balance issues. Usher syndrome has three different types:
Type 1: Individuals are born with severe to profound hearing loss and experience early-onset balance problems. Vision loss typically starts in early childhood and worsens over time.
Type 2: Hearing loss is moderate to severe from birth, and balance is typically unaffected. Vision loss starts in adolescence.
Type 3: Hearing, vision, and balance issues progressively worsen.
Hearing aids and cochlear implants can significantly improve the quality of life for individuals with Usher syndrome by helping them manage hearing loss. In contrast, vision and balance issues are handled separately.
Pendred Syndrome
Pendred syndrome is another genetic disorder associated with hearing loss. In addition to hearing impairment, this condition often causes thyroid problems and can lead to the development of goitre (an enlarged thyroid gland). The hearing loss in Pendred syndrome is usually sensorineural, resulting from damage to the inner ear or the nerve pathways from the inner ear to the brain. Hearing loss may be present at birth or develop early in childhood.
Depending on the severity of their hearing loss, patients with Pendred syndrome may benefit from hearing aids or cochlear implants. Additionally, regular monitoring of thyroid function is important for managing the condition.
Waardenburg Syndrome
Waardenburg syndrome is a rare genetic disorder that affects hearing, pigmentation, and facial features. People with Waardenburg syndrome may have varying degrees of hearing loss, from mild to profound, often in one or both ears. Additionally, individuals may have distinctive facial features, such as a broad nasal bridge, and unique pigmentation patterns, such as bright blue eyes or different coloured eyes (heterochromia).
Hearing aids are often helpful for individuals with Waardenburg syndrome, mainly if the hearing loss is not profound. Cochlear implants can be considered to improve hearing and communication abilities in cases of more severe hearing loss.
Alport Syndrome
Alport syndrome primarily affects the kidneys, leading to progressive kidney disease. However, hearing loss is another major feature of this syndrome, along with vision problems. The hearing loss associated with Alport syndrome usually begins in childhood or adolescence and progresses. The type of hearing loss is typically sensorineural, which means it is due to problems in the inner ear or the auditory nerve.
Hearing aids can help manage hearing loss in individuals with Alport syndrome, especially in the early stages. As the condition progresses, cochlear implants may be an option for more severe hearing loss.
The Role of Genetic Testing in Hearing Loss
Genetic testing has become an important tool for diagnosing the cause of hearing loss, particularly for newborns and children. It can help identify specific genetic mutations that are responsible for hearing impairment. This knowledge provides multiple benefits:
Identification of cause: Genetic testing can help confirm whether hearing loss is due to a genetic mutation, which can be reassuring for families who may have no history of hearing loss.
Guidance on treatment: Knowing the genetic cause of hearing loss can guide decisions about treatment. Certain genetic causes of hearing loss are associated with better outcomes from cochlear implants.
Family planning: Parents who have a child with genetic hearing loss can use this information to understand the likelihood of having another child with hearing impairment.
Genetic testing is beneficial when there are additional health concerns, such as in syndromic hearing loss, as it can provide insight into how the condition might progress.
Benefits of Hearing Aids for Genetic Hearing Loss
Improved Communication: Hearing aids can make it easier to hear speech, enhancing communication in quiet and noisy environments.
Better Quality of Life: Individuals with hearing loss often experience social isolation or difficulty participating in conversations. Hearing aids can help alleviate these challenges, improving overall quality of life.
Early Intervention for Children: For children with genetic hearing loss, early use of hearing aids can support language development and academic success. Research has shown that children who receive treatment early on, including hearing aids, often have better language and communication skills outcomes.
Cochlear Implants for Severe Cases: For individuals with severe to profound genetic hearing loss, hearing aids may not be enough to provide sufficient sound amplification. In these cases, cochlear implants may be recommended. A cochlear implant is a surgically implanted device that bypasses damaged parts of the inner ear and directly stimulates the auditory nerve, allowing individuals to hear sounds they otherwise wouldn’t.
Cochlear implants have been particularly effective for individuals with specific genetic mutations, such as those affecting the GJB2 gene. Studies have shown that patients with this type of genetic hearing loss often have excellent outcomes after receiving cochlear implants, mainly when the procedure is performed early in life.
Future Directions: Precision Medicine and Gene Therapy
As our understanding of the genetics of hearing loss grows, the field is moving towards precision medicine—an approach that tailors treatment to the individual based on their genetic makeup. This could mean better hearing aid programming based on the specific genetic cause of hearing loss or selecting patients for cochlear implants based on their likely outcomes.
Gene therapy is also an exciting area of research. In the future, correcting genetic mutations that cause hearing loss may be possible, potentially restoring normal hearing function. Although this is still in the experimental stages, it offers hope for individuals with genetic hearing loss who may one day benefit from targeted treatments that address the root cause of their condition.
References
Adone A, Anjankar A. (2023). Alport Syndrome: A Comprehensive Review. Cureus. ;15(10):e47129. doi: 10.7759/cureus.47129.
Lahlou G, Calvet C, Giorgi M, Lecomte MJ, Safieddine S. (2023) Towards the Clinical Application of Gene Therapy for Genetic Inner Ear Diseases. J Clin Med;12(3):1046. doi: 10.3390/jcm12031046.
Liao EN, Taketa E, Mohamad NI, Chan DK. (2022) Outcomes of Gene Panel Testing for Sensorineural Hearing Loss in a Diverse Patient Cohort. JAMA Netw;5(9):e2233441. doi: 10.1001/jamanetworkopen.2022.33441.
Read, A.P. (1998). Waardenburg Syndrome. In: Jameson, J.L. (eds) Principles of Molecular Medicine. Humana Press, Totowa, NJ. https://doi.org/10.1007/978-1-59259-726-0_112
Simón Angeli, René Utrera, Samar Dib, Edgar Chiossone, Carlos Naranjo, Oswaldo Henríquez, Magdalena Porta. (2000). GJB2 Gene Mutations in Childhood Deafness. Acta Oto-Laryngologica, 120(2), 133–136. https://doi.org/10.1080/000164800750000766
Snoeckx, Rikkert L. et al. (2005) GJB2 Mutations and Degree of Hearing Loss: A Multicenter Study The American Journal of Human Genetics, Volume 77, Issue 6, 945 – 957
Toms M, Pagarkar W, Moosajee M. (2020) Usher syndrome: clinical features, molecular genetics and advancing therapeutics. Ther Adv Ophthalmol;12:2515841420952194. doi: 10.1177/2515841420952194.
Wémeau JL, Kopp P. (2017) Pendred syndrome. Best Pract Res Clin Endocrinol Metab. 31(2):213-224. doi: 10.1016/j.beem.2017.04.011.
