Posted: November 7th, 2023
Sensorineural Hearing Loss
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November 7, 2023
Sensorineural Hearing Loss
The human inner ear is a fragile and complex organ to study, underpinning why medical research on Sensorineural Hearing Loss (SNHL) has been slow and multifactorial. The neurological condition has the attributes of impaired speech discrimination, sound identification, speech recognition, and sound localization. Contemporary medicine still categorizes SNHL as an incurable neurological condition since not all the potential mechanisms contributing to the disease have been identified and expounded on. Most studies consider the condition to be caused by an interplay of environmental and genetic factors. The challenge of SNHL to neurological science is the inability to affirm early diagnosis and difficulties in managing the symptoms during treatment. Unfortunately, no treatments have been developed to address SNHL’s neurodegenerative process. Given that SNHL is the most common cause of hearing loss in adults, there is a growing and pressing need for further research. Even though SNHL remains largely popular, primary care practitioners often under-recognized the neurological condition, which implies practice gaps in diagnosis and management.
SNHL occurs following damage to structures of the human inner ear. Ren et al. (2022) specify that SNHL is caused by damage to the auditory nerve within the inner ear. The spiraling organ contains the cochlea, which has tiny hairs (stereocilia) that are responsible for converting sound vibrations into neural waves that the auditory nerve can transmit to the brain (Ren et al., 2022). A variety of factors can cause damage to these hairs. Depending on the degree of injury, the patient might experience mild, moderate, or severe hearing loss (Tanna et al., 2023). While SNHL is not a life-threatening condition, it does impede a person’s quality of life. The condition will result in communication difficulties due to trouble recognizing sounds. The damage to the inner ear underpinning SNHL is also associated with dizziness and balance problems (Tanna et al., 2023). Therefore, patients with onset SNHL should undergo a comprehensive audiometric evaluation to determine the extent of damage and to introduce speech therapy.
Several studies identify the existence of predisposing genetic factors for SNHL, making the disease hereditary. According to Ren et al. (2022), more than one hundred gene mutations have been discovered to account for the inception of SNHL. The genome mutation explains why the neurological condition is relatively common in newborns. One in a thousand births will result in SNHL (Ren et al., 2022). However, research is yet to outline the specific role of gene mutation in adult SNHL. The gap has made it difficult to capture whether disabling the genes causing SNHL could be a feasible medical approach for preventing the neurological condition.
Noise exposure is the most common cause of SNHL in adults. Noise-induced hearing loss occurs in many recreational places, such as concerts and clubs, and in occupations, such as mining (Khosravipour & Rajati, 2021). Individuals exposed to loud noises may experience early onset SNHL, presenting symptoms such as the loss of hair cells. Lateral wall histopathology will occur with continuous exposure to loud noises (Ren et al., 2022). While hair cell loss is a more immediate consequence of noise exposure, the degradation of primary auditory neurons is progressive. The difference in symptomology suggests that noise-induced SNHL might not be detected for years.
Aging is one of the primary factors behind the degenerative processes of the human auditory system. Age contains the cumulative biological effect of noise, physical trauma, ototoxic drugs, hormones, and metabolic changes (Khosravipour & Rajati, 2021). Age-related SNHL cannot be explained because there is equally a decline in perception and cognition. Another cause of SNHL is the use of ototoxic drugs. Medical research finds that the use of platinum antitumor medications, such as glutamic acid and aminoglycoside antibiotics, cause SNHL (Ren et al., 2022). Exposure to heavy metals is also associated with ototoxic-induced SNHL (Ruhl et al., 2019). Certain compounds in these drugs are known to damage the spiral ganglion neurons, with the extent of the damage being dependent on the dosages.
SNHL is diagnosed and evaluated using a combination of examination methods. The first step for physicians is to differentiate SNHL from other auditory complications. According to Jung and Hoegeri (2022), an otoscopic exam is applied since SNHL patients tend to have normal auditory canals and tympanic membranes. The Tuning Fork test is applied to differentiate SNHL from conductive hearing loss (CHL). The Rinne and Weber test is the choice of Tuning Fork examination applied, and the process entails a bedside investigation done using a 512 Hz tuning fork (Tanna et al., 2023). The Weber test is applied to identify the ear that can recognize the sound the loudest. In SNHL, the loud voice is heard in the good ear, but in CHL, the loud voice lateralizes to the deaf ear (Tanna et al., 2023). The Rinne test focuses on assessing sound bone and air conduction in the ear.
Auditory Brainstem Response (ABR) testing is another alternative diagnostic method. The test equally assesses bone and air conduction pathways. The threshold for the two pathways is plotted on a graph at different sound frequencies (Tanna et al., 2023). The physician will increase the sound frequencies to the degree where the bone and air conduction worsens. The worst scenario is when there is a gap between the air and bone (Tanna et al., 2023). In SNHL, both bone and air conduction pathways should exhibit a downward-sloping graph. Other minor tests for SNHL include tympanometry, which assesses the mobility of the tympanic membrane, and electrophysiological tests, which determine whether tumors are compressing nerves in the inner ear (Tanna et al., 2023). It is often the case that multiple diagnostic approaches will be used to confirm SNHL and its degree of inner ear damage.
Course of Progression
SNHL is progressive and might take several years for the true symptoms to manifest. There are multiple pathways to the degeneration of the sensory epithelium cells of the cochlea. The organ contains sensory cells, cochlea-stimulating outer hair cells (OHCs), and inner ear hair cells (IHCs) that are tonotopically positioned (Tanna et al., 2023). SNHL slowly alters the alignment of OHCs and IHCs. The stereocilia will slowly fail to grow in one direction, resulting in deflection (Tanna et al., 2023). As a result, the hair cells will not activate their receptors, meaning no sound signals are sent to the auditory nerve. However, there are cases where the deflection of hair cells is instantaneous and not degenerative. Trauma, such as loud music, can alter the arrangement of the hair cells, deactivating the receptors. Such immediate progression of SNHL results in what is termed as Sudden SHNL (Tripathi & Deshmukh, 2022). The different causes of SNHL and their divergent pathways increase the difficulty in determining the standard course of progression for the neurological disorder.
Neurological Functions Affected by the Disorder
SNHL leads to the loss of auditory neuropathy, which is the disruption of the nerve impulses that move from the inner ear to the brain. The loss of this function in one ear ultimately results in the loss of the same function in the other ear. As a result, SHNL is considered a form of neural deafness. When the neurological complication results in complete deafness, the cortical brain areas associated with the processing of sound, such as Heschl’s gyrus, become taken over by the other four senses (Alfandari et al., 2018). The change underlies the adaptive strategy to be used by SNHL patients. The brain informs that they will need to use their other four senses to offset hearing loss.
The treatment and management of SNHL are dependent on the degree of hearing loss and the timing of onset. In cases where the root cause cannot be identified, a brain MRI is done to uncover the status of the internal auditory meatus. Such patients are prescribed oral corticosteroids with a dosage of 1mg/kg/day for seven days (Tanna et al., 2023). An audiogram is done after the week to assess if there are any improvements to the inner ear. The use of hearing aids remains a common practice in SHNL (Tanna et al., 2023). The medical device is normally used for moderate hearing loss.
Bone conduction hearing systems are used in cases where there is a big gap between bone and air conduction (Tanna et al., 2023). The treatment device entails titanium fixtures placed on the bone through osseointegration (Tanna et al., 2023). The treatment approach bypasses air conduction through the use of the attached sound processor. Cochlear implants are other common devices used in the management of SHNL. The device is surgically fixed to provide the patient with speech perception (Tanna et al., 2023). Cochlear implants are used to bypass the functions of the auditory hair cells.
Challenges in Treatment Development
SNHL research and drug development requires high human expertise and intense capital investments. According to Crowson et al. (2017), efforts to create drug-based treatments for the disorder have led to over 25 active randomized clinical drug trials. It is still impossible to conclude whether medical science will make available drugs that address SNHL. Physicians are also uninformed and untrained on the mechanisms underpinning the drug therapies, making it more difficult for the drugs to reach the market (Crowson et al., 2017). Further research into the different SNHL pathways, causes, and processes is required to inform the development of treatments.
Key Logs of the Thinking Process
Research, especially for sciences, demands the substantial use of critical thinking. The first step was to investigate the research topic and identify a problem. The identification of these two variables plays an essential role in the formulation of the thesis statement. Once I identified the research topic I wanted to work on from the class materials, I attempted to create an outline. The activity provides a brief glimpse into the type of resources I should look for online. The outline also helps reinforce the quality of the thesis statement. I initially wanted to create an annotated bibliography to summarize and compare the resources that I would gather online. However, the approach would have been cumbersome due to the additional writing. Therefore, I decided to focus on the technicalities that convey the quality of a paper, such as the number of peer reviews, date of publication, and institutions behind the publication, as part of the selection criteria.
I used library catalog search engines to explore and gather journal articles with scientific evidence regarding the research topic. PubMed, ESCU and PMC Medical were some search engines used. I clicked on the advanced search options in all databases used to allow a keyword search and the use of limiters. Some of the databases do not limit the search by discipline. Sensorineural hearing loss, cochlea, inner ear loss, and auditory nerves were some of the keywords used in the search process. I realized combining two keywords outputs a more comprehensive and relevant list of applicable scholarly articles. I set limiters for content type (article and book), discipline, and date. Out of the list, I selected books and articles published within the last seven years. Selected resources had to have active links for the authors for the reader to identify their qualifications and the institutions behind the research.
Ren, H., Hu, B., & Jiang, G. (2022). Advancements in prevention and intervention of sensorineural hearing loss. Therapeutic Advances in Chronic Disease, 13. https://doi.org/10.1177/20406223221104987
Khosravipour, M., & Rajati, F. (2021). Sensorineural hearing loss and risk of stroke: a systematic review and meta-analysis. Scientific Reports, 11(1), 11021. https://doi.org/10.1038/s41598-021-89695-2
Ruhl, D., Du, T. T., Wagner, E. L., Choi, J. H., Li, S., Reed, R., Kim, K., Freeman, M., Hashisaki, G., Lukens, J. R., & Shin, J. B. (2019). Necroptosis and Apoptosis contribute to cisplatin and aminoglycoside ototoxicity. The Journal of Neuroscience: The Official Journal of the Society for Neuroscience, 39(15), 2951–2964. https://doi.org/10.1523/JNEUROSCI.1384-18.2019
Jung, W. W., & Hoegerl, C. (2022). Sudden sensorineural hearing loss and why it’s an emergency. Cureus, 14(1), e21418. https://doi.org/10.7759/cureus.21418
Tripathi, P., & Deshmukh, P. (2022). Sudden Sensorineural Hearing Loss: A review. Cureus, 14(9), e29458. https://doi.org/10.7759/cureus.29458
Alfandari, D., Vriend, C., Heslenfeld, D. J., Versfeld, N. J., Kramer, S. E., & Zekveld, A. A. (2018). Brain volume differences associated with hearing impairment in adults. Trends in Hearing, 22. https://doi.org/10.1177/2331216518763689
Crowson, M. G., Hertzano, R., & Tucci, D. L. (2017). Emerging therapies for sensorineural hearing loss. Otology & Neurotology, 38(6), 792–803. https://doi.org/10.1097/MAO.0000000000001427
Tanna, R., Lin, J. W. & Jesus, O. (2023, February 12). Sensorineural hearing loss. StatPearls Online, https://www.ncbi.nlm.nih.gov/books/NBK565860/
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