According to experts, approximately 466 million people worldwide suffer from "disabling" hearing loss, representing nearly 6.1% of the global population. This figure is likely underestimated, as many individuals with hearing impairment delay seeking medical attention, often attributing their symptoms to allergies or illness, or simply adapting to them over time. This is why understanding the different types of hearing loss is so important.
First, what is hearing loss? Clinically, it is defined as the inability to perceive sounds at an intensity of 20 decibels (dB HL) or higher in one or both ears. It can range from mild to profound and affect one or both ears.
This article focuses on a particular type: asymmetric sensorineural hearing loss (ASHL).
WHAT IS ASYMMETRIC HEARING LOSS?
Hearing loss that progresses comparably in both ears is called symmetric. Conversely, asymmetric hearing loss occurs when the hearing in one ear is significantly worse than in the other. To meet the criteria for asymmetric hearing loss, the difference in hearing thresholds between the two ears is generally ≥ 15 dB at at least two frequencies. Both ears are most often affected (bilateral impairment), but unequally.
Asymmetric hearing loss can be conductive, sensorineural, or mixed. Its causes include:
Age-related hearing loss (presbycusis)
Noise-induced hearing loss
Genetics
Certain diseases affecting the auditory system
Asymmetric exposure to loud noises
WHAT IS SENSORINEURAL HEARING LOSS?
Sensorineural hearing loss (SNHL) occurs when the inner ear (specifically the hair cells of the cochlea) or the auditory nerve pathways leading to the brain are damaged. SNHL is generally permanent and most often bilateral, although it can also be unilateral or asymmetric.
Its symptoms include:
Difficulty hearing in noisy environments
Difficulty understanding speech, especially high-pitched consonants
Voices sounding muffled
The feeling of hearing someone speak without being able to distinguish their words
Tinnitus or balance disorders in some cases
SNHL accounts for approximately 90% of all cases of hearing loss.
ASYMMETRIC SENSORINEURAL HEARING LOSS (ASHL)
ASHL specifically refers to sensorineural hearing loss that is significantly more pronounced in one ear than in the other. While it shares causes with classic SNHL, such as aging and noise exposure, ASHL can also signal a more serious underlying problem and warrants a thorough evaluation.
POSSIBLE CAUSES OF ASHL
Noise-induced or age-related hearing loss
Vestibular schwannoma (acoustic neuroma): a benign tumor that develops on the vestibular portion of the eighth cranial nerve, often originating in the internal auditory canal. Although rare (approximately 2% of ASHL cases), it accounts for over 85% of cerebellopontine angle tumors, which is why an MRI is frequently recommended.
Autoimmune inner ear disease
Ménière's disease: an inner ear condition that can cause fluctuating hearing loss, vertigo, tinnitus, and a feeling of fullness in the ear. Its management may involve dietary changes, medications, or, in severe cases, surgery.
Idiopathic sudden sensorineural hearing loss: a sudden and unexplained hearing loss that may or may not regress with rapid treatment, usually corticosteroids.
Often, the exact cause remains unknown. This is why a thorough assessment by a hearing professional is essential.
TREATMENT OF ASHL
The first step in managing ASHL is a complete audiological evaluation, followed by imaging (most often an MRI) to rule out retrocochlear involvement, such as an acoustic tumor. The diagnosis may also reveal a familial characteristic or an autoimmune component.
Treatment depends on the cause and severity:
Hearing aids are the most common intervention for ASHL. The goal is to restore binaural hearing as much as possible, to help the user localize sounds and better understand speech.
Cochlear implants may be considered in cases of total unilateral deafness (SSD) or when one ear has profound loss and the other is only partially functional.
CROS/BiCROS systems reroute sound from the poorer ear to the better ear when amplification alone is not sufficient.
THE DIFFERENT CASES OF ASHL
Hearing loss in both ears, but one is more affected → Both ears can benefit from hearing aids, with different amplification levels.
One deaf ear, the other hearing impaired → Cochlear implants may be an option. Otherwise, a hearing aid can still provide some benefit, with variable results.
One normal ear, the other hearing impaired → This case is particularly delicate. The fitted ear must harmonize with the normal hearing on the other side, which may seem artificial at first. Many users adapt over time and benefit from improved clarity and directional cues.
CONCLUSION
Asymmetric sensorineural hearing loss is a complex and often underdiagnosed condition. It results in uneven hearing loss between the two ears, due to damage to the inner ear or auditory nerve, and may reveal an underlying medical problem, such as a tumor. While it cannot always be cured, solutions such as hearing aids, CROS systems, or cochlear implants provide considerable help.
If you suspect you have signs of ASHL, do not wait. Early diagnosis is key to better outcomes. Make an appointment with your audiologist or ENT today.
