Hearing loss is a diminished ability to perceive sounds. Sensorineural hearing loss involves damage to the inner ear or auditory nerve. When this type of hearing loss affects each ear differently, it is known as asymmetrical sensorineural hearing loss. This article explains its characteristics, potential causes, and identification.
Understanding Sensorineural Hearing Loss
Sensorineural hearing loss (SNHL) originates from issues within the inner ear or the auditory nerve pathways leading to the brain. The inner ear’s cochlea contains tiny hair cells that convert sound vibrations into electrical signals, which are then sent to the brain via the auditory nerve. Damage to these hair cells or the nerve disrupts this process, making it difficult to hear soft sounds and often causing louder sounds to appear muffled or unclear.
SNHL differs from conductive hearing loss, which involves problems in the outer or middle ear preventing sound from reaching the inner ear efficiently. Conductive issues include earwax blockages, fluid buildup, or damage to the eardrum or middle ear bones. Unlike many conductive hearing losses, SNHL is permanent because damaged inner ear structures or nerve fibers cannot regenerate.
The Asymmetrical Distinction
Asymmetrical sensorineural hearing loss means one ear experiences worse hearing than the other, or the type of hearing loss varies between them. A common clinical definition involves an interaural asymmetry of at least 10 dB at two consecutive frequencies or 15 dB at a single frequency.
This imbalance affects sound localization, as the brain relies on input from both ears to determine sound direction and distance. When one ear hears much better, this spatial processing becomes challenging. Speech understanding in noisy environments also becomes more difficult, as the brain struggles to filter out background noise without balanced input. This asymmetry can also present challenges when fitting hearing aids, requiring careful adjustments to balance sound signals to each ear.
Causes of Asymmetrical Sensorineural Hearing Loss
Several factors can lead to sensorineural hearing loss affecting one ear more than the other. Sudden sensorineural hearing loss (SSNHL) is characterized by a rapid onset of hearing loss, often within 72 hours, and typically affects only one ear. While the exact cause is frequently unknown, it can be linked to viral infections, blood flow issues, or autoimmune responses affecting the inner ear. Prompt medical attention for SSNHL is important, as early intervention can improve outcomes.
Meniere’s disease is an inner ear disorder presenting with fluctuating hearing loss, often affecting one ear, alongside episodes of vertigo, tinnitus, and aural fullness. An acoustic neuroma, also known as a vestibular schwannoma, is a non-cancerous tumor growing on the nerve connecting the inner ear to the brain. This slow-growing tumor causes gradual, unilateral hearing loss, sometimes accompanied by tinnitus and balance problems, as it presses on auditory and balance nerves.
Exposure to loud noise can also result in asymmetrical hearing loss if one ear is subjected to higher sound levels or is more susceptible to damage. For instance, certain occupations or hobbies might expose one side of the head to more intense noise, leading to differential damage to cochlear hair cells. Unilateral ear infections, especially chronic or severe cases, can sometimes extend beyond the middle ear to affect the inner ear, causing SNHL. Similarly, a traumatic head injury to one side can directly damage inner ear structures or the auditory nerve, resulting in SNHL primarily in the affected ear.
Diagnosis and Next Steps
Identifying asymmetrical sensorineural hearing loss begins with a comprehensive hearing evaluation performed by an audiologist. The audiogram, a key diagnostic tool, measures hearing thresholds across different frequencies for each ear separately. This test illustrates any significant differences in hearing sensitivity between the two ears, confirming the asymmetry.
Following the audiogram, further medical evaluation is often necessary to determine the underlying cause, especially when the hearing loss is unilateral or progressing rapidly. This may involve imaging studies, such as magnetic resonance imaging (MRI), to rule out conditions like an acoustic neuroma or other retrocochlear pathologies affecting the auditory nerve or brain. Blood tests might also be conducted to investigate potential systemic conditions contributing to the hearing loss. The initial approach after diagnosis focuses on identifying the specific cause, which then guides discussions about monitoring the condition or referring to other medical specialists for targeted management.