Hearing loss, a common sensory deficit, affects millions globally. While hearing can decline in both ears at similar rates, it can also manifest unevenly, a condition known as asymmetrical hearing loss.
Understanding Asymmetrical Hearing Loss
Asymmetrical hearing loss refers to a noticeable difference in hearing ability between a person’s two ears, where one ear may have normal hearing while the other experiences mild to profound loss. A diagnosis of asymmetrical sensorineural hearing loss (ASNHL) is often made when there is a difference of greater than 10 decibels (dB) at two consecutive frequencies or greater than 15 dB at one frequency between the ears. This unevenness can make it challenging to locate sounds and understand speech, especially in noisy environments.
Acoustic Neuroma The Most Common Cause
An acoustic neuroma, also known as a vestibular schwannoma, is a primary cause of asymmetrical hearing loss, especially when it is progressive and sensorineural. This benign, non-cancerous growth develops on the eighth cranial nerve, which connects the inner ear to the brain and has branches responsible for both hearing and balance. Acoustic neuromas are relatively rare, accounting for roughly 5-10% of all intracranial tumors.
The hearing loss associated with an acoustic neuroma occurs in one ear and worsens gradually over months or years. The tumor causes hearing loss by pressing on the vestibulocochlear nerve, disrupting nerve signal transmission to the brain. In over 90% of individuals with acoustic neuromas, one-sided hearing loss is the initial symptom. This can make speech sound muffled, particularly high-pitched parts, as hearing is no longer “in stereo.”
Beyond hearing loss, an acoustic neuroma can also cause other symptoms because it affects the balance portion of the nerve. Patients may experience tinnitus, a ringing or buzzing sound in the affected ear. Dizziness or unsteadiness can also occur due to the tumor’s impact on balance. As the tumor grows larger, it might press on other nearby cranial nerves or brain structures, potentially leading to facial numbness, weakness, or coordination problems.
Other Potential Causes
While an acoustic neuroma is a primary cause, other factors can also lead to asymmetrical hearing loss. A simple unilateral earwax blockage or a foreign object in one ear can cause a temporary hearing difference. Unilateral ear infections, such as otitis media or externa, can affect hearing in one ear. Repeated infections over time may even result in permanent damage to structures within a single ear.
Sudden Sensorineural Hearing Loss (SSNHL) is a rapid, unexplained loss of hearing in one ear. While the exact cause is unknown, it may be linked to viral infections, autoimmune diseases, or circulatory issues affecting the inner ear. Meniere’s disease, an inner ear disorder, causes vertigo, tinnitus, and progressive hearing loss, typically starting unilaterally. Uneven exposure to loud noise, such as from firearms or certain occupational tools, can cause asymmetrical noise-induced hearing loss. Other less common causes include otosclerosis, a condition involving abnormal bone growth in the middle ear, or certain genetic factors.
Diagnosis and Investigation
When asymmetrical hearing loss is suspected, a thorough diagnostic process begins to identify the underlying cause. It begins with a comprehensive medical history and a physical examination of the ear. Understanding a patient’s symptoms, such as the onset and progression of hearing loss, presence of tinnitus, or balance issues, provides clues.
The next step involves audiometry, a series of hearing tests measuring the type and degree of hearing loss in each ear. These tests help determine if the hearing loss is conductive (problems with sound transmission), sensorineural (inner ear or nerve problems), or mixed. If audiometry reveals specific patterns of asymmetry, such as a difference of at least 15 dB at 1000 Hz, further investigation with imaging studies is recommended. Magnetic Resonance Imaging (MRI) of the brain and inner ear, particularly with contrast, is the primary imaging method to rule out or confirm conditions like an acoustic neuroma. This imaging shows tumors or other abnormalities affecting the auditory nerve or brain structures.
Management Approaches
Management for asymmetrical hearing loss is tailored to the identified cause. For an acoustic neuroma, the treatment strategy depends on factors like tumor size, growth rate, and the patient’s overall health. One approach is watchful waiting, especially for small tumors that are not growing or causing bothersome symptoms. This involves regular MRI scans and hearing tests to monitor the tumor’s behavior.
If the tumor is growing or causing bothersome symptoms, active treatment options include microsurgery to remove the tumor or radiation therapy to stop its growth. Stereotactic radiosurgery, such as Gamma Knife surgery, uses highly targeted radiation beams to inactivate tumor cells, often in a single session. Fractionated stereotactic radiotherapy delivers lower daily doses over weeks. These radiation techniques aim to preserve cranial nerve function, including hearing, while controlling tumor growth. For other causes of asymmetrical hearing loss, such as earwax blockage or infections, treatments like earwax removal or antibiotics are effective.