Hearing loss often affects both ears at a similar rate, known as symmetrical hearing loss. Asymmetrical hearing loss describes a condition where a measurable difference exists between the hearing ability of the left and right ears. This disparity requires attention because the underlying reason can range from minor, treatable issues to more serious medical conditions. The condition presents a unique challenge to the auditory system, which relies on balanced input from both sides to function optimally.
Defining Asymmetry and Measurement
The clinical classification of asymmetrical hearing loss relies on precisely measuring the difference in hearing thresholds between the two ears. This measurement is determined through an audiogram, a graph that plots the softest sounds a person can hear across different frequencies, measured in decibels (dB). Audiologists classify the loss as asymmetrical when there is a notable difference in the pure-tone average (PTA) between the ears.
A commonly accepted criterion for defining asymmetry is a difference of 15 dB or more in the air-conduction PTA across the speech frequencies. Another metric is the speech recognition score, where a difference of 15% or greater in the ability to understand words between the two ears indicates significant asymmetry.
The audiogram also helps differentiate between sensorineural and conductive hearing loss in each ear, guiding the investigation into the cause of the asymmetry. Sensorineural loss involves damage to the inner ear or auditory nerve, while conductive loss relates to problems in the outer or middle ear that block sound transmission. Understanding the type of loss in each ear is a fundamental step in determining the condition’s origin.
Common Causes of Asymmetrical Hearing Loss
The origins of asymmetrical hearing loss are diverse, requiring a thorough medical evaluation. One serious, though rare, cause is an acoustic neuroma (vestibular schwannoma), a benign tumor growing on the nerve connecting the inner ear to the brain. This slow-growing tumor often presents with gradually worsening sensorineural hearing loss and unilateral tinnitus, making it a priority to rule out during diagnosis.
Sudden sensorineural hearing loss (SSNHL) typically affects only one ear, involving a rapid loss of hearing over 72 hours or less. Although often idiopathic (unknown), it is thought to relate to viral infections or circulatory issues within the inner ear. Prompt medical treatment is important for SSNHL, as early intervention can sometimes improve the chance of hearing recovery.
Inner Ear and Noise Exposure
Other causes include inner ear disorders like Ménière’s disease, which causes episodes of vertigo, fluctuating hearing loss, and tinnitus, often predominantly affecting one ear. Unilateral noise exposure, common in professions or hobbies involving firearms, can also damage inner ear hair cells, leading to permanent, asymmetrical sensorineural loss.
Conductive Causes
Asymmetry can also result from conductive hearing loss, which involves the outer or middle ear. These causes include temporary issues like earwax buildup or fluid behind the eardrum from an infection. Otosclerosis, where abnormal bone growth affects the middle ear bones, may also cause a conductive or mixed hearing loss worse in one ear.
Recognizing the Signs and Symptoms
Individuals with asymmetrical hearing loss often experience distinct subjective symptoms because their brain receives two very different sound signals. A primary complaint is difficulty with sound localization, which is the inability to accurately determine the direction or origin of a sound. The brain uses the slight time and intensity differences between the two ears to pinpoint a sound source, and this process is significantly impaired when one ear is much weaker.
Many people also find that their listening effort increases substantially, especially in environments with background noise, like crowded restaurants. This struggle occurs because the poorer ear cannot contribute effectively to filtering unwanted sounds. People may instinctively turn their “better ear” toward a speaker during conversations to maximize sound input. Furthermore, a persistent ringing or buzzing sound, known as tinnitus, is a common accompanying symptom, frequently localized to the ear with the greater hearing loss.
Diagnosis and Treatment Options
Diagnosis begins with a comprehensive audiometric evaluation to confirm the degree and type of loss in each ear. If the audiogram confirms significant sensorineural asymmetry, further testing is necessary to investigate the underlying cause. Imaging studies, most commonly a magnetic resonance imaging (MRI) scan, are often ordered to rule out a retrocochlear lesion, such as an acoustic neuroma. Specialized electrophysiological tests, like Auditory Brainstem Response (ABR) testing, may also assess the function of the auditory nerve and brainstem.
Once the cause is identified, treatment is tailored to the specific pathology, involving medical management for conditions like SSNHL or Ménière’s disease. For permanent loss, management focuses on improving hearing awareness and communication.
Specialized Amplification Systems
Specialized amplification systems manage the functional effects of asymmetrical loss. CROS (Contralateral Routing of Signals) or BiCROS hearing aids are designed for individuals with a significant, non-aidable loss in one ear. The CROS system uses a microphone on the poorer ear to wirelessly transmit sound to a receiver worn on the better ear. BiCROS systems are used when both ears have hearing loss but one is significantly worse, transmitting sound from the poorer ear while also amplifying the better ear. In cases of profound single-sided deafness, a cochlear implant may be considered.