Asymmetrical sensorineural hearing loss (SNHL) is a type of hearing impairment where hearing ability differs significantly between the two ears. While hearing loss is common, a noticeable difference between the ears raises distinct medical concerns. This condition requires professional evaluation because it can be the first sign of an underlying health issue. The presence of asymmetry mandates a thorough diagnostic approach to determine the precise cause and appropriate management options.
Understanding the Components of Asymmetrical Sensorineural Hearing Loss
The term “sensorineural” refers to damage within the inner ear, specifically the cochlea or the auditory nerve. The cochlea contains delicate hair cells that translate sound vibrations into electrical signals. Damage to these hair cells or the auditory nerve results in sensorineural hearing loss, causing a permanent reduction in hearing clarity and volume.
The “asymmetrical” component defines the significant difference in hearing ability between the left and right ears. Clinically, this condition is diagnosed when the hearing threshold difference between the ears is 15 decibels (dB) or greater at any one frequency. Alternatively, a difference of at least 10 dB across two consecutive frequencies is used for diagnosis.
Asymmetry is measured by pure-tone thresholds and the ability to understand speech. A difference in the maximum speech discrimination score of greater than 15% between the two ears is also considered significant. Establishing this difference guides the need for further medical investigation. The distinction can range from one ear having normal hearing and the other having a mild loss, to one ear having profound deafness.
Common Underlying Causes of Asymmetry
The presence of asymmetrical SNHL signals the need to rule out specific, unilateral conditions. A primary cause to investigate is a Vestibular Schwannoma, a benign tumor on the vestibular portion of the eighth cranial nerve (acoustic neuroma). This slow-growing tumor causes hearing loss by compressing the cochlear nerve or releasing toxic molecules into the inner ear. Though rare, Vestibular Schwannoma may present with ASNHL as its only symptom.
Sudden Idiopathic Sensorineural Hearing Loss (SSNHL), which develops within a 72-hour period, is another common cause of asymmetrical presentation. Although the exact cause is often unknown, SSNHL is thought to result from viral infections, vascular issues, or an autoimmune response affecting the inner ear. Since SSNHL is almost always unilateral, it creates a significant difference in hearing between the two sides.
Unilateral Meniere’s Disease, characterized by vertigo, tinnitus, and aural fullness, frequently presents with asymmetrical hearing loss. This inner ear disorder causes fluctuating hearing loss, particularly in the lower frequencies, due to excessive fluid pressure buildup. Furthermore, specific infections like viral labyrinthitis, syphilis, or Lyme disease can affect only one ear, resulting in an asymmetrical pattern of SNHL.
Asymmetry can also result from environmental factors, particularly noise exposure or trauma that disproportionately affects one ear. Individuals frequently exposed to loud, unilateral noise, such as hunters or machinery operators, may sustain greater damage to the hair cells in the ear closest to the sound source. This localized trauma leads to the asymmetrical pattern observed on the audiogram.
Essential Steps in Diagnosis
The initial diagnostic step is a comprehensive audiogram performed by a licensed audiologist. This test establishes the degree and type of hearing loss in each ear, using pure-tone testing to measure the softest sounds a person can hear across different frequencies. The audiogram compares bone conduction thresholds between the two ears to confirm the decibel difference meeting the criteria for asymmetrical SNHL.
The audiogram also includes speech recognition testing, where the patient repeats words presented at different volumes. A significant finding is a disproportionately poor speech understanding score in the poorer ear compared to the pure-tone loss. This disparity indicates a problem with the auditory nerve itself, known as a retrocochlear pathology.
If asymmetrical SNHL is confirmed and the cause is not obvious, the professional will order magnetic resonance imaging (MRI) of the inner ear and brain. The MRI’s purpose is to exclude a Vestibular Schwannoma or other intracranial tumors pressing on the auditory nerve. Since MRI is the standard for identifying these structural lesions, it ensures that a potentially serious condition is not missed.
Managing Asymmetrical Hearing Loss
The management strategy depends on the underlying cause and the degree of hearing difference. If the hearing loss in the better ear is mild, traditional hearing aids may be used to amplify sound in both ears. If the asymmetry is severe (one ear having profound loss), specialized devices are necessary.
One common solution is the Contralateral Routing of Signal (CROS) system, designed for single-sided deafness. A microphone on the non-hearing ear captures sounds and wirelessly transmits the signal to a receiver worn on the better-hearing ear. This system allows the patient to hear sounds coming from their poorer side, eliminating the head-shadow effect.
For patients with some hearing loss in their better ear, a BiCROS system is used (“Bi” stands for bilateral). This device routes sound from the poorer ear while also providing amplification to the better ear. In cases of profound hearing loss not helped by traditional amplification, a cochlear implant may be considered to directly stimulate the auditory nerve. Long-term monitoring is a component of care, tracking the stability of hearing and checking for progression of the original condition.