Asymmetric Sensorineural Hearing Loss: Definition and Clinical Significance
Asymmetric sensorineural hearing loss (ASNHL) is a hearing impairment affecting the inner ear, cochlear nerve, or central auditory pathways where one ear demonstrates significantly worse hearing than the other, most commonly defined as a difference of ≥15 dB at 2 or more frequencies or a ≥15% difference in speech recognition scores between ears. 1
Defining Asymmetry
The definition of ASNHL varies across clinical practice, but the most widely accepted criteria include: 1
- ≥15 dB difference at 2 or more frequencies between ears 1
- ≥15% difference in speech recognition scores 1
- ≥20 dB difference at 2 contiguous frequencies 1
- ≥15 dB difference at any 2 frequencies between 2000-8000 Hz 1
- ≥10 dB difference at 2000 Hz in patients with loud noise exposure history (e.g., military personnel) 1
The American Academy of Otolaryngology-Head and Neck Surgery Foundation position statement defines symmetric hearing loss as audiometric results within 15 dB for pure tone average between ears with word recognition score differences ≤15%, making anything beyond these thresholds asymmetric. 1
What "Sensorineural" Means
Sensorineural hearing loss refers to abnormal functioning of the cochlea (inner ear), auditory nerve, or higher central auditory centers—colloquially termed "nerve hearing loss." 1 This contrasts with conductive hearing loss, which involves pathology in the external ear canal or middle ear structures. 1
Clinical Importance and Red Flags
ASNHL is atypical and warrants immediate further evaluation because it can indicate serious retrocochlear pathology, particularly vestibular schwannoma or meningioma. 1 Age-related hearing loss should be symmetric—both ears exhibiting similar degrees of hearing loss—making asymmetry a critical warning sign. 1
The prevalence of acoustic neuroma in patients presenting with ASNHL is approximately 2.1% in community-based populations, making systematic evaluation essential despite the relatively low yield. 2
Mandatory Workup
When ASNHL is identified, the standard evaluation includes: 1
- MRI of the internal auditory canals to exclude vestibular schwannoma or meningioma 1, 3
- CT of the temporal bone if head trauma preceded the hearing loss, to evaluate for fractures or inner ear anatomic abnormalities 1, 3
- Serological testing only if specific conditions are suspected by history (Lyme disease, syphilis, HIV, autoimmune disease) 1, 3
Potential Causes
ASNHL can result from multiple etiologies: 1, 3
- Retrocochlear tumors: vestibular schwannoma (most common), meningioma 1, 3
- Sudden sensorineural hearing loss: an otologic emergency requiring immediate steroid therapy 1, 3
- Trauma: temporal bone fractures, inner ear anatomic abnormalities including superior canal dehiscence 1, 3
- Infections: Lyme disease, syphilis, HIV 1, 3
- Autoimmune inner ear disease 1, 3
- Asymmetric noise exposure: occupational or recreational 3
Despite extensive workup, the etiology remains unclear in the majority of ASNHL cases. 1
Critical Timing Considerations
If ASNHL develops within 72 hours, this constitutes sudden sensorineural hearing loss—an otologic emergency requiring immediate steroid therapy. 1 Patients should be educated and offered oral steroids as soon as possible, with intratympanic steroid therapy offered for treatment failures or patients presenting 2-6 weeks after onset. 1