Causes of Asymmetric Sensorineural Hearing Loss
Asymmetric sensorineural hearing loss (ASNHL) most commonly results from retrocochlear pathology—particularly vestibular schwannoma or meningioma—which must be excluded with MRI imaging of the internal auditory canals in all cases. 1
Primary Retrocochlear Causes (Require Urgent Workup)
The most critical causes to identify are structural lesions affecting the auditory nerve or cerebellopontine angle:
- Vestibular schwannoma is the leading retrocochlear cause that must be ruled out with MRI in every patient presenting with ASNHL 1
- Meningioma represents another important intracranial tumor that can present with asymmetric hearing loss 1
Sudden-Onset Causes
When ASNHL develops rapidly (within 72 hours), consider:
- Idiopathic sudden sensorineural hearing loss (SSNHL), which is an otologic emergency requiring immediate steroid therapy 1
- Autoimmune inner ear disease, which can cause rapid unilateral or bilateral hearing loss 1
- Infections including Lyme disease, syphilis, and HIV, all of which can produce asymmetric patterns 1
- Herpes zoster oticus (Ramsay-Hunt syndrome) with associated otalgia and vesicles 1
Trauma-Related Causes
Head trauma preceding ASNHL warrants specific evaluation:
- Temporal bone fractures should be evaluated with CT imaging when head trauma precedes the hearing loss 1
- Inner ear anatomic abnormalities including superior canal dehiscence or other third window lesions 1
- Barotrauma from pressure changes 1
Other Acquired Causes
- Noise-induced hearing loss can be asymmetric, particularly with unilateral occupational or recreational noise exposure (military personnel may show asymmetry with ≥10 dB difference at 2000 Hz) 1
- Ototoxic medications including aminoglycosides, cisplatin, loop diuretics, and phosphodiesterase-5 inhibitors, though these more commonly cause bilateral loss 1, 2, 3
- Meniere disease may present with fluctuating unilateral hearing loss, particularly affecting low frequencies 1
Idiopathic Cases
Despite extensive workup including MRI and directed serologic testing, the majority of ASNHL cases remain unexplained. 1 This represents a critical clinical reality—even with comprehensive evaluation, most patients will not have an identifiable etiology.
Diagnostic Definitions
ASNHL is variably defined but commonly includes: 1
- ≥15 dB difference at 2 or more frequencies between ears
- ≥15% difference in speech recognition scores
- ≥20 dB at 2 contiguous frequencies
- ≥10 dB at 2000 Hz in noise-exposed populations
Critical Clinical Pitfall
The most dangerous error is failing to obtain MRI imaging to exclude vestibular schwannoma or other retrocochlear pathology. 1 All patients with documented ASNHL require imaging regardless of the degree of asymmetry. Serologic testing (syphilis, Lyme, HIV, thyroid function) should only be ordered when history or examination suggests specific infectious or autoimmune etiologies, as routine screening has low yield. 1, 4