Acoustic Tumors
An acoustic tumor (acoustic neuroma/neuroma or vestibular schwannoma) is a rare disorder resulting from the over proliferation of schwann cells into a mass. Once identified, these tumors are usually removed surgically, because they can compress the brainstem and threaten life. Early diagnosis and removal lessen the risk of complications during surgery and increase the opportunity to preserve hearing if that approach is pursued. Magnetic resonance imaging (MRI) is the definitive test for acoustic tumors. Unfortunately, it is expensive and only becomes cost effective when a screening test is used to assess which patients should receive an MRI. Pure-tone audiometry should be considered as part of that screening procedure. When the auditory nerve is compressed by the tumor, it often, but not always (Magdziarz et al., 2000), results in a unilateral or asymmetrical hearing loss. Because the fibers on the outside of the auditory nerve code high frequencies, the hearing loss is associated with the high frequencies (Schlauch et al., 1995). Studies have shown that a screening test that compares the average threshold difference between ears for 1, 2, 4, and 8 kHz is most effective (Schlauch et al., 1995). Threshold differences between ears for this PTA that exceed 15 dB or 20 dB maximize identification of persons with these tumors while minimizing false-positive diagnoses of persons with cochlear losses. The pass–fail criterion (e.g., requiring a 20-dB difference between ears) may differ depending on the money available for follow-up tests. A pass–fail criterion requiring 15-dB or greater differences between ears identifies more tumors than one requiring 20-dB or larger differences, but the smaller difference also yields more false-positive responses. False-positive responses (in this case, persons with cochlear losses identified incorrectly as having tumors) place a burden on the healthcare system, because follow-up tests such as MRI or auditory-evoked potentials are expensive.The effectiveness of a screening test based on the thresh- old asymmetries between ears is dependent on the clinical population. This test was found to be ineffective in a Veterans Administration hospital where many patients are males who have presbycusis and noise-induced hearing loss (NIHL) (Schlauch et al., 1995). By contrast, preliminary data from young women with normal hearing in their better ear suggest that true-positive rates and false-positive rates for this test are comparable to those for auditory brainstem response (Schlauch et al., 1995). It should also be noted that a small percentage of persons (<3%) with acoustic tumors have no hearing loss or hearing threshold asymmetry (Magdziarz et al., 2000).
Practical Audiology
Signs from the Case History
Site of Lesion
Severity of Loss
Tympanometry
Acoustic Reflexes (tests whether a reflex decay is maintained or weakens during continuous stimulation; 10 seconds @ 500 & 1000 conducted at 10dB above ART)
Acoustic Decay
OAEs
Pure Tone Audiometry
Speech Audiometry (SRT/WRS/QuickSIN)
Additional Tests/Recommendations
- Hallmark: Unilateral Hearing Loss & Tinnitus
- Imbalance/Unsteadiness, dizziness, vertigo
- Facial Numbness
- Headaches
Site of Lesion
- Retrocochlear/Nerve
- Vestibilar (VVIII) or Acoustic (AVIII)
- Possible additional damage to Inner Ear
Severity of Loss
- Progressive and Slow
- Sudden in only 10% of cases
- Typically takes time for tumor to grow
- High Frequency loss due to auditory nerve anatomy
Tympanometry
- Normal Type A
Acoustic Reflexes (tests whether a reflex decay is maintained or weakens during continuous stimulation; 10 seconds @ 500 & 1000 conducted at 10dB above ART)
- Reflexes elevated beyond 95th percentile
- Reflex decay positive for 1000Hz and below
- Look for retrocochlear pattern
Acoustic Decay
- Useful in detecting retrocochlear pathology
- Negative decay is normal, either fully for 10 seconds or less than 50% in 10 seconds
- Positive decay is at least 50% decay in 10 seconds
OAEs
- Depends on degree of loss
Pure Tone Audiometry
- Unilateral or Bilateral SNHL
Speech Audiometry (SRT/WRS/QuickSIN)
- SRT/PTA in agreement
- Word Recognition Scores poorer than anticipated
- difficulty in speech regions
- May show rollover indicated retrocochlear pathology
Additional Tests/Recommendations
- ABR
- not sensitive to small tumors
- abnormal with large tumors
- I-III prolognation, thus I-V prolongation
- Wave V > 2 standard deviations from norms indicating retrocochlear
- MRI for confirmation of tumor
- Retest when released from medical management
- Fit for amplification with medical clearance