Acoustic neuroma audiogram

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  1. Complete exam includes pure-tone audiometry, acoustic reflex testing with reflex decay, and speech reception audiometry
    1. Pure-tone audiometry – unilateral sensorineural hearing loss is associated with acoustic neuromas and can be detected with this test
      1. Test done by varying the sound intensity at octave intervals from 250 to 8000 Hz – patient indicates when he is able to hear the pulsed tone
      2. In acoustic neuromas, 60% have high frequency hearing loss, 20% complete deafness, 15% flat loss and 5% normal
      3. there is an absence of loudness recruitment as is seen in Meniere’s disease
      4. an aidable ear is one that has a pure tone average (PTA) of at least 70dB and a speech discrimination score (SDS) of 70% with normal dynamic range
      5. the most common cause of high frequency perception loss is sensorineural damage (most often in the elderly)
    2. Acoustic reflex testing – measures response of the stapedius muscle to the presenation of a loud sound
      1. Measures reflex arc from cochlear ganglion cells to ipsilateral and contralateral ventral cochlear nucleus to superior olivary complex then to CN 7; CN 7 then activates the stapedius muscle that stiffens the ossicular chain and tympanic membrane
      2. Acoustic tumors interfere with this reflex arc and cause delay or complete absence (absent in 75%)
    3. Speech discrimination – measures the ability to recognize familiar words; <90% is abnormal
      1. Poor speech discrimination out of proportion to the degree of pure tone loss is the audiologic hallmark of acoustic neuroma
        1. Probably due to the fact that 75% of auditory fibers may be damaged before change of pure-tone threshold is evident
      2. Incidence of abnormal speech discrimination with acoustic neuroma varies from 20-70%
    4. Brainstem auditory evoked potentials – changes with acoustic neuromas are usually ipsilateral even though most fibers cross to the contralateral side
      1. Wave 1 – auditory nerve or organ of Corti
      2. Wave 2 – cochlear nuclei (pons)
      3. Wave 3 – superior olive
      4. Wave 4 – lateral lemniscus
      5. Wave 5 – inferior colliculus
      6. Wave 6 – medial geniculate body
      7. Wave 7 – auditory radiations
      8. NOTE: brain localizes sound in three ways: 1) interval time difference and 2) phase shift measured by the medial superior olive; 3) interval intensity difference measured by the lateral superior olive; superior olive is the most proximal source of tertiary auditory fibers
      9. In acoustic neuroma, there is a stretching of the cochlear nerve causing a delay between Wave 1 and 5 (wave 5 is the largest and most reproducible of the waves); waves are compared between ears on the same patients – if there is a difference of more than 0.2 msec between ears, acoustic neuromas are present in 90% of cases
      10. BAEPs are abnormal in many patients with MS that have no clinical evidence of brainstem involvement
      11. High doses of barbiturates do not alter BAEPs but do cause cerebral vasoconstriction which may lead to cerebral hypoxia
      12. Comatose patients who have no wave 4 or wave 5 are less likely to survive a coma
      13. BAEPs may be slightly delayed in hypothermic patients and are also slightly delayed in men compared to women
      14. Remember: Hearing in each ear is represented bilaterally thus an infarction of the dominant temporal lobe will not result in unilateral deafness
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