Stimulation

Foam ear inserts connected to a transducer are used in the OR to deliver broad-band clicks.

Care should be taken to ensure that the tubing is not kinked or obstructed in any way.  Prior to placing the inserts the ear canal should be inspected to ensure that there is not excessive ear wax, which could  interfere with monitoring.

Once the inserts are placed in the ear, the ear canal should be sealed with bone wax and a Tegaderm (or any waterproof adhesive tape) should be placed over the ear to prevent fluid from entering.

Clicks should be delivered monaurally (one ear at a time).   Clicks are condensation or rarefaction clicks.  The alternating polarity clicks improve the amplitude and the peaks, while reducing artifact.

  • When using ABR with click stimuli 40 dB of broad band masking below the stimulus should be used
  • Click intensity of 100dB pe SPL or 60-70 dB HL is common
  • Stimulus rate: 5 – 12/s show the best resolution of waves I, III, and V.  Faster stim rates will show wave I and V
    • If stim rate is changed new baselines must be set
  • Condensation clicks  – when the movement of the membrane moves towards the ear drum and condenses the air in the space.
  • Rarefaction clicks – when the movement of the membrane moves away from the ear drum and reduces air pressure.
    • Condensation clicks enhance wave V
    • Rarefaction clicks enhance wave I
      • Wave I can be easily differentiated based upon click polarity
  • Sound intensity at the tympanic membrane depends on the acoustic coupling between the sound stimulus generator and the ear
  • The ipsilateral ear to the surgery site can be stimulated and monitored continually.
  • Deceased synchronization of the auditory nerve firing will likely result in a decrease in wave amplitude
  • The contralateral ear to the surgery site should be stimulated at regular intervals.
  • The non-stimulated ear should be masked with white noise to avoid crossover responses.
    • White noise should be delivered at 60dB pe SPL or 30-35 dB HPL
  • Recruitment of the cochlea is an abnormal sensation of loudness

Recording

Bandpass

  • 100 – 150 to 2,500 – 3,000 Hz

Analysis Time

  • Lower temps in the OR can cause the BAEP waves to be prolonged beyond 10ms
  • Analysis time should be 15ms from stim onset

Number of Trails

  • 500-1000 trails depending on the amount of noise present and the amplitude of the waves

Electrode Type and Placement

  • Subdermal or disk EEG electrodes
    • EEG electrodes should be applied with collodion and sealed to protect from fluids.
  • Recording electrodes:
    • Cz
    • M1 and M2 or A1 and A2
    • Ground is typically at Fz

Montage

  • Channel 1: Cz – Mi/Ai
  • Channel 2: Cz – Mc/Ac
    • The i and c reference ipsilateral and contralateral

Near-Field Recordings

  • NAP – Nerve Action Potential can be recorded from the nerve after exposure
  • Electrode is placed directly on the proximal part of the auditory nerve by the  surgeon
  • Very few trails needed because the SNR is large

Parameters

  • Measurements of amplitude and latencies of waves I and V and inter peak latency should be made at baseline
  • Continuous monitoring of the amplitude and latency of wave V throughout the procedure
    • If wave V is poorly defined, the latency and amplitude of wave IV should be followed
  • Alert criteria:
    • 1 ms latency delay of wave V
    • 50% drop in amplitude of wave V

Changes in BAEPs can occur due to:

  • Technical
    • Problems with the recording or stim electrodes
    • Kink in the tubing
    • Equipment malfunction
    • Operator error
  • Physiological
    • Temperature and blood pressure decreases can increase the latency and decrease the amplitude
  • Surgical anesthesia
    • Propofol can increase the latencies of wave I, III, and V without changing the amplitudes
  • Injury to the auditory system
    • Compression
    • Traction
      • Retraction of the cerebellum can increase the latency of wave V during posterior fossa surgery
    • Thermal injury
      • Drilling can cause a temporary threshold shift during CPA surgery
    • Ischemia
      • Ischemia of the cochlea  = sudden loss of all BAEP waveforms
    • Injury to the distal auditory nerve
      • Increase latency and decrease amplitude in wave I
      • Increased latency of waves III and V
      • Wave I-III and I-V inter peak latency remains stable
    • Injury to the proximal area of the auditory nerve
      • Increase latency and decreased of wave III and V
      • Inter peak latency latencies I-III and I-V are increased
    • Injury to the brainstem
      • Increase latency and decreased amplitude of waves III and V

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