Neurosurgery notes/Procedures/Cranial procedures/Neurophysiology/Cranial nerve monitoring/BAER

BAER

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Cranial nerve monitoring

General

  • AKA
    • Auditory brainstem response (ABR)
    • Auditory evoked potential (AEP)
  • The BAER/ABR measures the neural response of the auditory system to sound stimulation.

Indication

  • Hearing preservation vestibular schwannoma surgery
    • Once used for assistance in diagnosing vestibular schwannomas, their use for intraoperative monitoring is limited and has been largely replaced by direct eighth cranial nerve monitoring which provides more rapid information for the surgeon.
  • Microvascular decompression for hemifacial spasm

Tech

  • Auditory clicks are delivered to the patient by earphones.
  • The BAER audio stimulator is placed inside ear, reinforced with bone wax on top to hold in place.
  • The ground electrode is placed in close proximity, away from the surgical incision
  • About 1000 EEG responses are averaged, with electrodes normally placed at the mastoid and vertex
  • To locate auditory fibres within VS
    • A direct response to a 500 Hz tone can be recorded directly in theatre
Vestibulocochlear (Vlll) Al/A2 Lead O Xian Marie Boles 2022

Interpretation

  • Normal
    • Waveform: E Coli MRI,
    • See CN 8 anatomy
        • Early waves with III and V often the clearest.
        Wave Vill Cortical Vil VI V Brainstem IV Peripheral Anatomic Correlation Primary Auditory Cortex Thalamocortical Radiation Medial Geniculate Nucleus Inferior Colliculus Lateral Lemniscus Superior Olivary Complex Cochlear Nucleus Cochlear Nerve
  • Pathological
    • Potentially damaging manipulations or retraction of the auditory nerve increases the latency of waves III and V.
    • V wave
      • Increase more than 0.9 ms gives a measurable alteration of audiogram: 50% amplitude loss
      • 1 mSec latency shift of wave V normally is considered likely to show postoperative hearing deficit.
      • Reducing retraction or resting usually allows the response to recover
    • In head injury, longer latency auditory evoked responses are good indicators of outcome

Vestibulocochlear nerve

  • General
    • Auditory sensory function through measurement of auditory evoked potentials.
    • Auditory clicks applied and EEG recordings over average time obtained (A₁,A₂).
    • Changes in amplitude may reflect a spectrum of nerve injury or cochlear injury, not specific.
    • Less affected by anaesthetic agents.
  • Techniques
      • Wave 5 is the most important as it is most stable
      • Pathology
        • Increase latency >1ms → reduce amplitude > 50%
      Vestibulocochlear Nerve - Techniques
      Vestibulocochlear Nerve - Techniques Brainstem Thalamo-cortical Sensory association areas pathways Cochlear nerv Inferior colliculus (mid-brain) 5.0 Secondary auditory cortices 2.0 1.0 0.5 0.2 0.5 Vertex 1.0 plus 2.0 5.0 1 2 5 10 20 so 100 200 soo 1000 Latency (ms)