Intraoperative Neurophysiological Monitoring Flashcards
What is intraoperative monitoring?
- Continuous, on-line electrophysiologic assessment, measurement, and interpretation of neural responses to intraoperative events
- Modality-specific, controlled stimulation during the course of surgery
- Assesses the integrity of a sensory (CN VIII) or motor (CN VII) modality during a surgical procedure
What is the audiologist’s job during IOM?
- Administer and interpret electrophysiologic measurements of neural function including:
- Sensory and motor evoked potentials
- Tests of nerve conduction velocity
- Electromyography
-These measurements are useful in differential diagnosis, pre- and post-operative evaluation of neural function, and neurophysiologic IOM of CNS, spinal cord, and cranial nerve function
What are indications for IOM?
- When there is a risk of surgical complications involving the CNS or PNS
- When surgical or anesthesia intervention (indicated by monitoring) could help prevent, reverse, reduce consequences
- PURPOSE: to improve post-operative by correlating changes in neurophysiological responses with intraoperative responses
Describe the pre-operative assessment.
1) Pre-op audiologic workup
- Audiogram
- Immittance
- OAEs
- Speech discrimination
- Auditory electrophysioloy
2) Pre-incision ABR/ECochG
- Check equipment
- Identify any noise
- Patient baseline
What personnel should be in the operating room?
- Surgeon
- Anesthesiologist
- Nurses
- Audiologist
Describe the electrical and magnetic environment of the operating room.
- Operating microscope
- Anesthesia machine
- Pulse oximeter machine
- Electroacutery machine
- Automated blood pressure machine
- Heart rate monitor
- Respirator rate monitor
What are some hearing preservation strategies?
- Cerebellopontine angle tumor removal
- Vestibular nerve resection
- Microvascular decompression of CN V or CN VII
- Cochlear implantation (electric ABR and facial nerve)
What types of damage could occur in the operating room?
- Mechanical: compression, tearing, cutting, stretching
- Ischemic: interrupted blood supply to cochlea or brainstem
- Thermal: electrocautery
What are the three surgical approaches?
1) Retrosigmoid (suboccipital)
2) Translabyrinthine
3) Middle Fossa
Describe the retrosigmoid surgical approach.
- Craniotomy is made behind the ear in the occipital bone
- Bone overlying the IAC is removed to expose and remove the tumor
- Best candidates: patients with tumors that do not involve the lateral 1/3 of the IAC and do not impinge on the brainstem
What are advantages of the retrosigmoid surgical approach?
- Allows removal of tumors of different sizes
- Offers the possibility of hearing preservation
- Approach offers the surgeon a wide view of the cisternal component of the tumor and thus good access to the root entry zone of the acoustic nerve
What are disadvantages of the retrosigmoid surgical approach?
- Necessity for cerebellar retraction
- Less access to the facial and cochlear nerves in the distal IAC
- Increases the potential to leave a residual tumor fragment behind
Describe the translabyrinthine surgical approach.
- Craniotomy is made through the ear in the mastoid bone
- SCCs are removed to expose the tumor in the IAC
- Because the canals are removed, complete hearing loss occurs in the affected ear
- Use: patients who already have hearing loss or when preservation of hearing is not possible
What are advantages of the translabyrinthine surgical approach?
- Size of the tumor not a limiting factor
- It offers early identification of the facial nerve in the auditory canal
- There is absolutely no need for cerebellar retraction
What are disadvantages of the translabyrinthine surgical approach?
-No hearing preservation
Describe the middle fossa surgical approach.
- Craniotomy is made above the ear in the temporal bone
- Bone overlying the IAC is removed to expose and remove the tumor
What are advantages of the middle fossa surgical approach?
- Exposes the IAC and its contents from a superior trajectory
- Chosen for small tumors located primarily within the IAC
- Offers the possibility of hearing preservation
- Excellent approach for small tumors that predominantly occupy the IAC with a minor component in the cistern (usually < 10 mm)
- Provides exceptional access to the lateral end of the canal
Compare far- and near-field recordings.
1) Far field: ABR, ECochG
- ABR tracks amplitude and latency of waves I-V
- ECochG tracks N1
2) Near field: Auditory Nerve-Compound Action Potential (AN-CAP)
- AN-CAP tracks P1 and N1
Describe P1 and N1 in the AN-CAP.
- P1: depolarizing front of AP as it approaches the electrode
- N1: AP as it passes over the electrode
Describe CN VIII Monitoring: Stimulus.
- Goal is to obtain the largest, most reliable response
- Transducers: inserts (seal with ear with bone wax or bioclusive patches)
- Stimulus: clicks
- Stimulus intensity: 70-95 dB nHL
- Stimulus rate: as high as possible (limited by sampling rate); odd (minimize 60 Hz noise)
Why are benefits of using inserts for CN VIII IOM?
- Better placement to reduce stimulus artifact
- Reduces contamination by external acoustic noise
- Minimizes risk for noise-induced hearing loss
Why are clicks used for CN VIII IOM?
-Broad spectrum stimuli to assess the integrity of the system
Describe CN VIII Monitoring: Recording.
- Goal is to optimize speed and SNR
- Electrodes: subdermal (ABR), peri-tympanic or promontory/transtympanic (ECochG)
- Filter: 100-3000 Hz
- Amplifier: differential (common mode rejection)
- Gain: 75,000-100,000x
- Artifact reject: on
- Signal averaging: variable (usually < 10 ms)
What should be monitored during CN VIII IOM?
1) ABR/ECochG
- Absolute latencies (I, III, V)
- Interwave latencies (need wave I as a reference; indicator of neural conduction time)
2) AN-CAP
- Amplitude (# active ANFs)
- Loss of N1 suggests asynchronous firing and/or conduction block
What are potential threats to hearing during CN VIII IOM?
- Fluid in the EAC/ME (conductive component)
- Mechanical displacement of the nerve
- Misplaced electrodes (especially for ECochG)
What should the audiologist communicate to the surgeon regarding CN VIII IOM?
-Any significant changes in response, such as:
- Prolongation in latency
- Obliteation of waveforms
- Severe reductions in amplitude
- Appearance of abnormal activity
What factors influence the auditory evoked potential during IOM?
1) Pharmacological agents
- EX: Isoflurance, Lidocaine
- Wave V amplitude reduction
- Absolute latency delay
- Prolongation of wave I-V interpeak latency
2) Temperature
- Decrease in body temp below 35 degrees C (95 degrees F) results in a prolongation of absolute and interwave latencies and a decrease in amplitude of all waves
What knowledge should audiologists have in the OR?
- Basic neurophysiology
- General human neuro/anatomy and physiology
- Clinical neurophysiology (EPs, EEG, EMG, etc.)
- Recording neurophysiological signals in the OR
- Anesthesia effects
- Surgical neuro/anatomy
- Surgical procedures that utilize IOM
How is CN VII IOM performed?
- Continuous monitoring of ongoing EMG of CN VII
- Use of electrical or mechanical stimulation to obtain a compound facial muscle action potential
What stimulus is used during CN VII IOM?
- Direct electrical stimulation of CN VII (0.1-0.2 mA at 4 pulses/s)
- OR mechanical stimulation
Describe CN VII IOM recording.
- Electrodes: subdermal (Cz, lateral orbicularis oculi, orbicularis oris)
- Filters: HP: 5-30 Hz; LP: 2000-3000 Hz
- Response latency usually < 20 ms
- Automatic muting devices (aka artifact rejection) are often available on some machines
What factors may influence facial nerve monitoring?-
- Local anesthetics: delayed latencies and reduced amplitudes due to impaired propagation of potentials
- Neuromuscular blockers: spontaneous, free running and evoked responses may be abolished for lengthy periods
