Interoperative Monitoring Flashcards
What is the definition of interoperative monitoring?
- Continuous, on-line electrophysiologic assessment, measurement, and interpretation of neural responses to intraoperative events
- Modality-specific, controlled stimulation during the course of surgery
- Assess the integrity of a sensory or motor modality during a surgical procedure
What are indications for IOM?
- When there is a risk of surgical complications involving the CNS or peripheral nervous system
- When surgical or anesthesia intervention (indicated by monitoring) could help prevent, reverse, reduce consequences
What is the purpose for IOM?
To improve postoperative outcome by correlating changes in neurophysiological responses with intraoperative events
What personnel and equipment are in the operating room?
Personnel: surgeon, anesthesiologist, nurses, audiologist
Electrical and magnetic environment:
- Operating microscope
- Anesthesia machine
- Pulse oximeter machine
- Electrocautery machine
- Automated blood pressure machine
- Heart rate monitor, respiratory rate monitor
What are the different kinds of hearing preservation surgery?
Cerebellopontine angle tumor removal
Vestibular nerve section
- Bad vertigo and no treatment methods have worked
- Don’t want to complete surgery and loose auditory hearing
Microvascular decompression of CN V or CN VII
Cochlear implantation (electric ABR and facial nerve)
What are the different types of damage that occur during surgery?
- Mechanical (compression, tearing, cutting, stretching)
- Ischemic (interrupted blood supply to cochlea or brainstem)
- Thermal (electrocautery)
What is included in preoperative audiologic workup?
- Audiogram
- Immittance measurements
- Otoacoustic emissions
- Speech discrimination
- Auditory electrophysiology
What is included in pre-incision ABR/ECochG?
- Check equipment
- Identify any noise
- Patient baseline
What is the suboccipital (retrosigmoid) craniotomy?
- Made behind the ear in the occipital bone
- Bone overlying the internal auditory canal is removed to expose and remove the tumor
- This approach may be used for all tumor sizes, but especially large ones, while preserving facial nerve function and useful hearing if possible
What are the advantages to the retrosigmoid approach?
- Allows removal of tumors of different sizes
- Offers the possibility of hearing preservation
- Offers the surgeon a wide view of the cisternal component of the tumor and good access to the root entry zone of the acoustic nerve
What are the disadvantages of the retrosigmoid 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
What is the translabyrinthine craniotomy approach?
- Made through the ear in the mastoid bone
- The semicircular canals are removed to expose the tumor in the internal auditory canal
- B/c the canals are removed, complete hearing loss occurs in the affected ear
- This approach may be used for patients who already have hearing loss or when preservation of hearing is not possible
What are the advantages of the translabyrinthine craniotomy?
- Size of the tumor is not a limiting factor for this approach
- It offers early identification of the facial nerve in the auditory canal
- There is absolutely no need for cerebellar retraction
What is the disadvantage of the translabyrinthine craniotomy?
No hearing preservation
What is the middle fossa craniotomy?
- Made above the ear in the temporal bone
- Bone overlying the internal auditory canal is removed to expose and remove the tumor
- Approach may be used for small tumors and when preservation of hearing is optimal
- Exposes 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 predominately occupy the IAC with a minor component in the cistern (usually <10 mm)
- Provides exceptional access to the lateral end of the canal
What are far-field measurements?
- Averaged Auditory Brainstem Response (ABR)
- Electrocochleography (ECochG)
What is a near-field measurement?
- Auditory Nerve-compound action potential (AN-CAP)
What is the protocol for 8th nerve monitoring?
Transducers: insert earphones
- Seal the ear with bone wax or bioclusive patches
- Better placement
- Reduce stimulus artifact
- Reduces contamination by external acoustic noise
- Minimizes risk for noise-induced hearing loss
Stimuli
- Clicks (broad spectrum stimulus to assess the integrity of the system
- 70-95 dB nHL
Stimulus rate
- As high as possible (will be limited by the sampling rate)
- Should be odd (so as not to reinforce 60-Hz noise)
Recording (optimize speed and SNR)
- Electrodes: subdermal electrodes are ideal for ABR
- Peri-tympanic or promontory (transtympanic) electrodes for ECochG contact electrode on CN VIII for CAP
Signal Acquisition
- Filters: 100-3000 Hz
- Amplifier: differential (common mode rejection)
- Gain: 75,000-100,000x
- Artifact rejection: on
- Signal averaging: variable (averaging can stop when response is clear)
- Response is usually <10 ms
What should the AuD monitor during ABR/ECochG measurements?
Absolute latencies (I, III, V) - Wave III is an early predictor of post-op deafness
Interwave latencies (need Wave I as a reference)
- Indicators of neural conduction time
- I-III: auditory nerve function
- III-V: brainstem function
What should the AuD measure during AN-CAP measurement?
- Amplitude (# of active auditory nerve fibers)
- Loss of N1 suggests asynchronous firing and/or conductive block
What are potential threats to the AN-CAP response?
- Fluid in the ear canal/middle ear (conductive component)
- Mechanical displacement of the nerve
- Misplaced electrodes (especially for ECochG)
What should the AuD communicate to the surgeon?
- Any significant change in response
- Prolongations in latency
- Obliteration of waveforms
- Severe reductions in amplitude
