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Skull Based Tumors - E Medicine
notes:
Important factors for obtaining ABR:
1. adequate stim strength 2. optimal stim rep rate 3. optimal electrode placement 4. reduction of noise that reaches amplifiers 5. optimal filtering.
Rectangular waves of 100µs duration
Electrocochleogram (ECochG )-
generated by depolarization within the distal (cochlear) ends of the auditory nerve axons, which have been activated by excitatory synaptic input from the cochlear hair cells. It is recorded as a phasic negativity in the middle ear or extratympanic recording site irrespective of the stimulus polarity. Multiple volleys are typically illicited (N1 and N2, sometimes N3 components).Cochelar microphonic- generated in the receptor cells (hair cells) within the cochlea. Its waveform approximates the sound pressure waveform of the acoustic stimulus (if waveshape of the acoustic stimulus is reversed in polarity, the cochlear microphonic is also inverted).
8th CN action potential-
Auditory Evoked Potentials:
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Short Latency- under 10 ms. BAEPs |
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Middle Latency- 10-50 ms. Most likely generatedwithin the cerebral cortex, including the primary auditory cortex. Affected by anesthesia, so not useful for IOM, but they have been used to assess the depth of anesthesia. |
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Long Latency- over 50 ms. Predominantly generated in the cortex including cortical association areas. Suppressed by anesthesia- not useful in IOM. |
BAEPs
BAEPs can be used to monitor acoustically evoked activity within the auditory pathways up through the level of the mesencephalon; they are not useful for monitoring the pathways rostral to this. They are most often used to monitor surgery for eighth nerve tumors such as vestibular schwannomas (formerly called acoustic neuromas) and for tumors or vascular abnormalities within the posterior fossa, both extra-axial and within the substance of the brainstem. BAEP monitoring can help to avoid excessive eighth nerve stretch from cerebellar retraction, which can cause hearing loss, during surgery in the cerebellopontine angle.
Almost identical in walking and sleeping states.
Sedation & surgical anesthesia produce only minor changes
Intraoperative changes are likely to be caused by changes in body temperature rather than anesthetic affects
Recordings should include: vertex-to-ipsilateral-ear (Cz- Ai) recording. Vertex positive peaks in the channel are labeled with Roman numerals.
Other recordings: vertex-to-contralatera-ear (Cz-Ac) may help in identification of components.
Most BAEP components are far-field recordings- therefore small displacements of the recording electrodes don't significantly alter the BAEP waveform. The exception to this is Waves I & II, which are near field recordings from the distal auditory nerve recorded in the vicinity of the stimulated ear. Changes in the position of the recording electrodes at Ai can significantly affect waves I & II and can be manipulated to obtain improved recordings.
BAEP
components are composites of contributions of multiple generators. The
complexity of the generators of human BAEPs derives in part from the pattern of
connections within the auditory pathways, with ascending fibers both synapsing
in and bypassing various relay nuclei.
It also reflects the presence of at least two bursts of activity in the auditory nerve (corresponding to the N1 and N2 components of the eighth nerve compound action potentials in the ECochG), which can drive the more rostral pathways.Several different structures within the infratentorial auditory pathways may be active and generating field potentials simultaneously.
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Wave I |
Arises from the 1st volley of action potentials in the auditory nerve in the most distal portion of the nerve. Represents the N1 component of the 8th CN compound action potential in the ECochG. Because wave I arises from the most distal portion of the auditory nerve, it may persist after the nerve is sectioned at a more proximal location, such as during surgery for eighth nerve tumors. |
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Wave II |
Originates, in part, in neural activity that began as the N1 component of the eighth nerve compound action potential and has propagated from the distal auditory nerve to its proximal end and to the cochlear nucleus. However, the activity at this point in the auditory pathway occurs simultaneously with the second auditory nerve volley, the N2 component of the eighth nerve compound action potential, in the distal nerve (Gersdorff, 1982). The latter contributes to the scalp-recorded BAEP in the same manner as the N1 component did when it was at the same location. This can cause persistence of a wave II in cases where the proximal eighth nerve has been destroyed (Legatt, 2005). With regard to the more proximal generator of wave II, the relative contribution of activity in auditory nerve fibers within the proximal nerve and of activity in cochlear nucleus neurons has been a subject of controversy (Legatt et al., 1988). This proximal generator is the major determinant of the scalp topography of this BAEP component over the dorsal part of the head |
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Wave III |
Predominantly originates in the caudal pontine tegmentum, including the region of the superior olivary complex, though a contribution from continued activity at the level of the cochlear nucleus cannot be ruled out (Legatt, 2005). Ascending projections from the cochlear nucleus are bilateral, so wave III may receive contributions from brainstem auditory structures both ipsilateral and contralateral to the stimulated ear. In patients with asymmetrical lesions of the brainstem, wave III abnormalities are usually most pronounced following stimulation of the ear ipsilateral to the lesion (Brown et al., 1981; Oh et al., 1981; Faught and Oh, 1985), though occasionally they aremore pronounced following contralateral stimulation (Stockard and Rossiter, 1977). |
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Waves IV and V |
Waves IV and Vare often fused into a IV–V complex, and their anatomical generators are most likely in close anatomical proximity or overlapping, since they are usually either both affected or both unaffected by brainstem lesions. They may, however, be differentially affected by intraoperative brainstem damage. Wave IV appears to reflect activity predominantly in ascending auditory fibers within the dorsal and rostral pons, just caudal to the inferior colliculus Wave V predominantly reflects activity at the level of the inferior colliculus, perhaps including activity in the rostral portion of the lateral lemniscus as it terminates in the inferior colliculus. As is the case with wave III, wave V abnormalities due to unilateral brain stemlesions are usually most pronounced following stimulation of the ear ipsilateral to the lesion , though there are exceptions |
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Waves VI |
Waves VI and VII are absent in some normal subjects. While they may in part reflect activity in more rostral structures such as the medial geniculate nucleus, they also receive contributions from activity in the inferior colliculus; the latter generator may cause persistence of these waves in patients with auditory pathway damage rostral to the inferior colliculus. Therefore, BAEPs cannot be used to assess or monitor the auditory pathways rostral to the mesencephalon. |
STIMULATION
Click stimuli produced by delivering trains of 100 µs duration electrical square pulses. Brief tone pips can also be used.
Since responses to rarefaction and compression clicks may differ.
Alternating click polarities are useful to cancel a large stim artifact and/or the cochlear microphonic.
The time required for the acoustic signal to propagate through the tubing typically prolongs the latencies of all BAEP components by approximately 0.9–1.0 ms.
The delay helps to prevent obscuration of wave I by the electrical stimulus artifact, because
(1) it prolongs the latency of wave I, helping to separate it in time from the electrical stimulus artifact (which remains simultaneous with the activation of the acoustic transducer)
(2) it permits increasing the distance between the acoustic transducer and the recording electrodes, thus reducing the amplitude of the electrical stimulus artifact.
Stim Intensity - Should be high, but not too high so that it can cause noise induced hearing loss (NIHL). Clicks at 105 dB peak equivalent sound pressure level (PeSPL) are commonly used. This correlates to 65 dB hearing level (HL) (dB above the average threshold for hearing in individuals with normal hearing).
A stimulus rate of 10/s is typical, but a rate of exactly 10 Hz or another submultiple of the power line frequency should be avoided. If a line-frequency or harmonic artifact appears in the averaged BAEPs and examination of the raw data does not show increased line frequency artifact, the stimulus rate should be adjusted slightly, because timing circuits can drift. If rep rate is too high, amplitude of responses will be reduced. Peaks I-III are more affected by rate than V. Commonly used rep rates are 10-20 pps, although higher rep rates up to 40 or 50 pps can be used
Recording
An Ac-Ai recording channel can improve the detection of wave I; although this component is picked up mainly as a near-field negativity at the Ai electrode, the horizontal orientation of its dipole projects a small positivity to the contralateral ear.
Cz-Ai and the Cz-Ac waveforms. Wave V is identifiable in both of these recording linkages, and recording both of them provides a measure of redundancy should one of the ear electrodes become unusable.
A typical analog filter bandpass for BAEP recording is 100 or 150 Hz to 3,000 Hz (3 dB points).
While line-frequency (e.g., 60 Hz) “notch” filters should not be used for somatosensory evoked potential (SEP) recordings because they can cause a “ringing” oscillatory artifact
The analog gain depends on the input window of the analog-to-digital converter; a value of 100,000 is typical.
Epoch duration, typically 15 ms, is preferable during intraoperative BAEP monitoring because component latencies can be prolonged by preexisting pathology, hypothermia, or intraoperative compromise of the auditory system. The choice of the number of sweeps per average will depend on the signal-to-noise ratio of the raw data.
A value of 1,000 sweeps per average is typical, but more may be required if the raw data are noisy and the BAEPs are small. Near-field recording from the proximal eighth nerve may permit averaging using a much smaller number of sweeps.
Analysis and interpretation of BAEPs