Brainstem Auditory Evoked Potentials Part II

For intraoperative monitoring, it is most
important to know how the various nuclei of the
ascending auditory pathways are connected and
how these nuclei together with the fiber tracts
that connect them produce electrical activity
when the ear is stimulated with transient sounds.

1. B rainstem A uditory E voked P otentials II Anurag Tewari MD

2. Auditory Pathway Anurag Tewari MD

3. Anatomy and Physiology Anurag Tewari MD

4. Ossicles of middle ear Anurag Tewari MD The purpose of the ossicles is to overcome the impedance mismatch between air and water

5. Ossicles of middle ear The ossicular chain converts the large displacement, low pressure signal in air to the small displacement, high pressure signal Anurag Tewari MD Oval Window

6. Cochlea Has three fluid-filled compartments ( scalae ) Separated by the cochlear partition (or basilar membrane ) and the Reissner’s membrane The cochlea separates sounds according to their spectrum and transforms each sound into a neural code in the individual fibers of the CN VIII Anurag Tewari MD At the Cochlear Nuclei important decoding of the basic signal occurs: Duration, Intensity And Frequency

7. Frequency Analysis in the Cochlea The in -and- out motion of the stapes footplate sets the basilar membrane into motion This causes vibration of the fluid in the cochlea The motion of the basilar membrane causes a traveling wave Anurag Tewari MD

8. Frequency Analysis in the Cochlea The traveling wave starts at the base of the cochlea and progresses relatively slowly toward the apex of the cochlea and at a certain point along the basilar membrane, its amplitude decreases abruptly A low-frequency sound travels a long distance before being extinguished High-frequency sound travels a short distance before its amplitude decreases Anurag Tewari MD

9. Frequency Analysis in the Cochlea Low frequencies at the apex of the cochlea High frequencies at the base of the cochlea Anurag Tewari MD

10. Frequency Analysis in the Cochlea The basilar membrane is more frequency selective for low-intensity sounds than high-intensity sounds The highest audible frequencies produce maximal vibration amplitude of the basilar membrane near the base of the cochlea Anurag Tewari MD

11. Sensory Transduction in the Cochlea Sensory cells; known as Hair cells ( stereocilia ) Two types INNER HAIR CELLS OUTER HAIR CELLS Arranged along the basilar membrane as one row of inner hair cells and three to five rows of outer hair cells Anurag Tewari MD

12. Sensory Transduction in the Cochlea Each INNER hair cell connects with SEVERAL axons Several OUTER hair cells connect with ONE nerve fiber About 95% of the nerve fibers of the cochlear nerve connect to inner hair cells While, about 5% connect to outer hair cells Anurag Tewari MD

13. Sensory Transduction in the Cochlea The motion of the basilar membrane deflects the hairs on the hair cells. Deflection of the hair cells in one direction causes the intracellular potentials to become less negative ( depolarization ), whereas a deflection in the opposite direction causes hyperpolarization Anurag Tewari MD

14. Hair Cells & Basilar Membrane Motion Inner hair cells function as Electric-mechanical transducers Transform sound energy to neural signals that can be interpreted by the brain Can not regenerate Outer hair cells function as Motors Amplify the motion of the basilar membrane Anurag Tewari MD

15. Ascending Auditory Pathway Anurag Tewari MD

16. Primary Auditory Cortex ( Herschl’s Gyrus) Anurag Tewari MD

17. Resources How Ear Works Organ of Corti Anurag Tewari MD

18. Break Questions? Anurag Tewari MD

19. Sound Anurag Tewari MD

20. Sound Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave SI unit is Pascal (Pa) =1 Newton of Force/ m 2 SILENCE AUDIBLE SOUND ATMOSPHERIC PRESSURE SOUND PRESSURE Anurag Tewari MD

21. Sound is a Pressure Wave Anurag Tewari MD

22. Sound Sound Pressure : is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave (SI Units=Pascal) Sound Power : Measure of sound energy per unit of time (SI Unit=Watts) Sound Intensity : Sound power per unit area (Watts/Area) Anurag Tewari MD

23. Sound Anurag Tewari MD

24. What is DECIBEL ? The Deci -BEL (dB) is 1/10 Bel. dB is a logarithmic scale, describing a ratio, [it is always a comparison] Zero dB does not mean absence of sound, but sound level of intensity that is equal to that of a standard Human can hear from 20 – 20,000 Hz Decibel is NEGATIVE if the signal is ATTENUATED POSITIVE if AMPLIFIED Engineers use the unit to show if the signal has lost or gained power/strength Anurag Tewari MD

25. What is DECIBEL ? dB is a logarithmic scale, describing a power ratio If P 2 is twice the P 1 ; the ratio is 10*log (2) = 3dB If P 2 is half the P 1 ; the ratio is 10*log (1/2) = -3dB If P 2 is 1,000,000 times the P 1 ; the ratio is 10*log (10 6 ) = 60dB log (10 n ) = n*log (10) = n log (1) = 0 Power being measured Power of the reference Anurag Tewari MD

26. What is DECIBEL ? Actuality, when sound level is measured, it is done in terms of pressure or voltage (through a microphone) Pressure or voltage is the square root of their power Therefore, in describing a change in pressure or voltage, the formula for the decibel changes as follows So doubling the sound pressure would increase it by approximately 6dB: Anurag Tewari MD

27. How to tackle Decibel conversion Questions Loss or Gain in Power Ratio Loss or Gain in Decibels { P output / P input } 10 log { P output / P input } 1000 30 100 20 10 10 1 0.1 -10 0.01 -20 0.001 -30 0.0001 -40 Anurag Tewari MD

28. How to tackle Decibel conversion Questions Anurag Tewari MD

29. DECIBEL-POWER GAIN AMPLIFIER Amplifier is an electronic device for increasing the amplitude of electrical signals Anurag Tewari MD

30. DECIBEL-POWER GAIN AMPLIFIER The power gain ( A p ) of an Amplifier is the logarithmic ratio between Input Power (P i ) and output power (P o ) Anurag Tewari MD

31. DECIBEL-POWER GAIN AMPLIFIER 100mW 200mW Anurag Tewari MD

32. DECIBEL-POWER GAIN AMPLIFIER 100mW 200mW Thus a 3 dB GAIN causes a DOUBLING of POWER Anurag Tewari MD

33. DECIBEL-POWER GAIN AMPLIFIER 100mW 1mW Decibel – Attenuation: A reduction of one hundred times is expressed as -20dB Anurag Tewari MD

34. Sound Intensity Loudness of a sound is determined by its INTENSITY Anurag Tewari MD

35. Sound Intensity If energy E passes through the area A in the time t the intensity ( I ) if the wave is I = E/(At) Since Power (P) = E/t I = P/A (SI Unit = Watts/Meter 2 [W/m 2 ] Anurag Tewari MD

36. Sound Intensity The INTENSITY falls off with the square of the distance: Anurag Tewari MD

37. Sound Intensity Anurag Tewari MD

38. Sound Intensity The quantity that we use to relate the intensities of different sounds to each other is the DECIBEL In terms of sound intensity, a dB is a measure of relative sound pressure Anurag Tewari MD

39. Sound Intensity The ear perceives sound in a nonlinear fashion; in fact, it perceives sounds logarithmically Therefore the decibel is a logarithmic measure By the rule of thumb: loudness is only DOUBLED with a TEN–fold increase in intensity Anurag Tewari MD

40. Quantifying Auditory Stimuli ACNS guidelines Decibels peak-equivalent sound pressure level (dB pe SPL) Decibels above normal hearing level (dB HL) Decibels above sensation level (dB SL) Anurag Tewari MD

41. Quantifying Auditory Stimuli ACNS guidelines Anurag Tewari MD

42. dB pe SPL Decibels peak-equivalent sound pressure level (dB pe SPL) A measure of the actual sound pressure delivered by the auditory stimulator In ABR it is the unit used in calibrating auditory stimulators The unit of pressure used in this measure is the micropascal , or dyne 2 /cm 2 Sound pressure levels are compared to an arbitrary zero pressure level which is 20 micropascals , or 0.0002 dyne 2 /cm 2 Anurag Tewari MD

43. dB HL Decibels above normal hearing level (dB HL) A measure comparing sound intensity to the hearing threshold of NORMAL, HEALTHY YOUNG ADULTS Therefore a sound level of 6 dB HL is twice as loud as the threshold of a population of normal subjects hearing the same sound. This measure is usually default in IONM machines and most commonly used during IONM. Anurag Tewari MD

44. dB SL Decibels above sensation level (dB SL) A measure using each patient as his or her own control The reference point for this measure is the patient’s own hearing threshold This is an important measure in clinical BAEPs in dealing with a patient with a hearing deficit Anurag Tewari MD

45. Quantifying Auditory Stimuli ACNS guidelines NIOM machines should have the ability to provide stimuli in both dB HL and dB SL dB pe SPL is impractical to use as a means of quantifying sound intensity in the operating room. It is imperative to know what measurement scales are available on one’s equipment and how to select the appropriate measure so that the expected stimulus intensity is delivered. Anurag Tewari MD

46. Resources Decibels What is a Decibel? Anurag Tewari MD

47. Break Questions? Anurag Tewari MD

48. How to Obtain an Interpretable BAEP in the Shortest Possible Time Important factors for obtaining an interpretable record in the shortest possible time are: The use of adequate stimulus intensity The use of optimal stimulus repetition rate Optimal electrode placement Reduction of electrical noise that reaches the amplifiers Use of optimal filtering of recorded potentials Anurag Tewari MD

49. Stimulus Intensity The stimulus intensity should be adequately high without risking noise-induced hearing loss (NIHL), so that the amplitude of the recorded ABR is as high as possible Clicks at an intensity of 105 dB peak equivalent sound pressure level ( dBpeSPL ) have been used for intraoperative monitoring for many years without experiencing any problems. This intensity corresponds to the approximately: 65-dB hearing level (HL) Anurag Tewari MD

50. Stimulus Repetition Rate When the stimulus repetition rate is increased, the number of responses that can be collected within a certain period of time increases If the amplitude of the responses was independent of the repetition rate Then the time it would take to obtain an interpretable record would be inversely proportional to the repetition rate Thus a doubling of the repetition rate would shorten that time by a factor of two Anurag Tewari MD

51. Stimulus Repetition Rate However, this is only the case below a certain repetition rate, because the amplitude of the peaks decreases with increasing repetition rate above a certain repetition rate, diminishing the gain of increasing the repetition rate Above a certain repetition, there would be no advantage to increasing the repetition rate. Anurag Tewari MD

52. Stimulus Repetition Rate Peaks I–III are much more affected by an increased repetition rate than peak V, which is the most robust of the peaks of the ABR with regard to high repetition rate of the stimulus Hearing loss of cochlear origin does not seem to affect the way that the amplitude of the ABR peaks decrease with increasing repetition rate of the click stimuli, but if the hearing loss is of retrocochlear origin, (an injury to the auditory nerve), then the amplitude of peak V deceases more rapidly with increasing repetition rate of the stimulus. Anurag Tewari MD

53. Auditory Stimulators The most common type of auditory stimulator used is a piezoelectric transducer that delivers broadband clicks via a plastic tube and foam ear insert These stimulators are commonly known as Insert Earphones Traditional headphones are not recommended difficult to affix can dislodged during surgery Anurag Tewari MD

54. Auditory Stimulators Using insert earphones, all BAEP waveform latencies are delayed by 1 ms because of the time that it takes for the stimulus click to travel from the transducer through the tubing to the eardrum. Many IONM machines automatically subtract this 1-ms delay Sound travels at a speed of about 340 m/s, corresponding to a delay of 1 ms per 34 cm. Anurag Tewari MD

55. Auditory Stimulators IONM auditory stimulators typically deliver a broadband click produced by a 100μs electrical pulse to the transducer The resultant click oscillates up to 2 ms in duration The polarity of the click should be selectable Anurag Tewari MD

56. Auditory Stimulators Three types of polarity are available CONDENSATION : produces a sound wave that pushes the eardrum inward , away from the stimulator RAREFACTION: produces a sound wave that pulls the eardrum outward , toward the stimulator ALTERNATING CLICKS : as the name implies, consist of alternating condensation and rarefaction stimuli Most common used polarity for BAEP Anurag Tewari MD

57. Auditory Stimulators The intensity of the auditory stimuli, should be selectable The stimulator should be capable of producing a broadband click with an intensity of up to 120 dB pe SPL Most machines use dB HL as the default method of measuring intensity Therefore the auditory intensity should be selectable from 0 to 100 dB HL in 1-dB steps Anurag Tewari MD

58. Auditory Stimulators The availability of white noise masking is helpful in eliminating crossover of the auditory stimulus Transmission of one-sided stimuli to the opposite ear by air or bone conduction can result in a BAEP that reflects activity in both ears Deliver white noise at 60 dB SPL to the contralateral ear, to eliminate crossover component Anurag Tewari MD

59. BAEP Parameters Monoaural or Binaural Stimulation? BAEP usually measured by stimulating each ear independently Simultaneous stimulation produces a response that differs from the sum of the BAEP produced by stimulating each ear independently and is not of use Electrode placement Cz : Recording electrode: at Vertex A1 : Reference Electrode: Ipsilateral Ear Lobule or Mastoid Process A2 : Ground Electrode: Contralateral ear Lobule Montage Channel 1: Cz -ipsilateral ear or mastoid (Cz-A1/M1) Channel 2: Cz -contralateral ear or mastoid (Cz-A2/M2) Anurag Tewari MD

60. BAEP Parameters Stimulus Characteristics Type : Broadband clicks @ 100microsecond , electric square wave pulses into audiometer ear speaker Rate : 8-10/second Polarity : Rarefaction, alternating Intensity : 90-120dBpeSPL or 60-70dB SL Masking : 60db SPL of white noise to c/l ear Filter Settings : LFF: 10-30Hz in case of artifacts 100-200Hz HFF: 2500-3000Hz Number of averaged responses : 2000 (1000-4000) Sweep Length : at least 15mSec Sampling Rate : min: 10k/sec max: 100microSec S ilence PL ease Anurag Tewari MD

61. Interpretation of results When interpreting the ABR, we look at AMPLITUDE (the number of neurons firing) LATENCY (the speed of transmission) INTERPEAK LATENCY (the time between peaks) INTERAURAL LATENCY (the difference in wave V latency between ears) Anurag Tewari MD

62. Alerts A latency prolongation of 1 ms or an amplitude decrement of at least 50% historically been considered significant and indicative of traction or heat related injury to the vestibulocochlear nerve Anurag Tewari MD

63. BAEP Waveforms Anurag Tewari MD

64. BAEPs Short Latency Response < 10mS Middle Latency Response 10 – 100mS Long Latency Response >100ms Anurag Tewari MD

65. The neural pathway for ABR response purposed by Hall, J. W., III. Handbook of auditory evoked responses. (1992) Anurag Tewari MD

66. BAEP waveform Measurements Measurements must include the following: wave I peak latency wave III peak latency wave V peak latency I-III interpeak interval III-V interpeak interval I-V interpeak interval wave I amplitude wave V amplitude wave IV-V/I amplitude ratio Anurag Tewari MD

67. BAEP Waveforms and its Physiological Generators Wave Generator I Acoustic nerve (segment near cochlea) II Acoustic nerve (segment near brainstem) or cochlear nuclei III Superior olive and projections to the lateral lemniscus; medial nucleus of trapezoid body might generate a part of wave III IV Most likely the lateral lemniscus , but data is not definitive V High pontine or lower midbrain structures: probably the lateral lemniscus, inferior colliculus , or a combination thereof VI Most likely the medial geniculate nucleus or projections from the inferior colliculus VII Most likely the auditory radiations to primary auditory cortex Anurag Tewari MD

68. BAEP Waveforms and its Physiological Generators Anurag Tewari MD Mnemonic: E COLI E:  eighth nerve action potential (wave I), C:  cochlear nucleus (wave II) O:  olivary complex (superior) (wave III) L:  lateral lemniscus (wave IV) I:  inferior colliculus (wave V)

69. Prepared by Anurag Tewari MD Wave I First consistent wave seen in BAEP with ipsilateral stimulation It is the far-field representation of the compound auditory nerve action potential in the distal portion of CN VIII Believed to originate from afferent activity of the CN VIII fibers ( first-order neurons ) as they leave the cochlea and enter the internal auditory canal Prolongation implies a lesion of the VIII th cranial nerve This can be observed in sensorineural or conductive hearing loss Most schwannomas spare wave I Anurag Tewari MD

70. Wave I If peak I changes or disappears during an operation and there also is a change in all other peaks (or total obliteration of the ABR), it is a sign that the blood supply to the ear (cochlea) has been compromised If peak I is largely unchanged while there are changes in both peaks III and V: it is likely that there has been injury to the intracranial portion of the auditory nerve, with the blood supply to the cochlea remaining intact Anurag Tewari MD

71. Wave II The ABR wave II is generated by the proximal VIII nerve as it enters the brain stem Usually identified between I and III which are more consistent It can be ABSENT in normal subjects Anurag Tewari MD

72. Wave III The ABR wave III arises from second-order neuron activity (beyond CN VIII) in or near the cochlear nucleus Literature suggests wave III is generated in the caudal portion of the auditory pons Almost equidistant between I and V in normal subjects Peak III might be a more reliable (clean) indicator of changes in neural conduction of the auditory nerve than peak V Identification may be difficult in Absence of wave II Partial fusion of wave IV and V Bifid wave III Anurag Tewari MD

73. Wave IV The ABR wave IV, which often shares the same peak with wave V Thought to arise from pontine third-order neurons mostly located in the superior olivary complex but additional contributions may come from the cochlear nucleus and nucleus of lateral lemniscus Anurag Tewari MD

74. Wave V Most consistent peak of BAEP Last of the core Short Latency waveforms Requires lowest threshold for stimulation Highest amplitude of all waveforms Because peak V of the ABR is the most prominent and most easily identified peak, it seems natural to use changes in the latency of this peak as an indication of injury to the auditory nerve. If there is a change in peak V but peak III is unchanged, there is reason to assume that the brainstem has been affected by surgical manipulations or that there is ischemia because of impaired blood supply. Anurag Tewari MD

75. Wave V Changes in the function of structures of the ascending auditory pathways that are located rostral to the generators of peak III (the cochlear nucleus) causes Increase in the latency of peak V The latency of peak III remains unchanged The interval between peaks III and V increases (increased interpeak latency III–V). Anurag Tewari MD

76. Wave VI & VII Thalamic (medial geniculate body) origin is suggested for generation of waves VI and VII but the actual site of generation is uncertain Anurag Tewari MD

77. WAVE I TO III INTERPEAK LATENCY Prolongation implies a lesion between the proximal segment of the eighth cranial nerve and the superior olivary nucleus Often this reflects damage to structures at the CPA Meningiomas and schwannomas at the CPA are typical examples of neoplasms that can cause prolonged I to III interpeak latency Anurag Tewari MD

78. WAVE III TO V INTERPEAK LATENCY Prolongation suggests a lesion in pathways from the caudal pons to midbrain Demyelinating plaques, infarcts, and neoplasms in the brainstem are often associated with increased III to V latency Anurag Tewari MD

79. Prepared by Anurag Tewari MD WAVE III TO V INTERPEAK LATENCY Increased IPL III–V might also be caused by general changes in, cerebral circulation or from changes in oxygenation from other causes If this occurs in operations in the CPA, the anesthesiologist should be informed because such changes might be a result of cardiovascular changes or other changes that the anesthesiologist can correct Anurag Tewari MD

80. BAEP Waveforms and its Physiological Generators BAEP FINDING Lesion Prolonged wave I latency Distal CN VIII Prolonged I–III interpeak latency Between proximal CN VIII and pons (CPA masses) Prolonged III–V interpeak latency Lesion between caudal pons and midbrain (stroke, tumor, MS, ICH, AVM, etc.) Prolonged I–III and III–V latencies Both rostral pons or midbrain and acoustic nerve or caudal pons Absent wave I with normal III and V Mild to moderate peripheral hearing loss Absent wave III with normal I and V Normal variant Absent wave V with normal I and III Above the caudal pons Absence of all waves Severe hearing loss Absence of all waves except I (and possibly II) Brain death Anurag Tewari MD

81. Interpretation of results Wave I should also be observed but will only be present ipsilaterally Anurag Tewari MD

82. Interpretation of results Waves III and V should be detectable in all healthy individuals Anurag Tewari MD

83. Interpretation of results Wave II is often absent and Wave IV is frequently buried and, therefore, indistinguishable from wave V . Anurag Tewari MD

84. Interpretation of results Wave III often has decreased amplitude on the side contralateral to that being stimulated Anurag Tewari MD

85. Prepared by Anurag Tewari MD Interpretation of results Waves VI and VII appear variably after wave V . Anurag Tewari MD

86. Indications for ABRs Microvascular decompression (MVD) operations Surgery to relieve trigeminal neuralgia (TGN), hemifacial spasm (HFS), glossopharyngeal neuralgia (GPN) Spine (extradural, decompression/fusion): High Posterior Cervical (risk to vertebral artery). Spinal Cord (intradural): High Cervical Extramedullary tumor. High Cervical Intramedullary tumor, syrinx. Intracranial (infratentorial): Brain stem (CPA, acoustic neuroma, cranial nerves, etc ). Intracranial Vascular: Brainstem aneurysm clipping. Brainstem repair of arteriovenous malformation (AVM). Brainstem aneurysm coiling (interventional radiology; IR). Diagnostics (non-surgical): Clinical Audiology Anurag Tewari MD

87. PATIENT-RELATED VARIABLES AFFECTING BAEPS A number of patient-related factors impact the BAEP recordings, these include Individual’s age Gender Level of arousal Body position Temperature Medications Preexisting hearing loss Can be adjusted to optimize results Anurag Tewari MD

88. Effects of Anesthetics BAEPs have the advantage of being relatively insensitive to anesthetics. Regardless of the type of inhalation or intravenous anesthetic agent used, there is little effect on BAEP waveforms. These agents can cause up to a 0.1-to 0.2-ms prolongation of absolute latency, but it occurs at the start of the case and does not confound interpretation. Temperature and blood pressure have a more significant effect. Wave V latency increases by 0.2 ms per degree (centigrade) drop in temperature Similarly, if anesthetics result in a drop in blood pressure, wave V amplitude drops and latency increases. Neuromuscular blocking agents do not have a significant impact on BAEPs. Anurag Tewari MD

89. Limitations of ABRs in Surgery Delayed results due to averaging (seconds to minutes) Affected by severe hearing loss, may not be recorded May remain unchanged in face of evolving long-track motor/sensory deficit Cannot tell you about the function of other cranial nerves and their nuclei Susceptible to contamination from electrical and mechanical noise Anurag Tewari MD