as an auditory rehabilitation device , the auditory brainstem implant ( abi ) has been used to treat deafness in individuals with neurofibromatosis type 2 since the first device was implanted in 1979 . in the following years
, progressively more effective multichannel abis have been developed to palliate the deficits due to loss of integrity of the auditory nerves .
furthermore , the indications for abi have also been expanded to other treatments , such as the treatments for nontumor patients , ossification or major cochlear malformations , aplasia or avulsion of the cochlear nerves , vestibular schwannoma on the only hearing ears , and bilateral temporal bone fractures .
thus , an increasingly greater number of patients have acquired auditory perception again with the help of abi , and their quality of life has improved . despite the enormous potential of this increasingly applied treatment , the auditory performance of many implanted patients is limited , and the variability between cases hinders a complete understanding of the roles played by the multiple parameters related to the efficacy of the implant .
although several researchers have committed to improve the efficacy of the implant , the predicament has not been resolved . to solve this problem
, we attempted to establish a large animal model for abis in a rhesus macaque monkey to explore the array of the cochlear nucleus ( cn ) complex , which would thus lay the foundation for the investigation of the mechanisms of auditory recovery after implant surgery .
the care and handling of animals were conducted in compliance with the chinese animal welfare act , the guidance for animal experimentation of capital medical university , and the beijing guidelines for the care and use of laboratory animals .
this study was approved by the animal ethics committee of beijing neurosurgical institute , capital medical university ( no . 20131114 ) .
the animals were in good medical condition at the time of testing and had no history of exposure to known ototoxic drugs .
all monkeys were treated in the same way ( e.g. , diet and cage status ) .
they were maintained in individual cages in a controlled environment of 120 cm ( h ) 80 cm ( w ) 75 cm ( d ) .
the front , roof , and walls were composed of organic glass , whereas the bottom of the cage was composed of metal net .
the monkeys were routinely released into a semi - closed environment that had environmental enrichment objects , such as toys , branches and climbing materials , mirrors , and tvs , as well as other objects . in the abi surgical procedure ,
general anesthesia was induced via intramuscular injections of ketamine ( 15 mg / kg , gutian pharmaceutical co. , ltd . ,
china ) , midazolam ( 0.5 mg / kg , enhua pharmaceutical co. , ltd . , china ) and atropine ( 0.02 mg / kg , jinyao amino acid co. , ltd . ,
orotracheal intubation was performed after an intravenous ( iv ) injection of fentanyl ( 0.001 mg / kg , astrazeneca ltd . , uk ) and rocuronium ( 0.3 mg / kg , n.v .
organon , the netherlands ) . lumbar puncture and urethral catheterization were performed prior to surgery . during the surgery , the electrocardiogram ,
blood pressure , pulse rate , spo2 , end - tidal carbon dioxide partial pressure and temperature were monitored . for the magnetic resonance imaging ( mri ) , computed tomography ( ct ) scanning , auditory brainstem response ( abr ) , electrical abr
( eabr ) or lumbar puncture , the monkeys were initially anesthetized with intramuscular ketamine ( 15 mg / kg ) , midazolam ( 0.5 mg / kg ) and atropine ( 0.02 mg / kg ) .
all of these experiments were performed under iv propofol ( propoflo , abbott laboratories , usa ) anesthesia with an initial bolus of 1.5 mg / kg and a continuous infusion of 0.5 mgkgmin .
the electrode pad was comprised by medical silicone , slightly elliptical , and measured 12 mm  4 mm  2 mm .
the 24 surface electrodes were attached to one side , and the other side was covered with a 1-cm - diameter polyethylene terephthalate mesh pad for stabilization purposes .
we performed mri / ct scanning for all six monkeys prior to the surgery to evaluate the intracranial status and gasification degree of the mastoid , and to obtain a reference for surgical planning ; abr testing was conducted to assess auditory function .
a lumbar puncture was performed , the cerebrospinal fluid ( csf ) was tested , and the results were subsequently reserved as a baseline .
general anesthesia was performed via endotracheal intubation , and a left bench - park position was subsequently placed .
a postauricular inverted l - shaped skin incision was performed under aseptic technique , and a right modified suboccipital retrosigmoid ( rs ) craniotomy was performed from the level of the superior nuchal line down to the foramen magnum . after the bone flap was removed , the transverse sinus , sigmoid sinus , and their intersection were exposed , and the foramen magnum was also opened [ figure 1 ] .
a line incision was then performed on the dura mater along the sigmoid sinus . under surgical microscopic guidance ,
the nerves in the cerebellopontine angle ( cpa ) ( xi , x , ix , viii , and vii ) were detected and distinguished from bottom to top [ figure 2 ] .
the abr was used again to verify the normality of neural function . the right cochlear nerve ( viii )
was subsequently cut under intraoperative neuromonitoring . by the anatomic markers of the glossopharyngeal nerve ( ix ) , choroid plexus of the fourth ventricle and brainstem stub of cochlear nerve , the right luschka 's foramen of the fourth ventricle
once the anatomical location was confirmed , the electrode assays were implanted in the lateral recess through luschka 's foramen .
the electrode assays were subsequently fixed with autologous muscle , followed by dural closure , bone replacement and suturing of muscles and skin . at the end of the surgery , eabr and head
ct were used to evaluate the validity of the electrophysiological function and anatomical placement . during the operation , the prophylactic use of iv ceftriaxone sodium
when the animals awoke from the anesthesia , they were transported to the observation room for postoperative monitoring .
( a ) an intra - operative bone window ( marked with bold black line marked in figure b ) , the solid arrow points to the transverse sinus , and the sigmoid sinus is marked with hollow arrow .
the stars point to viii cranial nerve , the solid arrow points to vii cranial nerve , and the hollow arrow points to xi , x and ix cranial nerves .
the animals were housed in a separate cage and monitored for food intake , skin wound healing condition , behavioral and neurological changes , and postoperative complications .
the modified tarlov classification and the modified canada disability scale were used for neural defects and general status evaluation , respectively .
intramuscular ceftriaxone sodium 10 mg / kg was administered for the first 3 postoperative days .
a lumbar puncture was again performed during the 1 week postoperation or as indicated by signs of intracranial infection .
abr and eabr testing were performed every 2 weeks from the 1 month after the implant surgery as the brain edema subsided .
neural function in the monkeys was assessed using abr before and 1-month after the abi surgery .
the animal 's ear canals were inspected and cleaned of debris if necessary , and the condition of the tympanic membrane was determined .
following sedation , the skin was wiped clean with alcohol wipes , and 0.22 gauge skin electrodes were placed subcutaneously behind each ear , on the forehead , and on the back of the neck the electrode impedance was checked after placement into the skin and was below 2 k ohm for all recordings .
the abr recordings were obtained using a bio - logic auditory evoked potentials system ( version 6.2.0 , natus medical incorporated , usa ) controlled by a laptop computer .
the evoked responses were amplified 100 k times , and our physiological filters were set to pass at 100500 hz . for the abr recordings , rarefaction clicks were delivered through insert earphones ( er-3a , etymotic research , inc . , il , usa ) at a rate of 13.3 stimuli / s and at a level of 80 db nhl ( peak sound pressure level ) . the abr waveforms were averaged over a minimum of 1500 repetitions . clicks and tone bursts were presented at 80 db nhl and decreased in 10 db nhl increments from 80 to 30 db nhl and 5 db nhl until the abr response was no longer detectable .
the latencies and amplitudes of peaks i , ii , and iv were measured at 80 db nhl .
the monaural responses were recorded for each monkey and averaged ( biologic traveler express ) in a 10 ms time window .
the averages for 1000 sweeps were collected , and the responses were replicated to determine the waveform reliability .
the first eabr test was conducted every 2 weeks since the 1 month after the abi procedure .
the noninverting electrode was placed at the midline ( cz ) , and the inverting electrode was placed subcutaneously behind the ipsilateral ear .
the recording electrodes were passed through an external low pass filter ( fc = 32 khz ) in an attempt to eliminate the frequency - modulated signal sent from the external to internal components of the abi .
frequencies outside the 1003000 hz range were filtered out , and the response was sampled at 20,000 hz , which provided data points at 0.05 ms intervals and interpolation between the points .
the waveforms were marked using the bio - logic auditory evoked potentials system , which provided the latency values to the 0.01 ms .
signal averaging was performed between 5 ms and 80 ms relative to the stimulus onset ; however , a more limited time window of  210 ms was used for the analyses .
sweeps that contained large signals between 2 ms and 80 ms latency were rejected from the average .
at least 500 sweeps were typically averaged , and the responses were recorded at least twice .
monopolar pulses ( mp 1 + 2 ) were delivered by the multichannel implants at the basal end of the array .
the stimulation levels were increased in steps of 1030 clinical units until the maximum levels were reached ; in most cases , the eabrs recorded at least three increasing levels during this process .
the measurement data were presented as the mean  standard deviation ( sd ) . for the analysis of the comparison of latencies and amplitudes of peaks i , ii and iv between the bilateral ears in the abr test , a dependent samples t - test was used . a paired samples t - test
the p level was set to 0.05 for all analyses to determine the statistical significance .
the care and handling of animals were conducted in compliance with the chinese animal welfare act , the guidance for animal experimentation of capital medical university , and the beijing guidelines for the care and use of laboratory animals .
this study was approved by the animal ethics committee of beijing neurosurgical institute , capital medical university ( no . 20131114 ) .
the animals were in good medical condition at the time of testing and had no history of exposure to known ototoxic drugs .
all monkeys were treated in the same way ( e.g. , diet and cage status ) .
they were maintained in individual cages in a controlled environment of 120 cm ( h ) 80 cm ( w ) 75 cm ( d ) .
the front , roof , and walls were composed of organic glass , whereas the bottom of the cage was composed of metal net .
the monkeys were routinely released into a semi - closed environment that had environmental enrichment objects , such as toys , branches and climbing materials , mirrors , and tvs , as well as other objects . in the abi surgical procedure ,
general anesthesia was induced via intramuscular injections of ketamine ( 15 mg / kg , gutian pharmaceutical co. , ltd . ,
china ) , midazolam ( 0.5 mg / kg , enhua pharmaceutical co. , ltd . , china ) and atropine ( 0.02 mg / kg , jinyao amino acid co. , ltd . ,
orotracheal intubation was performed after an intravenous ( iv ) injection of fentanyl ( 0.001 mg / kg , astrazeneca ltd . , uk ) and rocuronium ( 0.3 mg / kg , n.v .
organon , the netherlands ) . lumbar puncture and urethral catheterization were performed prior to surgery . during the surgery , the electrocardiogram ,
blood pressure , pulse rate , spo2 , end - tidal carbon dioxide partial pressure and temperature were monitored . for the magnetic resonance imaging ( mri ) , computed tomography ( ct ) scanning , auditory brainstem response ( abr ) , electrical abr
( eabr ) or lumbar puncture , the monkeys were initially anesthetized with intramuscular ketamine ( 15 mg / kg ) , midazolam ( 0.5 mg / kg ) and atropine ( 0.02 mg / kg ) .
all of these experiments were performed under iv propofol ( propoflo , abbott laboratories , usa ) anesthesia with an initial bolus of 1.5 mg / kg and a continuous infusion of 0.5 mgkgmin . the pulse rate , spo2 , respiratory rate and temperature were monitored .
the electrode pad was comprised by medical silicone , slightly elliptical , and measured 12 mm  4 mm  2 mm .
the 24 surface electrodes were attached to one side , and the other side was covered with a 1-cm - diameter polyethylene terephthalate mesh pad for stabilization purposes .
we performed mri / ct scanning for all six monkeys prior to the surgery to evaluate the intracranial status and gasification degree of the mastoid , and to obtain a reference for surgical planning ; abr testing was conducted to assess auditory function .
a lumbar puncture was performed , the cerebrospinal fluid ( csf ) was tested , and the results were subsequently reserved as a baseline .
general anesthesia was performed via endotracheal intubation , and a left bench - park position was subsequently placed .
a postauricular inverted l - shaped skin incision was performed under aseptic technique , and a right modified suboccipital retrosigmoid ( rs ) craniotomy was performed from the level of the superior nuchal line down to the foramen magnum . after the bone flap was removed , the transverse sinus , sigmoid sinus , and their intersection were exposed , and the foramen magnum was also opened [ figure 1 ] .
a line incision was then performed on the dura mater along the sigmoid sinus . under surgical microscopic guidance
, the nerves in the cerebellopontine angle ( cpa ) ( xi , x , ix , viii , and vii ) were detected and distinguished from bottom to top [ figure 2 ] .
the abr was used again to verify the normality of neural function . the right cochlear nerve ( viii )
was subsequently cut under intraoperative neuromonitoring . by the anatomic markers of the glossopharyngeal nerve ( ix ) , choroid plexus of the fourth ventricle and brainstem stub of cochlear nerve , the right luschka 's foramen of the fourth ventricle
once the anatomical location was confirmed , the electrode assays were implanted in the lateral recess through luschka 's foramen .
the electrode assays were subsequently fixed with autologous muscle , followed by dural closure , bone replacement and suturing of muscles and skin . at the end of the surgery , eabr and head
ct were used to evaluate the validity of the electrophysiological function and anatomical placement . during the operation , the prophylactic use of iv ceftriaxone sodium
when the animals awoke from the anesthesia , they were transported to the observation room for postoperative monitoring .
( a ) an intra - operative bone window ( marked with bold black line marked in figure b ) , the solid arrow points to the transverse sinus , and the sigmoid sinus is marked with hollow arrow .
the stars point to viii cranial nerve , the solid arrow points to vii cranial nerve , and the hollow arrow points to xi , x and ix cranial nerves .
the animals were housed in a separate cage and monitored for food intake , skin wound healing condition , behavioral and neurological changes , and postoperative complications .
the modified tarlov classification and the modified canada disability scale were used for neural defects and general status evaluation , respectively .
intramuscular ceftriaxone sodium 10 mg / kg was administered for the first 3 postoperative days .
a lumbar puncture was again performed during the 1 week postoperation or as indicated by signs of intracranial infection .
abr and eabr testing were performed every 2 weeks from the 1 month after the implant surgery as the brain edema subsided .
neural function in the monkeys was assessed using abr before and 1-month after the abi surgery .
the animal 's ear canals were inspected and cleaned of debris if necessary , and the condition of the tympanic membrane was determined .
following sedation , the skin was wiped clean with alcohol wipes , and 0.22 gauge skin electrodes were placed subcutaneously behind each ear , on the forehead , and on the back of the neck the electrode impedance was checked after placement into the skin and was below 2 k ohm for all recordings .
the abr recordings were obtained using a bio - logic auditory evoked potentials system ( version 6.2.0 , natus medical incorporated , usa ) controlled by a laptop computer .
the evoked responses were amplified 100 k times , and our physiological filters were set to pass at 100500 hz . for the abr recordings , rarefaction clicks were delivered through insert earphones ( er-3a , etymotic research , inc . , il , usa ) at a rate of 13.3 stimuli / s and at a level of 80 db nhl ( peak sound pressure level ) .
clicks and tone bursts were presented at 80 db nhl and decreased in 10 db nhl increments from 80 to 30 db nhl and 5 db nhl until the abr response was no longer detectable .
the latencies and amplitudes of peaks i , ii , and iv were measured at 80 db nhl .
the monaural responses were recorded for each monkey and averaged ( biologic traveler express ) in a 10 ms time window .
the averages for 1000 sweeps were collected , and the responses were replicated to determine the waveform reliability .
the first eabr test was conducted every 2 weeks since the 1 month after the abi procedure .
the noninverting electrode was placed at the midline ( cz ) , and the inverting electrode was placed subcutaneously behind the ipsilateral ear .
the recording electrodes were passed through an external low pass filter ( fc = 32 khz ) in an attempt to eliminate the frequency - modulated signal sent from the external to internal components of the abi .
frequencies outside the 1003000 hz range were filtered out , and the response was sampled at 20,000 hz , which provided data points at 0.05 ms intervals and interpolation between the points .
the waveforms were marked using the bio - logic auditory evoked potentials system , which provided the latency values to the 0.01 ms .
signal averaging was performed between 5 ms and 80 ms relative to the stimulus onset ; however , a more limited time window of  210 ms was used for the analyses .
sweeps that contained large signals between 2 ms and 80 ms latency were rejected from the average .
at least 500 sweeps were typically averaged , and the responses were recorded at least twice .
monopolar pulses ( mp 1 + 2 ) were delivered by the multichannel implants at the basal end of the array .
the stimulation levels were increased in steps of 1030 clinical units until the maximum levels were reached ; in most cases , the eabrs recorded at least three increasing levels during this process .
the measurement data were presented as the mean  standard deviation ( sd ) . for the analysis of the comparison of latencies and amplitudes of peaks i , ii and iv between the bilateral ears in the abr test , a dependent samples t - test was used .
a paired samples t - test was applied for the pre- and post - operative csf analysis .
the p level was set to 0.05 for all analyses to determine the statistical significance .
the average time for the abi procedure was 5.2 h ( range , 4.56 h ) , which included nearly half of the time for the eabr testing .
the preoperative abrs indicated that all animals possessed adequate auditory function , and the thresholds were 515 db nhl [ figure 3 ] .
the peaks i , ii and iv in the abr testing were distinguished at 80 db nhl , and their latencies and amplitudes are shown in table 1 .
the latency and amplitude of peaks i , ii and iv in the abr testing for six monkeys ( mean  sd ) * the left ears were tested twice , and one for the right ; p : comparison between left and right ears .
only one - peak eabr waves were induced during the abi operation [ figure 4 ] .
however , both one - peak and two - peak eabr waves were observed during the postoperative testing [ figure 5 ] .
the intra- and post - operative eabr waves confirmed that the devices were accurately placed where the stimulus could excite the complete auditory pathway .
for both one - peak and two - peak eabr waves , the latencies did not appear to change with the stimulus intensity ; however , the amplitudes had a positive correlation with the intensity .
the postoperative ct scans confirmed the anatomic site of the implant electrode arrays [ figure 6 ] . throughout the eabr testing , all monkeys remained stable , and no abnormal changes in the pulse rate , spo2 , respiratory rate , or abnormal limb movements were identified .
the one intra - operative electrical auditory brainstem response wave ( ep i ) followed a stimulus artifact ( sa ) , and the stimulus intensity from top to bottom of the five lines were 200 , 150 , 100 , 50 and 0 current levels .
( a ) one - peak postoperative electrical auditory brainstem response ( eabr ) wave ( ep i ) , the stimulus intensities from top to bottom were 120 , 100 , 80 , 50 , 30 and 0 current levels ( cls ) ; ( b ) two distinguishable eabr waves ( ep i and ii ) , the stimulus intensities from top to bottom were 150 , 150 , 150 , 120 , 110 , 100 , 80 , 50 and 30 cls .
a group of bone windows in postoperative computed tomography scans showed auditory brainstem implants ( arrows ) in the lateral recess of the fourth ventricle , which was the target anatomic location .
all wounds healed well , and no csf leakage , subcutaneous hydrops , wound infection , intracranial infection , or death occurred . according to the tarlov classification
the animals recovered to preoperative health status in 5.7 days ( range , 57 days ) , which was evaluated by the modified canada disability scale . by the time of the last test , no monkeys had died or were sacrificed ; they were all maintained for use in a subsequent study .
one animal presented with transient facial nerve paralysis postoperation , which was evidenced by a reduced ipsilateral less blink , a shallow forehead , and facial wrinkles .
the csf samples of the monkeys at different time points were colorless and clear , with a negative reaction in pandy 's test .
the csf items were not significant differents between the pre- and post - operative tests , including intracranial pressure ( 80.0 mmh2o vs. 81.7 mmh2o , p = 0.742 ) , chloride ( 118.0 mmol / l vs. 120.0
mmol / l , p = 0.522 ) , glucose ( 2.3 mmol / l vs. 2.4 mmol / l , p = 0.771 ) , protein ( 17.2 mmol / l vs. 17.6 mmol / l , p = 0.946 ) , and cells ( 1.7 cells / visual field vs. 0.3 cells / visual field , p = 0.184 ) .
the average time for the abi procedure was 5.2 h ( range , 4.56 h ) , which included nearly half of the time for the eabr testing .
the preoperative abrs indicated that all animals possessed adequate auditory function , and the thresholds were 515 db nhl [ figure 3 ] .
the peaks i , ii and iv in the abr testing were distinguished at 80 db nhl , and their latencies and amplitudes are shown in table 1 .
the latency and amplitude of peaks i , ii and iv in the abr testing for six monkeys ( mean  sd ) * the left ears were tested twice , and one for the right ; p : comparison between left and right ears .
only one - peak eabr waves were induced during the abi operation [ figure 4 ] . however , both one - peak and two - peak eabr waves were observed during the postoperative testing [ figure 5 ] .
the intra- and post - operative eabr waves confirmed that the devices were accurately placed where the stimulus could excite the complete auditory pathway .
for both one - peak and two - peak eabr waves , the latencies did not appear to change with the stimulus intensity ; however , the amplitudes had a positive correlation with the intensity .
the postoperative ct scans confirmed the anatomic site of the implant electrode arrays [ figure 6 ] . throughout the eabr testing , all monkeys remained stable , and no abnormal changes in the pulse rate , spo2 , respiratory rate , or abnormal limb movements were identified .
the one intra - operative electrical auditory brainstem response wave ( ep i ) followed a stimulus artifact ( sa ) , and the stimulus intensity from top to bottom of the five lines were 200 , 150 , 100 , 50 and 0 current levels .
( a ) one - peak postoperative electrical auditory brainstem response ( eabr ) wave ( ep i ) , the stimulus intensities from top to bottom were 120 , 100 , 80 , 50 , 30 and 0 current levels ( cls ) ; ( b ) two distinguishable eabr waves ( ep i and ii ) , the stimulus intensities from top to bottom were 150 , 150 , 150 , 120 , 110 , 100 , 80 , 50 and 30 cls .
a group of bone windows in postoperative computed tomography scans showed auditory brainstem implants ( arrows ) in the lateral recess of the fourth ventricle , which was the target anatomic location .
all wounds healed well , and no csf leakage , subcutaneous hydrops , wound infection , intracranial infection , or death occurred . according to the tarlov classification
the animals recovered to preoperative health status in 5.7 days ( range , 57 days ) , which was evaluated by the modified canada disability scale . by the time of the last test , no monkeys had died or were sacrificed ; they were all maintained for use in a subsequent study .
one animal presented with transient facial nerve paralysis postoperation , which was evidenced by a reduced ipsilateral less blink , a shallow forehead , and facial wrinkles .
the csf samples of the monkeys at different time points were colorless and clear , with a negative reaction in pandy 's test .
the csf items were not significant differents between the pre- and post - operative tests , including intracranial pressure ( 80.0 mmh2o vs. 81.7 mmh2o , p = 0.742 ) , chloride ( 118.0 mmol / l vs. 120.0 mmol / l , p = 0.522 ) , glucose ( 2.3 mmol / l vs. 2.4 mmol / l , p = 0.771 ) , protein ( 17.2 mmol / l vs. 17.6 mmol / l , p = 0.946 ) , and cells ( 1.7 cells / visual field vs. 0.3 cells / visual field , p = 0.184 ) .
to the best of our knowledge , this is the first report regarding the rhesus macaque monkey ( m. mulatta ) implanted with abi placement .
the successful induction of an abi model in the rhesus macaque monkey paves the way for future studies .
an appropriate surgical approach is vital for the exposure of the implant site and following abi implantation .
two approaches have been used in clinical practice for abis : the translabyrinthine ( tl ) and rs approaches .
the tl approach was originally advocated by edgerton et al . and is currently the only method approved by the us food and drug administration ( fda ) in abi clinical trials .
however , an anatomical study by kuroki and moller on cadaver specimens has demonstrated that the tl approach to the cpa offers a limited view ; thus , it is necessary to medically retract the sigmoid sinus to obtain a medial view of the root entry zone of the cochlear nerve . in another anatomical investigation , friedland and wackym have demonstrated that the rs approach provides excellent visualization of the lateral recess of the fourth ventricle when a 30 endoscope is used , which provides a more direct appreciation of the implant site compared with the tl approach .
previous studies have proven that the rs approach has various advantages , such as the adequate control of bleeding , dissection of the entire tumor under direct view , easy identification of the facial and cochlear nerves at their root entry zone and the distal end in the internal auditory canal , and adequate exposure of the foramen of luschka .
in addition , the rs approach provides a unique opportunity for real - time intraoperative monitoring by direct recording of cochlear nerve action potentials , which could prove useful in attempts at hearing preservation .
compared with humans , the rhesus macaque monkey possesses a more flat , smaller ( approximately 2 cm  1.5 cm on one side ) posterior fossa and relatively strong , thick head and neck muscles
. the regular linear postauricular incision can not provide satisfactory posterior fossa bone exposure , which , therefore , makes it difficult to distinguish nerves during the abi procedure .
after considering the advantages of the two approaches , the modified rs approach was adopted for this study . the inverted l - shaped flap and muscles , which had been fully freed ,
were turned outward , and the external occipital protuberance , middle line , superior nuchal line , part of superior border of the foramen magnum and mastoid process were completely exposed . during the nerve identification stage ,
the lower cranial nerves were recognized first , followed by the cochlear and facial nerves .
the choroid plexus of the fourth ventricle and luschka 's foramen could then be located by anatomic landmarks .
prior to electrode array implantation , the cerebellum and brainstem were separated from the bottom to top , which proved to be easier than the operation on the locale of luschka 's foramen .
the experiences with the six monkeys suggested that the modified approach is feasible for multichannel abi implantation .
several researchers have studied auditory evoked potentials in rhesus macaque monkeys . they have concluded that peak i in rhesus macaque monkeys ( which is analogous to peaks i and ii in humans ) was generated by eighth nerve fibers , whereas peak ii ( that is analogous to peak iii in humans ) was generated by the cn .
the neural generator of peak iii ( which is analogous to peak iv in humans ) was not determined by a specific near - field recording , although the researchers assumption was that this peak had its origin in the superior olivary complex .
finally , peak iv in monkeys ( which is analogous to peak v in humans ) was determined to originate from the contralateral lateral lemniscus . in our study , the latencies and amplitudes of peaks i , ii and iv were accurately calculated .
the latencies of the waves were similar to a previously published study ; however , the amplitudes appeared lower than the data from the same research .
intraoperative electrophysiology during abi implantation demonstrated that electrical stimulation from the implanted electrode elicits a response from the ascending auditory pathway , which is reflected in the recorded eabr . as the full surface of the cn is not visible , even an experienced neurosurgeon may have difficulty placing the electrode array into an optimal position every time .
in our study , preoperative abr in the rhesus macaque monkey contains three explicated waves [ figure 3 ] ; however , only one - peak eabr waves can be observed during surgery [ figure 4 ] , and one - peak or two - peak eabr waves can be observed postoperatively [ figure 5 ] . in humans ,
the eabr elicited by the electrical stimulation of the cn is similar to the abr ; however , it typically does not contain components that correspond to peaks i or ii in the abr .
the latencies of the peaks that are generated in the cn and the lateral lemniscus are shorter than the abr . referred the relevant information of humans and analyzed the waves in the abr and eabr of rhesus monkey
, we concluded that the eabr waves in rhesus macaque monkeys do not contain components that correspond to peak i in abr , and ep i and ep ii [ figures 4 and 5 ] correspond to peaks ii and iv [ figure 3 ] , respectively .
furthermore , peaks ii and iv were generated in the cn and contralateral lateral lemniscus .
in addition , the latencies of ep i and ep ii were less than peaks ii and iv [ figures 4 and 5 , and table 1 ] .
of particular note was that the waveform of the recorded eabrs differed considerably among individuals .
during testing , we could observe that as the stimulus intensity increased , the amplitude of the eabr wave in the same electrode also increased in a certain scope .
however , no similar correlation was identified between the latency and stimulus intensity . in the current study , we successfully established an abi animal model in the rhesus macaque monkey , and short - term data supported its feasibility and safety
. however , the long - term survival state and electrophysiology of monkeys with implants requires further investigation .
although eabr testing and postoperative ct could prove the effectiveness of this procedure in electrophysiology and anatomy , respectively , relevant behavioral studies should be considered in future implantations . any disorder that affects
the secretion , absorption or transportation will cause changes in the components of the csf and/or neural activity .
the short recovery time and stable csf components of all animals may demonstrate the safety of the abi .
we concluded that the rhesus macaque monkey is a valid abi model , and the modified suboccipital rs approach can provide sufficient exposure for the implant surgery . during the intra- and post - operation of abis , the eabr test can guide us to the optimal implant sites , and the postoperative ct can confirm the implant location .
ep i and ep ii of eabr wave were generated in the cn and contralateral lateral lemniscus .
numerous studies have clearly established this species as an essential model for neurophysiologic investigations in conjunction with sophisticated behavior .
we suggested that the demonstration of the feasibility of abi procedures in the macaque monkey opens a promising field of research regarding the mechanisms of auditory recovery after abi surgery .
 background : the auditory brainstem implants ( abis ) have been used to treat deafness for patients with neurofibromatosis type 2 and nontumor patients . 
 the lack of an appropriate animal model has limited the study of improving hearing rehabilitation by the device . 
 this study aimed to establish an animal model of abi in adult rhesus macaque monkey ( macaca mulatta).methods : six adult rhesus macaque monkeys ( m. mulatta ) were included . under general anesthesia , 
 a multichannel abi was implanted into the lateral recess of the fourth ventricle through the modified suboccipital - retrosigmoid ( rs ) approach . the electrical auditory brainstem response ( eabr ) 
 waves were tested to ensure the optimal implant site . 
 after the operation , the eabr and computed tomography ( ct ) were used to test and verify the effectiveness via electrophysiology and anatomy , respectively . 
 the subjects underwent behavioral observation for 6 months , and the postoperative eabr was tested every two weeks from the 1st month after implant surgery.result:the implant surgery lasted an average of 5.2 h , and no monkey died or sacrificed . 
 the averaged latencies of peaks i , ii and iv were 1.27 , 2.34 and 3.98 ms , respectively in the abr . 
 one - peak eabr wave was elicited in the operation , and one- or two - peak waves were elicited during the postoperative period . 
 the eabr wave latencies appeared to be constant under different stimulus intensities ; however , the amplitudes increased as the stimulus increased within a certain scope.conclusions:it is feasible and safe to implant abis in rhesus macaque monkeys ( m. mulatta ) through a modified suboccipital rs approach , and eabr and ct are valid tools for animal model establishment . 
 in addition , this model should be an appropriate animal model for the electrophysiological and behavioral study of rhesus macaque monkey with abi . 
I
M
Preparation
Auditory brainstem implant instrument
Surgical procedure
Postoperative observation
Auditory brainstem response test
Electrical auditory brainstem response recordings
Statistical analysis
R
Preoperative auditory brainstem response
Electrical auditory brainstem response waves
Complication
Laboratory reports of cerebrospinal fluid
D