spinal anesthesia is a frequently used anesthetic technique . whereas postoperative urinary retention and post - dural - puncture headache ( pdph ) represent the most often reported complications of spinal anesthesia , the post - anesthesia loss or transient deficit of hearing are of markedly rarer evidence [ 213 ] . however , contrary to usually transient hearing loss , which was reported in individual cases only , the deficit of hearing , also transient , is observed more often . the incidence of hearing deficits in patients undergoing spinal anesthesia can be estimated at 0.20.8% based on patient - reported symptoms [ 35 ] . however , studies using pure tone audiometry in small groups of patients showed that the prevalence of hearing deficits after spinal anesthesia can range from 0% up to 27% . deficits of hearing in patients after spinal anesthesia are defined as a transient sensorineural hearing loss , mostly at low frequencies , which resolves spontaneously within up to 3 weeks [ 29,15,16 ] . functional impairment of cochlear outer hair cells ( ohcs ) is postulated as a primary mechanism of transient hearing loss related to spinal anesthesia . the impairment results from a decrease in the perilymph pressure in the cochlea , occurring secondarily to a drop - off in cerebrospinal fluid resulting from spinal puncture . the cochlear duct forms an anatomical connection between these 2 fluid spaces , allowing even distribution of their hydrostatic pressures . otoacoustic emissions ( oaes ) are sounds that arise in the ear canal when the tympanic membrane receives vibrations transmitted backward through the middle ear from the cochlea . oae tests constitute an excellent , non - invasive and objective , instrument for determining cochlear activity . they can detect very early changes , which are still imperceptible for a patient , and thus allow detection of even slight alterations of cochlear micromechanics . ohcs constitute the most sensitive cochlear structure , being most prone to injuries caused by exogenous factors , such as decreased oxygen saturation of the blood , ototoxic drugs , toxins , noise , and toxic metabolites . oae is an objective , accurate , frequency - specific , easy and rapid test , which can be used to detect both transient and permanent alterations . as most previous studies of spinal anesthesia - related hearing loss were based on a subjective hearing test , namely pure tone audiometry ( pta ) , the pathomechanism of this complication is still not completely understood . the principal aim of this study was to examine the effect of spinal anesthesia of the function of ohc determined by means of distortion product otoacoustic emissions ( dpoae ) testing . to the best of our knowledge , apart from the study by karatas et al . , this was the first experiment employing both subjective ( pta ) and objective ( tympanometry and dpoae ) tests to assess the organ of corti prior to and after spinal anesthesia . ethical approval for this prospective study was provided by the local bioethical committee at the medical university of silesia in katowice ( chairperson prof . initially , we enrolled 36 consecutive patients ( 28 men and 8 women ) qualified to a scheduled uretherorenoscopic lithotripsy . the age of the patients ranged between 18 and 45 years ; all of them represented american society of anesthesiologists ( asa ) physical status i , and gave their written informed consent to participate in the study . exclusion criteria from the study were : present or past history of ear diseases , injuries of the head with resultant loss of consciousness , concomitant central nervous system disorders , meningitis , administration of ototoxic agents , contraindications to general anesthesia ( severe spinal deformations , history of injuries or surgeries on the spine , purulent lesions or tattoos on the skin at a site of the puncture , use of anticoagulants , anti - aggregants , or fibrinolytic agents ) . normal tympanic membrane in otoscopy , normal results of tympanometry ( tympanometric peak pressure between + 25 dapa and 160 dapa , and maximum compliance of 0.31.4 ml ) , pure - tone auditory threshold of less than 30 db hl , documented at 2506000 hz constituted inclusion criteria to the study . the tympanometry was conducted with minitymp interacoustics as dk-5610 impedance bridge , and tympanometric curves were determined for each ear separately . the pure - tone audiometry was performed with maico st 20 audiometer , with bilateral determination of auditory threshold expressed as db hl , at 2506000 hz for air - conduction and at 2504000 hz for bone - conduction audiometry . both tests were conducted in the course of preoperative qualification of patients , in a soundproof room at the department of urology . eventually , a group of 20 patients ( 18 men and 2 women ) was identified based on the above mentioned inclusion and exclusion criteria , and qualified for further analysis . dpoaes were collected in a quiet prep room of the surgical department , immediately before surgery . the otoacoustic emissions from the ear with the better result of pure - tone audiometry were recorded , with a patient placed in a supine position . the dpoae tests were performed using an ilo 2.92 otodynamics analyser system , with non - linear distortion products evoked by 2 tonal signals of different frequencies , f1 and f2 . a 2-channel probe with 2 loudspeakers and one microphone was inserted within the external ear canal , and fit with a soft adapter to provide precise adaptation to the canal wall . prior to the test , the software automatically checked the resonance of the external ear canal and the probe sealing . the data were collected using a dp - gram paradigm , i.e. during a step change in f1 and f2 frequencies , at a constant f2/f1 frequency ratio , and with constant levels of primary tones , l1 and l2 . pairs of primary tones ( l1=l2=70 db spl ) were delivered via the probe at a frequency ratio f2/f1=1.22 . the non - linear distortions f3=2f1 f2 were recorded at various f2 frequencies ( 1001 , 1184 , 1416 , 1685 , 2002 , 2380 , 2832 , 3369 , 4004 , 4761 , 5652 , and 6299 hz ) . only the recordings with the signal to noise ratio of at least 3 either the dpoae response across all studied frequencies , or the levels of background noise , recorded under various experimental conditions , was subjected to analysis . following the baseline dpoae testing , each patient was transferred to the surgical theater , placed on an operating table and subjected to the endoscopic urological procedure under spinal anesthesia . immediately after the procedure , each patient was transferred to the prep room , and dpoaes were collected from the same ear as before , with a subject placed in a supine position . the emissions were recorded no later than 7 minutes after the surgery ( mean 5.21.8 min ) . the last ( third ) dpaoe testing and the second pta took place on postoperative day 2 , in a quiet room at the department of urology ; again the same ear was examined , with the patient in a supine position . following the baseline dpoae testing , each patient was transferred to the surgical theater , and placed on an operating table . an 18-gauge ( g ) intravenous cannula ( venflon , becton - dickinson , sweden ) was inserted into the antebrachial vein , and an intravenous drip infusion of 500 ml 0.9% nacl ( natrium chloratum 0.9% , fresenius kabi , poland ) was started at 2 ml / kg b.w./h , along with continuous monitoring of the electrographic curve , heart rate , and pulse oximetry . moreover , a non - invasive measurement of the baseline arterial blood pressure ( t0 ) was taken . subsequently , spinal anesthesia was delivered : a 27-g ( 0.490 mm ) pencil - point whitacre spinal needle ( becton - dickinson , spain ) was inserted into l3/l4 intervertebral space through a 22-g ( 0.732 mm ) introducer from the midline approach , with an opening oriented cranially . upon obtaining a leakage of the cerebrospinal fluid , 10 mg ( 2 ml ) of 0.5% hyperbaric bupivacaine ( marcaine spinal 0.5% heavy , astrazeneca ab , sweden ) were administered , along with 50 g ( 1 ml ) of fentanyl ( fentanyl , warszawskie zakady farmaceutyczne polfa s.a . , poland ) at about 1 ml/45 s. subsequently , a patient was placed in a supine position , and another measurement of arterial blood pressure ( t1 ) was taken . during the surgery , oxygen was delivered via a nasal cannula , at a 6 l / min flow rate . the hemodynamic parameters of each patient , i.e. heart rate , systolic , diastolic and mean blood pressure , and pulse oximetry were recorded after 5 , 10 , 15 , 20 , 25 , 30 , 35 , 40 , 45 , 50 , 55 , 60 , 65 , 70 , 75 , 80 , 85 , and 90 minutes of anesthesia , excepting the individuals who left the surgical theater earlier . any drop off in arterial blood pressure ( at least 20% decrease from the baseline value ) was compensated with intravenous ephedrine ( ephedrinum hydrochloricum , polfa warszawa , poland ) in 5-mg fractions , given every 3 minutes until normalization . intravenous atropine ( atropinum sulfuricum , polfa warszawa , poland ) was administered in each case of decreased heart rate ( at least 20% decrease from the baseline level ) , at 0.01 mg / kg b.w . until the cumulative dose of 3 mg was reached . the results were subjected to statistical analysis with statistica 10.0 package ( statsoft , usa ) . normal distribution of the analyzed variables was verified with the shapiro - wilk test . due to non - normal distribution of most study parameters , the significance of differences between pairs of dependent variables was verified with the wilcoxon s test , and the mann - whitney u test was used for intergroup comparisons of independent variables . the relationships between pairs of variables were tested with spearman s coefficients of rank correlation . dpoaes were collected in a quiet prep room of the surgical department , immediately before surgery . the otoacoustic emissions from the ear with the better result of pure - tone audiometry were recorded , with a patient placed in a supine position . the dpoae tests were performed using an ilo 2.92 otodynamics analyser system , with non - linear distortion products evoked by 2 tonal signals of different frequencies , f1 and f2 . a 2-channel probe with 2 loudspeakers and one microphone was inserted within the external ear canal , and fit with a soft adapter to provide precise adaptation to the canal wall . prior to the test , the software automatically checked the resonance of the external ear canal and the probe sealing . the data were collected using a dp - gram paradigm , i.e. during a step change in f1 and f2 frequencies , at a constant f2/f1 frequency ratio , and with constant levels of primary tones , l1 and l2 . pairs of primary tones ( l1=l2=70 db spl ) were delivered via the probe at a frequency ratio f2/f1=1.22 . the non - linear distortions f3=2f1 f2 were recorded at various f2 frequencies ( 1001 , 1184 , 1416 , 1685 , 2002 , 2380 , 2832 , 3369 , 4004 , 4761 , 5652 , and 6299 hz ) . only the recordings with the signal to noise ratio of at least 3 either the dpoae response across all studied frequencies , or the levels of background noise , recorded under various experimental conditions , was subjected to analysis . following the baseline dpoae testing , each patient was transferred to the surgical theater , placed on an operating table and subjected to the endoscopic urological procedure under spinal anesthesia . immediately after the procedure , each patient was transferred to the prep room , and dpoaes were collected from the same ear as before , with a subject placed in a supine position . the emissions were recorded no later than 7 minutes after the surgery ( mean 5.21.8 min ) . the last ( third ) dpaoe testing and the second pta took place on postoperative day 2 , in a quiet room at the department of urology ; again the same ear was examined , with the patient in a supine position . following the baseline dpoae testing , each patient was transferred to the surgical theater , and placed on an operating table . an 18-gauge ( g ) intravenous cannula ( venflon , becton - dickinson , sweden ) was inserted into the antebrachial vein , and an intravenous drip infusion of 500 ml 0.9% nacl ( natrium chloratum 0.9% , fresenius kabi , poland ) was started at 2 ml / kg b.w./h , along with continuous monitoring of the electrographic curve , heart rate , and pulse oximetry . moreover , a non - invasive measurement of the baseline arterial blood pressure ( t0 ) was taken . subsequently , spinal anesthesia was delivered : a 27-g ( 0.490 mm ) pencil - point whitacre spinal needle ( becton - dickinson , spain ) was inserted into l3/l4 intervertebral space through a 22-g ( 0.732 mm ) introducer from the midline approach , with an opening oriented cranially . upon obtaining a leakage of the cerebrospinal fluid , 10 mg ( 2 ml ) of 0.5% hyperbaric bupivacaine ( marcaine spinal 0.5% heavy , astrazeneca ab , sweden ) were administered , along with 50 g ( 1 ml ) of fentanyl ( fentanyl , warszawskie zakady farmaceutyczne polfa s.a . , poland ) at about 1 ml/45 s. subsequently , a patient was placed in a supine position , and another measurement of arterial blood pressure ( t1 ) was taken . during the surgery , oxygen was delivered via a nasal cannula , at a 6 l / min flow rate . the hemodynamic parameters of each patient , i.e. heart rate , systolic , diastolic and mean blood pressure , and pulse oximetry were recorded after 5 , 10 , 15 , 20 , 25 , 30 , 35 , 40 , 45 , 50 , 55 , 60 , 65 , 70 , 75 , 80 , 85 , and 90 minutes of anesthesia , excepting the individuals who left the surgical theater earlier . any drop off in arterial blood pressure ( at least 20% decrease from the baseline value ) was compensated with intravenous ephedrine ( ephedrinum hydrochloricum , polfa warszawa , poland ) in 5-mg fractions , given every 3 minutes until normalization . intravenous atropine ( atropinum sulfuricum , polfa warszawa , poland ) was administered in each case of decreased heart rate ( at least 20% decrease from the baseline level ) , at 0.01 mg / kg b.w . until the cumulative dose of 3 mg was reached . the results were subjected to statistical analysis with statistica 10.0 package ( statsoft , usa ) . normal distribution of the analyzed variables was verified with the shapiro - wilk test . due to non - normal distribution of most study parameters , the significance of differences between pairs of dependent variables was verified with the wilcoxon s test , and the mann - whitney u test was used for intergroup comparisons of independent variables . the relationships between pairs of variables were tested with spearman s coefficients of rank correlation . the basic characteristics of the study 20 participants ( age , sex , body height and weight , asa physical status , prevalence of cigarette smoking , and duration of the surgery ) are presented in table 1 . none of the participants showed any adverse events , such as pain , dizziness , nerve injuries , hypotension , bradycardia , or decreased arterial oxygen saturation during or after the anesthesia ( table 2 ) . we did not observe a significant effect of spinal anesthesia on the results of pta conducted on postoperative day 2 . comparative analysis of dpoaes at 10006000 hz did not reveal significant differences between values recorded prior to and immediately after the anesthesia . the only exception pertained to f2=5652 hz , for which a significantly higher level of dpoaes was observed immediately after anesthesia ( p=0.007 ; figure 1 ) . moreover , the post - anesthesia levels of dpoaes at frequencies higher than 2000 hz turned out to be higher than the respective baseline levels , but the differences did not prove statistically significant . to exclude the effect of noise on the results of dpoae testing , we compared the levels of background noise recorded prior to and after the anesthesia ( figure 1 ) . as no significant differences between the pre- and post - anesthesia levels of background noise were documented at any of the studied frequencies , one can exclude the confounding effect of noise on the dpoae level . the levels of otoacoustic emissions collected on postoperative day 2 were higher than at the baseline , for nearly all analyzed frequencies ( except f2=1001 hz and f2=1184 hz ) , with statistically significant differences observed at f2=2002 hz ( p=0.016 ) and f2 = 2380 hz ( p=0.026 ; figure 2 ) . irrespective of the frequency , we did not document any significant differences between the levels of background noise recorded prior to anesthesia and on the 2 day post - surgery ( p>0.05 ; figure 2 ) . comparative analysis of dpoaes at 10006000 hz did not reveal significant differences between values recorded prior to and immediately after the anesthesia . the only exception pertained to f2=5652 hz , for which a significantly higher level of dpoaes was observed immediately after anesthesia ( p=0.007 ; figure 1 ) . moreover , the post - anesthesia levels of dpoaes at frequencies higher than 2000 hz turned out to be higher than the respective baseline levels , but the differences did not prove statistically significant . to exclude the effect of noise on the results of dpoae testing , we compared the levels of background noise recorded prior to and after the anesthesia ( figure 1 ) . as no significant differences between the pre- and post - anesthesia levels of background noise were documented at any of the studied frequencies , one can exclude the confounding effect of noise on the dpoae level . the levels of otoacoustic emissions collected on postoperative day 2 were higher than at the baseline , for nearly all analyzed frequencies ( except f2=1001 hz and f2=1184 hz ) , with statistically significant differences observed at f2=2002 hz ( p=0.016 ) and f2 = 2380 hz ( p=0.026 ; figure 2 ) . irrespective of the frequency , we did not document any significant differences between the levels of background noise recorded prior to anesthesia and on the 2 day post - surgery ( p>0.05 ; figure 2 ) . the inner ear is composed of a number of interconnected spaces that form a membranous and bone labyrinth . the bone labyrinth consists of the vestibule , cochlea , and three semicircular canals , and is filled with a fluid , referred to as perilymph . in turn perilymph , being a filtrate of blood and cerebrospinal fluid , is a substrate for the inner ear cells . both , an imbalance between endolymph and perilymph probably underlies the hearing loss observed after spinal anesthesia , resulting from a drop - off in cerebrospinal fluid pressure . in most persons , the exchange of fluid between endolymphatic and perilymphatic space is low and well - balanced . according to michel et al . , the mechanism of transient hearing loss after spinal anesthesia can be associated with an anatomical anomaly of the cochlear aqueduct , leading to enhanced flow of cerebrospinal fluid through the latter . this anomaly is observed in approximately 2 per 1000 patients . due to a decrease in cerebrospinal fluid pressure and the presence of post - puncture opening in the dura mater , larger volumes of perilymph get into cerebrospinal space , which is reflected by a decreased perilymphatic pressure in the labyrinth . this leads to an increase in the endolymphatic pressure and development of endolymphatic hydrops , similar to that observed in the mnire s disease . the presence of hydrops causes a translocation of ohcs settled on the basilar membrane , which results in low - frequency hearing loss . nevertheless , some authors point to potential ischemic etiology of the loss of hearing or suggest that it is associated with microembolism or infection . hearing loss , observed after spinal anesthesia may by induced by a number of factors . according to literature , the volume of cerebrospinal leakage , and resultant incidence of hearing loss and other concomitant morbidities , especially post - dural - puncture headache , are determined by the type and diameter of the spinal needle . hearing loss was reported more frequently following a puncture with a 22- or 23-g needle , than after using a 25- or 26-g needle , and was not observed in patients who received anesthesia with a 27-g needle . compared the risk of hearing loss due to single - shot spinal puncture and continuous spinal anesthesia , with spinal catheter left in the cerebrospinal space for 24 hours . the incidence of hearing impairment after continuous anesthesia turned out to be higher ( 43% ) than after the single shot ( 37% ) . erol et al .   observed that the incidence of hearing loss among patients in whom a 25-g quincke spinal needle was used for spinal anesthesia was higher , than in those who received spinal anesthesia through a 25-g pencil - point or ball - pen needles . the same authors did not document significant differences in the incidence of hearing loss among patients subjected to anesthesia with 25-g pencil - point and ball - pen spinal needles . we used a 27-g pencil - point spinal needle in order to minimize the leakage of the cerebrospinal fluid through the post - puncture opening in the dura mater . none of our patients required repeated puncture of the cerebrospinal space , cerebrospinal fluid withdrawal , or barbotage . the degree of circulating blood loss and administration of infusion solutions also seem to be associated with the incidence of low - frequency hearing loss . acute changes in the blood volume , intracranial pressure , and osmolality of the plasma were postulated to be the risk factors of hypoacousia . proper and adequate fluid therapy can prevent loss of hearing as it improves perfusion of the inner ear . they observed that the incidence of hearing loss in patients who received lactated ringer s was higher than in those administered gelatin polysuccinate ( gelofusine ) , but the difference turned out to be insignificant . furthermore , low - frequency hearing loss seemed to correlate with intraoperative volume replacement . in this study , 500 ml of crystalloids were used for the initial intravenous volume replacement , followed by a colloid infusion at an individually adjusted dose . the endoscopic procedure is not associated with the loss of blood and does not require irrigation of the bladder with distilled water . all our patients received infusions of physiological saline during the 2 initial postoperative days , and the surgeries lasted no longer than 50 minutes . none of the subjects required administration of any additional medications , and no cases of hypotonia or decreased arterial blood oxygen saturation were documented . the influence of age on the incidence of post - spinal anesthesia hearing loss still raises controversies . we did not observe any negative effects of spinal anesthesia on hearing in younger patients ( i.e. , between 18 and 42 years of age ) . nevertheless , our participants showed higher levels of dpoaes , both immediately after the anesthesia and on postoperative day 2 . however , this finding seems clinically unrelated , as none of the subjects reported subjective improvement of hearing . due to limitations of otoacoustic emission testing ( physiological loss of response with age ) , this method can not be used in older patients . the results , similar to our findings , were documented in 1 previous study of women subjected to cesarean section , who received spinal anesthesia through a 24-g pencil - point needle or a 25-g qiuncke needle . furthermore , this group did not present with negative effects of spinal anesthesia on hearing , and a slight , albeit significant , transient improvement of auditory threshold was documented on pta . however , none of the patients reported subjective improvement of hearing . similar transient improvement of pta hearing threshold was also reported by ok et al . , who used pure tone audiometry and tympanometry to examine 20- to 40-year - old patients receiving anesthesia through a 22-g or 25-g spinal needles . in contrast , gultekin and ozcan   observed higher incidence of hearing loss in younger patients ( i.e. , below 30 years of age ) . they explained this phenomenon as a consequence of a more severe leak of the cerebrospinal fluid after dural puncture in the young patients , which was also associated with higher incidence of post - dural - puncture headache in this group . the same authors highlighted the potential confounding effect of preexisting hypoacousia on the incidence of post - spinal anesthesia hearing loss in older subjects ( > 60 years of age ) . in order to neutralize this potential confounder , we analyzed only the patients with pta evidence of normal auditory threshold at 2506000 khz . consequently , our results seem to be more objective than those documented in previous studies . the abovementioned phenomenon of transient improvement of auditory parameters immediately after spinal anesthesia , documented on both pta   and during otoacoustic emission testing ( present study ) , is an intriguing and still unexplained finding . the presence of enhanced amplitude of otoacoustic emissions after spinal anesthesia is difficult to explain . it may be associated with the influence of medications used for spinal anesthesia ( bupivacaine and fentanyl ) on the function of the efferent system ( ie , the medial olivocochlear ( moc ) system ) . the principal role of the moc system is reduction ( inhibition ) of the cochlear enhancement , which is reflected by a decrease in the basilar membrane motility and the resultant slight decrease in the otoacoustic emission level . efferent inhibition of ohc is mediated by nicotinic acetylcholine receptors ( nachr ) that form channels for calcium ions , thus allowing the activation of calcium - dependent potassium channels . blockade or decreased responsiveness of these receptors lead to withdrawal of the inhibitory effect of the moc system and resultant transient increase in the otoacoustic emission level . the influence of bupivacaine and fentanyl on the cholinergic receptors of the moc system is still not understood ; therefore , it can not be excluded that they impair neurotransmission in the moc system , which causes blockade or decrease in the efferent inhibition of the cochlea and resultant transient increase in oae amplitude . available data on the objectively evaluated spinal anesthesia - induced changes taking place in the organ of hearing are sparse . most of the published research was based on subjective hearing tests ie , pure tone audiometry ) . we evaluated the organ of hearing both by means of otoacoustic emission testing and pta . oae testing is an objective method and its results are tightly correlated with the degree of cochlear injury . only karatas et al . used a similar method in order to determine the objective effect of spinal anesthesia on the organ of hearing . they used passive oxygen therapy with a nasal cannula at 3 l / min flow rate and intravenous sedation with midazolam . the study revealed a transient negative effect of spinal anesthesia on cochlear function ( i.e. , a decrease in the otoemission level ) , with progressive return to its pre - anesthesia values within 48 hours post - surgery . in our patients , oxygen was delivered at a higher flow rate ( approximately 6 l / min ) ; furthermore , they received the anesthesia via a smaller needle and were not premedicated . our findings point to the lack of a detrimental effect of spinal anesthesia on objectively evaluated cochlear function . consequently , this method of anesthesia seems to be safe in the case of short bloodless endoscopic urological procedures , even in the context of ohcs , which are extremely susceptible to exogenous and endogenous injuries .