low - level laser therapy ( lllt ) has been practiced for over 20 years in europe and has more recently been introduced in the united states as a treatment for pain and postsurgical tissue repair .
it has been proposed that laser energy in the red and near - infrared light spectrum may aid in the repair of tissue damage .
a proposed mechanism for this therapeutic effect is the stimulation of mitochondria in the cells to produce more energy through the production of adenosine triphosphate [ 13 ] .  
animal studies have found that laser stimulation can induce anatomic and physiologic changes in the cochlea .
rhee et al .   reported that rat hair cells were repaired with lllt following noise exposure .
.   found that laser stimulation increased basilar membrane stiffness ( and therefore resonant frequency ) in guinea pigs .
the authors suggested that this could allow lower - frequency regions of the cochlea ( where auditory function is typically less compromised ) to respond to higher frequency sounds .
studies in humans have investigated the effects of lllt on both hearing loss and tinnitus , with equivocal results .
some studies have found an improvement in hearing thresholds and tinnitus symptoms ( e.g. , [ 610 ] ) , while others have found no significant effect of lllt ( e.g. , [ 1115 ] ) .
the reason for the discrepancy in findings is not known , but likely involves multiple factors such as study design , subject characteristics , lllt methodology , and outcome measures used to assess the effects of lllt .  
although some studies showed improvements in hearing thresholds , no published study to date has examined the effect of lllt on speech understanding and only one has examined the effect on cochlear function via otoacoustic emissions .
pilot data from hearingmed ( unpublished ) found that lllt improved word recognition scores in subjects with hearing loss relative to a placebo group , which motivated the current study .
the purpose of the current study was to assess the effect of lllt on hearing in terms of auditory sensitivity , speech understanding , and cochlear function . a randomized , double - blind
, placebo - controlled trial design was implemented using the laser therapy protocol suggested by the hearingmed study .
in order to accurately detect possible changes in hearing status due to laser treatment , it was necessary to avoid using subjects whose hearing might fluctuate due to other factors . before potential subjects were enrolled in the study , they were asked a list of screening questions to determine eligibility .
the questions were chosen to ensure stable hearing , as well as to address possible safety issues .
all subjects were also required to have normal middle ear function , as assessed by 226  hz tympanograms .
two subjects withdrew from the study due to loss of interest and/or scheduling difficulty . the data from three additional subjects were not included in the analysis .
one subject yielded unreliable audiometric and speech understanding data , speech scores could not be obtained from one subject with a profound hearing loss , and calibration problems compromised data from the third subject .
the experimental protocol was approved by the institutional review board of the university of iowa , and written informed consent was obtained from all participants . an erchonia ehl laser ( erchonia medical , inc . )
the device was a portable ( 9  5  1 ) unit that consisted of a hand - held probe and a main body .
one diode produced light in the green part of the visible light spectrum ( 532  nm wavelength ) , and the other diode produced light in the red part of the visible light spectrum ( 635  nm wavelength ) . both diodes produced energy levels of 7.5  mw ( class iiib ) .
the laser beams from both diodes were dispersed through lenses to create parallel line - generated beams , rather than spots . the 532  nm light was constant , and the 635  nm light was pulsed , with frequencies of 15 and 33  hz . the pulsing alternated between frequencies every 30 seconds .
it was identical to the treatment device , except that the laser diodes were replaced with nonfunctioning standard light - emitting diodes .  
initial group assignment was random with occasional adjustment to ensure that the three groups were similar in terms of number of participants , female / male ratio , mean age of participants , and mean pure - tone audiometric thresholds .
the treatment group received the laser treatment protocol ( described in section 2.4 ) using the functional laser device .
the placebo group also received the laser treatment protocol , but using the nonfunctioning laser device .
the control group made similarly timed visits to the laboratory but received no real or feigned  treatment . 
the study used a repeated - measures design , with each subject taking a battery of pretests , followed by treatment ( laser , sham laser , or nothing ) , followed by a battery of posttests .  
the pretest , treatment sessions , and posttest took place in three scheduled visits within a 710-day period for each subject . on the first visit ,
subjects in the control group simply sat in a comfortable chair and had a short conversation with the researcher .
all subjects returned for a second treatment session 2 - 3 days after the first visit .
subjects returned for the third and final treatment 2 - 3 days after the second treatment session . immediately after the third treatment
subjects did not know whether they were in the treatment or placebo groups , and the researchers administering the laser treatments did not know whether they were using the treatment or placebo device .
appropriate laser safety goggles ( laservision , style f12 , filter 000131.000 ) were worn by subjects and the researchers administering the laser treatment .
the goggles had lenses rated od 6 + @ 510680  nm , which blocked all visible laser light .
the goggles also hugged the face firmly , preventing laser light from entering from the side .   to further avoid potential bias in the test results ,
two team members administered the laser therapy independently of two other team members that administered the battery of auditory tests . the members who administered the auditory tests also performed scheduling and group assignments .
groups were simply identified by colors ; so , the testers did not know which treatment ( laser , placebo , or control ) any subject was receiving .
the team members giving the laser therapy did not know which laser device was functioning and which was the placebo . a fifth member of the research team , not otherwise involved in the study , assigned the functioning and placebo laser devices to two of the color groups and was responsible for checking the devices weekly to ensure that the functioning laser device was working .  
the lllt treatment protocol was based on a pilot study conducted by hearingmed ( unpublished ) showing improvement of word recognition scores following lllt .
subjects in the treatment group had the low - level laser applied for approximately 4 minutes to the area around both pinnae , the back of the neck , and the top of the head .
subjects in the placebo group received the same protocol , except that the disabled laser device was used .
subjects in the control group simply sat in a comfortable chair and conversed with the research team member for a few minutes , and no treatment of any kind was administered .
the laser was applied as described in the following steps and as shown in figure 1 . 

step 1the laser was centered on the right temporomandibular joint , just anterior to the external auditory meatus of the ear , at a distance of approximately 2 inches from the surface of the skin .
the hand - held probe was rotated from vertical to horizontal and back continuously for 15 seconds . 
 
the laser was centered on the right temporomandibular joint , just anterior to the external auditory meatus of the ear , at a distance of approximately 2 inches from the surface of the skin .
the hand - held probe was rotated from vertical to horizontal and back continuously for 15 seconds . 

step 2the laser was positioned on midline of cervical spine with the beams running vertically from external occipital protuberance to the seventh cervical vertebrae .
the hand - held probe was held at a distance of approximately 3 inches from the surface of the skin and continuously swept horizontally back and forth for 30 seconds . 
 
the laser was positioned on midline of cervical spine with the beams running vertically from external occipital protuberance to the seventh cervical vertebrae .
the hand - held probe was held at a distance of approximately 3 inches from the surface of the skin and continuously swept horizontally back and forth for 30 seconds . 

step 4the laser was positioned on top of the head with the beams running across the head from ear to ear .
the probe was held at a distance of approximately 2 inches from the surface of the head and continuously swept back and forth from the forehead to the occipital protuberance for 30 seconds . 
 
the laser was positioned on top of the head with the beams running across the head from ear to ear .
the probe was held at a distance of approximately 2 inches from the surface of the head and continuously swept back and forth from the forehead to the occipital protuberance for 30 seconds . 

step 5the laser was centered on the right external auditory meatus , with the probe held at a distance of approximately 2 inches from the surface of the pinna .
the probe was rotated from vertical to horizontal and back continuously for 60 seconds . 
 
the laser was centered on the right external auditory meatus , with the probe held at a distance of approximately 2 inches from the surface of the pinna .
the probe was rotated from vertical to horizontal and back continuously for 60 seconds . 

the probe was held at a distance of approximately 2 inches from the surface of the skin and continuously swept up and down from the occipital protuberance to the top of the shoulders for 15 seconds . 
 
the probe was held at a distance of approximately 2 inches from the surface of the skin and continuously swept up and down from the occipital protuberance to the top of the shoulders for 15 seconds . 

step 7the left external auditory meatus was stimulated , as described in step 5 . 
 
the auditory test battery consisted of three assessments : pure - tone audiometry , speech understanding , and transient - evoked otoacoustic emissions ( teoaes ) .
these tests were chosen to examine different aspects of hearing ; pure - tone audiometry assessed auditory sensitivity in quiet , speech testing assessed speech processing in noise , and otoacoustic emissions assessed the physiological state of the cochlea .
pure - tone thresholds were measured in 5  db steps at six audiometric frequencies ( 0.25 , 0.5 , 1 , 2 , 4 , and 8  khz ) .
audiometry was conducted using custom software written in matlab ( mathworks ) that implemented a method of adjustment psychophysical paradigm , with stimuli presented via er-2 insert earphones ( etymotic research ) .
the pure - tone average ( pta ) was the average of the thresholds at 0.5 , 1 , and 2  khz ( figure 2 ) .
the high - frequency average ( hfa ) was the average of the thresholds at 2 , 4 , and 8  khz ( figure 3 ) .  
the hearing in noise test ( hint )   was used to determine the signal - to - noise ratios ( snrs ) for the subsequent speech testing .
this test is a prerecorded , adaptive measure of sentence speech reception thresholds in noise .
subjects are asked to repeat 20 sentences presented in a background of speech - shaped noise . for this experiment ,
the noise was fixed at a level of 65  dba , and the speech level was adjusted adaptively based on the listener 's responses . the speech presentation level across sentences
5 through 20 was averaged to obtain the level at which the listener achieved a 50% correct performance ( snr50 ) .
the snr50 score obtained during pretesting ( figure 4 ) was used to set the snrs for the connected speech test ( cst ) for each subject , as described below .  
the cst [ 18 , 19 ] provides objective quantification of the intelligibility of connected speech .
each passage contains recordings of 9 or 10 sentences spoken by a female talker of average intelligibility .
scoring is based on the number of key words repeated by the listener out of 25 key words per passage .
the snr used in the present study was the snr50 obtained from the pretested hint , plus 4  db .
transient - evoked otoacoustic emissions ( teoaes ) are physiologic measures of the cochlea 's response to a click - like acoustic stimulus .
a double - evoked paradigm   was used to cancel the stimulus and extract teoaes .
probe clicks were presented at a level of 87  db peak - equivalent spl , and the higher - level suppressor was presented 12  db higher than the probe .
approximately 2000 recordings were made from each ear , and the recordings were averaged after artifact rejection .   
teoaes were filtered into two bands : 1 - 2  khz and 28  khz .
these frequency bands were chosen to correspond with the audiometric analyses and in this paper are referred to as the teoae pta ( 1 - 2  khz ) and the teoae hfa ( 28  khz ) . the distribution of pretest teoae amplitudes is shown for each frequency band in figures 5 and 6 .
it was discovered during data analysis that some of the earlier subjects were tested using probe and suppressor levels that were much lower than intended .
these subjects were therefore omitted from the data shown later and from subsequent analyses involving teoaes .
data were obtained from both ears of each subject . since no obvious differences were seen between left and right ears , data from both ears were combined in the following analyses . strictly speaking ,
this likely violates the statistical assumption of independent sampling , since the test results from left and right ears of a single subject are likely to be highly correlated .
none of the statistical tests used in the analyses are robust to the assumption of independent sampling , and the effect of including both ears is likely to be that of artificially increasing the sample size , making it more likely that a statistically significant result will be found .
changes in the low - frequency pure - tone thresholds ( pta ) were calculated by subtracting each subject 's pretest pta from their posttest pta .
changes in the high - frequency pure - tone thresholds were computed in the same way using the hfa thresholds .
negative values indicated an improvement in thresholds after treatment , and positive values indicated a worsening .
figures 7 and 8 show the distribution of change in thresholds for pta and hfa , respectively .  
analysis of variance was used to test the null hypothesis that the population means of the groups are all equal .
use of anova requires four assumptions : ( 1 ) data are from groups with normally distributed populations ; ( 2 ) data are from groups with equal population variances ; ( 3 ) groups are independent ; ( 4 ) data within groups are independent and randomly sampled .
the test is robust to the first and second assumptions if the number of samples in each group is large and equal or nearly equal .
the sample sizes in this data set ( n = 18 , 20 , and 22 ) were probably large enough and close enough to the same size to meet the first two assumptions . additionally , the sampled data did not suggest high skewness or kurtosis .
the third assumption was assumed to have been met . as discussed at the beginning of section 3
an analysis of variance showed no difference between group means for changes in pta ( f(2,57 ) = 0.09 , p = 0.913 ) or for hfa ( f(2 , 57 ) = 1.33 , p = 0.274 ) .
these results are shown in tabular form in tables 2 and 3 .   before computing changes in cst performance ,
scores were first transformed into rationalized arcsine units ( rau ) [ 19 , 22 ] .
changes in rau scores were calculated by subtracting each subject 's score obtained during the pretest from their score obtained during the posttest .
positive values indicate an improvement in speech understanding after treatment , and negative values indicate a decline .  
analysis of variance was used to test the null hypothesis that the population means of the groups are all equal .
the assumptions required by anova were discussed previously . as they apply to the rau difference data ,
the sample sizes were probably large enough and close enough to the same size to meet the first two assumptions .
however , the sampled data do suggest the possibility that the groups are differently skewed ( sk = 0.471 , 1.40 , 0.55 for the treatment , placebo , and control groups , resp . ) .
a kruskal - wallis test was therefore also performed to compare the medians of the groups .
the kruskal - wallis test technically requires the assumption that the populations of the different groups are identical .
the test is reasonably robust to differences in variability if the sample sizes are equal . while the sample sizes in this data set were not exactly equal , they were close to the same . additionally , the standard deviations , which are reasonable estimates of variability , were reasonably similar ( sd = 13.06 , 16.04 , 10.55 for the treatment , placebo , and control groups , resp . ) .
an analysis of variance showed no difference between group means for changes in rau score ( f(2,57 ) = 2.20 , p = 0.120 ) .
the kruskal - wallis test showed no difference between group medians for changes in rau score ( kw(2 ) = 4.04 , p = 0.133 ) .
changes in the lower - frequency teoae amplitudes were calculated by subtracting each subject 's teoae pta obtained during the pretest from their teoae pta obtained during the posttest .
changes in the higher - frequency teoae amplitudes ( teoae hfas ) were computed in the same way . because teoaes are generated as a byproduct of outer hair cell function , significant positive values
would theoretically be indicative of an improvement in outer hair cell function after treatment , and significant negative values would indicate a worsening .
figures 10 and 11 show the distribution of change in teoae amplitude for the lower and higher frequency bands , respectively .  
analysis of variance was used to test the null hypothesis that the population means of the groups are all equal . regarding the assumptions required by anova ,
the smaller sample sizes of the teoae data set were probably not large enough to make the test robust to the assumption of normality .
the groups in the teoae pta data set ( figure 7 ) appeared to be normally distributed and had equal variances ( skew = 0.67 , 0.06 , 0.5 ; kurtosis = 0.47 , 1.2 , 0.8 ; sd = 2.8 , 2.8 , 3.0 , for the treatment , placebo , and control groups , resp . ) .
as discussed at the beginning of section 3 , the fourth assumption was likely violated .   
there is no theoretical reason to expect the higher - frequency teoae data to be distributed differently from the lower - frequency data ; however , the groups in the teoae hfa data set ( figure 8) were skewed , with the control group being skewed in the opposite direction to the other two groups ( skew = 0.95 , 0.94 , 0.92 , for the treatment , placebo , and control groups , resp . ) .
because of this , a kruskal - wallis test was also performed to test the null hypothesis that the population medians of the groups are all equal , in addition to computing a standard analysis of variance .   
analysis of variance showed no difference between group means for changes in teoae pta ( f(2,31 ) = 0.133 , p = 0.876 ) or for teoae hfa ( f(2,31 ) = 0.20 , p = 0.819 ) .
in addition , the kruskal - wallis test showed no difference between group medians for changes in teoae hfa ( kw(2 ) = 4.04 , p = 0.133 ) .
none of the three measures of hearing ( audiometric thresholds , speech recognition test , or otoacoustic emissions ) showed a statistically significant difference between the treatment , placebo , or control groups .
although a statistically significant difference between the groups might be detected with a larger sample size , such statistical significance may not necessarily be clinically meaningful .
as discussed later , no individuals showed any clinically significant changes on any of the auditory tests .   from a clinical standpoint , a pure - tone threshold change of 10  db or greater
changes of a smaller magnitude are considered to be within normal clinical variability and are not suggestive of any significant alteration in hearing ability . as can be seen in figures 6 and 7 , changes in pta and hfa
were less than 10  db for all subjects in all groups , and the changes were evenly split in the positive and negative directions .
it is worth noting that a similar pattern was seen in individual frequencies : of the 300 audiometric measurements made ( 5 audiometric frequencies ( 0.5 to 8  khz ) , left and right ears , 30 subjects ) , only one measurement showed a change greater than 10  db . a single subject in the treatment group showed an improvement of 25  db at 0.5  khz in one ear .
the most plausible explanation is that the large threshold improvement was artifactual , in that the audiometric testing was always under the control of the subject . to summarize the audiometric test results , in addition to group
means being the same , no individual subjects showed a clinically significant change in thresholds .
when considering clinically significant changes in cst scores , it is necessary to know the critical difference of the scores , expressed in rau .
cox et al .   suggested that critical differences for this test should be derived from the measured within - subject standard deviation .
they reported a 95% critical difference for hearing - impaired subjects of about 15.5  rau .
humes et al .   pointed out that the subjects in the cox et al .
study , in which subjects received no practice with the cst , a 95% critical difference of 32.2  rau was determined .
further , the standard deviation of the rau differences in control group of the present study was 10.55  rau , which is similar to the standard deviation of 11.5  rau found in the humes et al .
examination of the distribution of rau scores for the control group ( figure 9 ) also supported using a 95% critical difference of somewhere between 25 and 30  rau .   from figure 9
, it can be seen that there were three data points that exceeded a critical difference value of 25 .
two of these were in the negative direction ( suggesting a decrease in speech intelligibility ) , and one was in the positive direction . since the critical value represents a 95% confidence interval , it can be expected that in any given sample , approximately 5% of the scores will exceed the critical value simply by chance . in the current sample ,
about 3 scores would be expected to exceed the critical value ( 0.05  60 = 3 ) .
further , these large changes occurred in only one ear of three different subjects , while changes in their other ears did not approach significance ( 5.48 , 11.56 , and 5.49  rau ) .
finally , these large rau changes did not cooccur with significant changes in audiometric thresholds or teoaes . to summarize the cst results
, in addition to group means being the same , no subgroup of individual subjects showed a clinically significant change in speech understanding .
when considering clinically significant change in teoaes , an amplitude change of 6  db or greater might be considered significant given the test - retest reliability in normal populations .
as can be seen in figures 10 and 11 , none of the measurements made a change of at least 6  db in the positive direction , which would indicate significant improvement in teoaes .
as with the other tests , in addition to group means being the same , no subgroup of individual subjects showed a clinically significant change in teoae amplitude . as described in section 1 ,
the current results are further evidence that lllt does not have an effect on hearing status . in the current study ,
care was taken to blind the subjects , the researchers administering the treatment , and the researchers administering auditory testing .
it appears that some previous studies were less careful about controlling for placebo effect and potential researcher bias .
future studies should also implement double - blinding , as well as control and placebo groups .
some previous studies may have achieved much greater laser penetration by using animal models   and isolated cochleae .
the current study , involving external irradiation of human subjects , likely involved less stimulation of structures associated with hearing . while a transmeatal approach to irradiation   would have achieved greater penetration , such an approach represents a less practical delivery method and is not commonly used by holistic practitioners of lllt .
differences in laser wavelength and dosage may also contribute to variable results across studies .    as discussed previously
, several statistical assumptions must hold true in order to report valid statistics ( normality , equal variance , independence , and random sampling ) .
since the current study was intended to be a feasibility study , it was anticipated that by randomly sampling individuals with documented sensorineural hearing loss , some evidence of an intervention effect would be measureable , if any existed . since no effect could be demonstrated across a number of outcomes for any individual subjects , the study was terminated .   in this feasibility study ,
the timeline was fixed as per the pilot data from the manufacturer of the device .
it is possible that the treatment number , treatment protocol , or even the duration of the entire data gathering was insufficient to show an intended effect .   
it is possible , though unlikely , that the laser diode or the total output power varied between subject applications , all of which took place within a 710-day period .
  no statistically significant effect of lllt on auditory function was found , as assessed by pure - tone audiometry , speech understanding , and teoaes .
further research elucidating the anatomic and physiologic bases for therapeutic effects of lllt on hearing are needed before further clinical testing is warranted .
 one purported use of low - level laser therapy ( lllt ) is to promote healing in damaged cells . 
 the effects of lllt on hearing loss and tinnitus have received some study , but results have been equivocal . 
 the purpose of this study was to determine if lllt improved hearing , speech understanding , and/or cochlear function in adults with hearing loss . using a randomized , double - blind , placebo - controlled design , 
 subjects were assigned to a treatment , placebo , or control group . 
 the treatment group was given lllt , which consisted of shining low - level lasers onto the outer ear , head , and neck . 
 each laser treatment lasted approximately five minutes . 
 three treatments were applied within the course of one week . 
 a battery of auditory tests was administered immediately before the first treatment and immediately after the third treatment . 
 the battery consisted of pure - tone audiometry , the connected speech test , and transient - evoked otoacoustic emissions . 
 data were analyzed by comparing pre- and posttest results . 
 no statistically significant differences were found between groups for any of the auditory tests . additionally , no clinically significant differences were found in any individual subjects . 
 this trial is registered with clinicaltrials.gov ( nct01820416 ) . 
1. Introduction
2. Materials and Methods
3. Results
4. Discussion
5. Conclusions