Therapeutic effects of photobiomodulation in the speech-language-hearing clinic: an integrative literature review

Purpose: to conduct a literature review on the therapeutic effects of photobiomodulation applicable to speech-language-hearing therapy. Methods : the databases searched were the Cochrane Library, Virtual Health Library, Medical Literature Analysis and Retrieval System Online (MEDLINE) via PubMed, and Web of Science/ISI.The selected articles were original ones whose abstracts were available and that evaluated the therapeutic effect of photobiomodulation in situations related to speech-language-hearing practice. Results: the sample comprised 23 articles, most of them being indexed in PubMed. The fields of health with the largest number of publications were Physical Therapy and Medicine. The sample size ranged from1 to 99 people, aged 15 to 77 years, and the most applied wavelength was the infrared one. Most of the studies had positive photobiomodulation application results – although in a few publications the effects of this treatment modality were assessed in a combination with rehabilitation exercises. Conclusion: photobiomodulation benefits different disorders treated by speech-lan -guage-hearing therapists, however, there is a broad methodological diversity, lacking specific protocols for the ideal dosimetry for each disorder.


INTRODUCTION
Low-level laser (LLL) therapy to modulate cell and tissue physiology can be applied from light sources such as a light-emitting diode (LED) and low-level light amplification by stimulated emission of radiation (laser) 1 .
Photobiomodulation (PBM) is the application of light to a biological system to induce a photochemical process -especially in the mitochondria, stimulating the production of energy as adenosine triphosphate (ATP) 2 . Hence, cell metabolism is increased, producing effects such as analgesia 3 , tissue regeneration, wound scarring 4 , decreased muscle fatigue 5 , and so forth. Besides these, evidence from the application of neuromodulation to different regions of the nervous system points to an increase in brain perfusion, resulting in cognitive and behavioral improvement in neurological diseases such as dementia, traumatic conditions, and Parkinson's disease, besides possible cognitive improvements in healthy people 6 .
Speech-language-hearing therapists are increasingly interested in joining the group of professionals who use PBM, as it is a noninvasive and nontoxic therapeutic resource with promising results. Some studies have recently evaluated its results in speech-languagehearing rehabilitation in the fields ofAudiology and Oral-Motor Function and observed, respectively, reduced tinnitus 7 and improved electrical activity in the orbicularis oris muscle 8 .
Despite the scarcity of studies in speech-languagehearing pathology, the application of PBM in disorders also treated by speech-language-hearing therapists has already been reported in the literature. In Audiology, there are studies on tinnitus 9,10 and hearing loss 11 . In Language, there is research on the use of neuromodulation in healthy people 12 andischemic 13 , neurodegenerative 13,14 , and traumatic diseases 15 reporting improved naming capacity in aphasic patients, as well as improvements in other cognitive skills 16 . In Oral-Motor Function, which concentrates a large part of the research, temporomandibular disorder (TMD) [17][18][19][20][21][22] , nipple fissures [23][24][25] , and facial palsy (FP) [26][27][28][29][30] treatment is highlighted. Lastly, in the field of Voice, a study has been conducted on vocal fatigue 31 .
Even though PBM has been used in the speechlanguage-hearing clinic, there is no robust evidence of its effects as a therapeutic technique, considering clinical aspects such as 1) most effective application (before, during, after exercises); 2) wavelength to be used; 3) application time; and 4) dosimetric parameters. Therefore, speech-language-hearing interventions with PBM application need to be standardized, using structured clinical protocols for the rehabilitation of different clinical conditions.
Given the above, this study aimed to conduct a literature review on the PBM therapeutic effects applicable to speech-language-hearing therapy.

Research strategies
This is an integrative literature review. The studies were selected based on the following steps: development of the research question, search in the literature, and critical analysis of the studies 32 .
The following research question guided the search on which the review was based: What are the therapeutic effects of PBM on disorders treated by speechlanguage-hearing therapists?
The articles were selected from the Cochrane Library, Virtual Health Library (VHL), Medical Literature Analysis and Retrieval System Online (MEDLINE) via PubMed, and Web of Science/ISI databases. The descriptors used were "Audiology", "Speech, Language and Hearing Sciences", "Stomatognathic System", "Low-Level Light Therapy", and "Voice", and their combinationsin English, Portuguese, and Spanish. The other speech-language-hearing specializations, as they are not descriptors, were used in the search as free terms.

Selection criteria
After finding the studies, they were screened by title and abstract. This stage was independently carried out by two speech-language-hearing evaluators. Considering the inclusion and exclusion criteria,23 articles were selected to be read in full.
Original articles whose full text was available, published between 2010 and 2020, assessing the therapeutic effects of PBM in situations applicable to speech-language-hearing therapy were selected. Duplicated articles, found in more than one database, were excluded, as well as those evaluating only analgesia or other PBM effects, not considering functional aspects, studies in animals, and ongoing research, whose final results had not yet been published.
The texts were fully read and analyzed considering year and place of publication, language, first author's professional education, study design, classification of the level of scientific evidence by the Oxford scale 33 , objectives, sample, types of PBM, wavelength, number of sessions, areas of application, energy per point, time of application, and outcomes in each identified theme. The Oxford scale 33 classifies, particularly, the study designs; the evidence is classified in levels, from best to worst, as 1a, 1b, 1c, 2a, 2b, 2c, 3a, 3b, 4, and 5.

LITERATURE REVIEW
Based on the methodology applied, 23 references were selected -10 national and 13 international articles. Regarding language, 21 articles had been published in English (91.3%), one in Portuguese (4.35%), and one in Spanish (4.35%). Brazil led the number of articles, followed by the United States (    Concerning the type of PBM, 16 articles usedlaser (69.6%), six used LED (26.1%), and one (4.3%) did not describe it. The infrared wavelength was applied in 13 pieces of research (56.5%), the red one was applied in five (21.7%), and a combination of both was applied in four publications (17.4%).
The analysis of the publications led to the identification of the following main themes: Therapeutic PBM effects in Audiology, highlighting publications on hearing loss and tinnitus (n=3); in Oral-Motor Function, highlighting the treatment of TMD (n=6), nipple fissures (n=3), and facial palsy (n=5); and in Language (n=5) and Voice (n=1).
The results were described separately, by fields of specialization (Figures 4 to 9).
The sample size ranged from one to 99 people, aged 15 to 77 years. Only one article was conducted with children. No significant difference was found between the groups in any of the hearing tests.
Captions: G -group; J -Joules; J/cm2 -Joules per square centimeters; LLL -low-level laser; LED -light-emitting diode  The combined LLL and exercise treatments presented a better facial recovery than exercises alone.
Captions: TMD -temporomandibular disorder; G -group; J -Joules; J/cm2 -Joules per square centimeters; LLL -low-level laser; BP -Bell's palsy Transcranial laser stimulation is a new, safe, and noninvasive brain PBM method that has benefits such as increased blood flow, decreased edema, neuroprotection, neurogenesis, and an anti-inflammatory effect 5 . The frontal cortex was stimulated in most of the studies, while there was no association between language exercises and neurostimulation in any of them.
One study revealed an improvement in the neuropsychological scores in traumatic brain lesions after transcranial stimulation 15 . There is a growing need for rehabilitation strategies, as the different cognitive rehabilitation techniques and combinations can be useful to treat various cognitive deficits due to traumas in the nervous system 34 .
Research with PBM in cases of ischemic, neurodegenerative, and traumatic diseases show cognitive improvement, probably due to factors such as increased brain blood flow and neurogenesis 5 .
The ideal energy wavelength, duration, dosage, flow, and density for each treatment have not yet been established, and it is not clear how long the

Therapeutic PBM effects in Audiology
In the field of Audiology, PBM has been used in treatments for various decades, mainly to treat tinnitus 9,10 .
In the two studies found, the number of treatment sessions ranged from three to 10 9-11 , and two studies used the red wavelength 9,11 . The areas of application were the external acoustic meatus and tympanic membrane, and the application time ranged from 4 to 20 minutes.
The LLL transmeatal stimulation in 43 patients was not an effective therapeutic means to treat tinnitus 9 . In another research, though, the tinnitus duration decreased significantly in the group treated with laser 10 . Studies are scarce in the national literature, as well as treatment protocols. The discrepancy in results may be due to the small sample and the different types of treatment protocols used. These factors hinder the adequate comparison between the results.
Although one piece of research did not reveal any improvement in hearing, speech comprehension, or cochlear function after applying LLL 11 , there is evidence proving the increase in ATP production after PBM 2which could sustain the theory of beneficial effects in cases of hearing loss.

Therapeutic PBM effects in Language
Five selected studies evaluated the effects of transcranial neurostimulation in both healthy participants 12  The red light proved to be more effective in all the evaluated parameter markers. There were better responses one hour after the treatment.
Captions: G -group; LLL -low-level laser; LED -light-emitting diode Figure 9. Therapeutic effects of photobiomodulation related to the field of Voice

Therapeutic PBM effects in Oral-Motor Function Nipple fissure treatment
In the present study, three selected pieces of research evaluated the PBM effects on the treatment of nipple fissures due to breastfeeding. The number of sessions ranged from 1 to 12, the applied energy ranged from 0.6 to 2 J per point, and the application time ranged from 5 to 79 seconds on the affected region of the breast.
The selected articles aimed to evaluate the scarring and the eased pain when breastfeeding [24][25][26] ; two of them evidenced positive effects when using either LED or laser in the red and infrared wavelengths 23,24 . A study that did not identify any improvement conducted only one session, which may have influenced the results 25 .
Research shows positive PBM effects on scarring and on the reduction of the inflammatory process, even after a surgical procedure 3 . The scarring effect may be due to physiological processes that take place when ATP production increases 2 in the cells, which stimulates mitosis and metabolism, and consequently increases the endothelium cell proliferation, angiogenesis, and speeds tissue reparation.
The literature lacks randomized controlled clinical trials that objectively assess whether the area with nipple fissures has decreased, as well as the ideal dosimetry and time of treatment to achieve the desired results.

Therapeutic PBM effects in Oral-Motor Function Facial palsy treatment
Five selected publications approached the use of PBM on peripheral facial palsy, four of which were cases of Bell's palsy (BP). In idiopathic facial palsy or BP, there may be functional sequelae such as oral incompetence, contractures, dysgeusia, synkinesis, and hemifacial spasms 35 .
Regarding the study designs, there were two case reports 26,27 and three clinical trials [28][29][30] . Only one study was conducted with children 28 . The LLL has been proposed as a painless, noninvasive treatment modality, with no side effects, and with faster clinical improvement 30 .
The infrared laser was the most used, whereas the irradiated points, the dosage, laser contact application time, and the number of sessions varied widely between the studies. The number of stimulated points ranged from eight to 80, the energy density ranged neurostimulation effects last, although the evidence suggests that transcranial stimulation is an important medical tool to treat cognitive deterioration.

TMD treatment
The studies presented various objectives, such as assessment of bite strength 17 , masticatory performance and analgesia 18 , effects of different dosimetry on the treatment of TMD 19 in oral-motor function 20 , comparison of PBM effects, either alone or in combination with manual therapy 21 , and impact on the amplitude of mandibular movements 22 .
In only one study, electromyography was used to assess muscle electrical activity 22 . In four studies, the interventions were applied to experimental and placebo groups [17][18][19]23 . In one, the PBM effects were combined with manual therapy, which did not reveal any increase in therapy effectiveness when the treatment modalities were combined 21 . These data agree with a study that demonstrated eased pain and improved masticatory function and orofacial myofunctional conditions with myofunctional therapy, both alone and in combination with laser therapy 8 . Another research, on the other hand, did not evidence any changes in mandibular movements or electrical activity of the masticatory muscles 22 with laser application. The lack of a standardized protocol may contribute to discrepancies in the findings, as the dosimetry, time of exposure, wavelength, irradiation points, and the number of applications are important factors to determine the use of LLL therapy on damaged tissues.
In the last research carried out by speech-languagehearing pathologists 20 , it was found that speechlanguage-hearing therapy conducted after analgesia with LLL balanced the oral-motor functions in TMD. However, that therapy began one month after finishing LLL therapy, which was not described.
The number of sessions ranged from six to 10, and the most used wavelength was the infrared. Five studies applied laser, and the main application points were the masticatory musculature, temporomandibular joint, and external acoustic meatus [17][18][19][20][21] . The dosimetry ranged from 0.96 to 12.64 J, and the application time per point ranged from 10 to 60 seconds.
Thus, the evidence does not make it possible to establish an application dosage window, and the lack of consensus on the measures limits the conclusions about PBM. from 4 to 20 J/cm 2 , the irradiation time ranged from 10 seconds to 2.7 minutes, and there was an average of 11 to 18 treatment sessions.
A study conducted with 48 patients with BP demonstrated that low-and high-level laser combined with orofacial exercises is more effective in treatment than exercises alone 29 . This corroborates another research with 46 subjects which also presented better results when applying combined treatments 30 . Intervention can be enhanced by applying LLL, as it speeds nerve regeneration acting as a cell stimulant and connective tissue modulator 36 .
A systematic review with meta-analysis evaluated pre-exercise PBM effects on large muscle groups and evidenced that laser therapy is effective to improve the exercise capacity of skeletal muscles 4 .
Two studies did not present full motor recovery 26 or any difference between the experimental and placebo groups 28 . The results may vary when different dosages or application techniques are used -as there is no consensus in the literature concerning dosage, application time, and the irradiation points in peripheral facial palsy procedures 37 .
Even though the studies evaluated facial nerve recovery using orofacial exercises combined with laser therapy, the first author was not a speech-languagehearing therapist in any of them.
Therefore, laser can be used as a complementary therapy for facial palsy recovery. It should be highlighted, though, that the literature lacks great randomized controlled clinical trials that consider the patients' characteristics -such as their skin color and condition, whether it is acute, subacute, or chronic-, besides the ideal stimulation window, in order to prove the effectiveness of this therapeutic resource.

Therapeutic PBM effects in Voice
There is a scarcity of studies on the effectiveness of PBM to treat voice disorders. Only one study assessed the effectiveness of low-level light therapy with LED irradiation to treat vocal fatigue in 16 vocally healthy people 31 .The results showed that the red light proved to be more effective in the acoustic, aerodynamic, and auditory-perceptual measures, with better responses one hour after the procedure. Recent research has demonstrated beneficial PBM effects on muscle fatigue based on the metabolic and photochemical effects, which help increase cell energy 5 . Also, there is evidence of greater proliferation and migration of epithelial cells of the human vocal fold in culture, as well as increased expression of some genes involved in tissue scarring, after PBM application 38 .
Further studies are necessary to establish the ideal light dosages, the best effectiveness of the wavelengths (whether alone or in combination), and the time when to apply the dosages (before, during, or after the vocal technique) to furnish resistance to fatigue, speed recovery, or improve muscle performance.

CONCLUSION
This integrative review identified pieces of research that approached therapeutic PBM effects in situations related to speech-language-hearing practice. There is a concentration of studies on Oral-Motor Function, particularly on TMD treatment. There are also studies in the fields of Audiology, Language, and Voice, although in almost all of them the first author's professional training was in some other field of health, especially Physical Therapy, Medicine, and Dentistry.
The results suggest that PBM benefits different disorders treated by speech-language-hearing therapists. However, considering the diversity of methodologies, scarcity of studies conducted by professionals of the field, lack of specific and standardized protocols for the ideal dosimetry for each disorder, the data in the literature are controversial, with questionable evidence of the application of this resource.
Hence, it is suggested that randomized controlled clinical trials be conducted by speech-languagehearing therapists of different fields of practice, with a detailed description of the dosimetric parameters and speech-language-hearing techniques used in the procedures, to prove the results, better guide their use by professionals of the field, and establish evidence within the Speech-Language-Hearing Sciences.