Light therapy, also known as phototherapy, consists of exposure to light for the treatment of health conditions. It has been used in different health-care fields such as neurosciences,1 dental therapy,2 dermatologic conditions,3 psychological disorders as depression,4 brain injuries5 and even, not so many years ago, for the treatment of dry age-related macular degeneration (AMD).6 The type of light applied is usually very varied, ranging from visible, infrared to ultraviolet radiation.1

Several studies have shown that phototherapy achieves therapeutic effects changing its intensity and wavelength.7 Infrared light has also been used in patients who have undergone trauma or developed degenerative diseases.8,9 The scientific literature has shown that this technique may produce beneficial effects for several conditions such as seasonal affective disorders,10-13 sleep disorders14 or psoriasis.15 Several studies suggest that the application of phototherapy on these alterations improves some of the symptoms suffered by the subjects.16,17

Related to vision, phototherapy has been used in amblyopia.18–20 Evans et al.19 have shown an improvement of one line of visual acuity in patients between 10 and 57 years old, using an intermittent photopic stimulation. Their results show an improvement for those who present strabismic amblyopia and anisometropic amblyopia. Other authors as Ivandic et al.20 have used a low frequency laser which irradiates the macula from 1 cm while the subject is in eye adduction position. Their results yield an improvement of three or more lines of visual acuity in 50% of subjects over 12 years old with strabismic and ametropic amblyopia.

Within the field of optometry, Syntonic optometric phototherapy began to be used a few decades ago. According to its followers, this therapy seeks the balance of the visual system associated with a general balance of the organism, through the autonomic nervous system and endocrine system. That is, they seek a balance between the sympathetic and parasympathetic nervous systems.21 It is a procedure that irradiates light in the retina noninvasively, usually incandescent, using selected filters with different wavelength.22 However, in the literature there is a controversy related to this procedure. Barret's literature review23 has shown that this technique has been used as a standalone treatment or as a part of a complete vision therapy program to increase the visual field size and visual memory. However, this literature review23 has also shown that the author could not find any evidence to support any of these assertions. The same has been reported by Suttle's literature review,18 which shows the lack of published studies on the effectiveness of this technique in amblyopia therapy. However, other studies such as Ibrahimi et al.1 have currently reported that the use of this light therapy may produce changes in the metrics of cortical activity in patients with strabismus and amblyopia. Argilés et al.24 have also reported that the use of Syntonic therapy can modify the functional connectivity of a broad range of visual and non-visual brain regions. Accordingly, even though some clinicians use phototherapy as a visual therapy for several visual conditions, there seems to be a lack of evidence about the efficacy of this technique.

The aim of this study is to analyse the scientific evidence about the efficacy of Syntonic phototherapy for producing changes in visual function.

A systematic review was performed to obtain all the clinical articles on the effects of Syntonic phototherapy on vision. It was conducted in accordance with the principles in the Cochrane Handbook.25

The methodological characteristics of the 8 studies included31,32,34–38,42 are shown in Table 2. The table indicates the author, year, characteristics of the sample, dysfunction analysed, study design, type of treatment, results and duration of treatment of each study. As it can be observed, this table shows the information of seven articles. The reason is that there were two studies34,38 which were published in different journals, with different tittles and years but both shared the same information. They had the same data but with different references.

The GRADE evidence profile for the studies through the Soft table31,32,34–38,42 was analysed and its results are shown in Table 3. As can be observed, the studies included in the systematic review represent to 433 subjects, where 244 subjects belong to 5 pseudo experimental studies with equivalent control group and 189 subjects are studied in 3 pre-post pseudo experimental studies. Seven outcomes were analysed: visual symptoms, functional visual fields, visual acuity, contrast sensitivity, deviation (phoria/ tropia), stereopsis and reading abilities. For each outcome, the number of studies, the number of subjects in experimental and control groups, commentaries and the certainty of evidence were included in the Table 3. Results of this analysis showed that although there were different results reported by each study, for all outcomes reviewed all studies yielded very low certainty of evidence.

These results may be observed by means Figs. 1 and 2. Fig. 1 represents the risk of bias summary. It contains the findings about each risk of bias item for each included study, assessed using the Cochrane risk of bias tool. Green, red and blue question mark pictures indicate low, high and unclear risk of bias respectively. Fig. 2 shows graphically these risk of bias as percentages across all included studies. Green, red and yellow pictures indicate low, high and unclear risk of bias respectively.

Fig. 1.

Risk of bias summary. Findings about each risk of bias item for each included study, assessed using the Cochrane risk of bias tool. Green, red and blue question mark pictures indicate low, high and unclear risk of bias respectively.

(0.4MB).

Fig. 2.

Risk of bias graph. Findings about each risk of bias item presented as percentages for included studies. Green, red and yellow pictures indicate low, high and unclear risk of bias respectively.

(0.23MB).

The results of this systematic review show a lack of scientific evidence about the efficacy of Syntonic optometric phototherapy to cause changes in the visual function. Accordingly, there is no scientific evidence to support its use in the treatment of visual anomalies.

The overall analysis of the studies included shows a lack of adequate methodology in clinical studies to provide good certainty evidence. Thus, according to the sample, there is no clinical trial which had analysed Syntonic therapy. Hence, there is no randomized sample and all patients studied are related to selected samples. This reduces the certainty of the evidence found in the systematic review as in no case the results of the treatment may be contrasted with a placebo control. Similarly, regarding the protocol for using the Syntonic technique, the studies also provide different methods. According to the application time, several authors31-33,36,40,41 use the technique during 20 min, other authors42 describe a variable duration and others do not describe the time of application.34,35,37,39 The same happens for the number of sessions needed. Several authors describe between 16 and 18 sessions,31 others 12 sessions,40 20 sessions,32,37 25 sessions,39 10 sessions,42 6 sessions,35 and other authors do not describe the number of sessions used.33,34,41

Furthermore, following the principles of Syntonic therapy21 different colour filters are used to improve certain visual dysfunctions, although without any scientific criteria. However, in most of the studies of this review, the different spectral and illuminance characteristics of the syntonic system are not specified. Only the studies of Ibrahimi et al.36 and Abbas et al.37 specify that they use a bulb that delivers 1.4 lx when unfiltered and only the Kaplan31 study shows the spectral distribution of filters. On the other hand, only 4 studies31,36,37,42 specify the selected filters used. Thus, one study uses the red and blue filters,31 other study only uses the red one,37 another study36 specify that they use 13 different colour filters and the other study42 uses 20 different filters. In addition to all these observations, it has been shown that filters have different transmission spectra when they are used with different lamps in the devices.44 All these aspects further limit the certainty of evidence of the Syntonic phototherapy.

The review also revealed different results reported by each study, according to the changes observed in the different visual outcomes analysed when the Syntonic therapy was used.

According to the visual field, of the 8 pseudo experimental studies of the review, 7 of them31,32,34–36,38,42 analysed this outcome and showed an improvement of visual field size. In some studies,36 this parameter even increased in the control group and in the case of Liberman32 it decreased in the controls. However, the vast majority of them have serious methodological errors that make the certainty of the evidence they provide for this outcome very low. For example, Heinrich42 only examined seven patients. Kaplan31 presents different baseline characteristics between the groups being compared in his study, that is, the experimental group presented writing difficulties, a control group binocular dysfunctions and the second control group presented inefficient reading. Ingersoll et al.34,38 have a different number of subjects between groups. Ibrahimi et al.36 used different coloured filters for the control subjects for their study, so the efficacy of each cannot be evaluated separately. Kondrot35 presented confusion biases in his study, since in addition to the Syntonic he applied 3 other therapies. Even the subjects included had 11 different baseline conditions. Furthermore, another factor that decreases the certainty of the evidence of these 7 articles31,32,34–36,38,42 is that they present very heterogeneous populations, which means that there is indirect evidence (children with low academic performance,32,34,38 children with academic difficulties,31 athletes,42 people with eye diseases35 and patients with strabismus and amblyopia).36

The studies that investigated effects of Syntonic phototherapy on visual acuity (VA) were three pseudo experimental studies.35–37 In the study by Ibrahimi et al.36 VA increased both in subjects with strabismus and in subjects with amblyopia (it did not change in controls). However, it is not analysed whether there were differences between the treatment group and the control group. In addition, for the control group, different types of coloured filters were applied, making it difficult to compare them with the treatment. For all this, the certainty of the evidence is very low. On the other hand, Kondrot35 refers to improvement in VA (VA lines). However, 3 therapies are applied (intravenous vitamins, oxidative therapy and stimulation by electrical currents) plus Syntonic, with which there is a confusion bias and the effect of only the Syntonic cannot be known. In this study, another outcome is also analysed, contrast sensitivity, in which an increase was produced. Lastly, in the study of Abbas et al.37 an improvement was found in VA (they did not specify how many log MAR VA lines improved), both in subjects with anisometropic amblyopia and in subjects with strabismic amblyopia. In addition, they report that subjects with anisometropic amblyopia improved more than those with strabismic amblyopia. However, there is no control group, nor has a placebo effect been controlled. In addition, it is a study with a very small sample. Therefore, the results obtained cannot be generalized to a population, since a larger and more significant sample would be required.

The studies that investigated effects of Syntonic phototherapy on contrast sensitivity were two pseudo experimental studies.35,37 Despite the fact that Kondrot35 refers to increases in this parameter, it cannot be confirmed that these changes are due to the application of Syntonic, since 3 different therapies were used at the same time. On the other hand, Abbas et al.37 refer to increases but does not offer numerical values ​​to be able to quantify how many lines of contrast sensitivity the subjects improved by.

In relation to deviations (tropias/phorias), Ibrahimi et al.36 found that the deviation in strabismus is reduced, although by a very small value and it does not disappear. However, curiously, this parameter increases in the control group. Again, the certainty of the evidence is very low due to the methodological errors that it presents, since in the control group the subjects present phoric values, which could be classified with subjects that present dysfunctions. Therefore, the value of the phorias not only does not decrease but increases.

After applying Syntonic phototherapy, with respect to stereopsis, Ibrahimi et al.36 reported an increase in the treatment group and a decrease in the control group. The certainty of the evidence is very low because the control group does not apply the same filter to all subjects. In addition, when it says that the value of stereopsis increases, it can be seen that the values ​​of the subjects are already within normality, so there is a diagnostic problem, since it is not logical that patients who have tropias present stereopsis.

In relation to visual symptoms, it is noteworthy that any of the eight studies31,32,34–38,42 have reported data about this outcome. However, in all studies the authors usually refer to symptoms and anybody use a symptoms questionnaire to know about them.

Lastly, regarding reading abilities, in the study by Ingersoll et al.34,38 they found that Syntonic diminished reading abilities. However, the certainty of the evidence is again very low. In this case, due to the great difference between groups that the study presents, since the experimental group has 9 subjects and the control 66 subjects.

The results of this systematic review are difficult to compare with other reviews, as, to our knowledge, this is the first systematic review on reports of the efficacy of this method. Other reviews have been published but they are bibliographic reviews without a systematic search of the literature.22,26-28 The Barret bibliographic review,23 although without a systematic search in the study, shows similar results to this systematic review, concluding that there is no scientific evidence to confirm that Syntonic phototherapy applied as visual therapy improves any aspect of visual function.

In conclusion, in this systematic review we found no consistent evidence for the efficacy of Syntonic phototherapy to cause changes in visual function. Accordingly, there is no scientific evidence to support its clinical use for treating any type of visual anomalies. This suggests that optometrists should not recommend its use in the clinical practice as it has been proved that this technique do not offer any improvement in visual function. Future studies in the field of light therapy should therefore clearly report visual outcomes based on studies adequately designed for the purpose. This is necessary not only for professionals as they will be able to provide a validated treatment, but also for patients, as they will benefit from receiving a treatment option based on scientific evidence.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.