Synchronous digital mental health interventions provide effective clinical outcomes (see Figure 3). Some systematic reviews identified that digital mental health interventions based on CBT (i.e., telephone, internet-based, videoconferencing, online) were equally effective as face-to-face CBT in the treatment of specific mental health conditions (e.g., social anxiety disorder, PTSD, panic, depressive symptoms, body dissatisfaction, insomnia, specific phobias) [30–33, 39, 44, 46, 47]. In addition, the different theoretical models used in CBT-based digital mental health interventions (i.e., classical, Mindfulness, trans-diagnostic, non-specific) and non-specific digital mental health interventions had a moderate to large effect in reducing depressive, anxious and PTSD symptoms, compared to control situations [32, 34–37, 41–43, 45, 46, 48]. Furthermore, different formats of individual and group electronic interventions (i.e., telephone, videoconferencing), and guided self-help treatment had comparable effectiveness in depression and anxiety treatment [32, 42, 43]. In addition, digital interventions have shown to be effective in different population groups, such as adults and elder people [30, 31, 37, 42], veterans [44, 46], and people with multiple sclerosis .
The advantages of interventions using technology are allowing the inclusion of add-ups to the therapy (e.g., written, audio or visual materials to access online or download, diary-keeping, chats , emails [29, 44], online forums [29, 40, 43], new platforms or existing ones, such as Skype or Zoom, etc.) . These interventions also promote better coordination of care and early treatment [39, 46].
Guided synchronous components are essential elements in digital interventions to reduce anxiety. They are more effective and improve adherence significantly compared to unguided interventions or those with only asynchronous components . Also, it is unclear which guided synchronous components are the most effective or whether there are cumulative effects when combining them . Of note, CBT-based and heterogeneous digital mental health interventions (not CBT-based) showed no difference in their effectiveness in reducing PTSD symptoms .
Synchronous digital mental health interventions reach populations otherwise unable to have access through face-to-face interventions, since they do not require the physical presence of the therapist nor the patient, thereby tackling geographic barriers posed by in-person therapy (To expand access).
Synchronous digital interventions in mental health reach populations that would not have access through face-to-face interventions, such as children, veterans, refugees, and people living in rural areas [47, 49], because these interventions do not require the physical presence of both the patient and the therapist (see Figure 3). We also found that these interventions can reduce geographical barriers to access (e.g. mobilization for several hours). In addition, they can interact in real-time , and tackle the geographic barriers of travel required to receive care, thereby being able to access even from remote areas [34, 38, 39, 43–45, 47].
Some aspects need to be taken into consideration for the delivery of successful therapy through synchronous digital mental health interventions. First, to find a quiet area in home or at the usual environment of the patient to receive the session, which could represent a challenge for many . Second, the platform should be as stable as possible since ineffective internet service could lead to withdrawing the therapy , and the quality of the image and sound could be associated with satisfaction . Third, the possibility to expand the use of telepsychiatry will require the development or improvement of a software specially designed for that purpose . Finally, the presence of technical support when needed, as one systematic review found that scheduled guidance showed better outcomes on anxiety symptom severity at post intervention and follow-up .
The presence of synchronous human support seems to improve the delivery of digital mental health sessions, although the evidence is not conclusive [29, 40]. Guided interventions were superior to completely unguided interventions for symptom severity across mental disorders and presented higher treatment adherence . In studies that used local clinics rather than home-based teletherapy, it was recommended to have local staff on hand to assist, such as to receive homework and other materials via fax machine and disseminate them to participants . However, in the future, artificial intelligence could replace human support to generate computer responses .
Additionally, we found some barriers: First, the absence of physical contact. One review identified that patients receiving in-person treatment were more likely to complete the home assessments and tasks given . Second, the safety of the patient could be compromised. It is worth noting a potential issue with interventions using technology. The distance between patient and therapist could put patients’ safety at risk, who could not receive the necessary care in the event of a crisis or emergency . Some studies also suggested the presence of an extra person to provide in-person support in case of emergencies [40, 46], although not all studies showed favorable results [29, 30, 40]. Finally, the presence of technical issues could impose a potentially modifiable barrier. Some flaws found during the therapy delivery were limited connectivity, the lack of human resources and telepsychiatry equipment , low image resolution, difficulties for establishing the connection, slight audio delays, and problems with the internet connection . Moreover, a systematic review assessing mindfulness-based cognitive therapy for stress reduction found that the users’ dissatisfaction was linked to technical issues .
Synchronous digital mental health interventions reach populations otherwise unable to have access via face-to-face interventions because they can be successfully delivered by non-specialists, which makes them more cost-effective to implement in health services (To expand access).
A second reason why these interventions reach populations that otherwise would not have access to face-to-face interventions is that they are an accessible and cost-effective treatment in the short term . This may lead to reductions in mental health costs, at least in depression . It should be noted that CBT-based digital interventions tend to be slightly more expensive compared to usual treatment at baseline. Because their cost-effectiveness improves when considering their positive effect on quality-adjusted life years  and their costs in the long-term, since they require limited interaction between patient and therapist [31, 39].
This higher cost-effectiveness is associated with different components. Regarding phone sessions, they adhere to a more structured format and focus on problem solving and tasks, resulting in more efficient and direct sessions ; and shorter durations than in-person therapies [31, 33]. It should be noted that the session duration of these interventions was not associated with better outcomes in cases of anxiety and depression, although the therapy duration varied from 19 to 150 minutes .
Evidence suggests that physicians, psychiatrists, psychologists, or nurses trained for various mental health problems could perform digital interventions such as telepsychiatry or teleconsultations . This enables optimization for using available human resources when there is a reduced number of specialists for large populations, since non-specialists with adequate training and supervision are as effective as specialists for this purpose [38, 40]. For this outcome, it is important to consider some barriers. A potential barrier was the provision of care by non-specialists, highlighting the importance of having appropriate training and supervision to provide long-distant care. Training for therapists providing interventions using technology should include contents on good clinical practices [36, 49], the use of technology  and telepsychiatry , the management of risk or crises , as well as potential ethical and/or legal conflicts . Second, distrust of the health personnel. One study pointed out that therapists showed greater preference for face-to-face interventions compared to online interventions , while another found that some professionals may be reluctant to apply electronic interventions using telephones to treat mental health problems, arguing that it could harm the interactions with the user . However, evidence suggests that the use of electronic interventions using telephones does not change interaction patterns in consultations (duration, alliance, disclosure, empathy, attention, and participation) .
Some relevant aspects to consider are clinicians’ satisfaction, the lack of training for providers, and ethical challenges. For example, a systematic review of teletherapy for veterans with PTSD found high fidelity to the intervention and good therapist competence, as well high levels of satisfaction among clinicians in terms of their confidence for the delivery of these forms of therapies . However, as mentioned before, proper training is needed for successful delivery [36, 46, 49], and the ethical and legal aspects should be established  .
Synchronous digital mental health interventions are acceptable by patients and show good results in satisfaction, because they require less need of disclosure and provide more privacy, comfortability, and participation, enabling the establishment of rapport with the therapist (User's satisfaction).
Telepsychiatry for PTSD patients shows the advantage of diminishing the risk of stigmatization. Since patients are treated from their own homes and are no longer required to visit a psychiatric facility, they feel more motivated to seek mental health care . One systematic review found that patients presented more active participation at distance-delivered therapies compared to face-to-face interviews. This may be due to the feeling of “safety” that being at a different location from the therapist could produce. They found that neither empathy, attention nor participation diminished when using telephone interventions . Additionally, telephonic interventions offer the patient a potentially immediate, anonymous, and easy to access option . Another author pointed out that patients felt that the therapist could understand them better during face-to-face therapies. However, there were no differences for the ability of the therapist to guide the patients to “open themselves” between modalities . It was reported that the efficacy of interventions was similar across modalities and although the interaction between patient and therapist was lower , the therapeutic alliance was able to be achieved without limitations [39, 44], except for the difficulties at reading corporal language .
Telephone and videocall interventions were usually acceptable and efficient for digital mental health . This was probably because more access to care was allowed for children and adults with comorbid psychiatric and complex medical illnesses in various settings, age spans, and demographic characteristics, including rural areas . However, although there is greater satisfaction on the users’ side (and therefore an improvement in mood state), this does not imply that there are improvements in the quality of life, since recovery (the relief of depressive symptoms) does not necessarily amount to parallel improvements in quality-of-life measures . In addition, it should be considered that those two outcomes do not follow the same recovery rate.
Worth noting, during telephone therapies, the patients could develop an awareness of their own emotional and affective changes by listening to their own voice. Moreover, since there is no difference in the measure of how “closely” the therapist could be listening as usual in face-to-face communication, the patients could easier feel the “connection” with their therapist and enhance disclosure of feelings and emotions . It was found that the use of technology did not influence the therapeutic alliance with their patients [36, 44, 46]. This could be explained since, in this context, the therapist’s validation is not based on non-verbal communication but their listening capacity, their verbal clarity, the tone of voice used by the therapist, and how the patient experiences it . Indeed, telephone therapy could work better for introverted patients because it provides more anonymity, creating a sense of safety [31, 40].
Some aspects to consider include barriers such as awkward silence, concerns about privacy and constraint communication. Some patients had expressed their privacy concerns. For instance, veterans with PTSD mentioned questions about the confidentiality of the video transmissions, and the data they shared during the consultation . In that same review, constraint communication for detecting body language and non-verbal communication by clinicians when doing teletherapy for veterans with PTSD was reported. However, they still could develop rapport . Finally, during communications where there is no video of the patient, as in telephone therapy, silences during the patients’ speech were more challenging to interpret. 
One review noted that only two studies reported to providing ongoing technical support during interventions . In addition, none of the studies included in their review mentioned videoconferencing-specific good practice guidelines, training of facilitators to conduct online psychological interventions, or contingency plans to support remote participants . Also, few studies reported on the frequency of technical problems .
Limitations of digital mental health reported in reviews
Lastly, even though technology interventions have proven to be as effective as in-person and have 2.13 times more probability of achieving an appointment once a month , some limitations should be noted. First, their effectiveness will depend on treatment adherence . Second, there is limited information on whether CBT-based electronic interventions maintain their beneficial effects over time; two systematic reviews did not identify sufficient evidence to support the benefits of this therapy after 3- or 6-months post-treatment for PTSD cases [35, 44]. Third, most of these studies did not use randomization and their sample sizes were small, therefore more research is needed [29, 32–34, 36, 38, 41–43, 45, 46, 48]. Finally, most of the available evidence comes from high-resource countries with integrated health systems and larger research budgets . Hence some results may not be extrapolated to low- or middle-income countries.