Study description

A total of 957 related studies were identified, with 65 duplicates removed, leaving 892 studies for title and abstract screening. Following title and abstract screening, 48 studies remained and full text articles of those studies were screened for eligibility. Out of these 48 studies, 28 were discarded as 11 studies were found to have the wrong study design (three review articles, one book chapter, six methodological or guideline papers and one protocol paper) and one had the wrong study population. The other 16 studies were considered duplicates as they were conference abstracts related to full text studies already included in the review. Thus, in total, this systematic review included 20 studies. One additional study [37] was excluded from this review as it was identified through communication with the author that the data came from a trial that was already included within the review [38]. As B Dobbs, N Pawlak, M Biagioni, S Agarwal, M Shaw, G Pilloni, M Bikson, A Datta and L Charvet [38] reported all outcomes of interest to this review, it was decided that data would be extracted from this study. As a result, there were 19 studies identified with data from unique trials (Fig. 1).

Description of included studies

All 19 studies are summarised in Table 1. Although the inclusion criteria for this review included several variations of tES, all identified studies used tDCS as the form of brain stimulation, with none using tACS or tRNS. The studies were relatively recent, all being published within the past 5 years (2013–2018), while 84% were published within the last 2 years (2016–2018). Of the included studies, five were identified as randomised controlled trials [39,40,41,42,43], two randomised cross-over trials [44, 45], two observational studies [46, 47], ten case series [38, 48,49,50,51,52,53,54,55,56].

Table 1 Descriptions of studies based on sample populations and stimulation parameters Full size table

Risk of bias in included studies

Review of internal validity using the Cochrane risk of bias assessment tool in seven domains is summarised in Fig. 2. In keeping with the low level of research design, which likely reflects the preliminary nature of current work, the majority of studies had a high risk of bias. Few studies (26.3%) demonstrated low risk for random sequence generation [39,40,41, 43, 45], allocation concealment (15.8%) [40, 41, 45], blinding of personnel and participants (42.1%) [39,40,41,42,43,44,45,46] and blinding of outcome assessments (26.3%) [40, 41, 43,44,45]. However, a large proportion of studies (94.7%) demonstrated low risk of bias for incomplete outcome data, with only one study identified as high risk of bias due to an increased number of participants who withdrew from the study [44]. Selective reporting was generally (68.4%) identified as an unclear level of bias [40, 42,43,44, 46,47,48,49, 51,52,53,54,55], with two studies identified as high risk of bias [38, 39], and one as low risk of bias [41] in this category. One study was identified as high risk of bias under the domain of other sources of bias due to substantial variation in duration and intensity of stimulation based on response to treatment [52]. However, we note that the overall purpose of this study was different compared to all other included studies as it was a maintenance program for symptoms of schizophrenia.

Fig. 2 Cochrane risk of bias tool was used to assess quality of included studies Full size image

Participants’ characteristics and stimulation protocols

There were various patient populations included in this review for in-home tES (Table 1). Four studies were performed with people who had Multiple Sclerosis (MS) [40, 47, 48, 54], two with Parkinson’s disease (PD) [38, 43] and two with stroke [41, 56]. Other populations included tinnitus [46], dementia [42], minimally conscious state [45], Mal de Debarquement syndrome [39], trigeminal neuralgia [44], neuropathic pain [55], depression [49], multimodal hallucinatory perceptions [53], schizophrenia [52], various neurological pain conditions [51] and a case series of four chronically ill patients which included myasthenia gravis, depression, chronic pain and stroke [50].

In-home tES treatment was provided over a wide range of different durations from 4 [42] to 400 sessions [53], with the most common approach to apply in-home tES for 10–20 sessions [38,39,40, 43,44,45,46,47,48,49,50,51]. One study did not report the number of treatment sessions which ranged from once to twice daily over a period of 3 years [52]. The duration for each treatment was 20–30 min for all included studies, with the majority applying stimulation at 1-2 mA. Only one study exceeded 2 mA, with stimulation intensity increased up to 3 mA to control symptoms of Schizophrenia [52].

Approaches to achieve optimal treatment fidelity for in-home brain stimulation

Across the 19 included studies, there were a range of strategies used both prior to, and during, the treatment period to implement in-home tES (Table 2). The most common approach was to conduct training sessions prior to beginning in-home tES. This frequently included practicing the placement and positioning of electrodes on the scalp, sponge preparation, starting the stimulator, troubleshooting common problems and provision of training videos [39, 46, 53]. Furthermore, several studies extended the training sessions to include a caregiver, or support person, who was able to assist during the home treatment phase. For some studies, the assistance of a caregiver, or support person, was a requirement for all participants [44, 48, 50, 55], and others specifying it only for those participants with higher disability [38, 40, 47].

Table 2 Strategies identified to implement in-home tES Full size table

During delivery of in-home tES, several monitoring approaches were identified as strategies to achieve optimal treatment fidelity (Table 3). The most common treatment monitoring approach was to use videoconferencing to observe, in real time, the in-home treatment being conducted by the patients or their caregivers [38,39,40, 43, 47,48,49,50, 54,55,56]. This provided researchers opportunity to visualise tES set-up, correct electrode placement and troubleshoot issues that arose. Monitoring in-home stimulation in this manner may also assist with compliance with the treatment protocol. Five studies which utilised videoconferencing as a method to monitor in-home tES also used a remote desktop access approach for each treatment [38, 43, 47, 48, 50, 56]. This allowed research staff to have strict dose control for the delivery of tES and remotely solve any technology-based issues that arose. Four studies used passive (not in real time) monitoring approaches which included recording use of tES through websites such as Survey Monkey [39] or with self-report treatment diaries [44,45,46].

Table 3 Monitoring approaches and protocol compliance of the studies Full size table

Protocol compliance

Table 3 shows that most studies reported a high level of compliance with the in-home tES treatment program which was defined as the percentage of correctly completed stimulation sessions relative to the total number of intended sessions. Seven studies had 100% compliance, with three additional studies having 95% compliance or greater and a further three studies reporting 90–95% compliance. These results suggest that it is possible to implement an in-home tES study and obtain an excellent level of compliance. An observation from this review is that studies which provided regular and repeated real-time videoconferencing to monitor each in-home treatment session achieved compliance levels of 93% or greater [38,39,40, 43, 47, 48, 50, 54,55,56]. One study delivered in-home tES with research staff attending a participant’s home as opposed to the study participant performing stimulation independently, achieving 100% compliance [41].

The studies which did not use real-time videoconferencing to monitor each in-home treatment session reported different approaches that achieved comparatively lower compliance levels (see Table 3). These strategies included a single real-time videoconference call for the first in-home treatment session only, daily phone calls and emails and self-reported treatment diaries [44,45,46, 49]. Although there may be some indication that strategies to monitor in-home tES may influence protocol compliance, there are likely to be additional factors which contribute to this outcome. For example, it is important to acknowledge that protocol compliance may be affected by the duration of the experiment with high levels of compliance likely to be more difficult to achieve with more home stimulation sessions. Within this review, those studies with relatively higher levels of protocol compliance generally conducted 5–20 sessions, while those with relatively lower levels of compliance conduced 10–20 sessions.

As opposed to monitoring strategies, where videoconferencing appears to be a factor that may enable high levels of protocol compliance, there did not appear to be any association between a particular strategy to implement in-home tES and protocol compliance (Table 2). For example, some of the studies which reported relatively high levels of compliance used various strategies to implement in-home tES such as training sessions for participants and caregivers [38, 43, 47, 48, 50, 55, 56], customised headbands [38,39,40, 43, 47, 48, 50, 56] and remote computer access [38, 43, 47, 48, 50, 56], while others did not report any strategies [54]. However, it may be that use of multiple strategies is best. Of the studies which used three or more strategies to prepare the participant and deliver the in-home tES program, reported compliance levels were between 93 and 100% [38, 40, 43, 47, 48, 50, 56]. For studies which used two strategies or less, compliance levels appeared more variable and were as low as 76% [44].

Adverse events

Eighteen studies reported outcomes for adverse events (Table 4). The most common event was tingling sensations during the stimulation, which was reported in 12 studies [38,39,40,41, 43, 44, 46, 48, 52, 54,55,56]. Other common adverse events were itching or skin irritation, burning sensation, head pain, difficulties in concentrating, blurred vision, facial muscle twitching and changes in mood. There did not appear to be any association between adverse events and strategies to implement or monitor in-home tES. One study reported occurrence of a seizure, however the authors suggest this was not associated with stimulation as the participant was receiving a sham condition (no stimulation) and had a history of epilepsy [45]. Excluding the occurrence of this seizure which did not appear to be associated with delivery of tDCS, none of the reported adverse events would be considered severe or requiring medical attention.

Table 4 Reported adverse events for in-home tES Full size table

Participants’ satisfaction

Participants’ satisfaction towards the treatment was only reported by six studies [39, 46, 48, 50, 55, 56]. For five studies where real-time monitoring was provided, but a range of different strategies were employed to prepare participants for home stimulation, authors reported that participants generally had positive experiences with using in-home tES [39, 48, 50, 55, 56]. These studies were performed in various clinical populations that included neuropathic pain, Mal de Debarquement Syndrome, stroke, depression, myasthenia gravis, chronic pain and Multiple Sclerosis. Themes that emerged included users reporting that there were no difficulties in the treatment set-up, being comfortable with the stimulation device, being satisfied with the overall experience and expressing a desire to continue this home-based treatment after the study [48, 50, 55, 56] with purchase of their own stimulator [39]. However, some participants with Mal de Debarquement Syndrome, a condition characterised by feelings of rocking or swaying after exposure to motion, reported feeling uncomfortable applying the stimulation independently and reported frustration setting up the device and achieving appropriate levels of impedance to start stimulation [39]. The fourth study reported the perspectives of people with tinnitus who used in-home tES and were provided with self-reported treatment diaries to monitor stimulation [46]. The authors reported six out of 35 participants felt it was difficult to apply stimulation despite being providing with a one-day training session and instruction notes.

Registered clinical trials

The search of trial registries (clinicaltrials.gov, anzctr.org.au and who.int/ictrp/en/) identified ten clinical studies which are currently ongoing or completed, but not yet published (Table 5). These studies implemented in-home tES in various neurological and psychiatric conditions. Four studies stated that videoconferencing (or telemedicine) will form part of the monitoring strategy to implement in-home tES. Few studies have identified that they will be reporting on the occurrence of adverse events, protocol compliance or patients’ perspective. However, it is worth noting that limited information is required to be provided in clinical trial registries.