1. Introduction
Multiple Sclerosis (MS) is an inflammatory demyelinating disease of the Central Nervous System (CNS) [
1]. It is caused by an autoimmune condition which largely leads to the loss of myelin in the white matter of the brain, spinal cord and optic nerves, with the resulting pathological features being diffuse and focal areas of inflammation, demyelination, gliosis, and neuronal injury [
2]. MS is the most common non-traumatic cause of neurological disability in younger adults [
3] and estimates suggest 2.8 million people world-wide are living with the disease [
4]. Given the widespread nature of the lesions within the CNS [
5], MS symptoms can be quite heterogeneous, with patients showing impairment in motor activity, sensory functions, visual functions, cognition, and behaviour. MS disease modifying therapy aims at slowing down the progression of the disease and treating the symptoms, while rehabilitation is primarily targeted towards some degree of recovery of motor and cognitive functions [
3].
Around 40%-65% of individuals with MS suffer from Cognitive Impairment (CI) [
6,
7], with deficits manifesting during the disease, even in patients with probable MS, early MS and clinically isolated syndrome [
8,
9]. The cognitive functions mostly affected are attention, information processing speed, verbal memory, visuospatial skills, and executive functions [
10,
11]. CI impacts social, work, and day-to-day living [
12], and it is related to lower quality of life [
13,
14]. Indeed, People with MS (PwMS) report lower chances for employment, a greater need for personal assistance, lower likelihood to engage in social activities [
7], more difficulties in parenting [
15] and greater impairment in some instrumental skills, such as driving [
16,
17]. Interestingly, the effects of pharmacological interventions are limited in treating cognitive symptoms in MS [
18]. Consequently, there has been a growing focus on neuro-behavioural approaches as a means of managing cognitive dysfunction in individuals with MS [
19]. The purpose of cognitive remediation techniques is mostly to strengthen residual capacities and promote the learning of new strategies, eventually leading to improved cognitive performance [
3].
Behaviourally based cognitive remediation provides many advantages (i.e., non-invasive, no side effects typical of medications), however, the traditional approach requires the patient to travel to the clinic for repeated one-to-one sessions with the clinician for a set period of time, which may last several weeks. This may be a costly approach and not entirely feasible for some patients with MS [
20,
21]. Chiu and colleagues examined the specific barriers to accessing healthcare services in PwMS and found geographical location and transportation to be a frequent issue [
22]. In summary, reported concerns in MS patients included: (a) living in remote and/or rural regions [
23]; (b) suffering from fatigue, which may increase the burden of travel [
23,
24]; (c) inefficiency of existent transit services [
24,
25,
26,
27,
28]; (d) needing to depend on family members or friends for assistance, thus, having to also rely on others' availability when scheduling appointments; (e) having to make appointments during working hours [
23,
24]. Home-based options for cognitive rehabilitation may offer a fundamental tool to overcome some of these issues, while potentially reducing health care costs by limiting in-person visits [
29].
Naturally, telerehabilitation has received much attention in the context of the Covid-19 pandemic, for which many health care services were limited to emergency care to reduce risk of contagion and due to lockdowns in many countries [
30]. This rendered even more evident the need for support services which could allow continuity of health care even when external circumstances may impede in-person medical assistance. Moreover, since the management of symptoms and impairment in MS often requires a comprehensive set of continuous treatments to promote patients’ well-being during the lifespan [
22], it may be valuable to have the possibility to extend medical care to home-based services during the disease course, particularly in the context of prolonged treatment and monitoring of outpatients.
Owing to the continuous technological advancement, new forms of technology-based programmes have simultaneously gained interest as potential tools for rehabilitation in PwMS. Examples of such techniques are robotic training, computerised serious games, virtual reality systems and video games. Technology-based rehabilitation provides many advantages, such as: (a) tasks can be built to closely resemble activities of daily living, (b) frequent repetitive training can be easily implemented, (c) multisensory feedback can be available, (d) training difficulty can be adapted to patient's ongoing performance, (e) training can provide an engaging and motivating environment [
31,
32]. Moreover, many technology-based tools allow for home-based asynchronous rehabilitation, meaning patients can complete the training at home at any time [
33]. This may be particularly advantageous in PwMS who struggle to schedule their appointments due to work and other commitments.
Given the growing interest in tele rehabilitative techniques based on the newest technologies, it remains to be established whether there is solid evidence regarding their efficacy in alleviating cognitive deficits in MS, and whether they can be a feasible option in these patients. Regarding the efficacy of cognitive rehabilitation in MS, different evidence-based reviews exist, including both evidence from studies investigating in-clinic rehabilitation and telerehabilitation [
34,
35,
36,
37,
38]. Conflicting findings are not uncommon and limited evidence seems to be available regarding the efficacy of different rehabilitative techniques in MS. Nonetheless, promising results have also emerged in favour of cognitive remediation in these patients, suggesting more rigorous studies should be implemented to overcome methodological issues of previous research [
35,
36,
37]. The aim of the current narrative review is to present and critically evaluate recent research findings uniquely about home-based digital cognitive rehabilitation in MS, in order to explore its feasibility and efficacy.
4. Discussion
The purpose of this narrative review was to explore the feasibility and efficacy of digital cognitive telerehabilitation in PwMS. Thirteen studies were presented, offering an understanding on the subject of interest. Regarding the feasibility of cognitive telerehabilitation, researchers commonly indicate adequate rates of adherence to at-home rehabilitation protocols [
32,
41,
43,
44,
45,
46,
47,
49]. However, it is worth noting that adherence metrics may vary, with most studies primarily providing the proportion of participants who adhered to a certain percentage of scheduled training [
41,
44,
45,
47,
49], lacking more detailed insights into the specific duration (minutes/hours) of completed training in comparison to the intended training duration. Only one study reported the number of days in which participants performed the intervention [
43] and only one study reported the percentage of completed training vs. the percentage of total training [
46]. Five studies failed to report a measure of adherence in their result section [
6,
39,
40,
48,
50]. Telerehabilitation interventions may depend heavily on individuals' adherence to the prescribed treatment, thus, for instance, only displaying the dropout rate of participants [
48] may not be sufficient. Given that many factors can influence compliance, such as degree of satisfaction with the tele-protocol [
53], vacation, technological issues, health issues, occupational issues, etc. [
32], researchers should monitor session attendance, completion of assigned tasks and actual use of the provided digital tools. Improvements could also be made in reporting the number of participants who successfully complete the training, and further research is warranted to determine the threshold of completed training that signifies satisfactory compliance with the study procedures. Some existent software (e.g., RehaCom) are able to provide accurate measures of treatment adherence offering the actual minutes of completed training vs. total minutes of programmed training for each session. Such tools should be exploited to gain more accurate insights into the feasibility of home-based interventions. Moreover, assessing reasons for non-adherence or dropouts, if possible, may also provide valuable information regarding the practicality and acceptability of telerehabilitation in real-world settings. Surely, it is crucial to report objective measures of adherence to minimise reliance solely on self-reports provided by patients and caregivers, as these have been reported to overestimate adherence in the context of pharmacological treatment [
54] and at-home exercise therapy [
55].
Regarding the efficacy of the different rehabilitative techniques in improving cognitive performance in MS, there is more inconsistency in the findings. Variable results may be attributed, to some extent, to a wide range of methodological shortcomings across different studies. For example, many studies lacked double-blinding or failed to report whether the study was double-blind, single-blind or open label [
39,
40,
41,
43,
45,
46,
49,
50]. Lack of double-blinding remains an issue as it may lead to increased cognitive bias and unreliability of findings [
56]. Lack of double-blinding is often a direct consequence of inadequate control groups. This is actually the case for various studies which did not involve active sham conditions and/or adopted wait-list control groups [
39,
40,
41,
43,
49,
50]. This is a concern, as lack of blinding may lead to an effect for the treatment group which is not a true effect per se but arises from different group expectations [
57]. Lack of controlled randomisation, which is the gold standard in science [
58] is also an issue, as a few studies failed to implement controlled randomisation [
39,
40,
41]. Moreover, most studies lacked a follow up assessment, rendering impossible to know whether any eventual benefits of training were maintained over time [
32,
39,
40,
41,
43,
44,
47,
48,
49]. On the positive note, a few studies tried to control for practice effects using alternate forms of neuropsychological testing, when possible [
6,
32,
43,
44], and adopting counterbalancing when administering the neuropsychological assessment [
44]. It should be noted, however, that some studies have demonstrated how alternate test version may not always be strictly equivalent [
59,
60]. Therefore, guidelines for the use of alternate test version should be established, in order to further reduce heterogeneity across studies. Interestingly, one study tried to control for practice effects by subtracting the difference of mean scores at the neuropsychological tests in the control group (no active sham condition) with the difference of mean scores in the intervention group [
49]. Furthermore, a few studies [
32,
39,
46,
47], also reported an intent-to-treat analysis which may be useful to help preserve randomisation, realistic evaluation of an intervention, and minimise the risk of biases due to non-compliance and dropouts [
61]. Two studies also provided a Reliable Change Index (RCI) analysis [
44,
49], which is optimal as RCI is a statistical measure that helps determine whether there was a significant change in score at the individual score level on a particular assessment test. This is useful to determine whether the observed effect is actually a real change or whether it is due to random variability or measurement error [
62].
As an additional limitation, studies on cognitive telerehabilitation in MS tend to have relatively small samples, which may reduce statistical power, robustness, and generalisability of the results [
63]. Furthermore, there is the issue of heterogeneity among PwMS, as they may exhibit distinct cognitive profiles depending on the location of lesions within the CNS [
10]. Consequently, grouping patients together based on diagnostic labels, could potentially overlook the variations in cognitive symptomatology. This challenge in MS research urges researchers to devise ideal strategies to effectively account for disease heterogeneity. Undoubtedly, controlling for baseline characteristics and cognitive profiles of participants is of utmost importance. A common approach is to ensure that the intervention and control groups have similar baseline characteristics through stratified controlled randomisation. If any discrepancies arise between the two groups, appropriate adjustment analysis should be employed to account for these differences [
64]. Of greater significance, future studies should strive to overcome recruitment challenges by aiming to increase sample sizes. Multicentre studies will be key in this respect. This expansion would enable grouping analysis, which can aid in identifying disease subtypes based on baseline characteristics [
65]. Indeed, a multicentric study on the effectiveness of cognitive rehabilitation and exercise in the clinic (N = 284) was recently published [
66]. Similar efforts should be encouraged in the framework of telerehabilitation, and indeed one could argue that telerehabilitation may afford a higher degree of standardisation across centres, thus facilitating multicentric and decentralised trials. Ultimately, such identification may facilitate predictions regarding which group of patients may be more likely to respond positively to a certain treatment. Similar work has been done to explore whether medications are more effective in some MS patients compared to others [
67]. Moreover, cluster analysis can provide support in this context, as it provides a data-driven approach to classify patients into homogeneous groups based on specific characteristics, allowing distinct patterns to be identified. This approach facilitates individualised adaptation of cognitive rehabilitation interventions to meet individual needs. While cluster analysis has already been successfully applied in some neurological disorders within the context of cognitive rehabilitation [
68,
69], there is still room for significant advancement in the field of MS and in the application of digital technologies. The possibility to extend this methodology to the cognitive field may help shed more light into the effectiveness of cognitive rehabilitation. In this regard, a multicentre cross-sectional study applied latent profile analysis to cognitive tests to identify cognitive phenotypes (N = 1212). Cognitive phenotypes can represent a more meaningful measure of the cognitive status of PwMS and can help tailor cognitive rehabilitation strategies [
70]. Another approach, instead, may be personalised rehabilitative intervention based on individual deficits emerged by the neuropsychological assessment. By identifying and targeting specific deficits that a person has, rehabilitation efforts can be tailored to their unique needs, hypothetically maximising the potential for improvement. One of the studies included in this review looked at personalised treatment regimens with the use of an app and found some improvements with moderate to large effects [
41]. More research will be needed to also explore the possibility of customised treatment.
The variability in findings within the field of tele-rehabilitative methods could also be partly attributed to the heterogeneity of these techniques as well as the heterogeneity of inclusion criteria employed in studies. For instance, some studies recruited participants based on self-reported cognitive deficits [
32,
40,
44] rather than objectively measured cognitive impairment using neuropsychological tests, or individuals with intact cognitive performance [
43]. While this approach may be driven by the primary focus on assessing the feasibility of a specific protocol, it becomes crucial, particularly when presenting the efficacy of a telerehabilitation intervention, to recognise that individuals with cognitive impairment could potentially respond distinctively compared to those with intact cognitive abilities. Interestingly, when Hancock and colleagues specifically examined subjects with cognitive impairment on the Symbol-Digit Modalities Test (SDMT), excluding cognitively intact individuals, the treatment effect became statistically non-significant [
44]. This may suggest that cognitively impaired individuals may respond less to cognitive training compared to those who are cognitively intact, possibly due to lower brain reserve and/or cognitive reserve. In this regard, however, there are conflicting results. The study performed by Whitlock and colleagues on a sample of 39 older adults aged 60–77 suggests that older adults with lower cognitive functioning may stand to benefit more from cognitive training [
71]. Nevertheless, not screening for cognitive impairment may also pose an ethical concern as it may result in 'treating' cognitively intact participants who might still benefit from cognitive boosting interventions but with limited clinical impact. Emphasising the prioritisation of enhancing clinical relevance in research is crucial, considering the importance of generalising and translating findings to real-world situations. Indeed, it is crucial to examine whether study results have a substantial impact that holds significance for patients in their daily lives. While self-report questionnaires can serve as an initial measurement of perceived improvements in functioning, exploring methods to objectively assess meaningful functional changes is an intriguing avenue to explore.
Selection of the appropriate outcome measures can also be challenging. A measure should be sensitive to the cognitive domain of interest and relevant for the intervention being evaluated. For instance, Campbell and colleagues used the subtests from the Brief International Cognitive Assessment for MS (BICAMS) to measure cognitive improvement after rehabilitation with RehaCom software modules targeting working memory, visuospatial memory and divided attention [
45]. BICAMS subtests are the SDMT (information processing speed and sustained attention), the California Verbal Learning Test-II (CVLT-II; verbal learning and memory) and the Brief Visuospatial Memory Test Revised (BVMT-R; visuospatial learning and memory) [
72]. Although working memory refers to a set of cognitive systems that are considered essential for retaining and manipulating information while engaging in complex tasks such as reasoning, comprehension, and learning [
73], thus it is likely involved in various neuropsychological tests, it is usually measured with tests that require the active maintenance and manipulation of information [
74]. As a result, perhaps, it would have been interesting to also include measures more sensitive than BICAMS to a possible improvement in working memory following rehabilitation. Similarly, for instance, it would have also been interesting to include a verbal memory module as cognitive training. Indeed, no significant improvement in BICAMS test CVLT-II was reported after cognitive training. Since no verbal memory training was involved in the intervention, this finding is perhaps not surprising. Interestingly, the authors showed that there was a functional imaging difference between control and intervention groups, with the intervention group showing more activation in prefrontal cortex and right temporoparietal regions in response to working memory tasks (in functional Magnetic Resonance Imaging - fMRI), which, however, was not reflected in the scores obtained by the BICAMS tests. Again, this is suggestive that BICAMS neuropsychological subtests may not be sensitive enough to working memory changes [
45]. This particular case serves as an illustrative example highlighting the challenge of selecting relevant outcome measures. When evaluating the cognitive gains resulting from an intervention, it becomes crucial to employ sensitive measures of change. Without such appropriate measures, the interpretation of cognitive improvements can be challenging. Some researchers, for instance, also reported a composite score of cognition derived from an average of the different neuropsychological tests used [
32,
47]. Combining multiple measures into a composite score can provide a more comprehensive assessment of cognitive function and it may increase the statistical power to detect treatment effects when interventions are designed to target multiple cognitive domains. Conversely, this approach could lead to loss of statistical power when assessing the efficacy of domain-specific interventions. Therefore, it remains important to carefully select outcome measures and apply appropriate statistical analyses to create and interpret composite scores.
A further challenge when designing telerehabilitation experiments lies in the decision of treatment intensity and duration. Only one study included in the review compared different training schedules, with no significant differences in pre-post intervention scores at neuropsychological testing, concluding that the effects of training were independent of training intensity. The only difference found was in the CORSI block backwards, for which there was an improvement only in the distributed training group [
40]. When qualitatively inspecting the apparent relationship between training results and the overall duration of training, it remains unclear what the optimal intensity and duration of training are [
45]. Moreover, in some research scenarios, it is possible that the duration of the intervention may have been too short to allow significant results to emerge in the cognitive measures under investigation. Certainly, further investigations are required to provide clearer insights into the ideal schedule of training for optimal efficacy.
In conclusion, it is imperative to address the methodological concerns discussed above to uphold the validity, reliability, and generalizability of research findings related to cognitive telerehabilitation in MS. Most notably, it is essential to a) pursue standardisation of intervention protocols in cognitive telerehabilitation to minimise study variability and facilitate comparison of results; b) favour the adoption of double-blind randomised controlled trials to achieve a high degree of reliability in assessments, and encourage the use of objective and correct cognitive screening measures; c) broaden and stratify the study sample by promoting multicentre studies to ensure greater representativeness of the study population; and (d) implement long-term follow-up to adequately evaluate the effectiveness of the intervention over time. The adoption of these proposed guidelines will be able to contribute significantly to the improvement of quality of scientific research in the context of cognitive telerehabilitation. Furthermore, ongoing and relentless technological research will enable the development of ever more cutting-edge digital solutions for cognitive telerehabilitation in MS, ensuring highly personalised and more accessible advanced treatment options to effectively manage cognitive challenges and improve quality of life.