A 71-year-old female patient with no relevant previous medical history and no family history of neurological diseases presented with a two-year history of postural instability and multiple falls that have progressed to confine her to a wheelchair most of the day. During the physical assessment, muscle strength was found to be normal, but hyperreflexia, predominantly in the lower limbs, was observed. Sensitivity, cranial nerves, and cerebellar function were normal. When testing lower limb mobility, she had difficulty performing tasks such as raising the left toe or tapping her foot, spontaneously noting that her leg did not respond to her commands and produced unintended spontaneous movements, leading to a diagnosis of alien limb syndrome. Over time, the impairment progressed to affect the upper left arm with occasional dystonic posture. An extensive neuropsychological study revealed alterations in executive functions, attentional components, and executive memory. The patient was aware of her deficits and displayed a depressive mood.

Brain and neuroaxis magnetic resonance imaging (MRI) showed no abnormalities. An electroencephalogram (EEG) and electromyography of the lower limbs produced normal results. Functional assessments included a normal 185 MBq FP-CIT (ioflupano) scan (DAT scan) in the early stages of her disease. This test was not repeated. On the other hand, a cerebral 18-Fluorodeoxyglucose PET scan (FDG-PET) revealed right frontal hypoactivity. The FDG-PET was repeated one year later, revealing worsened hypoactivity extending partly to the left hemisphere. Extensive genetic testing was performed and MAPT mutation was discarded along with other genetic causes of dementia or movement disorders. CBD was suspected.

Empirical therapy with levodopa was initiated at the onset due to slight left sided parkinsonism, reaching 900 milligrams per day with limited improvement and subsequent discontinuation without evident deterioration. The patient underwent conventional rehabilitation, including two weekly sessions of physical therapy from the onset of the condition, with limited or no improvement.

This particular patient was chosen for the study because her symptoms meet the criteria for probable CBD, characterized by asymmetric limb akinesia, limb dystonia, limb apraxia, alien limb phenomena, and the absence of MAPT mutations (Armstrong et al., 2013[4]). However, her neuroimaging findings were atypical, yet plausible; the DAT scan was normal, and the PET-CT scan did not show basal ganglia or thalamic impairment, thus not indicating subcortical involvement (Cilia et al., 2011[16]; Pardini et al., 2019[49]). This presents a unique opportunity to explore the potential benefits of therapeutic approaches that aim to modulate cortical excitability.

The neuromodulation protocol presented in this report was specifically designed for this patient with the goal of enhancing right frontal cortical activation due to the findings on neuroimaging functional tests.

The protocol consisted of 20 sessions of tDCS+AO+MI (Monday to Friday) while continued receiving her regular conventional physical therapy two days per week. Non-invasive neuromodulation techniques were combined and always applied “online” (i.e. AO+MI and tDCS were delivered simultaneously in each session).

Each session of tDCS consisted in 20 minutes of a 2 mA direct galvanic excitatory current with 30 seconds of ramp-up and 30 seconds of ramp-down over the primary motor cortex (M1) using a saline-soak pair of surface sponge electrodes (35 cm2). The anode electrode was placed over the right M1 (C3 position according to the EEG 10/20 system), and the cathode electrode over the contralateral supraorbital area (Fp2). A Starstim tDCS^® stimulator (Neuroelectrics, Barcelona, Spain) was used.

As the main complaint of this patient and the initial symptom was lower limb alteration, limiting walking and standing even with normal lower limbs strength, the AO+MI protocol was centered on lower extremities movements, encompassing both static and dynamic sitting, the transition from sitting to standing and vice versa, as well as gait. The intervention was delivered with graded weekly progression as shown in Figure 1(Fig. 1). Monthly progression was achieved as follows: In the first week, the intervention commenced with exclusive AO sessions. These AO sessions involved the patient watching videos of a physiotherapist (A.M.B.) demonstrating various movements and actions. The second week introduced the preparation for movement component alongside the ongoing AO. The patient was instructed to “prepare for the movement” but not to perform it or imagine it. During the third week, MI was incorporated, continuing the AO exposure, and therefore AO+MI was used. The patient was instructed to “try to imagine the movement at the same time you see it”, incorporating both visual and kinesthetic components. The final week integrated movement execution with AO. The patient was instructed to try to execute the movements at a similar pace as it was shown in the videos. Because of motor deficits, the speed of the movements showed in the videos was slightly adapted when the patient could not follow the same pace as the model.

Each week, the intervention was structured to follow a progression from uniplanar/monoarticular movements (see supplementary information; day 1; Supplementary Figure 1a) and multiplanar/multiarticular movements (day 2; Supplementary Figure 1b) while sitting, to transfers (day 3; Supplementary Figure 1c) and postural stability components through weight shifting (day 4; Supplementary Figure 1d), finishing always with gait practice (day 5; Supplementary Figure 1e). Each daily session commenced with a 30-second introduction and instructions, followed by two 10-minute working blocks separated by a 1-minute rest period. The intervention concluded with a 30-second final summary.

Outcome measures were assessed by two trained physical therapists and one neuropsychologist at three different time points: pre-intervention (day 1, measurement A), two days after the intervention was finished (day 32, measurement B) and after one month (day 60, measurement C). Between measurements B and C, the patient only received conventional physical therapy. Three tests were used to evaluate motor capacity as primary outcome measures. Secondary outcomes included functional, neuropsychological, and neurophysiological evaluations.

The 10-Meter Walk Test (10-MWT) was used to assess gait speed using a walker as assistive device. The patient was instructed to walk 10 meters at her normal pace. The 10 meters were divided into three parts by marks on the floor: 2 meters at the beginning, 6 meters in the middle and 2 meters at the end. The time taken to walk through the middle part was recorded to calculate gait speed (meters per second), considering that the first and last parts were for acceleration and deceleration, respectively (Watson, 2002[63]; Steffen and Seney, 2008[56]).

The Berg Balance Scale (BBS) was used to assess balance and risk of falling. The BBS consists of a series of 14 different-balance related tasks designed to simulate everyday activities and range in difficulty from sitting and standing to activities such as reaching and turning around. Higher scores indicate better performance (Berg et al., 1995[7]; Usuda et al., 1998[61]; Blum and Korner-Bitensky, 2008[10]; Downs et al., 2013[20]).

The 5-Times Sit-to-Stand Test (5TSST) was used to evaluated functional strength, as the time needed to stand up fully from the seated position and sit back again five times as fast as possible (Duncan et al., 2011[21]; Muñoz-Bermejo et al., 2021[40]).

Digits and Symbol Search subtests, which belong to the Wechsler Adults Intelligence Scale, fourth edition (WAIS-IV), the Trail Making Test and the STROOP test were utilized.

WAIS-IV's Digit subtests were administered to evaluate working memory and attentional span (phonological loop). Both subtests require the participant to learn and recall number sequences in different orders. The Symbol Search task was used to evaluate visual perception, working memory and processing speed, as it involves identifying a symbol's duplicate from among a list (Wechsler, 2012[64]).

The Trail Making Test (TMT) is a test in which the participant must draw a line, connecting a set of numbered dots in the correct order, as fast as possible. Part A is a number sequence, while part B is an alternating number-letter sequence. They measure processing speed, visual perception, eye-to-hand coordination and mental flexibility, attentional set shifting and inhibition, respectively (Tombaugh, 2004[59]).

The STROOP test is comprised of three parts: word, color, and word-color. The first two parts both measure visual perception and attention, processing speed, but also the visual routes of language in the form of reading and denomination. The third part evaluates the more executive-leaning processes of attention: attentional control, inhibition of an overlearned skill (reading) and interference control (Scarpina and Tagini, 2017[55]).

For neuropsychological tests, scores were normalized with the help of Spanish-validated scales (except WAIS-IV's symbol search). Scales for the TMT, both inverse and direct digits and STROOP test can be found in Peña-Casanova et al. (2009[50]), and scores for the WAIS-IV symbol search test were standardized using the WAIS' built-in scales. Age corrections were applied on every scale.

The Cortical Basal Ganglia Functional Scale (CBFS) was used to evaluate the functional impact of CBD. The CBFS is a self-report questionnaire that consists of 14 questions related to motor aspects in daily living, and 17 questions related to non-motor aspects in daily living. Higher scores indicate more severe impact (Lang et al., 2020[32]).

The Falls Efficacy Scale International (FES-I) questionnaire is a self-report assessment tool used to measure the perceived level of confidence in the ability to avoid falling during various activities of daily life, with lower values suggesting higher self-confidence (Lomas-Vega et al., 2012[36]).

The Short Form Health Survey-36 (SF-36) was used to measure health-related quality-of-life. The SF-36 is a 36-item questionnaire where the person rates, using a Likert type scale, 8 domains of quality of life: physical functioning, role physical, bodily pain, general health, vitality, social functioning, role emotional, and mental health. Higher scores indicate better quality-of-life (Alonso et al., 1995[3]).

The only neurophysiological measure was the Cortical Silent Period (CSP), collected using single-pulse Transcranial Magnetic Stimulation (TMS). Electromyography (EMG) recording coupled to TMS was collected using 9 mm diameter Ag-AgCl surface electrodes placed on the first dorsal interosseus muscle. EMG measurements were amplified 1000 times, filtered with a band pass of 20 Hz - 2.5 kHz using a Digitimer D440-2 amplifier (Digitimer Ltd., UK) and digitized with a CED Micro 1401-3 (Cambridge Electronic Design, UK) (Groppa et al., 2012[26]). All EMG data were pre-processed and analyzed using Signal software, version 6 (Cambridge Electronic Design, UK). For CSP, ten consecutive single-pulse TMS at 130 % RMT were applied during weak voluntary contraction (15-20 % of the participant's maximal voluntary contraction) of the FDI. The MEP-onset until return of voluntary EMG activity time (in milliseconds) was measured as CSP for each hemisphere separately as an average of all 10 measurements (Hupfeld et al., 2020[28]).

This case report did not include appropriately spaced subsequent interventions to compare with, and therefore no formal statistical analyses were implemented. Results are reported as changes from baseline to post-intervention and to follow-up for each outcome measure.

There was a good tolerance to the proposed protocol and the adherence to all the sessions was complete. No adverse effects were reported.

No significant increase in walking speed was observed either after treatment or during the follow-up period. Improvements were observed in balance and functional strength after treatment, as indicated by the BBS (16 to 23 points) and the 5TSTS (83 to 52.89 seconds). Improvements in postural control were partially sustained in the long term (5TSTS: 83 to 67 seconds), while changes in balance did not persist.

Regarding cognitive and neuropsychological variables, a sustained long-term enhancement in attentional set-shifting and inhibitory control (TMT-B: +0.33σ post-treatment, +0.66σ follow-up) was found. Additionally, a persistent improvement in verbal attentional span during the follow-up, relative to baseline, was evidenced (Direct Digits +1σ in the follow-up). A long-lasting post treatment improvement in working memory was observed, characterized by an increase of 1σ in Inverse Digits scores both post-treatment and during the follow-up. Furthermore, there was an enduring long-term rise in color denomination (STROOP-C +0.66σ in the follow-up). STROOP interference measures showed a significant increase in score after treatment.

When comparing pre-treatment and post-treatment evaluations for CBFS, no significant difference was observed in total scores (23 and 22 points), but a negative trend was noticeable during the follow-up evaluation (total score of 30 points).

Emotional well-being and general health dimensions of the SF-36 showed immediate increases after treatment. However, these improvements were not sustained during the follow-up period. The FES-I did not exhibit an immediate increase in scores after treatment, but a noticeable long-term improvement was observed during the follow-up.

In the most affected hemisphere (right hemisphere) the motor evoked response was not obtained after stimulating withing the ranges recommended in the safety recommended guidelines (70 % output). CSP increases from baseline to post intervention.

The main limitation of this study is that it is conceptualized as a single case report, and therefore the evidence provided is preliminary and exploratory. Although our intervention protocol was long enough to generate changes in motor, cognitive or functional outcomes, a longer protocol in terms of number of sessions, or a training with higher intensity in terms of repetitions per session, might have been able to increase effect sizes, in the context of a chronic and progressive disorder. The lack of a second or third period(s) without any treatment or sham treatment prevented from performing any formal statistical analysis, although taking the personal situation of this patient into account, we considered it unethical and inappropriate. Absence of neuroimaging data pre-post intervention impeded further insights into the neurobiological mechanisms of the intervention delivered.

The authors extend their sincere gratitude to the staff at the Texum Clinic in Madrid for diligently maintaining the conventional therapy for the patient involved in this study. The authors would also like to thank Beatriz Madroñero Miguel for her assistance in creating the figures. This work was supported by the Spanish Ministry of Science and Innovation grant (PID2020-113222RBC21/AEI/10.13039/501100011033).

The authors declare that they have no conflict of interest, because the beforementioned institution did not have any role in the study design, data collection and analysis, decision to publish or preparation of the manuscript.