Introduction

Alzheimer’s disease (AD) is the most common form of dementia worldwide (50–70%) [1], estimating that dementia will affect 65.7 million people by the year 2030 [2]. Despite advances in the pharmacological treatment of AD, no therapies currently exist that can modify the course of the disease [3]; transcranial magnetic stimulation (TMS) applied in combination with cognitive stimulation (CS) seems a promising approach [4, 5]. TMS is able to induce changes in cortical excitability, increasing brain plasticity and facilitating the recovery and/or reorganisation of affected neural networks in pathologies causing cognitive impairment [6,7,8,9,10].

The most encouraging results for the use of TMS have been obtained after applying high frequency stimulation (at 10–20 Hz) to increase patient’s cortical excitability over the left dorsolateral prefrontal cortex (DLPFC) [11, 12] or bilaterally [13,14,15]. Improvements have been found in general cognitive performance [13, 15], functional and depression scales [13], episodic memory and processing speed [12], and language skills [11, 14]. Newly developed protocols apply stimulation over several brain regions bilaterally, concurrently with CS, during 6 months, finding medium to large effect size improvements (0.4–0.7) in neuropsychological, clinical and functional assessments up to 4.5 months [16, 17]. Similar, but sorter interventions (6 weeks) have been employed also with encouraging results [18,19,20,21].

Despite the aforementioned promising results, to date there has been no randomized controlled trial with AD patients using the intermittent theta burst stimulation (iTBS) protocol. iTBS protocol allows an increase in cortical excitability in a much shorter time than conventional repetitive TMS (3 vs. 30 min) and has been effective, for example, in improving language deficits in Parkinson’s and post-stroke aphasia patients [22, 23].

To determine the target stimulation area, all the previous studies have looked at structural aspects, but none of them used brain functional information. Location based on the functional involvement of cortical areas in relevant cortico-subcortical networks allows a much more specific and individualized treatment approach, which might be the best option in this disease [9, 24, 25]. Finally, the underlying mechanisms explaining the observed improvements (e.g. functional and/or structural brain changes) and the possible influence of genetic factors (e.g. the presence of specific ApoE alleles) have not been explored [26, 27].

Therefore, the main goal of this clinical trial is to study the benefits of individually targeted short TMS protocol combined with CS in AD. We will assess the efficacy of iTBS protocol in the improvement of cognitive, functional and emotional deficits, as well as functional brain connectivity, and explore genetic modulatory factors. We hypothesize that a 2 weeks intervention (10 sessions every working day), stimulating the DLPFC and parietal cortex (PC) of both hemispheres, combined with CS, will be more effective than CS conducted alone.

Main text

We will conduct a randomized, double-blind, parallel clinical trial. The participants will be randomly allocated (1:1:1) to one of the three groups: (1) experimental group: TMS + CS; (2) sham control group: sham TMS + CS; (3) non-TMS control group: CS alone. The reporting of the trial outcomes will comply with the CONSORT guidelines (http://www.consort-statement.org/) for non-pharmacologic treatment [28], and it is registered in ClinicalTrials.gov (https://clinicaltrials.gov/; identifier NCT03121066).

All the necessary means for conducting the trial will be provided both by the Cognitive NeuroLab research group (Universitat Oberta de Catalunya) and by Consorci Sanitari de Terrassa.

Sample

The sample will consist of 54 volunteer patients (18 per group), aged 60–75 years old, with a diagnosis of AD according to the NIA-AA. To ensure the maximum homogeneity of the sample in terms of severity of the symptoms and current health condition that may interfere with the diagnostic, we will apply strict inclusion and exclusion criteria (see Table 1).

Table 1 Inclusion and exclusion criteria
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Participants will be selected from patients attending the Dementia Unit at Consorci Sanitari de Terrassa.

To calculate the sample size we used G*Power software (v 3.1.0.2) [35, 36], assuming a dropout risk of a 20% (so the withdrawal of participants does not undermine the clinical relevance of the results), a type I error probability (α) of 0.05, and a type II error probability or statistical power (1 − β) of 0.8. The effect size for a treatment consisting of CS is medium [37] and, given the recent results [5, 19] we expect TMS to increase the effects of CS alone, then assuming a Cohen’s effect size of at least 0.6. Thus, the total recommended sample size is 45, 15 per arm to which we added 3 more participants per group to cover the 20% dropout risk. The risk of clustering effect [38] is absent in this trial since the centre and the healthcare professionals providing the treatments (TMS and/or CS) will be the same for all patients.

Procedure

The intervention consists of a 2-week treatment during which TMS will be applied for 10 days over four different brain regions (see below). Since both, short single-region interventions and long multiple-region interventions have achieved positive outcomes in terms of cognitive and functional improvements [e.g., 13, 15, 18, 39] we have followed a cost-effectiveness approach proposing a short (2 weeks) multi-region (four brain areas) intervention to maximize the outcomes while reducing the costs. The stimulation protocol will be the iTBS (600 pulses in bursts of 3 pulses applied at 50 Hz administered every 200 ms -5 Hz- with intervals of 2 s of stimulation and 8 s of rest, lasting 3 min and 12 s). The stimulation will be delivered using a Magstim Super Rapid2 device, with a 70 mm, 8-figure coil and neuronavigated using Brainsight™ 2 device. The stimulation intensity will be set at the 80% of the active motor threshold [see safety guidelines, 10, 34].

Stimulation will be delivered over the DLPFC and the PC in both hemispheres (1 day left DLPFC and right PC with a 15 min interval, and the contralateral areas the following day). The specific target areas for stimulation will be determined individually based on the functional connectivity of each area with two subcortical regions related to AD cognitive dysfunction: the fornix and the hippocampus respectively [40,41,42,43]. The seeds used to compute the connectivity analysis, will be 10 mm radius spheres placed bilaterally on the fornix and the hippocampus and adjusted to individual anatomical landmarks. The selection of the specific TMS targets within the DLPFC and PC will be based on its functional connectivity with the seeds. Based on previous literature, the stimulation over the DLPFC is intended to improve participants performance in language tasks and general functioning [11, 13, 14] while stimulation in parietal is intended to improve performance in memory tasks [43, 44].

The placebo condition (sham TMS control group) will be performed using the same stimulation protocol as the active condition over the same areas (bilateral DLPFC and bilateral PC) but with the coil rotated 90° to prevent the magnetic field from inducing electrical activity in the cortex.

Before and after each session, mood and fatigue will be assessed using a visual analogue scale (VAS). At the end of each session, the side effects of TMS will be also assessed.

Along with the TMS treatment, all patients will undergo the CS intervention programme regularly provided by the Consorci Sanitari de Terrassa. The CS is based on Clare and Woods’ definition [45] and Bottino’s et al. model [46], and follows the basic principles of non-pharmacological interventions aiming to improve the quality of life through engagement in significant activities. The program includes 1-h cognitive stimulation and occupational therapy group sessions three times per week (10–12 patients per group). All sessions are conducted by an occupational therapist and supervised by a clinical neuropsychologist who design the CS for each patient. All sessions include reality orientation therapy for 10 min, and training in attention and concentration, memory, language, calculation, gnosias, praxias, or executive function for 50 min [for more details see, 47].

Neuropsychological, functional and emotional aspects will be assessed before and after the intervention (see Table 2 and Fig. 1). A neuropsychologist blinded to the treatment will manage the outcome measurements at all intervention time points.

Table 2 Outcome measures
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Given the need of multiple administrations, we avoided neuropsychological tests with a marked practice effect, and selected, when possible, tests with parallel versions. The length of each assessment session is ~ 2 h.

All participants will undergo an MRI scan before and after their participation in order to: (1) detect the presence of neurological disorders; (2) localize individual cortical targets for TMS based on their functional involvement in cortico-subcortical networks; (3) guide neuronavigated TMS; and (4) assess the functional and structural brain changes after the intervention.

First exploration, lasting ~ 30 min, will consist in a brain volumetric acquisition (3D) and a resting state acquisition. At the end of the study (three days after treatment), resting state will be acquired again to obtain reliable data on the effects of the intervention on brain activity.

The planning of the clinical trial following the SPIRIT guidelines is displayed in Fig. 1.

Fig. 1
figure 1

Trial timeline following SPIRIT recommendations

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Ethical and legal aspects

Patients’ participation will be voluntary after being informed about the objectives of the study and signing an informed consent form. The participants will be free to withdraw from the study at any time. The researchers agree to respect all the established current legislation regarding clinical research (WMA Declaration of Helsinki, 2004; Law 41/2002 on patient autonomy). The Institutional Review Board of the Consorci Sanitari de Terrassa has approved this project.

In accordance with Regulation (EU) 2016/679, on the protection of personal data, any data collected from the participants will be treated with strict confidentiality.

Possible risks, side effects and discomforts

TMS has been used in research for more than 20 years and safety guidelines have been developed [62]. In this study, all the safety recommendations will be followed and a doctor will always be on call during the TMS sessions.

Statistical analyses

Statistical analyses will be performed using SPSS (v. 23). We will perform a descriptive analysis of demographic and clinical variables (age, sex, years of schooling, diagnosis, and comorbid disorder) and multivariate analysis of variance for repeated measures of cognitive, emotional and functional variables included as a measure of efficacy. All statistical tests will be performed using a significance level of 0.05.

For processing and analysis of magnetic resonance images we will use different software packages: FSL (FMRIB Software Library, www.fmrib.ox.ac.uk/fsl), and FreeSurfer (http://surfer.nmr.mgh.harvard.edu).

Discussion

Nowadays, AD is the most common cause of dementia with no known cure. The cognitive decline increase as the disease progresses, and existing therapeutic approaches are not efficient in the improvement of cognitive deficits or functional limitations. TMS seems to be a promising tool for this purpose, given its ability to modulate cortical excitability and neural network activity.

Although research in this field has notably increased in recent years, it is still very scarce and the most effective stimulation parameters in terms of frequency, intensity, localization and length of stimulation, are unknown. Additionally, it is necessary to include functional and structural neuroimaging measurements to reveal the underlying neural mechanisms of the beneficial effects of TMS.

The expected results of this research will contribute to deepening the knowledge of the effectiveness of TMS as a therapeutic approach in AD, one of the most prevalent, disabling and incapacitating diseases nowadays.

Limitations

The main limitation of this study is the heterogeneity of AD patients. The variability in clinical symptoms can hinder the capacity to extract robust findings from clinical trials. To avoid this risk, a wide range of inclusion and exclusion criteria have been established. This strategy also comprises another limitation related to the recruitment process to achieve the required sample size. Thus, the strict exclusion criteria will prolong the recruitment process but it will ensure the detection of clinically meaningful effects.

Finally, another major possible limitation will be the experimental mortality due to the length of the study, which includes two follow-up assessments 1, 3, and 6 months after the intervention.

Trial status

This trial has not started the patient recruitment phase yet since no funding has been obtained to date.