Study design and participants
A randomized, placebo-controlled trial with a parallel group design was performed. Two groups were included in the intervention: an active tDCS group and a placebo tDCS group. The allocation ratio was 1:1.
Patients diagnosed with Alzheimer’s disease were invited to participate in the study via a letter from the Department of Geriatric Medicine at the University Hospital of North Norway, and healthy participants were recruited through a newspaper advertisement. The eligibility criteria were living at home and fulfillment of the research criteria for the likelihood of having Alzheimer’s disease according to the revised criteria of the National Institute of Neurological and Communicative Disorders and Stroke and Alzheimer’s Disease and Related Disorders Association criteria [7]. We followed section 4.2 in these criteria: “Probable Alzheimer’s disease with increased level of certainty.” This determination of eligibility for the study requires evidence of a progressive cognitive decline based on information from informants (relatives) and a cognitive and/or neuropsychological evaluation [7].
We excluded patients who scored <18 on the Mini Mental State Examination (MMSE) [8]. Other exclusion criteria included serious somatic disorders (cancer, chronic obstructive pulmonary disease, and heart failure) or neuropsychiatric disorders (e.g., severe depression and psychosis) that might reduce cognitive abilities. The patients with comorbid cerebral conditions, such as cerebrovascular injuries and/or stroke, brain tumor, or Parkinson’s disease, were not eligible to participate in the study. Patients using cholinesterase inhibitors had to have been using them for at least 3 months before enrolling in the study. A total of 25 patients with Alzheimer’s disease were included in the study.
A total of 22 healthy elderly volunteers, aged 59–83 years, served as controls for the neuropsychological test performance at baseline. None of them had cognitive impairment or other serious diseases. These healthy volunteers were recruited through an advertisement. The control group did not receive any tDCS stimulation. They completed the Hospital Anxiety and Depression Scale [9], a questionnaire used to screen for depression and anxiety.
The neuropsychological test battery used for healthy volunteers and patients with Alzheimer’s disease was identical. The study was executed in a research laboratory at the University of Tromsø Institute of Psychology. The study was ethically approved by the regional committee for medical and health research ethics (2012/1890) and was registered in the ClinicalTrials.gov database with the identifier NCT02518412. All of the patients and healthy control subjects signed a written informed consent form in line with the Declaration of Helsinki before participating in the study. Each patient received a gift card worth 600 NOK (67 EUR, 75 USD) for their participation. Figure 1 contains a flow diagram of the trial.
Outcome measures
The primary outcome measure was verbal memory function. We used a validated and standardized Norwegian version of the California Verbal Learning Test–Second Edition (CVLT-II) to assess three aspects of verbal memory function: immediate recall, delayed recall, and recognition [10]. CVLT-II is normed by age and gender and is widely used to assess patients with Alzheimer’s disease [10]. To reduce test-retest effects, the CVLT-II consists of two parallel versions: the CVLT-II standard and alternate forms, which contain two different and independent word lists. We used the standard form at baseline and the alternative form in the posttest.
The secondary outcome measures included the MMSE, clock-drawing test, and Trail Making Test parts A and B (TMT A and B). The MMSE is a screening tool used for assessing cognitive impairment (e.g., orientation, recall, arithmetic, language, and ability to follow simple instructions) [8]. The clock-drawing test is another screening tool used for detecting cognitive impairment and is also used to assess visuoconstructive ability [11]. The TMT consists of part A and part B. TMT A measures sustained attention, whereas TMT B assesses executive function [12].
To control for general cognitive abilities, we used the Wechsler Abbreviated Scale of Intelligence with the matrix reasoning and vocabulary subtests [13]. To screen for depressive symptoms, we used the Cornell Scale for Depression in Dementia [14], which is a questionnaire completed by an informant (i.e., a relative). A score above 13 indicates depression, which was an exclusion criterion in the present study. We documented progressive decline using the Informant Questionnaire on Cognitive Decline in the Elderly (IQCODE) [15], which was also completed by an informant. To assess for potential confusion during neuropsychological testing, the Confusion Assessment Method [16] was applied by a research assistant. This questionnaire is based on the observation of core symptoms of confusion (e.g., inattention, disorganized thinking, and altered level of consciousness).
Intervention
The intervention was treatment with tDCS using a direct current stimulator (neuroConn, Ilmenau, Germany), which is battery-driven and delivers a direct current. The current intensity was 2 mA, and the stimulation duration was 30 minutes. A pair of 35-cm2 rubber electrodes transferred the direct current. These electrodes were inserted into sponge pads soaked with 10 ml of sterile water. To stimulate the left temporal lobe, the anode (positive electrode) was placed at the T3 position in the 10–20 system for electroencephalographic electrode positioning. The cathode (negative electrode) was placed on the right frontal lobe at the Fp2 position. For the placebo tDCS, the electrode placement and session duration were identical to those for active tDCS. However, in the placebo tDCS, the current was delivered for 30 seconds at the beginning of the stimulation, then the current was turned off automatically.
Randomization and blinding
The patients were assigned to a list with five-digit codes provided by the manufacturer of the tDCS stimulator. Each patient had his or her own code. The codes instructed the stimulator to deliver either placebo or active stimulation. The order of the codes was randomized using the Random.org website (https://www.random.org/). To ensure double-blinding, the list of code assignments was not disclosed during the entire tDCS intervention. The list was decoded when the study was completed to identify the patients in the active and placebo groups. The tDCS stimulator did not display information that could be used to identify the placebo or active stimulation.
Procedure
After their inclusion in the study, the patients and their relatives visited the research laboratory and received information regarding the project. During this meeting, the patient completed an informed consent form. Subsequently, the patient underwent neuropsychological testing (baseline). The neuropsychological assessment lasted for approximately 60 minutes, including several short breaks. After the neuropsychological assessment was completed, the first tDCS stimulation commenced. Each patient underwent six sessions of tDCS or placebo tDCS stimulation for 10 days. Each tDCS stimulation session lasted 30 minutes. An experienced research assistant administered the tDCS stimulation. When the last tDCS stimulation was completed, the patient performed the neuropsychological posttesting and received a gift certificate. Figure 2 gives an overview of the procedure.
Power and statistical analyses
In previous studies in which tDCS was used to stimulate memory functions in patients with Alzheimer’s disease, researchers reported significant results (p < 0.05) with a total of ≤15 patients [3–5] in a within-group design. Thus, we aimed to include a larger sample than those described in previous studies [3–5] to ensure accurate analysis of the effects of the intervention.
We used IBM SPSS version 22 software (IBM, Armonk, NY, USA) to perform the statistical analysis. Because of a violation of the assumption of a normal distribution, a nonparametric Mann-Whitney U test was conducted to compare the placebo tDCS and active tDCS groups at baseline. A nonparametric Kruskal-Wallis test was used to assess the baseline characteristics for all three groups (placebo tDCS, active tDCS, and healthy control subjects at baseline).
For the primary analyses, the data had a normal distribution. However, because of a small sample size and a large variance, we decided to use a nonparametric Mann-Whitney U test for the analysis. With the Mann-Whitney U test, we examined the change from baseline to posttest. The raw scores for the neuropsychological tests (CVLT-II and WASI) were scaled according to standardized norm tables [13, 17]. The significance level was set at p < 0.05.