Remote Ischemic Conditioning for Non-Proliferative Diabetic Retinopathy (RIC-NPDR): study protocol for a randomized controlled trial

Abstract

Background

Diabetic retinopathy (DR) is a leading cause of blindness in diabetic patients. However, effective treatments for non-proliferative DR (NPDR) are limited, especially in the early stage. Remote ischemic conditioning (RIC) has shown great promise in protecting retinal tissues from ischemic damage, but its therapeutic potential in NPDR remains unclear. This study aims to evaluate the safety and efficacy of RIC in patients with mild to moderate NPDR and to provide preliminary data for the design of future multicenter clinical trials.

Methods

A single-center, randomized, controlled trial was conducted to assess the impact of RIC on patients with mild to moderate NPDR. Participants were randomly assigned to either the RIC group or the sham group. The primary outcome is the change in DR Severity Score from baseline to 1 year after RIC treatment. Secondary outcomes included incidence of PDR, changes in retinal vascular parameters, inflammatory markers, and visual function. Safety assessments included adverse events and any RIC-related complications.

Discussions

This study aims to conduct a pilot clinical randomized controlled trial to evaluate the safety and efficacy of RIC in treating NPDR. The findings are intended to provide preliminary data on the potential of RIC as an early intervention for NPDR, which may inform the design of future larger-scale clinical studies.

Trial registration

Clinicaltrials.gov, identifier: NCT06713720, Registration Date: 2024.11.21.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-026-05340-3.

Background

In recent years, the incidence and prevalence of diabetes mellitus have gradually increased yearly, and it has become one of the major diseases that endangers human health. According to the 2021 international diabetes federation statistics, the number of diabetic patients in China is more than 140 million, ranking first in the world [1]. A meta-analysis that integrated 35 global studies from 1980 to 2008 showed that approximately one in every three diabetic patients has diabetic retinopathy (DR) [2]. As one of the most common and severe microvascular complications of diabetes, DR has become a major public health problem with its high incidence and blindness rates [3–6].

DR can be divided into early non-proliferative DR (NPDR) in the early stage and proliferative DR(PDR) in the middle and late stages. At present, no safe and effective intervention methods are available for NPDR. Existing treatments for NPDR, such as drugs, anti-VEGF therapy, and laser treatments, have limitations including invasiveness and potential side effects. More importantly, these existing treatments are unable to delay or prevent the progression of the disease. In the stage of PDR, retinal ischemia and hypoxia lead to the formation of neovascularization and fibrous proliferative membrane, resulting in complications such as vitreous hemorrhage, tractional retinal detachment, or neovascular glaucoma, which cause severe vision impairment and require urgent surgical intervention, but vision is still difficult to restore to the ideal state. Therefore, effective interventions during the NPDR stage might be feasible for the prevention and treatment [7]. Therefore, we urgently need to explore a new method and a new strategy to delay the progression of NPDR safely and effectively.

The body has adaptive capabilities, and early ischemic intervention can enhance its ability to resist ischemic injury. Remote ischemic conditioning (RIC) is a simple and non-invasive physical therapy that stimulates the body’s endogenous protective mechanisms through repeated, moderate ischemia/reperfusion training of the limbs [8]. This process activates synergistic protective effects on vital organs, such as the heart and brain, through neural, humoral, and immune pathways [9–12]. Previous clinical studies have confirmed that RIC can promote normal angiogenesis, reduce abnormal vascular proliferation, increase blood flow perfusion, and effectively protect tissues from ischemic and hypoxic damage [13–16]. Animal experiments have shown that RIC can reduce the loss of retinal ganglion cells and improve the ischemic and hypoxic conditions in retinal tissues [17–19]. However, to our knowledge, the application of RIC in patients with NPDR has not yet been investigated in clinical trials.

This study aims to conduct a randomized, double-blind, sham-controlled clinical trial to preliminarily evaluate the safety and efficacy of RIC for NPDR. The findings are expected to provide a novel intervention strategy for NPDR, reducing the risk of blindness, improving patient outcomes, and laying the foundation for future large-scale multicenter clinical trials.

Methods

Study design

This project investigates the safety and efficacy of RIC treatment for NPDR through a randomized, double-blind, sham-controlled parallel clinical study. The design flowchart of the entire clinical trial is shown in Fig. 1, referring to protocols for RIC treatment for stroke patients [20]. And Fig. 2 presents the study timeline in the form of a SPIRIT figure.

Fig. 1.

Trial design flowchart. NPDR, non-proliferative diabetic retinopathy; DRSS, Diabetic Retinopathy Severity Score; RIC, remote ischemic conditioning

Fig. 2.

SPIRIT figure summarizing schedule of enrollment, interventions, and assessments

After randomization, participants will receive one-year RIC treatment or a sham treatment. During the study period, we will conduct monthly telephone visits to record the participants’ treatment progress. Face-to-face visits will be scheduled at 6 months and 1 year after treatment to complete comprehensive laboratory and ophthalmic examinations. We will perform full laboratory analyses at a central laboratory at the first visit, 6 months, and 1 year after treatment. A list of normal ranges for all variables will be obtained prior to the initiation of the study. Laboratory analyses will include fasting blood glucose, HbA1c concentration, red blood cell count, white blood cell count, platelet count, hemoglobin concentration, and packed cell volume. Blood chemistry measurements will be conducted on hepatic and renal function, electrolytes, and serum lipids. The indicators of hepatic and renal function are alanine aminotransferase, aspartate aminotransferase, γ-glutamyltransferase, alkaline phosphatase, creatine kinase, creatinine, urea, and uric acid. Electrolyte concentrations (sodium, potassium, and calcium) will also be measured. Lipid fractionation will be performed to determine levels of total cholesterol, HDL, LDL, and triglycerides. Urine analysis will include testing for glucose, total protein, and bilirubin. In addition, patients will undergo a complete bilateral eye examination, which will include best corrected visual acuity as tested with Early Treatment DR Study (ETDRS) charts, slit-lamp examination, intraocular pressure measurement, stereoscopic biomicroscopy (ophthalmic examination technique), optical coherence tomography (OCT), and OCT angiography. Seven-field stereoscopic fundus photographs (color images, 30° field) will be taken, according to the criteria described in the ETDRS protocol. Fluorescein angiography will also be performed to assist in evaluating the degree of DR.

Participants population: inclusion and exclusion criteria

Participants will be recruited from the outpatients of Xuanwu hospital. The inclusion criteria are (1) 40–80 years old; (2) type II diabetes mellitus (T2D); (3) diagnosis of mild to moderate NPDR with the DR Severity Scale (DRSS) developed in the Early Treatment DR Study was NPDR (grade 20-47D) [21]; (4) informed consent obtained from participants or their legal representative.

Exclusion criteria are as follows: (1) diabetic macular edema (macular thickness > 250 μm); (2) serious eye diseases affecting eye examination evaluation, including but not limited to high myopia, severe cataract, corneal leucoma, glaucoma, retinal detachment, retinal vein occlusion, congenital eye diseases, ocular tumors and severe infection; (3) history of ocular laser and intraocular surgery; (4) poor imaging quality due to opacity of the refractive stroma; (5) Contraindication of fluorescein fundus angiography; (6) unstable blood glucose (HbA1c ≥ 8.0%) after oral antidiabetic drugs; (7) severe diabetes complications occurred within 6 months; (8) severe, sustained hypertension (systolic blood pressure ≥ 180mmHg or diastolic blood pressure ≥ 110mmHg); (9) body mass index ≥ 28 kg/m²; (10) hepatic and renal insufficiency: alanine aminotransferase or aspartate aminotransferase > 2 times upper limit of normal, estimated glomerular filtration rate < 60 ml/min/1.73m²; urine albumin/creatinine ratio ≥ 30 mg/g; (11) myocardial infarction occurred within 6 months; (12) Alzheimer’s disease, Parkinson’s disease, cerebrovascular disease, intracranial tumor, cerebrovascular malformation, aneurysm and other brain diseases; (13) one side of subclavian artery stenosis, upper limb soft tissue injury, fracture or vascular injury, distal upper limb peripheral vascular disease, limb deformity and other contraindications of RIC; (14) severe organic diseases, such as malignant tumors, and life expectancy less than 24 months; (15) known pregnancy or breastfeeding; (16) participating in other experimental clinical studies; (17) other circumstances deemed by the investigator to be inappropriate for participation in the trial.

Randomization

In this study, randomization was performed using a computer-generated random sequence to allocate eligible participants into the RIC group or the sham group at a 1:1 ratio. The detailed process is as follows:

1. Generation of random sequence: A randomization sequence was generated using computer software (e.g., random number generators in R or Python) with block randomization to ensure balance and reduce predictability. The block size was either fixed or varied as required.

2. Allocation concealment: The randomization sequence was securely sealed or managed through methods such as numbered opaque envelopes, a central randomization system, or equivalent approaches to maintain allocation concealment. This ensured that both participants and investigators remained blinded to group assignments. A specifically trained physician sets the device’s target inflation pressure (200mmHg or 60mmHg) based on the randomization scheme, but this physician does not participate in any efficacy or safety assessments for the subjects and is prohibited from disclosing the treatment allocation to any other researchers or subjects. Furthermore, except for the research management department and the subject safety department, other investigators and data processors are not allowed to view the randomization plan during the study’s implementation.

3. Group Assignment: After confirming that participants met all inclusion criteria, they were randomly allocated to either the RIC group or the sham group based on the randomization sequence. Participants in the RIC group received the designated intervention, while those in the sham group were administered a matching sham intervention.

Interventions

All interventions were performed by participants at home. Participants in the RIC group underwent RIC therapy using a RIC device on both upper limbs. The inflation pressure was set at 200 mmHg, with each cycle consisting of 5 min of inflation followed by 5 min of deflation, completing 5 cycles per session, twice daily, for at least 5 days per week over a 1-year period. In the sham group, participants received sham treatment with the same device on both upper limbs, but the inflation pressure was set at 60 mmHg, following the same cycle and treatment frequency as the RIC group. To improve adherence to intervention protocols, all participants will accept monthly telephone follow-up to assure treatment completion status.

Both groups received standard care for diabetic retinopathy (DR) prevention and management according to clinical guidelines. And the investigator may suspend or discontinue the intervention in case of persistent, intolerable device-related adverse events or if emergency unblinding is medically necessary. Affected participants will receive required medical treatment, be withdrawn from the trial, and the event will be recorded and reported promptly. Participants are permitted and encouraged to continue all routine standard care, including glucose-lowering, antihypertensive, and lipid-lowering medications. Participation in any other interventional clinical trials is prohibited. Receiving protocol-prohibited treatments (e.g., anti-VEGF injections, laser) will be recorded as an endpoint event.

Outcomes

The primary safety outcome was the first sight-threatening retina hemorrhage or macular oedema, defined as clinically significant bleeding of retina or macular oedema that resulted in persistent visual loss and required urgent intervention, such as laser photocoagulation, vitreoretinal surgery, intraocular injection, or a combination of these treatments. The secondary safety outcome is any adverse event (AE) that requires medical treatment during RIC treatment, including local complications related to RIC such as upper limb pain, erythema, edema, skin injury, and ischemia.

The primary efficacy endpoint is the change in DRSS from baseline to 1 year after RIC treatment.

Secondary efficacy end points include: (1) evaluating the incidence of primary endpoint events (PDR) 1 year after RIC treatment to assess its effectiveness; (2) assessing changes in patients’ visual acuity after 1 year of RIC treatment compared to baseline; (3) observing changes in the retinal neurovascular unit 1 year after RIC treatment compared to baseline, including variations in superficial and deep capillary densities in the macular area, radial peripapillary capillary density, as well as ganglion cell complex (GCC) and retinal nerve fiber layer (RNFL) thickness; (4) monitoring changes in retinal oxygen saturation compared to baseline 1 year after RIC treatment; and (5) analyzing changes in serum levels of VEGF, CRP, IL-6, and occludin compared to baseline after 1 year of RIC treatment.

Data collection and management

Data collection will utilize standardized Case Report Forms (CRFs) to record all efficacy and safety outcomes, including DRSS scores, visual acuity, imaging metrics, and laboratory results. Quality assurance measures include independent dual assessment of key metrics with third-party adjudication, specialized staff training, and double-data-entry verification with query resolution. Participants will receive monetary compensation: 200 RMB after completing the 6-month follow-up and 200 RMB after completing the 1-year follow-up. Participant retention will be promoted through regular communication, flexible scheduling, and continued data collection even after intervention discontinuation for intention-to-treat (ITT) analysis. The data management process incorporates rigorous consistency checks, culminating in database locking after final audit. All electronic and physical records will be securely stored and archived with regular backups to ensure long-term preservation. Participant confidentiality is maintained through use of unique study IDs and separate storage of identifiable information. Biological specimens collected at predefined intervals will be processed and stored at -80 °C for analysis of specified biomarkers exclusively for this trial, with any future use requiring separate ethical approval and participant consent.

Sample size estimation

There are no prior clinical trials reported on RIC treatment for DR patients, making this study exploratory in nature, with no available parameters for sample size estimation. Dobkin has suggested that 15 patients per group are typically sufficient to determine the feasibility of proceeding with larger multicenter trials [22]. Similarly, Hertzog recommends that 10–20 patients per group are adequate for assessing the feasibility of pilot studies [23]. To ensure the primary objective of this study is met, 30 patients per group (a total of 60 patients) will be enrolled to evaluate the primary efficacy outcomes. Considering a 13% dropout rate, 68 patients will need to be recruited to ensure that 60 patients complete the trial.

Statistical analysis

The efficacy analysis will include both the ITT and per-protocol populations, with the ITT population serving as the primary analysis set. The primary outcome measure (DRSS score) will be analyzed using the Wilcoxon rank-sum test. For the secondary outcome, the comparison of endpoint event incidence rates between the two groups will use the chi-square test, and the point estimate and 95% confidence interval for the difference in effectiveness rates will be calculated using the Newcombe-Wilson method. Differences in retinal neurovascular unit parameters (macular superficial and deep capillary density, radial peripapillary capillary density, GCC, and RNFL thickness), visual acuity, retinal oxygen saturation, and blood biomarkers (VEGF, CRP, IL-6, occludin) between the two groups post-RIC treatment will be assessed using t-tests (for normally distributed data) or Wilcoxon rank-sum tests (for non-normally distributed data). Pre- and post-treatment comparisons of these parameters at baseline, 6 months, and 1 year will be analyzed using repeated measures ANOVA. For missing data, a sensitivity analysis will be conducted, and multiple imputations will be applied to impute missing values as continuous data; for clinical events, patients lost to follow-up will be regarded as non-events in both groups.

The safety analysis will include all patients who received RIC treatment and had at least one safety follow-up record. Safety evaluation will encompass all AE observed during the trial. Adverse event incidence rates between the two groups will be compared using the chi-square test or Fisher’s exact test if the chi-square test is inapplicable.

Statistical analysis was conducted using R software (version 4.3.2) and the GraphPad Prism software (version 10.0).

Interim analyses

Interim analysis for efficacy and futility will be performed by the independent Data Monitoring Committee (DMC) after 50% of the planned primary endpoint events have been observed. The stopping guidelines will be based on pre-defined statistical boundaries for overwhelming efficacy or a low conditional power for success (futility). Only the DMC will have access to the unblinded interim results, and it will make a confidential recommendation to the Trial Steering Committee, which holds the final decision regarding trial termination.

Oversight and monitoring

As a single-center trial, the coordinating centre and sponsor of the trial is Xuanwu Hospital, including investigators, coordinators, and data managers responsible for daily operations, quality, data, and regulatory communication. The sponsor will assign monitors for periodic on-site monitoring visits to verify data accuracy, protocol compliance, and GCP adherence. Important amendments require prior approval from the hospital Ethics Committee before implementation. The investigator is responsible for communicating amendments to all relevant parties (including ongoing participants if necessary, potentially requiring re-consent) and updating trial registries.

​​All AEs and serious AEs will be actively collected at each follow-up through inquiry and examination. The investigator will judge their relationship to the intervention. Any serious AEs must be reported to the sponsor and the Ethics Committee within 24 h of awareness. The incidence and severity of device-related local complications (e.g., pain, erythema, skin damage) will be specifically recorded and summarized.

Discussion

Previous studies have shown that RIC can not only improve retinal ischemia-reperfusion injury in rats and monkeys [17, 24] but also protect against DR in Streptozotocin-induced diabetic rats through anti-inflammatory and antioxidant mechanisms [18]. Our team’s most recent research has discovered that RIC can slow down the disruption of the blood-retinal barrier in type 1 diabetic rats by preserving tight junction proteins [25].

Our study included only patients with mild to moderate NPDR and excluded those with severe NPDR and PDR. Patients with severe NPDR and PDR experience rapid disease progression, often leading to vitreous hemorrhage and retinal detachment, which threaten vision and require aggressive, invasive treatments such as laser therapy or surgery. In contrast, mild to moderate NPDR progresses more slowly, and while managing blood glucose, blood pressure, and lipids, as well as improving microcirculation, are important, there are no strong therapeutic measures available to significantly delay or prevent further progression. Therefore, mild to moderate NPDR was chosen as the target population for RIC intervention in our study. The study is limited to patients with type II diabetes mellitus, which helps define a more homogeneous cohort for this initial investigation, given the often more heterogeneous and autoimmune-involved pathophysiology of type I diabetes.

In conclusion, the RIC-NPDR study is a pilot trial designed to assess the safety and explore the potential efficacy of RIC in patients with NPDR. The results from this study are expected to generate preliminary evidence that could guide the design and sample size calculation of future multicenter clinical trials.

Limitations

This study has several limitations that should be acknowledged. First, as a pilot study with a relatively small sample size (n = 60), it is primarily designed to assess feasibility, safety, and generate preliminary efficacy signals rather than to provide definitive evidence of effectiveness. Second, it is a single-center trial, which may limit the generalizability of the findings to broader populations with different demographic or clinical care settings. Third, the inclusion criteria are restricted to patients with well-controlled type 2 diabetes and mild-to-moderate NPDR, excluding those with more advanced retinopathy or other significant systemic conditions; therefore, the results may not apply to these populations. Lastly, despite the sham-controlled design, the possibility of an unmasking effect cannot be entirely ruled out due to the sensation difference between high (200 mmHg) and low (60 mmHg) cuff pressures, although this is mitigated by the use of identical devices and procedures.

Trial status

The trial protocol version is V2.0, dated July 9, 2024. Recruitment began on April 1, 2025, and is currently ongoing, with an anticipated completion date in December 2027.

Abbreviations

AE

Adverse events

DMC

Data Monitoring Committee

DR

Diabetic retinopathy

DRSS

Diabetic Retinopathy Severity Score

ETDRS

Early Treatment Diabetic Retinopathy Study

GCC

Ganglion cell complex

ITT

Intention-to-treat

NPDR

Non-proliferative diabetic retinopathy

OCT

Optical coherence tomography

PDR

Proliferative diabetic retinopathy

RIC

Remote ischemic conditioning

Authors’ contributions

X.W. and S.H. contribute equally to the article. X.Z. led the development of this protocol, with substantial study implementation input from X.W. and S.H. X.W. and S.H. prepared the initial manuscript draft based on the study protocols and C.R provided substantive feedback to the manuscript; Z.L reviewed and edited the manuscript. All authors were involved in developing and/or enacting the study protocol. All authors read and approved the final manuscript.

Funding

This study was supported by the Capital’s Funds for Health Improvement and Research (CFH2024-2-20113).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

This study was approved by the ethics committee of Xuanwu Hospital of Capital Medical University (Ethics Approval Number: Clinical Research [2024] 243-001). Trained and authorized investigators will obtain informed consent using the latest approved form through a face-to-face discussion, ensuring participants fully understand the study’s purpose, procedures, risks, and rights-including voluntary participation and withdrawal without penalty. The signed form is kept in duplicate by both the institution and the participant. Additionally, the form specifies that collected blood samples and clinical data will be used for analyzing specific serum markers in this study and may be utilized as anonymized data in future related academic research, with all uses strictly adhering to confidentiality principles.

Consent for publication

Not applicable—no identifying images or other personal or clinical details of participants are presented here or will be presented in reports of the trial results. The participant information materials and informed consent form are available from the corresponding author on request.

Competing interests

The authors declare no competing interests.