Incidence of Perioperative Hypoglycemia Assessed by Masked Continuous Glucose Monitoring in Patients With Diabetes Undergoing Vitrectomy: a Prospective Observational Cohort Study

Patients with diabetes are at increased risk of perioperative glycemic disturbances due to preoperative fasting, the surgical stress response, and the use of insulin or oral hypoglycemic agents. Hypoglycemia, in particular, is an important clinical concern associated with neurological injury and cardiovascular complications. In routine practice, intraoperative glucose monitoring often relies on intermittent fingerstick testing or blood gas analysis, and these intermittent methods may fail to detect hypoglycemic episodes that occur during and immediately after surgery.

Continuous glucose monitoring (CGM), which estimates glucose concentrations from interstitial fluid, allows continuous tracking of glycemic trends and may provide a more precise assessment of glycemic variability and hypoglycemia in the perioperative period. Although current guidelines recommend periodic intraoperative glucose measurement in patients with diabetes-especially those receiving insulin-glucose monitoring is frequently omitted during relatively short and stable ophthalmic procedures such as vitrectomy. Because vitrectomy is commonly performed in patients with diabetes, perioperative hypoglycemia in this population may go undetected.

This is a single-center, prospective, observational cohort study conducted in patients with diabetes undergoing pars plana vitrectomy (TPPV). A masked CGM device (FreeStyle Libre 2®, an approved continuous glucose monitor) is applied for observational purposes only and does not influence clinical care. The sensor is placed on the upper arm before surgery. After an approximately one-hour warm-up period, glucose data are collected at 15-minute intervals from the completion of CGM warm-up until discharge from the post-anesthesia care unit (PACU); this interval defines the perioperative period for data collection. Throughout this period the CGM operates in a masked mode-no reader is provided, the device is not linked to the participant's smartphone, and alarms are disabled-so that real-time glucose values are not displayed to clinicians or participants. All glycemic management follows the existing standard of care based on point-of-care testing (POCT), and CGM data do not contribute to clinical decision-making. After all participants have completed data collection, CGM data are unblinded and analyzed.

The primary objective is to estimate the incidence of perioperative hypoglycemia, defined as a CGM glucose value below 70 mg/dL (Level 1), during the perioperative period. Secondary objectives include the incidence of clinically significant hypoglycemia (below 54 mg/dL, Level 2); indices of glycemic variability (Time Below Range, maximum and minimum glucose, and glucose excursion); identification of clinical risk factors associated with perioperative hypoglycemia (such as age, diabetes characteristics, HbA1c, insulin use, fasting duration, and operative time); the exploratory association between perioperative hypoglycemia and postoperative ophthalmic outcomes (changes in best-corrected visual acuity and ophthalmic complications such as recurrent vitreous hemorrhage, retinal redetachment, elevated intraocular pressure, and the need for additional procedures); and an exploratory, opportunistic concordance analysis between standard-of-care POCT glucose values and time-matched CGM values (within ±5 minutes). Hypoglycemia is classified according to ADA/Danne et al. criteria.

Eligible participants are adults aged 19 years or older with diabetes who are scheduled for vitrectomy and able to provide written informed consent. Participants are excluded if CGM sensor placement is not feasible, if they have a known sensor allergy, if valid data cannot be obtained, or if voluntary informed consent is not possible. Based on the institution's monthly surgical volume and the estimated proportion of patients with diabetes, a target enrollment of 85 participants over a 12-month study period is considered feasible. Postoperative ophthalmic outcomes are assessed using medical records from routine outpatient follow-up visits (for example, at approximately one week and one to three months after surgery) without any additional study-specific visits.

Because the primary aim is to estimate the incidence of perioperative hypoglycemia, ophthalmic outcomes and POCT-CGM concordance are analyzed as exploratory endpoints. This study aims to characterize the limitations of current perioperative glucose monitoring strategies and to provide foundational data for future risk-based monitoring strategies and CGM-based interventional research.