A noninvasive glucose monitor: preliminary results in rabbits

Glucose levels between the anterior chamber of the eye and blood are correlated based on blood glucose dynamics

Glycemic control is essential to manage metabolic diseases such as diabetes. Frequent measurements of systemic glucose levels with prompt managements can prevent organ damages. The eye is a glucose highly demanding organ in our body, and the anterior chamber (AC) in the eye has been suggested for a noninvasive blood glucose monitoring site. However, calculating blood glucose levels from measuring glucose levels in AC has been difficult and unclear. In this study, we aimed to examine glucose levels from AC and find a correlation with blood glucose levels. A total of 30 patients with cataracts (men and women, study 1; 7 and 3, study 2; 9 and 11) who visited Keio University Hospital from 2015 to 2018 and agreed to participate in this study were recruited. Glucose levels from AC and the blood were examined by a UV-hexokinase or H2O2-electrode method before/during the cataract surgery. These values were analyzed with regression analyses depending on the groups (blood glucose-ascending and descending groups). In the blood glucose-descending group, glucose levels from AC were strongly correlated with blood glucose levels (a high R2 value, 0.8636). However, the relatively moderate correlation was seen in the blood glucose-ascending group (a low R2 value, 0.5228). Taken together, we showed different correlation ratios on glucose levels between AC and the blood, based on blood glucose dynamics. Stacking data regarding this issue would enable establishing noninvasive blood glucose monitoring from measuring glucose levels in AC more correctly, which will be helpful for proper and prompt managements for glucose-mediated complications.

The effect of the glycemic control on the aqueous humor glucose levels in diabetic patients undergoing elective cataract surgery

PURPOSE

Aqueous humor glucose levels play a role in the anterior segment disorders' pathophysiology, mainly in diabetics. Our purpose was to evaluate the ratio of aqueous humor glucose levels to blood glucose levels in diabetics and to evaluate the correlation between this ratio and blood glycated hemoglobin (HbA1c) levels.

METHODS

This prospective study was performed in Sheba Medical Center during 2016-2018. The study included type 2 diabetic patients admitted for elective cataract surgery. Blood glucose levels were measured immediately before surgery. HbA1c was obtained within 3 months preoperatively. At the beginning of surgery, 0.05-0.1 mL aqueous humor was drawn. Main outcome measures were aqueous humor glucose/blood glucose ratio and the correlation between HbA1c and aqueous humor glucose/blood glucose ratio.

RESULTS

Thirty-seven patients (mean age 75.2 ± 11.2 years) were recruited. The average aqueous humor glucose/blood glucose ratio was 0.69 ± 0.20. A significant positive correlation was found between aqueous humor and blood glucose levels, Pearson coefficient constant R = 0.63 (p < 0.01), and specifically stronger among older patients R = 0.89 (p < 0.01), females R = 0.74 (p < 0.01), patients with short-term disease (<10 years) R = 0.80 (p < 0.01), and patients treated with oral anti-diabetic treatment R = 0.74 (p < 0.01). A significant strong positive correlation was found between HbA1c levels and aqueous humor glucose/blood glucose ratio R = 0.62 (p < 0.01), and specifically stronger among older patients R = 0.82 (p < 0.01), males R = 0.70 (p < 0.01), patients with prolonged disease (⩾10 years) R = 0.540 (p < 0.05), and patients treated with oral anti-diabetic treatment R = 0.62 (p < 0.01).

CONCLUSION

A significant strong correlation was found between aqueous humor glucose levels and blood glucose levels. Poor glycemic control was strongly correlated with an increased ratio, reflecting an increased anterior chamber's glucose permeability. Older age group was found to have stronger correlation of poor glycemic control with this ratio.

Confocal Raman spectroscopy: Evaluation of a non-invasive technique for the detection of topically applied ketorolac tromethamine in vitro and in vivo

Current information about the pharmacokinetics of an ocular drug can only be achieved by invasive sampling. However, confocal Raman spectroscopy bears the potential to quantify drug concentrations non-invasively. In this project, we evaluated the detection and quantification of ocular ketorolac tromethamine levels with confocal Raman spectroscopy after topical administration. Confocal Raman spectroscopy and high-performance liquid chromatography (HPLC) were compared in terms of sensitivity of detection. Enucleated pig eyes were treated with different concentrations of ketorolac. Hereafter, ketorolac concentrations in the aqueous humor of pig eyes were analyzed by confocal Raman spectroscopy and HPLC. Subsequently, twelve rabbits were treated with Acular™ for four weeks. At several time points, ketorolac concentrations in aqueous humor of the rabbits were measured by confocal Raman spectroscopy followed by drawing an aqueous humor sample for HPLC analysis. In ketorolac treated pig eyes, both ex vivo Raman spectroscopy as well as HPLC were able to detect ketorolac in a broad concentration range. However, in vivo confocal Raman spectroscopy in rabbits was unable to detect ketorolac in contrast to HPLC. To conclude, confocal Raman spectroscopy has the capacity to detect ketorolac tromethamine in vitro, but currently lacks sensitivity for in vivo detection.

In Vivo Characterization of Corneal Changes in a Type 1 Diabetic Animal Model

Diabetes mellitus (DM) is a metabolic disease that affects 9% of the adult population, promoting an increase in glucose concentration that affects the corneal structure, namely, its thickness, as well as the constituents and flow of the aqueous humor. In this study, high-frequency transducers (20-MHz and 50-MHz) were used to measure and characterize changes in the corneal and aqueous humor in streptozotocin-induced type 1 diabetic rats followed over 8 weeks. Increases of 24.6 and 15.4 μm in central corneal thickness were measured with the 20-MHz and 50-MHz probes, respectively, in DM rats (p < 0.001). The increases in thickness of the different corneal layers ranged from 7% to 17%. Structural alterations of the aqueous humor were also studied by relating the amplitudes of the anterior lens and posterior cornea boundary signals, the result of which was denominated by pseudo-attenuation. The results revealed an increase of 49% at week 8 compared with the baseline values (p < 0.020, with the 50-MHz probe). This study illustrated that high-frequency ultrasound can be used to measure corneal layer thickness and study the alterations promoted by diabetes in the eye's anterior segment. Those assessments may allow early detection of DM, improving the monitoring of diabetic patients.

Non-invasive detection of antibiotics and physiological substances in the aqueous humor by raman spectroscopy

BACKGROUND AND OBJECTIVES

Laser Raman spectroscopy is an inelastic light scattering technique able to characterize molecules in aqueous environments. The purpose of this work is to develop a non-contact and non-invasive spectroscopic method to identify and eventually quantify the presence of medicines (e.g., antibiotics) and physiological substances (e.g., glucose) in the aqueous humor of the eye.

STUDY DESIGN/MATERIALS AND METHODS

A new laser light delivery probe has been developed and adapted to a Raman spectroscopic system with the ability of favorable collection of the Raman light at 90 degrees scattering geometry while scanning the anterior chamber of the eye. Different amounts of ceftazidime, amphotericin B, and glucose had been injected in the aqueous humor of porcine eyes, maximum 24 hours after death and extraction, in-vitro. Raman measurements were excited with a visible (514.5 nm) laser beam at a power of 25 mW and an exposure/acquisition time of 1 second.

RESULTS

The specific collection optics and Raman analysis components used in the present work have resolved the Raman signatures of probed molecules and low concentrations of ceftazidime (0.9 mg/mL), amphotericin B (9 microg/mL), and glucose (2 mg/ml) separately injected in the anterior chamber of porcine eyes were detected in vitro.

CONCLUSION

This special illumination design gives the opportunity of avoiding the direct exposure to the laser light of basic cordial tissues of the eye, like lens and retina, although an optimum collection of scattered light is accomplished. Concentrations close to the minimum inhibitory concentration (MIC) have been detected for ceftazidime and amphotericin b; the detection of glucose has been realized at concentrations close to the early pathological levels of patients with diabetes.

Validation of ICNIRP estimates of toxicity thresholds for NIR (785 nm) light in the retinas of pigmented rabbits

The International Commission on Non-Ionizing Radiation Protection (ICNIRP) established eye-safe exposure limits for laser light based on estimates of the 50% exposure dose (ED50) for retinal damage in animals. The ED50 has not been determined for continuous-wave near-infrared (NIR) light, as may be used in instruments for spectroscopic analysis of the eye. The purpose of this study is to determine the ED50 of 785-nm light delivered by a Raman spectroscopy instrument on the retinas of pigmented rabbits, and to compare that ED50 to the ICNIRP estimates. Light of varying power was focused into the anterior chambers of rabbits for 4, 20, or 300 s. Estimates of spot size on the retina were determined by modifying the schematic rabbit eye with actual axial length and refractive error measurements. Masked examiners determined retinal burns from fundus photographs and fluorescein angiograms made immediately after exposure and again 48-72 h later. Thirty-five of 86 exposed eyes had retinal burns. In 12 of these the burn was not visible until 48-72 h. In 6 the burn was visible on color photographs, but not on fluorescein angiography. The ED50 retinal power density from probit analysis was 14,513 mW cm(-2), 10,369 mW cm(-2), and 5,237 mW cm(-2) for 4-s, 20-s, and 300-s exposures, respectively. Thus, the ED50 for retinal burns from 785-nm light is approximately 7 and 7.5 times the ICNIRP permitted limits for 4- and 20-s exposures, and 5.6 times the limit for 300-s exposures. This is consistent with the ICNIRP intention to set power limits several times less than ED50.

Determination of glucose in human aqueous humor using Raman spectroscopy and designed-solution calibration

Glucose concentrations of in vitro human aqueous humor (HAH) samples from cataract patients were determined using 785 nm Raman spectra and partial least squares (PLS) calibration. PLS models were created from spectra of prepared calibration solutions rather than aqueous humor samples. Spectra were obtained with an excitation energy (100 mW for 150 s), which was higher than can be applied in vivo, to decrease the models' contribution to prediction uncertainty. The solutions contained experimentally designed levels of glucose, bicarbonate, lactate, urea, and ascorbate. Multiplicative signal correction of spectra helped compensate for the +/-20% drift in laser power observed at the sample over six noncontiguous days of data collection. Seventeen HAH samples containing 38-775 mg/dL of glucose exhibited a root-mean-square error (RMSEP) of 22 mg/dL, coefficient of determination (r(2)) of 0.989, and bias of 6 mg/dL when predicted from lower energy (30 s) spectra collected contemporaneously with fifty calibration spectra. Similar results were obtained even when spectral data were gathered separately for human aqueous humor samples and calibration samples: 10 HAH samples, calibrated on 25 solutions measured 3.6 weeks earlier, exhibited an RMSEP of 23 mg/dL, r(2) of 0.992, and bias of 9 mg/dL. The results demonstrate progress toward the determination of glucose levels in patient-derived aqueous humor using laboratory-derived "artificial aqueous humor" calibration solutions.

Glucose determination in human aqueous humor with Raman spectroscopy

It has been suggested that spectroscopic analysis of the aqueous humor of the eye could be used to indirectly predict blood glucose levels in diabetics noninvasively. We have been investigating this potential using Raman spectroscopy in combination with partial least squares (PLS) analysis. We have determined that glucose at clinically relevant concentrations can be accurately predicted in human aqueous humor in vitro using a PLS model based on artificial aqueous humor. We have further determined that with proper instrument design, the light energy necessary to achieve clinically acceptable prediction of glucose does not damage the retinas of rabbits and can be delivered at powers below internationally acceptable safety limits. Herein we summarize our current results and address our strategies to improve instrument design.

Non-invasive glucose measurement technologies: an update from 1999 to the dawn of the new millennium

There are three main issues in non-invasive (NI) glucose measurements: namely, specificity, compartmentalization of glucose values, and calibration. There has been progress in the use of near-infrared and mid-infrared spectroscopy. Recently new glucose measurement methods have been developed, exploiting the effect of glucose on erythrocyte scattering, new photoacoustic phenomenon, optical coherence tomography, thermo-optical studies on human skin, Raman spectroscopy studies, fluorescence measurements, and use of photonic crystals. In addition to optical methods, in vivo electrical impedance results have been reported. Some of these methods measure intrinsic properties of glucose; others deal with its effect on tissue or blood properties. Recent studies on skin from individuals with diabetes and its response to stimuli, skin thermo-optical response, peripheral blood flow, and red blood cell rheology in diabetes shed new light on physical and physiological changes resulting from the disease that can affect NI glucose measurements. There have been advances in understanding compartmentalization of glucose values by targeting certain regions of human tissue. Calibration of NI measurements and devices is still an open question. More studies are needed to understand the specific glucose signals and signals that are due to the effect of glucose on blood and tissue properties. These studies should be performed under normal physiological conditions and in the presence of other co-morbidities.

Noninvasive detection of ganciclovir in ocular tissue by Raman spectroscopy: implication for monitoring of drug release

Treatment of various pathological conditions in ophthalmology, such as cytomegalovirus (CMV) retinopathy and endophthalmitis, requires a local drug intervention rather than a systemic approach. Accurate knowledge of intraocular drug concentration can permit the ophthalmologist to maintain drug levels within the therapeutic levels necessary for an optimal prognosis, while preventing or minimizing toxicity associated with drug overdose.

PURPOSE

To develop a noninvasive/noncontact method for quantification of the local concentration of ganciclovir in the ocular media.

METHODS

An integrated CCD-based Raman spectroscopic system designed specifically for ophthalmic applications was used to noninvasively detect the presence of ganciclovir in the ocular media. Various known concentrations of ganciclovir were injected into the aqueous humor of rabbit eyes in a pilot study, in vivo. Raman spectra were then acquired by focusing an argon laser beam within the anterior chamber of the eye. The specific Raman signature of ganciclovir was assessed at several concentrations.

RESULTS

Spectral features unique to ganciclovir were identified and distinguished from those of ocular tissue. The amplitudes of the spectral peaks corresponding to ganciclovir exhibited a linear dependence on the local concentration of the drug in the anterior chamber of the eye.

CONCLUSION

Raman spectroscopy may offer an effective tool for the noninvasive assessment of the local concentration of ganciclovir in the ocular media. This technique offers the potential to determine both the amount and the rate of the drug release from implants designed to deliver antiviral drugs locally within the eye. The availability of such data could enable the ophthalmologist to improve treatment efficacy by avoiding premature or late surgical replacement of the implants.

Methods for quantifying self-monitoring blood glucose profiles exemplified by an examination of blood glucose patterns in patients with type 1 and type 2 diabetes

The maintenance of glycemic control in patients with type 1 or type 2 diabetes mellitus (T1DM and T2DM, respectively) is commonly assisted by devices for self-monitoring of blood glucose (SMBG) that store multiple BG determinations. However, besides average BG, no other SMBG characteristics are routinely computed. We describe several SMBG-based measures that quantify the extent and rate of patients' BG excursions into hypoglycemia and hyperglycemia and can be used as markers for patients' vulnerability to hypoglycemia and BG irregularity. These markers are applied to analyze data from patients with T1DM (n = 277) and T2DM (n = 323), all of whom used insulin. T1DM and T2DM patients were matched by HbA(1c), gender, and number of SMBG readings/day. On average, 230 SMBG readings and three HbA(1c) assays were collected for each subject over 3 months. Compared with T2DM, patients with T1DM diabetes had (1) more extreme low and high BGs, (2) greater risk for severe hypoglycemia as quantified by the Low BG Index, (3) faster descent into hypoglycemia as quantified by the risk rate of change/hour, and (4) greater BG irregularity as computed by BG rate of change/hour and BG SD (all p levels < 0.0001). SMBG data allow for computing and frequent updating of various idiosyncratic diabetes characteristics and risk factors. The use of such computations may assist in optimizing patients' glycemic control.

Noninvasive glucose monitoring with optical heterodyne technique

A novel optical polarimeter to sense the glucose level in vivo and noninvasively has been developed in the Institute of Radiological Science, National Yang Ming University. A 30-minute delay between the aqueous glucose and the blood glucose was observed in rabbit's eyeball. Currently, aqueous glucose in low concentration is being tested and discussed.

Evaluation of Measurement Sites for Noninvasive Blood Glucose Sensing with Near-Infrared Transmission Spectroscopy

Six putative measurement sites were evaluated for noninvasive sensing of blood glucose by first-overtone near-infrared spectroscopy. The cheek, lower lip, upper lip, nasal septum, tongue, and webbing tissue between the thumb and forefinger were examined. These sites were evaluated on the basis of their chemical and physical properties as they pertain to the noninvasive measurement of glucose. Critical features included the effective optical pathlength of aqueous material within the tissue and the percentage of body fat within the optical path. Aqueous optical paths of 5 mm are required to measure clinically relevant concentrations of glucose in the first-overtone region. All of the tested sites met this requirement. The percentage of body fat affects the signal-to-noise ratio of the measurement and must be minimized for reliable glucose sensing. The webbing tissue contains a considerable amount of fat tissue and is clearly the worse measurement site. All other sites possess substantially less fat, with the least amount of fat in tongue tissue. For this reason, the tongue provides spectra with the highest signal-to-noise ratio and is, therefore, the site of choice on the basis of spectral quality.

Introduction to concepts in laser technology for glucose monitoring

Lasers are important tools in many diabetes-related applications, both diagnostic and therapeutic. Despite its wide range of uses, the laser derives all of its advantages over more conventional light sources from a few basic principles. A brief introduction to the fundamental properties of lasers emphasizes these points and suggests ways in which they can be exploited in experiments. Short descriptions of particular laser-based methods for monitoring glucose and related molecules provide introductions to research articles on these subjects.

Optical radiation safety considerations for ocular glucose monitoring

The potential for noninvasive detection of blood glucose is an area of intense academic and commercial research and a subject of keen interest in the diabetic and healthcare communities. A number of techniques are under investigation that attempt to infer blood glucose levels from measurements of optical signals. Frequently, these techniques are based on laser sources that may, under certain circumstances, be capable of inducing ocular injury. This article provides an overview of ocular damage mechanisms and the international standards for laser exposure limits that have been developed. The application of relevant standards to specific implementations of lasers in optical glucose sensing is presented. In addition, the concept of risk versus benefit for consideration of new medical devices is also discussed.

Laser-based measurement of glucose in the ocular aqueous humor: an efficacious portal for determination of serum glucose levels

The potential for the noninvasive measurement of blood glucose concentration to improve the level of blood sugar control and the quality of life of diabetic patients has been recognized for nearly 3 decades. Because of the many challenges involved in directly sensing glucose in the bloodstream noninvasively, the approach of noninvasive measurement of glucose concentration in the ocular aqueous humor, which reflects blood glucose concentration, has been likewise pursued over those 3 decades. Moreover, studies of the dynamic relationship between blood glucose concentration and ocular glucose concentration in both animals and humans suggest minimal latency between variations in blood glucose and those in ocular glucose. In a review of technological approaches to such measurements, the most promising techniques appear to be laser-based, and include laser polarimetry and Raman spectroscopy. Results for techniques such as multiwavelength polarimetry and Raman spectroscopy have been quite good, and it is likely that one or both of these techniques, possibly combined with refractive index measurement, will yield a useful instrument for both clinical use and patient self-monitoring.