Monte-Carlo simulation and tissue-phantom model for validation of ocular oximetry

Retinal changes detected by diffuse reflectance spectroscopy in parkinsonian monkeys

Significance

Parkinson's disease (PD) is diagnosed when 50% neurodegeneration has occurred. The retina could provide biomarkers that would allow for earlier diagnosis. Retinal spectroscopy is a technique that could be used to find such biomarkers.

Aim

We aimed to find new diagnostic biomarkers for PD following detailed spectral examinations of the retina.

Approach

The newly developed Zilia Ocular device was used to perform spectrometric scans of the optic nerve head (ONH) and the retina of four cynomolgus monkeys (Macaca fascicularis) before and after the administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin used to produce the gold-standard animal model of PD. From the spectrometric data, the blood oximetry was calculated, and the diffuse reflectance spectra (DRS) were analyzed to find variations between the two experimental conditions. Post-mortem analyses were also performed on the retina of the four parkinsonian monkeys and four additional control animals.

Results

The analysis of the DRS indicated a lower slope between the 480- and 525-nm wavelengths in both the ONH and the retina. Post-mortem measurements of the retinal layer thicknesses showed that the outer nuclear layer was significantly thinner in MPTP-intoxicated monkeys, compared with controls. Altogether, these results indicate that MPTP altered the optical properties of the ONH and the retina and show that these variations might be explained by MPTP-induced structural changes in the eye fundus, as observed post-mortem.

Conclusions

Overall, our results indicate that spectroscopy could be used as a noninvasive method to detect changes in the retina that occur in PD and that such changes could represent retinal biomarkers for improved diagnosis.

Evaluation of macular retinal oximetry across different levels of diabetic retinopathy: a cross sectional study

BACKGROUND

This study evaluates retinal oxygen saturation and vessel density within the macula and correlates these measures in controls and subjects with type 2 diabetes (DM) with (DMR) and without (DMnR) retinopathy. Changes in retinal oxygen saturation have not been evaluated regionally in diabetic patients.

METHODS

Data from seventy subjects (28 controls, 26 DMnR, and 16 DMR were analyzed. For those with DMR,8 were mild/moderate diabetic retinopathy (NPDR) and 8 severe NPDR/proliferative (PDR). Subjects were categorized with glycosylated hemoglobin A1c and fundus photography. Retinal oximetry measurements were performed within a 300-400 μm region at four diagonal locations 3.1 degrees from the center of the fovea in the superior nasal, superior temporal, inferior nasal, and inferior temporal locations adjacent to the foveal avascular zone (FAZ). Optical coherence tomography angiography (OCTA) was performed and corrected for refractive error. Photoshop and ImageJ were utilized to calculate the superficial capillary plexus vascular density (SCP). Oximetry and OCTA vessel density were analyzed overall and by region.

RESULTS

Average retinal oxygen saturation was highest in DMR (p = 0.008). Average OCTA density was less in DMR compared to controls (p = 0.01), but not compared to DMnR subjects (p = 0.07). A significant inverse correlation was observed between averaged oxygen saturation and SCP vascular density for all subjects (p = 0.02). Duration of DM was positively associated with oxygen saturation (p = 0.01) and negatively with OCTA SCP vascular density (p = 0.009). There were no differential effects of retinal location.

CONCLUSION

To our knowledge, this study is the first to evaluate the relationship between macular oxygen saturation and SCP vascular density at different levels of retinopathy. This may be useful to track patients with DM as they move through stages of retinopathy.

Relationship Between Retinal Oxygen Saturation and the Severity of Visual Field Damage in Glaucoma

PRCIS

Increased oxygen saturation (StO 2 ) was significantly associated with the severity of visual field (VF) damage in patients with glaucoma.

OBJECTIVE

To investigate the association between retinal StO 2 percentage and the severity of VF loss in glaucoma.

METHODS

A total of 198 eyes from 131 patients with glaucoma were included in this cross-sectional study. Participants underwent imaging using ocular oximetry (Zilia) and 24-2 Swedish Interactive Threshold Algorithm standard VF (Carl Zeiss-Meditec). StO 2 (%) was measured at 2 locations of the peripapillary optic nerve head (superotemporal, and inferotemporal). Measurements were reported as the mean of at least 5 measurements in each location. Associations between the severity of VF loss, reported as mean deviation, and StO 2 (%) were calculated.

RESULTS

A total of 198 eyes of 131 patients (mean (95% CI) age, 71.1 (68.9,73.3) years, 68 females (51.9%), and 63 males (48.1%) were analyzed. In univariable analysis, higher StO 2 -0.06 (-0.12, 0.00) was associated with severity in all hemifields ( P = 0.047). Multivariate regression analysis showed that each 1% increase in StO 2 was associated with -0.06 (-0.12, -0.00) dB loss in mean deviation in all hemifields ( P = 0.043). In multivariate regression analysis in the superior hemifields, higher StO 2 -0.07 (-0.16, 0.01) tended to be associated with superior hemifield severity ( P = 0.09).

CONCLUSIONS

Retinal oximetry enabled the continuous quantitative measurement of retinal StO 2. Increased StO 2 was significantly associated with the severity of VF damage in patients with glaucoma.

Test-retest repeatability in macular retinal oximetry

CLINICAL RELEVANCE

Evaluation of retinal macular oxygen saturation in healthy controls can aid in understanding the pathological changes seen in similar locations of those with vascular diseases like diabetes.

BACKGROUND

The aim of this study was to determine the test-retest repeatability of localised retinal oximetry measurements in the macula on the Zilia Oximeter within healthy subjects of different races, 18-40 years old. Oxygen saturation was measured between three time points within the same locations of the right eye.

METHODS

Twenty seven subjects were included (aged 26.3 ± 3.6 years). All were confirmed to have healthy retinas and at least 6/9 vision. Oximetry measurements were taken using the Zilia to acquire local oxygen saturation measurements (300 µm diameter) at four points 3.1 degrees from the fovea in the superior/temporal, superior/nasal, inferior/temporal, and inferior/nasal locations. Oximetry measurements were taken twice on the same day 20 minutes apart and then again 1-2 weeks later. Oximetry data was analysed with intraclass correlation between visits. To assess intrasubject repeatability, the Bland-Altman repeatability coefficient and coefficient of variation were calculated.

RESULTS

Average Intraclass correlation for the three acquisition times of the right eye was 0.78. The averaged intrasubject repeatability coefficient for the three acquisition times was 8.4. The averaged coefficient of variation was 5.4%.

CONCLUSION

The Zilia oximeter has good macular test-retest repeatability; however, multiple measurements may be needed to ensure accuracy.

Variations in Retinal Oxygen Saturation in a Diverse Healthy Population

Purpose

Local retinal oxygen saturation is a research technique, which has the potential as a biomarker for diabetes. However, normative data has not been established. This study examined differences in oxygen saturation around the macula and characterizes the relationship between age, race, refractive error (RE), sex, blood pressure (BP), prediabetic status and oxygen saturation.

Methods

Fifty-nine subjects aged 22-69 (38.8 ± 14.7 years) were included who were racially diverse and with equal gender distribution. None had eye disease. Oxygen saturation was taken with the Zilia Ocular in 4 locations around the macula 3.1 degrees from the fovea and they were also averaged. BP, RE, and HbA1c were noted. Regression analyses for oximetry and other factors were completed as were t-tests with multiple comparison corrections.

Results

There were significant variations in oximetry measures by race, with higher pigmentation levels associated with lower oximetry values (p < 0.01). There was no relationship between oximetry and sex (p = 0.34), RE (p = 0.67), BP (systolic p = 0.61, diastolic p = 0.71) nor prediabetic status (p = 0.87). Oximetry was associated with age when controlling for race (P < 0.002). Nasal-temporal variations showed nasal oximetry to higher than temporal measures (P < 0.01).

Conclusion

This study revealed race/pigmentation is an important influence on oximetry measures. Retinal location also caused variations, likely due to proximity to larger vessels nasally. No differences in sex, RE nor BP were observed to alter local oxygen saturation. However, age was correlated when considered with race. This study will inform our future work in different disease states and is an important first step in evaluating this technology.

Targeted spectroscopy in the eye fundus

Significance

The assessment of biomarkers in the eye is rapidly gaining traction for the screening, diagnosis, and monitoring of ocular and neurological diseases. Targeted ocular spectroscopy is a technology that enables concurrent imaging of the eye fundus and analysis of high-quality spectra from a targeted region within the imaged area. This provides structural, compositional, and functional information of specific regions of the eye fundus from a non-invasive approach to ocular biomarker detection.

Aim

The aim of our study was to demonstrate the multimodal functionality and validation of targeted ocular spectroscopy. This was done in vitro, using a reference target and a model eye, and in vivo.

Approach

Images and spectra from different regions of a reference target and a model eye were acquired and analyzed to validate the system. Targeted ocular fluorescence spectroscopy was also demonstrated with the same model. Subsequently, in vivo imaging and diffuse reflectance spectra were acquired to assess blood oxygen saturation in the optic nerve head and the parafovea of healthy subjects.

Results

Tests conducted with the reference target showed accurate spectral analysis within specific areas of the imaging space. In the model eye, distinct spectral signatures were observed for the optic disc, blood vessels, the retina, and the macula, consistent with the variations in tissue composition and functions between these regions. An ocular oximetry algorithm was applied to in vivo spectra from the optic nerve head and parafovea of healthy patients, showing significant differences in blood oxygen saturation. Finally, targeted fluorescence spectral analysis was performed in vitro.

Conclusions

Diffuse reflectance and fluorescence spectroscopy in specific regions of the eye fundus open the door to a whole new range of monitoring and diagnostic capabilities, from assessment of oxygenation in glaucoma and diabetic retinopathy to photo-oxidation and photodegradation in age-related macular degeneration.

Sensitivity of visible range multi-wavelength algorithms for retinal tissue oximetry to acquisition parameters

This study examined the sensitivity of broadband spectroscopy algorithms for retinal tissue oximetry to spectral acquisition parameters. Monte Carlo simulations were conducted on a 4-layer retinal model to assess the impact of various parameters. The optimal spectral range for accurate measurements was determined to be 530 nm to 585 nm. Decreased spectral resolution below 4 nm significantly reduced accuracy. Using an acquisition area larger than the blood vessel resulted in an underestimation of oxygen saturation, especially for high values. A threshold was observed where increased light intensity had no significant impact on measurement variability. The study highlights the importance of informed parameter selection for accurately assessing retinal microcapillary oxygenation and studying local hemodynamics.