Retinal Oxygen Saturation in Patients with Primary Open-angle Glaucoma Using a Non-flash Hypespectral Camera.

Purpose/Aim: To investigate retinal vessel blood oxygen saturation in patients with primary open-angle glaucoma (POAG) and healthy controls. MATERIALS AND METHODS: A novel non-flash hyperspectral retinal camera was used to image 17 healthy individuals (mean age 69.3 ± 6.1 years) and 22 patients with stable POAG (mean age 69.2 ± 5.8 years) at 548, 569, 586, 600, 605, and 610 nm wavelengths. POAG patients were grouped as mild-moderate (n = 13) and moderate-severe (n = 9) based on Humphrey 24-2 visual field results (mean deviation [MD] < -6 and MD ≥ -6; respectively). Optical density values were extracted using Image-J software for blood oxygen saturation (SO2) determination. Arteriolar and venular SO2 were measured within 1.5 optic nerve head diameters from the disc margin along the vessels in the inferior temporal quadrant of the tested eye per subject. Analysis of variance (ANOVA), student t-test, and Pearson's correlation were used for statistical analysis of the data (p < 0.05). RESULTS: Venular, arteriolar, and arteriovenous (AV) differences in SO2 measurements were not significantly different between controls and the combined POAG groups (p > 0.06 for all). However, mean venular SO2 was significantly higher in the POAG with MD ≥ -6 dB when compared to controls and patients with mild glaucoma (p = 0.005). The AV differences were significantly lower in patients with more severe field defect (p = 0.006) compared to the remaining groups. No differences were found in the mean arteriolar SO2 between the groups (p = 0.155). Significant correlations were found only between higher visual field MD values and higher venular SO2 (p = 0.048) but not the remaining SO2 measurements. CONCLUSION: Patients with POAG and moderate-severe visual field defect had higher venular SO2 compared to those with mild-moderate defect and controls. This would indicate reduced oxygen consumption in more advanced glaucoma likely as a result of ganglion cell degeneration.

A rapid decline in corneal small fibers and occurrence of foot ulceration and Charcot foot.

We present clinical, neuropathy and corneal nerve morphology data in a participant with type 2 diabetes who developed diabetic foot ulceration, partial amputation and Charcot during a longitudinal observational study. While conventional measures of neuropathy did not deteriorate significantly, corneal nerve parameters showed a rapid reduction prior to the development of foot complications.

A case study of choroideremia carrier - Use of multi-spectral imaging in highlighting clinical features.

PURPOSE: To report the use of non-invasive multi-spectral imaging of a female choroideremia (CHM) carrier with mild visual symptoms and extensive fundus mottling. OBSERVATION: This was an observational case report study. A symptomatic 42-year-old female with a history of binocular CHM presented for routine ocular examination and underwent review of her clinical and photographic records, optical coherence tomography (OCT), intravenous fluorescein angiography (IVFA) and multi-spectral imaging (MSI). Dilated fundus examination and photography revealed similar outcomes of diffuse mottling with normal looking vessels. IVFA showed large irregular and confluent patches of RPE atrophy in the peripapillary and parapapillary areas as well as the midperiphery, corresponding to the OCT findings. The entire range of MSI imaging (520-940 nm) clearly illustrated the anomalies of the fundus including retinal pigment epithelium (RPE) mottling with melanin clumping not readily seen with the other imaging modalities. MSI fundus autofluorescence (MSI-FAF) showed a spotty hypo and hyperautofluorescent appearance of the fundus, consistent with the observations seen on IVFA and OCT images. CONCLUSION AND IMPORTANCE: MSI significantly improves visualization of the retinal pigment epithelium in choroideremia. The non-invasive nature of MSI technique is a valuable tool in monitoring the effect of retinal and choroidal presentation in patients with CHM.

Assessment of total retinal blood flow using Doppler Fourier Domain Optical Coherence Tomography during systemic hypercapnia and hypocapnia.

The purpose of this study was to investigate changes in total retinal blood flow (RBF) using Doppler Fourier Domain Optical Coherence Tomography (Doppler FD-OCT) in response to the manipulation of systemic partial pressure of CO2 (PETCO2). Double circular Doppler blood flow scans were captured in nine healthy individuals (mean age ± standard deviation: 27.1 ± 4.1, six males) using the RTVue(™) FD-OCT (Optovue). PETCO2 was manipulated using a custom-designed computer-controlled gas blender (RespirAct(™)) connected to a sequential gas delivery rebreathing circuit. Doppler FD-OCT measurements were captured at baseline, during stages of hypercapnia (+5/+10/+15 mmHg PETCO2), return to baseline and during stages of hypocapnia (-5/-10/-15 mmHg PETCO2). Repeated measures analysis of variance (reANOVA) and Tukey's post hoc analysis were used to compare Doppler FD-OCT measurements between the various PETCO2 levels relative to baseline. The effect of PETCO2 on TRBF was also investigated using linear regression models. The average RBF significantly increased by 15% (P < 0.0001) with an increase in PETCO2 and decreased significantly by 10% with a decrease in PETCO2 (P = 0.001). Venous velocity significantly increased by 3.11% from baseline to extreme hypercapnia (P < 0.001) and reduced significantly by 2.01% at extreme hypocapnia (P = 0.012). No significant changes were found in the average venous area measurements under hypercapnia (P = 0.36) or hypocapnia (P = 0.40). Overall, increased and decreased PETCO2 values had a significant effect on RBF outcomes (P < 0.002). In healthy individuals, altered end-tidal CO2 levels significantly changed RBF as measured by Doppler FD-OCT.

Longitudinal assessment of neuropathy in type 1 diabetes using novel ophthalmic markers (LANDMark): study design and baseline characteristics.

AIMS: Corneal nerve morphology and corneal sensation threshold have recently been explored as potential surrogate markers for the evaluation of diabetic neuropathy. We present the baseline findings of the 'Longitudinal Assessment of Neuropathy in type 1 Diabetes using novel ophthalmic Markers'(LANDMark) study. METHODS: The LANDMark study is a 4-year, two-site, natural history study of three participant groups: type 1 diabetes with neuropathy (T1W), type 1 diabetes without neuropathy (T1WO) and control participants without diabetes or neuropathy. All participants undergo a detailed annual assessment of neuropathy including corneal nerve parameters measured using corneal confocal microscopy and corneal sensitivity measured using non-contact corneal aesthesiometry. RESULTS: 76 T1W, 166 T1WO and 154 control participants were enrolled into the study. Corneal sensation threshold was significantly higher (i.e., sensitivity was lower) in T1W (1.0±1.1mbars) than T1WO (0.7±0.7mbars) and controls (0.6±0.4mbars) (p<0.001), with no difference between T1WO and controls. Corneal nerve fibre length was lower in T1W (14.0±6.4mm/mm(2)) compared to T1WO (19.1±5.8mm/mm(2)) and controls (23.2±6.3mm/mm(2)) (p<0.001). Corneal nerve fibre length was lower in T1WO compared to controls. CONCLUSIONS: The LANDMark baseline findings confirm a reduction in corneal sensitivity only in Type 1 patients with neuropathy. However, corneal nerve fibre length is reduced in Type 1 patients without neuropathy with an even greater deficit in Type 1 patients with neuropathy.

A prototype hyperspectral system with a tunable laser source for retinal vessel imaging.

PURPOSE: To describe the technology and determine the within-session repeatability of manual retinal reflectance measurements of arterioles and venules using a prototype hyperspectral retinal camera. METHODS: Six healthy young volunteers (three males, average age 26 ± 4 years) had five repeated sets of retinal images captured between 500 and 600 nm at 5-nm intervals using a newly developed hyperspectral retinal camera. Optical densities were manually extracted for first-degree arterioles and venules and the repeatability of retinal reflectance was compared sequentially. The SDs of the differences between sequential mean values were used as an indication of the variance, while the coefficient of repeatability (COR) and intraclass correlation coefficient (ICC) were used to assess repeatability. RESULTS: The mean difference between each sequential measure was calculated using 21 images from each of the five spectral cubes. The SDs of these values ranged from 0.01 to 0.06 OD units and from 0.01 to 0.07 OD units for first-degree arterioles and venules, respectively. The COR ranged from 0.02 to 0.11 OD units (relative to a mean OD of 0.15 [0.06-0.23] OD units) for arterioles and 0.03 to 0.14 OD units (relative to a mean OD of 0.25 [0.17-0.31] OD units) for venules. Good reliability (P < 0.001) was found for arterioles (ICC: 78.8%-94.4% with a Cronbach's α of 89.6%-97.6%) and for venules (ICC: 63.7%-92.1% with a Cronbach's α of 86.2%-98.1%). CONCLUSIONS: Manual optical density determination with this novel hyperspectral camera showed very good intrasession (and intraobserver) repeatability with a small degree of variance that should form the basis of reliable retinal oxygen saturation values in future imaging research studies. Future automation of retinal vessel reflectance image analyses will likely further improve this repeatability.

Cerebral oxygen saturation: graded response to carbon dioxide with isoxia and graded response to oxygen with isocapnia.

BACKGROUND: Monitoring cerebral saturation is increasingly seen as an aid to management of patients in the operating room and in neurocritical care. How best to manipulate cerebral saturation is not fully known. We examined cerebral saturation with graded changes in carbon dioxide tension while isoxic and with graded changes in oxygen tension while isocapnic. METHODOLOGY/PRINCIPAL FINDINGS: The study was approved by the Research Ethics Board of the University Health Network at the University of Toronto. Thirteen studies were undertaken in healthy adults with cerebral oximetry by near infrared spectroscopy. End-tidal gas concentrations were manipulated using a model-based prospective end-tidal targeting device. End-tidal carbon dioxide was altered ±15 mmHg from baseline in 5 mmHg increments with isoxia (clamped at 110±4 mmHg). End-tidal oxygen was changed to 300, 400, 500, 80, 60 and 50 mmHg under isocapnia (37±2 mmHg). Twelve studies were completed. The end-tidal carbon dioxide versus cerebral saturation fit a linear relationship (R(2) = 0.92±0.06). The end-tidal oxygen versus cerebral saturation followed log-linear behaviour and best fit a hyperbolic relationship (R(2) = 0.85±0.10). Cerebral saturation was maximized in isoxia at end-tidal carbon dioxide of baseline +15 mmHg (77±3 percent). Cerebral saturation was minimal in isocapnia at an end-tidal oxygen tension of 50 mmHg (61±3 percent). The cerebral saturation during normoxic hypocapnia was equivalent to normocapnic hypoxia of 60 mmHg. CONCLUSIONS/SIGNIFICANCE: Hypocapnia reduces cerebral saturation to an extent equivalent to moderate hypoxia.

Optimal image sample size for corneal nerve morphometry.

PURPOSE: Arbitrary numbers of corneal confocal microscopy images have been used for analysis of corneal subbasal nerve parameters under the implicit assumption that these are a representative sample of the central corneal nerve plexus. The purpose of this study is to present a technique for quantifying the number of random central corneal images required to achieve an acceptable level of accuracy in the measurement of corneal nerve fiber length and branch density. METHODS: Every possible combination of 2 to 16 images (where 16 was deemed the true mean) of the central corneal subbasal nerve plexus, not overlapping by more than 20%, were assessed for nerve fiber length and branch density in 20 subjects with type 2 diabetes and varying degrees of functional nerve deficit. Mean ratios were calculated to allow comparisons between and within subjects. RESULTS: In assessing nerve branch density, eight randomly chosen images not overlapping by more than 20% produced an average that was within 30% of the true mean 95% of the time. A similar sampling strategy of five images was 13% within the true mean 80% of the time for corneal nerve fiber length. CONCLUSIONS: The "sample combination analysis" presented here can be used to determine the sample size required for a desired level of accuracy of quantification of corneal subbasal nerve parameters. This technique may have applications in other biological sampling studies.

Corneal markers of diabetic neuropathy.

Diabetic neuropathy is a significant clinical problem that currently has no effective therapy, and in advanced cases, leads to foot ulceration and lower limb amputation. The accurate detection, characterization and quantification of this condition are important in order to define at-risk patients, anticipate deterioration, monitor progression, and assess new therapies. This review evaluates novel corneal methods of assessing diabetic neuropathy. Two new noninvasive corneal markers have emerged, and in cross-sectional studies have demonstrated their ability to stratify the severity of this disease. Corneal confocal microscopy allows quantification of corneal nerve parameters and noncontact corneal esthesiometry, the functional correlate of corneal structure, assesses the sensitivity of the cornea. Both these techniques are quick to perform, produce little or no discomfort for the patient, and are suitable for clinical settings. Each has advantages and disadvantages over traditional techniques for assessing diabetic neuropathy. Application of these new corneal markers for longitudinal evaluation of diabetic neuropathy has the potential to reduce dependence on more invasive, costly, and time-consuming assessments, such as skin biopsy.

Corneal sensitivity as an ophthalmic marker of diabetic neuropathy.

PURPOSE: The objective of this study was to explore the discriminative capacity of non-contact corneal esthesiometry (NCCE) when compared with the neuropathy disability score (NDS) score-a validated, standard method of diagnosing clinically significant diabetic neuropathy. METHODS: Eighty-one participants with type 2 diabetes, no history of ocular disease, trauma, or surgery and no history of systemic disease that may affect the cornea were enrolled. Participants were ineligible if there was history of neuropathy due to non-diabetic cause or current diabetic foot ulcer or infection. Corneal sensitivity threshold was measured on the eye of dominant hand side at a distance of 10 mm from the center of the cornea using a stimulus duration of 0.9 s. The NDS was measured producing a score ranging from 0 to 10. To determine the optimal cutoff point of corneal sensitivity that identified the presence of neuropathy (diagnosed by NDS), the Youden index and "closest-to-(0,1)" criteria were used. RESULTS: The receiver-operator characteristic curve for NCCE for the presence of neuropathy (NDS ≥3) had an area under the curve of 0.73 (p = 0.001) and, for the presence of moderate neuropathy (NDS ≥6), area of 0.71 (p = 0.003). By using the Youden index, for an NDS ≥3, the sensitivity of NCCE was 70% and specificity was 75%, and a corneal sensitivity threshold of 0.66 mbar or higher indicated the presence of neuropathy. When NDS ≥6 (indicating risk of foot ulceration) was applied, the sensitivity was 52% with a specificity of 85%. CONCLUSIONS: NCCE is a sensitive test for the diagnosis of minimal and more advanced diabetic neuropathy and may serve as a useful surrogate marker for diabetic and perhaps other neuropathies.

Repeatability of measuring corneal subbasal nerve fiber length in individuals with type 2 diabetes.

PURPOSE: To analyze the repeatability of measuring nerve fiber length (NFL) from images of the human corneal subbasal nerve plexus using semiautomated software. METHODS: Images were captured from the corneas of 50 subjects with type 2 diabetes mellitus who showed varying severity of neuropathy, using the Heidelberg Retina Tomograph 3 with Rostock Corneal Module. Semiautomated nerve analysis software was independently used by two observers to determine NFL from images of the subbasal nerve plexus. This procedure was undertaken on two occasions, 3 days apart. RESULTS: The intraclass correlation coefficient values were 0.95 (95% confidence intervals: 0.92-0.97) for individual subjects and 0.95 (95% confidence intervals: 0.74-1.00) for observer. Bland-Altman plots of the NFL values indicated a reduced spread of data with lower NFL values. The overall spread of data was less for (a) the observer who was more experienced at analyzing nerve fiber images and (b) the second measurement occasion. CONCLUSIONS: Semiautomated measurement of NFL in the subbasal nerve fiber layer is highly repeatable. Repeatability can be enhanced by using more experienced observers. It may be possible to markedly improve repeatability when measuring this anatomic structure using fully automated image analysis software.

Exploring retinal and functional markers of diabetic neuropathy.

Diabetic peripheral neuropathy (DPN) is one of the most debilitating complications of diabetes. DPN is a major cause of foot ulceration and lower limb amputation. Early diagnosis and management are key factors in reducing morbidity and mortality. Current techniques for clinical assessment of DPN are relatively insensitive for detecting early disease or involve invasive procedures such as skin biopsies. There is a need for less painful, non-invasive, safe evaluation methods. Eye-care professionals already play an important role in the management of diabetic retinopathy but recent studies have indicated that the eye may also be an important site for the diagnosis and monitoring of neuropathy. Corneal nerve morphology is a promising marker of diabetic neuropathy occurring elsewhere in the body. Emerging evidence tentatively suggests that retinal anatomical markers and a range of functional visual indicators could similarly provide useful information regarding neural damage in diabetes, although this line of research is less well established. This review outlines the growing body of evidence supporting a potential diagnostic role for retinal structure and visual functional markers in the diagnosis and monitoring of peripheral neuropathy in diabetes.