Spare the rods and spoil the retina: revisited

Retinal microcirculation: A window into systemic circulation and metabolic disease

The retinal microcirculation system constitutes a unique terminal vessel bed of the systemic circulation, and its perfusion status is directly associated with the neural function of the retina. This vascular network, essential for nourishing various layers of the retina, comprises two primary microcirculation systems: the retinal microcirculation and the choroidal microcirculation, with each system supplying blood to distinct retinal layers and maintaining the associated neural function. The blood flow of those capillaries is regulated via different mechanisms. However, a range of internal and external factors can disrupt the normal architecture and blood flow within the retinal microcirculation, leading to several retinal pathologies, including diabetic retinopathy, macular edema, and vascular occlusions. Metabolic disturbances such as hyperglycemia, hypertension, and dyslipidemia are known to modify retinal microcirculation through various pathways. These alterations are observable in chronic metabolic conditions like diabetes, coronary artery disease, and cerebral microvascular disease due to advances in non-invasive or minimally invasive retinal imaging techniques. Thus, examination of the retinal microcirculation can provide insights into the progression of numerous chronic metabolic disorders. This review discusses the anatomy, physiology and pathophysiology of the retinal microvascular system, with a particular emphasis on the connections between retinal microcirculation and systemic circulation in both healthy states and in the context of prevalent chronic metabolic diseases.

Imaging Biomarkers of Mesopic and Dark-Adapted Macular Functions in Eyes With Treatment-Naïve Mild Diabetic Retinopathy

PURPOSE

To investigate the relationship between imaging biomarkers and mesopic and dark-adapted (ie, scotopic) functions in patients with treatment-naïve mild diabetic retinopathy (DR) and normal visual acuity.

DESIGN

Prospective cross-sectional study.

METHODS

In this study, 60 patients with treatment-naïve mild DR (Early Treatment of Diabetic Retinopathy Study levels 20-35) and 30 healthy control subjects underwent microperimetry, structural optical coherence tomography (OCT), and OCT angiography (OCTA).

RESULTS

The foveal mesopic (22.4 ± 4.5 dB and 25.8 ± 2.0 dB, P = .005), parafoveal mesopic (23.2 ± 3.8 and 25.8 ± 1.9, P < .0001), and parafoveal dark-adapted (21.1 ± 2.8 dB and 23.2 ± 1.9 dB, P = .003) sensitivities were reduced in DR eyes. For foveal mesopic sensitivity, the regression analysis showed a significant topographic association with choriocapillaris flow deficits percentage (CC FD%; β = -0.234, P = .046) and ellipsoid zone (EZ) normalized reflectivity (β = 0.282, P = .048). Parafoveal mesopic sensitivity was significantly topographically associated with inner retinal thickness (β = 0.253, P = .035), deep capillary plexus (DCP) vessel length density (VLD; β = 0.542, P = .016), CC FD% (β = -0.312, P = .032), and EZ normalized reflectivity (β = 0.328, P = .031). Similarly, parafoveal dark-adapted sensitivity was topographically associated with inner retinal thickness (β = 0.453, P = .021), DCP VLD (β = 0.370, P = .030), CC FD% (β = -0.282, P = .048), and EZ normalized reflectivity (β = 0.295, P = .042).

CONCLUSIONS

In treatment-naïve mild DR eyes, both rod and cone functions are affected and they are associated with both DCP and CC flow impairment, which suggests that a macular hypoperfusion at these levels might implicate a reduction in photoreceptor function. Normalized EZ reflectivity may be a valuable structural biomarker for assessing photoreceptor function in DR. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Retinal and Choroidal Effects of Continuous Positive Airway Pressure as Treatment for Sleep Apnea: Results at 12 Months

BACKGROUND

To determine the impacts of continuous positive airway pressure (CPAP) treatment on retinal and choroidal thickness measurement in individuals with obstructive sleep apnea (OSA).

METHODS

Participants were 28 patients with OSA treated with CPAP who were enrolled immediately after diagnosis and graded according to the apnea hypopnea index (AHI) determined in an overnight polysomnography. Inclusion criteria were a new diagnosis of OSA and an indication for CPAP. Participants underwent a full ophthalmologic examination including standard automated perimetry (SAP) and optical coherence tomography (OCT) at the levels peripapillary, macular, and choroidal before CPAP onset, and after three and twelve months of CPAP. The data compared before and after treatment were intraocular pressure, SAP, and the thicknesses peripapillary retinal nerve fiber layer (pRNFL), total retinal (TR), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), photoreceptor layer (PL), and choroidal.

RESULTS

After 3 months of CPAP, we observed thickening of the pRNFL (in 5/6 subfields) (p < 0.004) and TR (in 5/9 subfields) (p < 0.010). At 12 months, thickening persisted in these layers, this time affecting 2/6 and 2/9 subfields, respectively (p < 0.012 and p < 0.001, respectively). Choroidal thinning was observed at the temporal level at both 3 and 12 months compared to measurements before starting CPAP treatment (p = 0.014 and p = 0.038, respectively). SAP remained unchanged. Intraocular pressure was higher at 12 months than at 3 months (p = 0.001).

CONCLUSIONS

12 months of CPAP avoids retinal thinning and normalizes choroidal thickness in OSA patients.

A preliminary observation on rod cell photobiomodulation in treating diabetic macular edema

Purpose

To evaluate the safety and effectiveness of photobiomodulation (PBM) in the treatment of diabetic macular edema (DME).

Methods

It was a single-center, self-controlled prospective study. The clinical records of 12 diabetic retinopathy patients (5 males and 7 females, 20 eyes in total) who were treated with PBM for DME at the Second Affiliated Hospital, Zhejiang University School of Medicine, were analyzed. The mean age was 56 (26-68) years. All the participants received PBM treatment during darkness at night in no less than 5 days per week and no less than 8 ​h per day. In the baseline check and follow-up checks (1, 2, 6, 10, and 12 months after the start of treatment), the best-corrected visual acuity, the thickness of the retina in the macula, and the changes of the fundus lesions were observed. Wilcoxon signed rank test was used to compare the results before and after treatment. P ​< ​0.05 was considered statistically significant.

Results

No fundus complication was observed during follow-up checks. In baseline and 12-month follow-up checks, the best-corrected visual acuity was 71.75 ​± ​12.47 and 79.50 ​± ​10.85, maximal retinal thickness in macular area was 390.95 ​± ​77.12 ​μm and 354.13 ​± ​55.03 ​μm, average retinal thickness in macular area was 334.25 ​± ​36.45 ​μm and 314.31 ​± ​33.28 ​μm, foveal thickness was 287.00 ​± ​46.79 ​μm and 265.63 ​± ​67.14 ​μm. The best-corrected visual acuity, average retinal thickness in macular area in consecutive follow-up results except that in the 1st month showed significant difference compared with baseline results. There were significant difference between every follow-up result and baseline result of maximal retinal thickness in macular area (P ​< ​0.05). All follow-up results of foveal thickness were not significantly different (P ​> ​0.05) from the baseline result, except that in the 6th month (P ​= ​0.049). Obvious improvement could be observed in retinal fundus fluorescein angiography images.

Conclusions

PBM is a safe and effective treatment of DME, which deserves further investigation.

Correlation of Optical Coherence Tomography Angiography Characteristics with Visual Function to Define Vision-Threatening Diabetic Macular Ischemia

The thresholds of macular microvasculature parameters associated with mild visual impairment in diabetic macular ischemia (DMI) patients are unclear. Therefore, this prospective observational study is aimed at demonstrating the optical coherence tomography angiography parameters that best correlate with mild visual impairment (<70 Early Treatment Diabetic Retinopathy Study (ETDRS) letters, Snellen equivalent 20/40) in DMI. The study was completed at the Moorfields Eye Hospital from December 2019 to August 2021. A total of 123 eyes of 87 patients with stable-treated proliferative diabetic retinopathy following panretinal photocoagulation were recruited. DMI was defined as an irregular foveal avascular zone (FAZ) area ≥ 0.5 mm2 or a smaller FAZ area with parafoveal capillary dropout in at least one quadrant. The analysis showed that the whole image deep vascular complex vessel density (DVC VD) in the 3 × 3 mm area had the best discriminatory ability to identify participants with mild visual impairment at 41.9% (area under the curve = 0.77, sensitivity 94%, specificity 54%, likelihood ratio [LR] = 2.04), and the FAZ area had the greatest post-test LR = 4.21 at 0.64 mm2. The 3 × 3 mm whole image DVC VD and FAZ area cutoffs are useful for screening vision-threatening DMI, but DVC VD has low specificity.

Retinal and choroidal thickness measurements in obstructive sleep apnea: impacts of continuous positive airway pressure treatment

PURPOSE

To examine retinal and choroidal thicknesses in individuals with obstructive sleep apnea (OSA) and determine the impacts of continuous positive airway pressure (CPAP) treatment.

METHODS

Prospective follow-up study conducted at a university hospital. 40 patients with OSA, 28 treated with CPAP, and 12 untreated, were enrolled immediately after diagnosis and graded according to the apnea hypopnea index (AHI) determined in an overnight polysomnography. Inclusion criteria were a new diagnosis of OSA and CPAP indicated. Participants underwent a full ophthalmologic examination including optical coherence tomography (OCT) at the peripapillary, macular, and choroidal levels and the same examination 3 months later. Outcome measures were peripapillary retinal nerve fiber layer (RNFL), total retinal (TRT), retinal ganglion cell layer (RGCL), inner plexiform layer (IPL), photoreceptor layer (PL), and choroidal thicknesses.

RESULTS

At 3 months, RGCL thickness was reduced at the inner nasal macula segment in the no-CPAP group (P = 0.016). In + CPAP, increases were produced in RNFL thickness (5/6 segments) and TRT (7/ 9 segments), while choroidal thinning was observed temporally (P = 0.003). At baseline, positive correlation was detected between choroidal thickness and AHI (r = 0.352, P = 0.005) and between IPL thickness (7/9 segments) and AHI (r = 0.414, P < 0.001).

CONCLUSIONS

Initial retinal and choroidal thickening was followed by RGCL thinning over 3 months. In patients receiving CPAP, we observed no thinning of any retinal layer and normalization of choroidal thickness.

SGLT2 Inhibitor-Induced Low-Grade Ketonemia Ameliorates Retinal Hypoxia in Diabetic Retinopathy-A Novel Hypothesis

Diabetic retinopathy (DR) is a well-recognized microvascular complication of diabetes. Growing evidence suggests that, in addition to retinal vascular damage, there is significant damage to retinal neural tissue in DR. Studies reveal neuronal damage before clinically evident vascular lesions and DR is now classified as a neurovascular complication. Hyperglycemia causes retinal damage through complex metabolic pathways leading to oxidative stress, inflammation, vascular damage, capillary ischemia, and retinal tissue hypoxia. Retinal hypoxia is further worsened by high oxygen consumption in the rods. Persistent hypoxia results in increases in vascular endothelial growth factor (VEGF) and other pro-angiogenic factors leading to proliferative DR/macular edema and progressive visual impairment. Optimal glucose control has favorable effects in DR. Other treatments for DR include laser photocoagulation, which improves retinal oxygenation by destroying the high oxygen consuming rods and their replacement by low oxygen consuming glial tissue. Hypoxia is a potent stimulator of VEGF, and intravitreal anti-VEGF antibodies are effective in regressing macular edema and in some studies, retinal neovascularization. In this review, we highlight the complex pathophysiology of DR with a focus on retinal oxygen/fuel consumption and hypoxic damage to retinal neurons. We discuss potential mechanisms through which sodium-glucose cotransporter 2 (SGLT2) inhibitors improve retinal hypoxia-through ketone bodies, which are energetically as efficient as glucose and yield more ATP per molecule of oxygen consumed than fat, with less oxidative stress. Retinal benefits would occur through improved fuel energetics, less hypoxia and through the anti-inflammatory/oxidative stress effects of ketone bodies. Well-designed studies are needed to explore this hypothesis.

Effects of emixustat hydrochloride in patients with proliferative diabetic retinopathy: a randomized, placebo-controlled phase 2 study

PURPOSE

To evaluate the effects of oral emixustat hydrochloride on pro-angiogenic and inflammatory cytokines in the aqueous humor, as well as other ophthalmic parameters, in subjects with proliferative diabetic retinopathy (PDR).

METHODS

Twenty-three patients with PDR, with or without diabetic macular edema (DME), were assigned to emixustat or placebo in daily oral doses ranging from 5 to 40 mg over a step-up titration period, for 84 days. The main outcome measures included levels of IL-1β, IL-6, IL-8, TGFβ-1, and VEGF in the aqueous humor.

RESULTS

Seven of 12 subjects (58%) who were randomized to emixustat and 11 of 12 subjects (92%) who were randomized to placebo completed the study. No statistically significant differences between treatment groups were observed for changes in any of the aqueous humor cytokines tested. However, median VEGF levels were slightly reduced in the emixustat but not the placebo group (- 70.0 pg/mL versus + 42.7 pg/mL, or - 11.8% versus + 6.7%). In a post hoc analysis of all subjects (with or without DME), statistically significant differences between treatment arms in mean changes from baseline in central subfield thickness (CST; emixustat - 11.9 μm, placebo + 36.2 μm; P = 0.076) and total macular volume (TMV; emixustat - 0.13 mm³, placebo + 0.23 mm³; P = 0.026) were observed, both favoring emixustat. Emixustat's safety profile was consistent with prior studies (i.e., the adverse events of delayed dark adaptation and visual impairment were more common in subjects treated with emixustat).

CONCLUSION

Although this pilot study did not demonstrate statistically significant differences in changes in aqueous humor cytokine levels between the emixustat and placebo groups, VEGF levels were slightly reduced in the emixustat but not in the placebo group. In addition, statistically significant differences favoring the emixustat group were observed in CST and TMV among all subjects. These data warrant further investigation of emixustat's potential therapeutic effects in diabetic retinopathy.

TRIAL REGISTRATION

ClinicalTrials.gov identifier: NCT02753400 (April 2016).

Regulation of blood flow in diabetic retinopathy

Blood flow in the retina increases in response to light-evoked neuronal activity, ensuring that retinal neurons receive an adequate supply of oxygen and nutrients as metabolic demands vary. This response, termed "functional hyperemia," is disrupted in diabetic retinopathy. The reduction in functional hyperemia may result in retinal hypoxia and contribute to the development of retinopathy. This review will discuss the neurovascular coupling signaling mechanisms that generate the functional hyperemia response in the retina, the changes to neurovascular coupling that occur in diabetic retinopathy, possible treatments for restoring functional hyperemia and retinal oxygen levels, and changes to functional hyperemia that occur in the diabetic brain.

Age-related change in flicker thresholds with rod- and cone-enhanced stimuli

PURPOSE

Rod and cone photoreceptor-specific tests can be time-consuming. A new non-invasive test is described. The test is based on the measurement of flicker modulation thresholds with rod- and cone-enhanced visual stimuli, which requires only minimum adaptation time. Here, we investigated how the rod-and cone-mediated flicker thresholds vary with age.

METHODS

Monocular thresholds with rod and cone-enhanced stimuli were measured in 140 healthy adults, (age range: 18-75 years), foveally (0°) and at four parafoveal locations, at an eccentricity of 5° in each of the four quadrants using five, adaptive, interleaved staircases. Temporal frequencies, stimulus sizes, background luminance and spectral composition, were adjusted appropriately to achieve approximately 1 log unit separation in sensitivity between the rod- and cone-enhanced stimuli. Spectrally calibrated, 'neutral density' filters were used to enable adequate control of display luminance for rod enhanced stimuli.

RESULTS

The magnitude of central and parafoveal rod thresholds was significantly higher than the central and parafoveal cone thresholds, respectively (p < 0.001) in both the age groups. However, the rate of increase in central rod thresholds (y = 0.45x-12.79; linear regression equation) was not significantly steeper than the rate of increase in central (y = 0.29x-8.53) cone thresholds (p = 0.15). Centrally, cone thresholds showed a better correlation with rod central thresholds for the age > 45 years (Spearman correlation, ρ = 0.74, p < 0.001) compared to age ≤ 45 years (ρ = 0.41, p < 0.001).

CONCLUSIONS

Thresholds with rod- and cone-enhanced stimuli are largely invariant below 45 years of age and increase rapidly above this age. This age-wise normative database can be used as an effective functional-marker to assess photoreceptor sensitivities in retinal diseases.

Hypoxic drive caused type 3 neovascularization in a preclinical model of exudative age-related macular degeneration

Hypoxia associated with the high metabolic demand of rods has been implicated in the pathology of age-related macular degeneration (AMD), the most common cause of adult blindness in the developed world. The majority of AMD-associated severe vision loss cases are due to exudative AMD, characterized by neovascularization. To further investigate the causes and histopathology of exudative AMD, we conditionally induced hypoxia in a novel preclinical AMD model (Pde6gcreERT2/+;Vhl-/-) by targeting Vhl and used multimodal imaging and immunohistochemistry to track the development of hypoxia-induced neovascularization. In addition to developing a preclinical model that phenocopies exudative AMD, our studies revealed that the photoreceptor hypoxic response initiates and drives type 3 neovascularization, mainly in the outer retina. Activation of the VHL-HIF1a-VEGF-EPO pathway in the adult retina led to long-term neovascularization, retinal hemorrhages and compromised retinal layers. Our novel preclinical model would accelerate the testing of therapies that use metabolomic approaches to ameliorate AMD.

Contributions of the Melanopsin-Expressing Ganglion Cells, Cones, and Rods to the Pupillary Light Response in Obstructive Sleep Apnea

Purpose

To investigate the impact of obstructive sleep apnea (OSA) on the contribution of inner and outer retinal photoreceptors to the pupillary light response (PLR).

Methods

Ninety-three eyes from 27 patients with OSA and 25 healthy controls were tested. OSA severity was graded according to the apnea-hypopnea index. PLR was measured monocularly with an eye tracker in a Ganzfeld in response to 1-second blue (470 nm) and red (640 nm) flashes at -3, -2, -1, 0, 1, 2, and 2.4 log cd/m2. Peak pupil constriction amplitude, peak latency, and the postillumination pupil response were measured. The Cambridge Colour Test, standard automatic perimetry, spectral domain optical coherence tomography, polysomnography, and the Pittsburgh Sleep Quality Index were used.

Results

OSA patients have a significantly decreased peak pupil constriction amplitude for blue stimuli at -3, -2, -1, 1 log cd/m2 and at all red flash luminances (P < 0.050), revealing reduction of outer retina contributions to PLR. OSA patients showed reduced peak latency for blue (-2, 0, 2, 2.4 log cd/m2) and red stimuli (-2, 0 log cd/m2; P < 0.040). No significant difference was found in the melanopsin-mediated PLR.

Conclusions

This study is the first to evaluate the inner and outer retinal contributions to PLR in OSA patients. The results showed that the outer retinal photoreceptor contributions to PLR were affected in moderate and severe OSA patients. In contrast, the inner retina contributions to PLR are preserved.

Dyslipidemia in retinal metabolic disorders

The light‐sensitive photoreceptors in the retina are extremely metabolically demanding and have the highest density of mitochondria of any cell in the body. Both physiological and pathological retinal vascular growth and regression are controlled by photoreceptor energy demands. It is critical to understand the energy demands of photoreceptors and fuel sources supplying them to understand neurovascular diseases. Retinas are very rich in lipids, which are continuously recycled as lipid‐rich photoreceptor outer segments are shed and reformed and dietary intake of lipids modulates retinal lipid composition. Lipids (as well as glucose) are fuel substrates for photoreceptor mitochondria. Dyslipidemia contributes to the development and progression of retinal dysfunction in many eye diseases. Here, we review photoreceptor energy demands with a focus on lipid metabolism in retinal neurovascular disorders.

The Relationship Between Retinal Vessel Oxygenation and Spatial Distribution of Retinal Nonperfusion in Retinal Vascular Diseases

Purpose

We study the relationship between retinal vessel oxygenation and the spatial distribution of retinal nonperfusion using ultrawide field angiography in eyes with retinal vascular diseases.

Methods

This prospective single center study recruited 57 eligible eyes from 44 patients with retinal vascular diseases. Retinal oximetry measurements were obtained using the Oxymap T1 device to determine the arteriovenous (AV) difference. Retinal nonperfusion was measured from ultrawide field angiography images taken with the Optos 200TX system and superimposing the images with the concentric rings template to determine the area and distribution of retinal nonperfusion.

Results

Seven (12.3%) eyes had a diagnosis of a branch or hemiretinal vein occlusion, 24 (42.1%) with central retinal vein occlusion and 26 (45.6%) with diabetic retinopathy (11 [19.3%] nonproliferative and 15 [26.3%] proliferative diabetic retinopathy). The correlation between the total area of retinal nonperfusion with the AV difference controlling for age was not statistically significant (R = -0.103, P = 0.449). However, when analyzing the correlation of AV difference with the area of retinal nonperfusion in the posterior pole controlling for age and peripheral nonperfusion, this was significant (R = -0.295, P = 0.029). This was not significant for the area of retinal nonperfusion in the periphery while controlling for posterior pole nonperfusion and age (R = 0.124, P = 0.368).

Conclusions

Retinal nonperfusion has a negative correlation with AV difference measured on retinal oximetry. This correlation is significant in the posterior pole, but not in the peripheral retina.

Flavin homeostasis in the mouse retina during aging and degeneration

Involvement of flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) in cellular homeostasis has been well established for tissues other than the retina. Here, we present an optimized method to effectively extract and quantify FAD and FMN from a single neural retina and its corresponding retinal pigment epithelium (RPE). Optimizations led to detection efficiency of 0.1 pmol for FAD and FMN while 0.01 pmol for riboflavin. Interestingly, levels of FAD and FMN in the RPE were found to be 1.7- and 12.5-fold higher than their levels in the retina, respectively. Both FAD and FMN levels in the RPE and retina gradually decline with age and preceded the age-dependent drop in the functional competence of the retina as measured by electroretinography. Further, quantifications of retinal levels of FAD and FMN in different mouse models of retinal degeneration revealed differential metabolic requirements of these two factors in relation to the rate and degree of photoreceptor degeneration. We also found twofold reductions in retinal levels of FAD and FMN in two mouse models of diabetic retinopathy. Altogether, our results suggest that retinal levels of FAD and FMN can be used as potential markers to determine state of health of the retina in general and more specifically the photoreceptors.

Clinical efficacy and safety of a light mask for prevention of dark adaptation in treating and preventing progression of early diabetic macular oedema at 24 months (CLEOPATRA): a multicentre, phase 3, randomised controlled trial

BACKGROUND

We aimed to assess 24-month outcomes of wearing an organic light-emitting sleep mask as an intervention to treat and prevent progression of non-central diabetic macular oedema.

METHODS

CLEOPATRA was a phase 3, single-blind, parallel-group, randomised controlled trial undertaken at 15 ophthalmic centres in the UK. Adults with non-centre-involving diabetic macular oedema were randomly assigned (1:1) to wearing either a light mask during sleep (Noctura 400 Sleep Mask, PolyPhotonix Medical, Sedgefield, UK) or a sham (non-light) mask, for 24 months. Randomisation was by minimisation generated by a central web-based computer system. Outcome assessors were masked technicians and optometrists. The primary outcome was the change in maximum retinal thickness on optical coherence tomography (OCT) at 24 months, analysed using a linear mixed-effects model incorporating 4-monthly measurements and baseline adjustment. Analysis was done using the intention-to-treat principle in all randomised patients with OCT data. Safety was assessed in all patients. This trial is registered with Controlled-Trials.com, number ISRCTN85596558.

FINDINGS

Between April 10, 2014, and June 15, 2015, 308 patients were randomly assigned to wearing the light mask (n=155) or a sham mask (n=153). 277 patients (144 assigned the light mask and 133 the sham mask) contributed to the mixed-effects model over time, including 246 patients with OCT data at 24 months. The change in maximum retinal thickness at 24 months did not differ between treatment groups (mean change -9·2 μm [SE 2·5] for the light mask vs -12·9 μm [SE 2·9] for the sham mask; adjusted mean difference -0·65 μm, 95% CI -6·90 to 5·59; p=0·84). Median compliance with wearing the light mask at 24 months was 19·5% (IQR 1·9-51·6). No serious adverse events were related to either mask. The most frequent adverse events related to the assigned treatment were discomfort on the eyes (14 with the light mask vs seven with the sham mask), painful, sticky, or watery eyes (14 vs six), and sleep disturbance (seven vs one).

INTERPRETATION

The light mask as used in this study did not confer long-term therapeutic benefit on non-centre-involving diabetic macular oedema and the study does not support its use for this indication.

FUNDING

The Efficacy and Mechanism Evaluation Programme, a Medical Research Council and National Institute for Health Research partnership.

Measurement of dark adaptometry during ISCEV standard flash electroretinography

PURPOSE

Dark adaptometry (DA) is a sensitive test for diagnosis of retinal disease. Patients requiring flash electroretinograms (ERGs) benefit from DA for proper diagnosis. Our purpose was to develop a DA test to be administered during the 20-min dark adaptation period of the flash ERG ( www.iscev.org ) using alternating red and blue stimuli to bias responses in favor of cones (red) and rods (blue).

METHODS

An ERG Ganzfeld was used to measure DA in 21 normal subjects and 21 patients with retinal disease. Subjects underwent 75 s of preadaptation followed by 20 min of DA using an adaptive staircase to detect alternating flashes of red and blue light. Normative red and blue DA was compared to patients at the 10-min midpoint and 20-min final threshold.

RESULTS

At 15 min, normals and patients showed asymptotic plateaus; hence, final thresholds were achieved within 15 min for parameters of this study. In RP and allied conditions (n = 11), 100% showed significantly elevated blue DA thresholds (> 2SD above normal) at 10 and 20 min. In cone diseases, 80% showed abnormal red DA at 10 min and 30% at 20 min, indicating that delayed adaptation is more typical of cone disease.

CONCLUSIONS

Two-color DA is expediently measured during the DA phase of ERGs providing clinical data which separate rod and cone dysfunction critical for accurate diagnosis of various retinal diseases. It provides an additional benchmark for monitoring function over time and invaluable guidance for patients functioning in dimly lit settings.

Do photoreceptor cells cause the development of retinal vascular disease?

The retinal vasculature is affected in a number of clinically important retinopathies, including diabetic retinopathy. There has been a considerable amount of research into the pathogenesis of retinal microvascular diseases, but the potential contribution of the most abundant cell population in the retina, photoreceptor cells, has been largely overlooked. This review summarizes ongoing research suggesting that photoreceptor cells play a critical role in the development of retinal vascular disease in diabetic retinopathy and other retinopathies.

Reduced metabolic function and structural alterations in inherited retinal dystrophies: investigating the effect of peripapillary vessel oxygen saturation and vascular diameter on the retinal nerve fibre layer thickness

PURPOSE

To evaluate the relationship between the peripapillary metabolic alterations [retinal vessel Oximetry (RO)] and the structural findings [retinal vessel diameter and retinal nerve fibre layer thickness (RNFL)] in patients with inherited retinal dystrophies (IRD).

METHODS

Patients with IRD [24 patients with rod-cone dystrophy (RCD), 15 patients with cone-rod dystrophy, 13 patients with inherited maculopathy] and 18 age-matched controls, who underwent RO imaging and spectral domain optical coherence tomography, were included. The average and quadrant oxygen saturation in all four major peripapillary retinal arterioles (A-SO₂ ) and venules (V-SO₂ ) were measured, and their difference (A-V SO₂ ) was calculated. The corresponding retinal vessel diameter of these arterioles (D-A) and venules (D-V) was measured. The data were compared to the peripapillary RNFL thickness within the IRD subgroups and to the data obtained in the controls.

RESULTS

In general, patients with IRD had higher average V-SO₂ values when compared to controls (p ≤ 0.029). Rod-cone dystrophy (RCD) patients differed from controls, but also from patients with other IRDs, when the average and quadrant oxygen saturation values (A-SO₂ and V-SO₂ ) were evaluated (p ≤ 0.026). Within the RCD group, the correlations of RNFL thickness to V-SO₂ , A-V SO₂ , D-A and D-V were significant (p ≤ 0.030), thus indicating a different relationship between the RNFL thickness and the examined parameters, when compared to the other groups.

CONCLUSION

It becomes evident from our combined metabolic-structural approach that a prediction model, to identify which individual is at risk of developing a photoreceptor degeneration of RCD type, can be proposed. It will take into account the peripapillary retinal oxygen saturation, the retinal vessel diameter and the RNFL thickness values.

Retinal oxygen: from animals to humans

This article discusses retinal oxygenation and retinal metabolism by focusing on measurements made with two of the principal methods used to study O₂ in the retina: measurements of PO₂ with oxygen-sensitive microelectrodes in vivo in animals with a retinal circulation similar to that of humans, and oximetry, which can be used non-invasively in both animals and humans to measure O₂ concentration in retinal vessels. Microelectrodes uniquely have high spatial resolution, allowing the mapping of PO₂ in detail, and when combined with mathematical models of diffusion and consumption, they provide information about retinal metabolism. Mathematical models, grounded in experiments, can also be used to simulate situations that are not amenable to experimental study. New methods of oximetry, particularly photoacoustic ophthalmoscopy and visible light optical coherence tomography, provide depth-resolved methods that can separate signals from blood vessels and surrounding tissues, and can be combined with blood flow measures to determine metabolic rate. We discuss the effects on retinal oxygenation of illumination, hypoxia and hyperoxia, and describe retinal oxygenation in diabetes, retinal detachment, arterial occlusion, and macular degeneration. We explain how the metabolic measurements obtained from microelectrodes and imaging are different, and how they need to be brought together in the future. Finally, we argue for revisiting the clinical use of hyperoxia in ophthalmology, particularly in retinal arterial occlusions and retinal detachment, based on animal research and diffusion theory.