Retinal topography and thickness mapping in atrophic age related macular degeneration

Lack of Delta-Sarcoglycan (Sgcd) Results in Retinal Degeneration

Age-related macular degeneration (AMD) is the leading cause of central vision loss and severe blindness among the elderly population. Recently, we reported on the association of the SGCD gene (encoding for δ-sarcoglycan) polymorphisms with AMD. However, the functional consequence of Sgcd alterations in retinal degeneration is not known. Herein, we characterized changes in the retina of the Sgcd knocked-out mouse (KO, Sgcd^-/-). At baseline, we analyzed the retina structure of three-month-old wild-type (WT, Sgcd^+/+) and Sgcd^-/- mice by hematoxylin and eosin (H&E) staining, assessed the Sgcd-protein complex (α-, β-, γ-, and ε-sarcoglycan, and sarcospan) by immunofluorescence (IF) and Western blot (WB), and performed electroretinography. Compared to the WT, Sgcd^-/- mice are five times more likely to have retinal ruptures. Additionally, all the retinal layers are significantly thinner, more so in the inner plexiform layer (IPL). In addition, the number of nuclei in the KO versus the WT is ever so slightly increased. WT mice express Sgcd-protein partners in specific retinal layers, and as expected, KO mice have decreased or no protein expression, with a significant increase in the α subunit. At three months of age, there were no significant differences in the scotopic electroretinographic responses, regarding both a- and b-waves. According to our data, Sgcd^-/- has a phenotype that is compatible with retinal degeneration.

A lasered mouse model of retinal degeneration displays progressive outer retinal pathology providing insights into early geographic atrophy

Early stages of geographic atrophy (GA) age-related macular degeneration is characterised by the demise of photoreceptors, which precedes the loss of underlying retinal pigment epithelial (RPE) cells. Sight-loss due to GA has no effective treatment; reflecting both the complexity of the disease and the lack of suitable animal models for testing potential therapies. We report the development and characterisation of a laser-induced mouse model with early GA-like pathology. Retinas were lasered at adjacent sites using a 810 nm laser (1.9 J/spot), resulting in the development of confluent, hypopigmented central lesions with well-defined borders. Optical Coherence Tomography over 2-months showed progressive obliteration of photoreceptors with hyper-reflective outer plexiform and RPE/Bruch’s membrane (BrM) layers within lesions, but an unaffected inner retina. Light/electron microscopy after 3-months revealed lesions without photoreceptors, leaving the outer plexiform layer apposed to the RPE. We observed outer segment debris, hypo/hyperpigmented RPE, abnormal apical-basal RPE surfaces and BrM thickening. Lesions had wedge-shaped margins, extended zones of damage, activated Müller cells, microglial recruitment and functional retinal deficits. mRNA studies showed complement and inflammasome activation, microglial/macrophage phagocytosis and oxidative stress providing mechanistic insights into GA. We propose this mouse model as an attractive tool for early GA studies and drug-discovery.

Correlation between microaneurysms and retinal thickness in diabetic macular edema

PURPOSE

Microaneurysms commonly are believed to be related causally to retinal thickening in diabetic retinopathy, especially by leaking. The hypothesis that thicker areas of retina in diabetic retinopathy have more microaneurysms per unit area than areas that are not as thick was tested.

METHODS

Retinal thickness analysis was performed with a prototype instrument for 27 eyes of 27 diabetic patients and 22 normal eyes of 22 healthy subjects. Maps of retinal thickness were created, and microaneurysms were counted in zones having four levels of retinal thickness.

RESULTS

There was no increase in either total microaneurysms or apparent leaking microaneurysms per unit area with increasing levels of retinal thickness (P = 0.77 and 0.87, respectively).

CONCLUSION

Some microaneurysms may not cause thickening, or other factors may contribute to retinal thickening in diabetic retinopathy. The results may have implications on the pathogenesis of diabetic macular edema.

A method for three-dimensional imaging of the retina in human eyes

BACKGROUND AND OBJECTIVES

An optical system for three-dimensional imaging of the retinal tissue in human eyes is described.

PATIENTS AND METHODS

A laser beam was projected at an oblique angle on the retina and scanned to acquire 40 optical section images in a 1.0 x 1.5 mm retinal area. Because the incident laser beam was not coaxial with the viewing system, structures at various retinal depths appeared laterally displaced according to their depth location on the optical section image. The optical section images were segmented to construct a series of en face retinal images, parallel to the retinal surface and displaced in depth. Imaging was performed in three control subjects.

RESULTS

A series of 8 depth-displaced en face images of retinal layers was reconstructed in each eye, which allowed enhanced visualization of the retinal structures and vasculatures. En face depth-displaced retinal images provided improved contrast compared with fundus images and delineated the foveal depression and the surrounding retinal vasculatures.

CONCLUSIONS

An optical system for three-dimensional retinal imaging was developed that has potential as a tool for evaluation of retinal pathologies associated with chorioretinal diseases.

Gelatin-based biomimetic tissue adhesive. Potential for retinal reattachment

An adhesive that cures under moist/wet conditions could facilitate surgical procedures for retinal reattachment. We are investigating an adhesive that mimics the factor XIIIa-mediated crosslinking of fibrin that occurs in the late stages of the blood coagulation cascade. Specifically, we use gelatin as the structural protein (in place of fibrin), and crosslink gelatin using a calcium-independent microbial transglutaminase (in place of the calcium-dependent transglutaminase factor XIIIa). Injection of gelatin and microbial transglutaminase (mTG) into the vitreous cavity of Sprague Dawley white rats did not elicit structural or cellular damage to the retina as evidenced from histological evaluation 2 weeks post-injection. Qualitative in vitro studies indicate that the gelatin-mTG adhesive binds to bovine retinal tissue under wet conditions. Quantitative lap-shear tests were performed with more robust bovine tissue from the choroid and sclera. The lap-shear strength of the biomimetic gelatin-mTG adhesive was independent of tissue-type and ranged from 15 to 45 kPa, which is comparable to the values reported for other soft-tissue adhesives. These studies suggest that the mTG-crosslinked gelatin may provide a simple, safe, and effective adhesive for ophthalmic applications.

MACULAR THICKNESS MAPPING IN EXUDATIVE AGE-RELATED MACULAR DEGENERATION

PURPOSE

To report the feasibility of retinal thickness mapping for evaluating thickness differences in retinal areas with and without leakage shown by fluorescein angiography for patients who have age-related macular degeneration with choroidal neovascularization.

METHODS

A custom-built version of the retinal thickness analyzer was used for thickness mapping. Retinal thickness was defined as the separation between vitreoretinal and pigment epithelium-choroid interfaces. Imaging was performed in 1 eye of 10 patients with the clinical diagnoses of age-related macular degeneration and choroidal neovascularization. Patients either had never undergone photodynamic therapy at the time of measurement (untreated) or had received one or more photodynamic therapy treatments (treated). Average retinal thicknesses in selected areas with and without the presence of leakage shown by fluorescein angiography were calculated and compared statistically.

RESULTS

Retinal thickness (mean +/- SD) in areas with leakage (315 +/- 54 microm) was significantly greater than that in areas without leakage (280 +/- 28 microm) (P = 0.03). In untreated patients, areas with leakage (345 +/- 45 microm) were significantly thicker than areas without leakage (289 +/- 23 microm) (P = 0.02). In treated patients, retinal thickness in areas with leakage (271 +/- 33 microm) and without leakage (267 +/- 34 microm) was similar.

CONCLUSION

Retinal thickness mapping may prove to be useful as an adjunct to fluorescein angiography to monitor choroidal neovascularization and its treatment.

Mapping of the retinal pigment epithelium in exudative age related macular degeneration

PURPOSE

To evaluate a novel technique for three-dimensional mapping of the retinal pigment epithelium (RPE) layer in patients with subfoveal choroidal neovascularization (CNV) due to age-related macular degeneration.

METHODS

Scanning with a recent generation retinal thickness analyzer (RTA) was performed in consecutive patients undergoing fluorescein angiography. From a 3 x 3 mm area centered on the fovea, three-dimensional area maps of the RPE level were calculated by external spreadsheet software. Included were 18 eyes with classic CNV, 18 eyes with occult CNV and 18 eyes from age-matched normal subjects. Repeatability was assessed by measuring 17 eyes with CNV 3 times. In ten additional patients, RTA imaging results were compared with cross-sections obtained by optical coherence tomography.

RESULTS

By both methods, distinctive changes in RPE level maps were observed in classic and occult CNV. In classic CNV with the lesion extending over the RPE, only focal irregularities in the anteriorly displaced RPE surface were observed. In contrast, mapping of occult CNV showed a more irregular displacement of the RPE layer. The RPE map standard deviation indicating surface irregularity differed statistically significantly between the groups, with coefficients of variance of 5.9% for controls, 6.1% for classic and 8.8% for occult CNV (P < 0.001). Regarding repeatability, RPE level maps showed 1.2% coefficient of variance and an intra-class correlation coefficient of 0.87 for triplicate measurements in CNV patients.

CONCLUSIONS

Topographic mapping of CNV lesions offers a fast, reproducible method for obtaining three-dimensional morphometric information on the RPE level and to quantify changes.

Optical section retinal imaging and wavefront sensing in diabetes

PURPOSE

To investigate differences in higher-order ocular aberrations and in optical section retinal image resolution between healthy normal and diabetic subjects.

METHODS

An optical imaging system was established for combined retinal optical section imaging and wavefront sensing. A laser beam was expanded and focused to a point on the retina by the optics of the eye. For optical section retinal imaging, a cylindrical lens was placed in the path of the incident laser beam to form a focused line on the retina. Because of the angle between the incident laser and imaging path, an optical section image of the retina was captured. For wavefront sensing, a Shack-Hartmann aberrometer was incorporated in the imaging system. Twenty-two subjects with diabetes (average age, 52 +/- 12 years) and 13 normal subjects (average age, 47 +/- 9 years) were imaged. Retinal depth resolution was determined from the width of the laser line on the retina. Higher-order ocular aberrations were determined from the root mean square of the third to seventh Zernike terms, characterizing the wavefront aberration function. The data were analyzed statistically using Student's t-test and linear regression.

RESULTS

Higher-order ocular aberrations in diabetic subjects were significantly higher than in normal subjects (p=0.03). The retinal image depth resolution in diabetic subjects was significantly lower than in normal subjects (p <0.001). The retinal image depth resolution was inversely correlated with higher-order aberrations (r=-0.5; p=0.007; N=35).

CONCLUSIONS

The results demonstrate disease-related increases in higher-order ocular aberrations that influence retinal image resolution in diabetic eyes. This information is useful for designing high-resolution retinal imaging systems applicable for eyes with retinal disease.

Measurements of Ocular Aberrations and Light Scatter in Healthy Subjects

PURPOSE

To report and validate an optical imaging system that provides measurements of higher order ocular aberrations and light scatter in human eyes.

METHODS

An optical imaging system has been established that provides for combined measurements of ocular aberrations and light scatter. A laser beam was expanded and focused to a point on the retina by the optics of the eye. Wavefront sensing was performed with a Shack-Hartmann aberrometer to determine the wavefront aberration function and calculate the point spread function, giving information on ocular aberrations. A cylindrical lens was placed in the path of the incident laser beam path, and the line spread function was derived from the laser slit, giving information on combined ocular aberrations and light scatter. A relative index for ocular light scatter was determined by subtracting the area under the two line spread functions. Measurements were performed in one eye of 20 normal healthy subjects. The subjects' ages ranged between 21 and 78 years, and the average for all the eyes was 43 +/- 17 years (mean +/- SD).

RESULTS

Higher order ocular aberrations were correlated with subjects' ages (r = 0.6; p = 0.01; N = 20). Combined higher order ocular aberrations and light scatter were correlated with age (r = 0.7; p = 0.0002; N = 20). Light scatter was correlated with age (r = 0.6; p = 0.002; N = 20).

CONCLUSIONS

A method was established to measure age-related changes in ocular higher order aberrations and light scatter. Differentiating the contribution of ocular aberrations and light scatter to the retinal image quality has potential value for anticipating the outcome of procedures that attempt to compensate for ocular aberrations and for providing information on factors that degrade the optical performance of the eye in health and disease.

Eccentricity and measurement variability and repeatability with the retinal thickness analyser

AIMS

To define the variability and repeatability of retinal thickness measurements using the retinal thickness analyser (RTA) and to elucidate any interaction between eccentricity (that is, position relative to the fovea) and variability and repeatability.

METHODS

The sample comprised 20 normal subjects of mean age 33 years. Each subject attended for two visits. Repeated RTA scans were acquired centred on the fovea and for any one of the four possible non-foveal scan areas. The mean retinal thickness (+SD) was calculated for a series of concentric circular bands centred on fixation. A repeated measures analysis of variance (ANOVA) was used to determine any significant interaction between the variability of RTA thickness values and eccentricity.

RESULTS

The group mean coefficient of variation and coefficient of repeatability were highest at the fovea. The repeated measures ANOVA revealed that the within test variability of RTA measurements varied significantly with eccentricity (p<0.0001). Similarly, the between test repeatability varied significantly with eccentricity (p = 0.045).

CONCLUSION

The significantly elevated within test variability and between test repeatability in the foveal area need to be considered when using the RTA to evaluate patients with macular disease.