Quantification of nerve fiber layer thickness in normal and glaucomatous eyes using optical coherence tomography

Optical coherence tomography scan circle location and mean retinal nerve fiber layer measurement variability

PURPOSE

To investigate the effect on optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) thickness measurements of varying the standard 3.4-mm-diameter circle location.

METHODS

The optic nerve head (ONH) region of 17 eyes of 17 healthy subjects was imaged with high-speed, ultrahigh-resolution OCT (hsUHR-OCT; 501 x 180 axial scans covering a 6 x 6-mm area; scan time, 3.84 seconds) for a comprehensive sampling. This method allows for systematic simulation of the variable circle placement effect. RNFL thickness was measured on this three-dimensional dataset by using a custom-designed software program. RNFL thickness was resampled along a 3.4-mm-diameter circle centered on the ONH, then along 3.4-mm circles shifted horizontally (x-shift), vertically (y-shift) and diagonally up to +/-500 microm (at 100-microm intervals). Linear mixed-effects models were used to determine RNFL thickness as a function of the scan circle shift. A model for the distance between the two thickest measurements along the RNFL thickness circular profile (peak distance) was also calculated.

RESULTS

RNFL thickness tended to decrease with both positive and negative x- and y-shifts. The range of shifts that caused a decrease greater than the variability inherent to the commercial device was greater in both nasal and temporal quadrants than in the superior and inferior ones. The model for peak distance demonstrated that as the scan moves nasally, the RNFL peak distance increases, and as the circle moves temporally, the distance decreases. Vertical shifts had a minimal effect on peak distance.

CONCLUSIONS

The location of the OCT scan circle affects RNFL thickness measurements. Accurate registration of OCT scans is essential for measurement reproducibility and longitudinal examination (ClinicalTrials.gov number, NCT00286637).

Tracking retinal nerve fiber layer loss after optic neuritis: a prospective study using optical coherence tomography

INTRODUCTION

Optic neuritis causes retinal nerve fiber layer damage, which can be quantified with optical coherence tomography. Optical coherence tomography may be used to track nerve fiber layer changes and to establish a time-dependent relationship between retinal nerve fiber layer thickness and visual function after optic neuritis.

METHODS

This prospective case series included 78 patients with optic neuritis, who underwent optical coherence tomography and visual testing over a mean period of 28 months. The main outcome measures included comparing inter-eye differences in retinal nerve fiber layer thickness between clinically affected and non-affected eyes over time; establishing when RNFL thinning stabilized after optic neuritis; and correlating retinal nerve fiber layer thickness and visual function.

RESULTS

The earliest significant inter-eye differences manifested 2-months after optic neuritis, in the temporal retinal nerve fiber layer. Inter-eye comparisons revealed significant retinal nerve fiber layer thinning in clinically affected eyes, which persisted for greater than 24 months. Retinal nerve fiber thinning manifested within 6 months and then stabilized from 7 to 12 months after optic neuritis. Regression analyses demonstrated a threshold of nerve fiber layer thickness (75 microm), which predicted visual recovery after optic neuritis.

CONCLUSIONS

Retinal nerve fiber layer changes may be tracked and correlated with visual function within 12 months of an optic neuritis event.

Detection of early glaucoma with optical coherence tomography (StratusOCT)

PURPOSE

To evaluate the performance of optical coherence tomography (StratusOCT) for discriminating eyes with early glaucoma from normal eyes.

METHODS

Thirty eyes with established early glaucomatous visual field defects (EGVF group), 30 eyes with evidence of early glaucomatous optic neuropathy with normal standard achromatic perimetry [early glaucoma by disc (EGD)], and 33 age-matched normal eyes with good quality StratusOCT nerve fiber layer (NFL) images were enrolled. Average NFL thickness and NFL thickness at quadrants and sectors, areas under receiver operator characteristic curves, and sensitivities at 80% and 90% specificity were evaluated.

RESULTS

The average (+/-SD) mean deviation in the EGVF group was -3.4 (+/-1.7) dB. Receiver operator characteristic curves showed areas under the curve (AUC) for NFL thickness in the superior quadrant (AUC=0.75+/-0.07) and in the inferior quadrant (AUC=0.94+/-0.03) to be the best StratusOCT parameters for discrimination of normal controls from EGD and EGVF eyes, respectively. The best parameter for detection of EGD eyes at 80% and 90% specificities was NFL thickness at superior quadrant (51% and 36% sensitivities, respectively). The best parameter for detection of EGVF eyes at 80% and 90% specificities was NFL thickness in the inferior quadrant (90% and 87% sensitivities, respectively).

CONCLUSIONS

Optical coherence tomography (StratusOCT) showed good sensitivity and specificity in a group of glaucoma patients with early visual field loss. In patients with normal visual fields in whom the optic disc appeared glaucomatous to glaucoma specialists, half were confirmed to have StratusOCT findings consistent with damage from glaucoma.

Retinal nerve fiber layer thickness in normal, ocular hypertensive, and glaucomatous Indian eyes: an optical coherence tomography study

PURPOSE

To determine retinal nerve fiber layer (RNFL) thickness measurements in normal, ocular hypertensive (OHT), and glaucomatous Asian Indian eyes.

METHODS

This prospective observational cross-sectional study included patients with OHT, primary open angle glaucoma (POAG), and age-matched normal controls. The global and 4-quadrant average RNFL thickness was measured using the Stratus OCT. The main outcome measures were differences in RNFL thickness measurements between the 3 groups. The discriminating power of each parameter was evaluated by calculating areas under receiver operating characteristic curves (AROCs).

RESULTS

Twenty-three eyes of 23 POAG patients, 24 eyes of 24 OHT, and 48 eyes of 48 normal controls were analyzed. The superior, inferior, and global RNFL measurements were significantly thinner in OHTs compared with normals (P=0.031, 0.019, and 0.022, respectively). All 5 RNFL parameters were significantly thinner in the POAG group compared with OHT group (P<0.001). Parameters with largest AROCs for distinguishing glaucoma from OHT were average and inferior average RNFL measurements (0.989 and 0.979, respectively). Inferior and superior RNFL measurements had largest AROCs (0.717 and 0.700, respectively) to distinguish OHT from normal eyes.

CONCLUSIONS

Stratus OCT detected significant quantitative differences in RNFL thickness between normal, OHT, and glaucomatous Asian Indian eyes.

Comparison of optical coherence tomography and scanning laser polarimetry in glaucoma, ocular hypertension, and suspected glaucoma

BACKGROUND AND OBJECTIVE

To compare the performance of the newest generation optical coherence topography (OCT) and scanning laser polarimetry with variable corneal compensation (SLP-VCC) in eyes with glaucoma, ocular hypertension, and suspected glaucoma.

PATIENTS AND METHODS

One eye each of 84 patients (30 with glaucoma, 26 with suspected glaucoma, and 28 with ocular hypertension) was included in the study. Retinal nerve fiber layer (RNFL) thickness was measured with both technologies and thickness parameters were compared in the three groups of eyes. The correspondence of RNFL thickness measurements with visual field function was also studied.

RESULTS

Average OCT-RNFL thickness was found to have a statistically significant difference between patients with glaucoma and either suspected glaucoma or ocular hypertension. A statistically significant correlation between the average RNFL thicknesses measured by the two different technologies was shown only in the glaucoma group. A significant correlation with visual field mean deviation was found for superior average RNFL thickness as measured by SLP and for nerve fiber indicator and average and inferior average RNFL thickness as measured by OCT in glaucomatous eyes. Regression analysis indicated nerve fiber indicator to be the most valuable factor in predicting mean deviation.

CONCLUSION

RNFL thickness measurements obtained with OCT and SLP-VCC correlate well only in eyes with more advanced glaucomatous damage. The nerve fiber indicator parameter derived by SLP correlated best with mean deviation.

Stratus-OCT imaging in early glaucomatous and in ocular hypertensive patients with and without frequency-doubling technology abnormalities

AIM

To compare Stratus-OCT measurements in controls, ocular hypertensive (OHT) patients with (FDT+) and without (FDT-) frequency-doubling technology (FDT) abnormalities, and in patients affected with early primary open-angle glaucoma (POAG).

METHODS

Thirty-two controls, 78 OHT patients (38 FDT- and 40 FDT+), and 45 early POAG patients (six FDT- and 39 FDT+) underwent the following tests within 3 months: standard automated perimetry (SAP) HFA 30-2; FDT N-30-F; and, Stratus-OCT imaging with retinal nerve fibre layer (RNFL) and optic nerve head (ONH) scans. One eye per patient was considered. Differences among groups were evaluated using the Kruskal-Wallis, analysis of variance, and Duncan's tests.

RESULTS

There were no significant differences in all Stratus-OCT parameters between POAG and OHT FDT+ patients. Statistically significant differences were found between the control group and both the POAG and OHT FDT+ groups for 15 of the 21 Stratus-OCT parameters. Control eyes compared to OHT FDT- showed significant differences in 13 of the 21 parameters. The comparison between the OHT FDT- group, and both the POAG and OHT FDT+ group resulted in 13 of the 21 parameters to be significantly different.

CONCLUSIONS

Stratus-OCT seems to show a higher ability in detecting significant differences between healthy, OHT, and early POAG eyes when compared to SAP and FDT. This suggests that the Stratus-OCT could show structural abnormalities before SAP or FDT visual field defects appear in patients at risk of developing glaucoma, which may be beneficial in making therapeutic decisions, especially in OHT patients.

Changes in visual fields and lateral geniculate nucleus in monkey laser-induced high intraocular pressure model

Monkey eyes are useful for ophthalmologic research into eye diseases because their histological and functional properties are very similar to those of humans. The monkey laser-induced high intraocular pressure (IOP) model is a common model for ophthalmologic research, especially into glaucoma. Although several studies using this model have focused on changes in visual field, retinal ganglion cells (RGC), and lateral geniculate nucleus (LGN), clear relationships among these changes in one and the same monkey have not been established. We therefore examined visual field changes, RGC and LGN numbers, and glial fibrous acidic protein (GFAP) immunohistochemistry in the LGN in each of two monkeys. Visual field sensitivity, RGC number, and neuronal density of LGN were all decreased by high IOP. The relationship between loss of RGC and decrease in visual field sensitivity depended on the eccentricity from the fovea. Moreover, LGN immunohistochemistry revealed greater increases in GFAP expression in the layers receiving a neuronal input from the high IOP eye than in those receiving a neuronal input from the contralateral untreated eye. From these results, we suggest that glaucoma may lead to changes in glial function not only in the retina, but also in the visual pathway, and that such central nervous system changes may be a hallmark of neuropathy in glaucoma, as in other neurodegenerative diseases.

Retinal nerve fibre layer of perimetrically unaffected eyes of glaucoma patients: an optical coherence tomography study

PURPOSE

The aim of this study is to evaluate whether optical coherence tomography (StratusOCT) may detect early changes in perimetrically unaffected (PU) fellow eyes of glaucomatous patients by assessing retinal nerve fibre layer (RNFL) thickness parameters.

METHODS

Thirty-seven glaucomatous patients with unilateral field loss and 34 age-matched controls were recruited. In glaucoma patients, PU and perimetrically affected fellow eyes were analysed separately. For each group, mean values (+/-SD) of RNFL thickness parameters were calculated and comparisons between fellow eyes of glaucoma patients and between healthy and PU eyes of glaucoma patients conducted with paired t-test and Mann-Whitney U-test, respectively. Proportion of clock-hour sectors flagged with probability <5% or <1% was collected and differences between healthy and PU eyes were evaluated on Fisher exact test.

RESULTS

Global (Average Thickness) and sectoral parameters (Inferior and Nasal Average), Maximum thickness-minimum thickness (Max-min), as well as 2-o'clock (nasal side) and 6-o'clock sectors resulted significantly thinner in PU eyes than in control group. Proportion of eyes with clock-hour position flagged with probability <5% or <1% was not significantly different between healthy and PU eyes.

CONCLUSION

Despite a standard automated perimetry within normal limits, the StratusOCT detected both localized and diffuse RNFL thinning in PU eyes of glaucoma patients. These eyes should be considered at risk of developing functional damage over time and consequently require thorough monitoring for detecting any sign of progression.

Clinical applications and new developments of optical coherence tomography: an evidence-based review

Optical coherence tomography (OCT) is a new imaging modality that has increasingly become an indispensable tool in clinical practice for the diagnosis and management of ocular diseases involving the macula, optic nerve and anterior segment. The instrument is an advanced imaging technique that provides unprecedented high resolution and cross-sectional tomographic images of the ocular microstructure in situ, and in real time. Since its introduction about four years ago, a multitude of advantages has made OCT an essential instrument in ophthalmic imaging. The technique has fast image acquisition speed and non-contact, non-invasive applicability, allowing a non-excisional 'optical biopsy' to be performed. The purpose of this paper is to provide an evidence-based review of the increasing role of OCT in the diagnosis and management of ocular disorders, particularly in age-related macular degeneration, diabetic macular oedema, macular hole, epiretinal membrane and glaucoma. Being one of the first users of OCT in Australia, our clinical experiences will be highlighted and clinical examples of various conditions will be presented to provide an overview of the immense implications of OCT in practice. The latest developments of the OCT revolution, in relation to combining OCT with fundus photography and scanning laser ophthalmoscopy, will also be described. New developments of three-dimensional visualisation of tissue morphology with future models of ultra-high speed, ultra-high resolution OCT may further enhance the early diagnosis, monitoring of disease progression and assessment of treatment efficacy, facilitated by this powerful technology.

The role of OCT in glaucoma management

Clinical examination of the optic nerve and achromatic automated perimetry is the gold standard for the management of glaucoma. However, there is an increasing need for an objective evaluation of the optic nerve structure, particularly for preperimetric glaucoma. Optical coherence tomography (OCT) is a noninvasive tool that measures retinal nerve fiber layer (RNFL) thickness based on its optical properties. Computer image processing algorithms estimate NFL thickness from circumpapillary OCT images that are acquired in cylindrical sections surrounding the optic disc. Average values of NFL thickness can be calculated in the four quadrants or the 12 o'clock position sectors around the optic disc. The mean NFL thickness around the entire disc can also be calculated. NFL thickness values may be compared to a normative database. Although this technique offers objectivity, rapidity, and reproducibility, it is largely influenced by the variability of optic disc size and the number of nerve fibers among individuals. At present, OCT is a good instrument to diagnose early glaucoma, but cannot be used to exclude it. New technologies, like spectral domain and ultra-high resolution, which are already available, will overcome the limitations of OCT.

Phase retardation measurement of retinal nerve fiber layer by polarization-sensitive spectral-domain optical coherence tomography and scanning laser polarimetry

Phase retardation of in vivo human retinal nerve fiber layer (RNFL) is quantitatively measured by two methods--polarization-sensitive spectral-domain optical coherence tomography (PS-SD-OCT) and scanning laser polarimetry (SLP). An en face cumulative phase retardation map is calculated from the three-dimensional (3-D) phase retardation volume of healthy and glaucomatous eyes measured by PS-SD-OCT. It is shown that the phase retardation curves around the optic nerve head measured by PS-SD-OCT and SLP have similar values except near the retinal blood vessels. PS-SD-OCT can measure the cumulative phase retardation of RNFL as well as SLP, which will allow the evaluation of RNFL for glaucomatous eyes.

The role of optical coherence tomography in multiple sclerosis: Expert panel consensus

Optic neuritis (ON), a common manifestation of multiple sclerosis (MS), often occurs as the initial manifestation of central nervous system demyelination or develops during the course of this disease. Since the retinal nerve fiber layer (RNFL) is composed only of unmyelinated axons, measuring RNFL thickness represents a viable method of monitoring axonal loss in these patients. Optical coherence tomography (OCT) is a noninvasive, noncontact, accurate, and reproducible technique that quantitates the thickness of the peripapillary RNFL, fovea, and macula. Because of its potential role in defining axonal loss in ON and in assessing longitudinal changes in the RNFL before and after MS treatment, a multidisciplinary expert panel was charged with the following tasks: assess the current capabilities of OCT; review the current data about OCT, ON, and MS; and determine whether OCT could be a primary or secondary outcome measure in future MS clinical trials. The panel concluded that: [1] OCT is valid and reproducible; [2] OCT has yielded some important limited data concerning cross-sectional studies with ON and MS; [3] more studies are required to correlate OCT results with other measures of MS disease activity; [4] after correlation with these other measures and upon agreement of standardized technical and statistical methods, OCT may evolve into a important primary or secondary outcome metric for MS clinical trials and patient care.

Retinal prostheses: current challenges and future outlook

Blindness from retinal diseases, including age-related macular degeneration (AMD) and retinitis pigmentosa (RP), usually causes a significant decline in quality of life for affected patients. Currently there is no cure for these conditions. However, over the last decade, several groups have been developing retinal prostheses which hopefully will provide some degree of improved visual function to these patients. Several such devices are now in clinical trials. Unfortunately, the possibility of electrode or tissue damage limits excitation schemes to those that may be employed with electrodes that have relatively low charge densities. Further, the excitation thresholds that have been required to achieve vision to date, in general, are relatively high. This may result in part from poor apposition between neurons and the stimulating electrodes and is confounded by the effects of the photoreceptor loss, which initiates other pathology in the surviving retinal tissue. The combination of these and other factors imposes a restriction on the pixel density that can be used for devices that actively deliver electrical stimulation to the retina. The resultant use of devices with relatively low pixel densities presumably will limit the degree of visual resolution that can be obtained with these devices. Further increases in pixel density, and therefore increased visual acuity, will necessitate either improved electrode-tissue biocompatibility or lower stimulation thresholds. To meet this challenge, innovations in materials and devices have been proposed. Here, we review the types of retinal prostheses investigated, the extent of their current biocompatibility and future improvements designed to surmount these limitations.

Application of shape-based analysis methods to OCT retinal nerve fiber layer data in glaucoma

PURPOSE

(1) To evaluate the performance of shape-based analysis [wavelet-Fourier analysis (WFA) and fast Fourier analysis (FFA)] applied to retinal nerve fiber layer (RNFL) thickness values obtained from the optical coherence tomograph (OCT) to discriminate healthy and glaucomatous eyes. (2) To compare the performance of the shape-based metrics to that of the standard OCT output measures (Inferior Average and Average Thickness).

METHODS

RNFL values were obtained from 152 eyes of 152 individuals (83 healthy and 69 "mild"-stage perimetric glaucoma). WFA and FFA were performed on the RNFL values and linear discriminant functions for both were obtained using Fisher linear discriminant analysis. Performance was evaluated by calculating sensitivity, specificity, and area under the receiver operating characteristic (ROC) curve (ROC area).

RESULTS

The ROC area of the shape-based methods [0.94 (WFA) and 0.88 (FFA)] was greater than that of OCT metrics [0.81 (Inferior Average) and 0.74 (Average Thickness)]. Specifically, WFAs performance was significantly better than both the FFA (P=0.009) and the Inferior Average (P=0.001). Inferior average performed significantly better than Average Thickness (P=0.006).

CONCLUSIONS

The ability to differentiate glaucomatous from healthy eyes using stratus OCT measurements is improved by using these analysis methods that emphasize the shape of the RNFL thickness pattern.

Retinal sensitivity and retinal nerve fiber layer thickness measured by optical coherence tomography in glaucoma

PURPOSE

To define the relationship between retinal light sensitivity and peripapillary retinal nerve fiber layer (RNFL) thickness as measured using the Stratus optical coherence tomograph (OCT).

DESIGN

Prospective study.

METHODS

SETTING

Institutional.

STUDY POPULATION

A total of 126 healthy subjects, 42 patients with ocular hypertension (OHT) and 64 patients with primary open-angle glaucoma (POAG) were examined by the 24/II program of the Humphrey Field Analyzer (Carl Zeiss Meditec, Inc, Dublin, California, USA), and the Fast RNFL Thickness examination of the OCT. Individual visual field (VF) test scores and peripapillary RNFL thickness measurements were grouped into six topographically corresponding sectors whose mean values were then calculated. One eye per patient was randomly chosen.

MAIN OUTCOME MEASURES

The correlations between mean retinal sensitivity (expressed both in decibel and unlogged scales) and RNFL thickness were described with linear and logarithmic regression analyses.

RESULTS

With reference to all 232 individuals, the equation that best explained the model was logarithmic when using the decibel scale, and either linear or logarithmic when using the unlogged scale. A statistically significant, age-adjusted correlation between function and structure was found in most sectors both using the decibel (logarithmic regression analysis r(2) 0.24 to 0.61, P < .001), and the unlogged scale (r(2) 0.37 to 0.53, P < .001 in both linear and logarithmic regression analyses).

CONCLUSIONS

When using the decibel scale, the logarithmic equation may better explain the function/structure relationship between retinal sensitivity and OCT-measured RNFL thickness along a wide spectrum of glaucoma continuum. These results support the need to combine functional and structural tests in the detection of early glaucoma.

A framework for comparing structural and functional measures of glaucomatous damage

While it is often said that structural damage due to glaucoma precedes functional damage, it is not always clear what this statement means. This review has two purposes: first, to show that a simple linear relationship describes the data relating a particular functional test (standard automated perimetry (SAP)) to a particular structural test (optical coherence tomography (OCT)); and, second, to propose a general framework for relating structural and functional damage, and for evaluating if one precedes the other. The specific functional and structural tests employed are described in Section 2. To compare SAP sensitivity loss to loss of the retinal nerve fiber layer (RNFL) requires a map that relates local field regions to local regions of the optic disc as described in Section 3. When RNFL thickness in the superior and inferior arcuate sectors of the disc are plotted against SAP sensitivity loss (dB units) in the corresponding arcuate regions of the visual field, RNFL thickness becomes asymptotic for sensitivity losses greater than about 10dB. These data are well described by a simple linear model presented in Section 4. The model assumes that the RNFL thickness measured with OCT has two components. One component is the axons of the retinal ganglion cells and the other, the residual, is everything else (e.g. glial cells, blood vessels). The axon portion is assumed to decrease in a linear fashion with losses in SAP sensitivity (in linear units); the residual portion is assumed to remain constant. Based upon severe SAP losses in anterior ischemic optic neuropathy (AION), the residual RNFL thickness in the arcuate regions is, on average, about one-third of the premorbid (normal) thickness of that region. The model also predicts that, to a first approximation, SAP sensitivity in control subjects does not depend upon RNFL thickness. The data (Section 6) are, in general, consistent with this prediction showing a very weak correlation between RNFL thickness and SAP sensitivity. In Section 7, the model is used to estimate the proportion of patients showing statistical abnormalities (worse than the 5th percentile) on the OCT RNFL test before they show abnormalities on the 24-2 SAP field test. Ignoring measurement error, the patients with a relatively thick RNFL, when healthy, will be more likely to show significant SAP sensitivity loss before statistically significant OCT RNFL loss, while the reverse will be true for those who start with an average or a relatively thin RNFL when healthy. Thus, it is important to understand the implications of the wide variation in RNFL thickness among control subjects. Section 8 describes two of the factors contributing to this variation, variations in the position of blood vessels and variations in the mapping of field regions to disc sectors. Finally, in Sections 7 and 9, the findings are related to the general debate in the literature about the relationship between structural and functional glaucomatous damage and a framework is proposed for understanding what is meant by the question, 'Does structural damage precede functional damage in glaucoma?' An emphasis is placed upon the need to distinguish between "statistical" and "relational" meanings of this question.

Objective analysis of retinal function in HIV-positive children without retinitis using optical coherence tomography

PURPOSE

To assess the retinal nerve fiber layer thickness in children with human immunodeficiency virus disease without cytomegalovirus retinitis or visual symptoms.

METHODS

Thirty-eight eyes of 19 human immunodeficiency virus-positive children (group A) with visual acuity of 20/20 or better, normal color vision testing and no ophthalmoscopically detectable disorders were prospectively examined. All subjects of group A had no history of cytomegalovirus retinitis and CD4 counts consistently above 100. Patients in group B (40 eyes of 21 patients) were human immunodeficiency virus-negative age-matched control subjects. Thickness of retinal nerve fiber layer along a 3.4-mm-diameter circle centered on the optic nerve head was evaluated using third-generation optical coherence tomography. CD8 T-lymphocyte count, presence of systemic infection, hemoglobin, hematocrit and serum beta-microglobulin levels were also recorded.

RESULTS

The mean overall retinal nerve fiber layer thickness in groups A and B were 89.2 +/- 24.01 microm and 102.82 +/- 29.168 microm (SD) respectively. The difference was considered extremely significant (P < 0.0001). Group A had significantly thinner average nerve fiber layer in temporal, nasal, superior and inferior retinal areas.

CONCLUSIONS

Significant retinal nerve fiber layer thinning occurs in human immunodeficiency virus-positive children with no visual impairment or ophthalmologic evidence or retinitis.

Improved visualization of glaucomatous retinal damage using high-speed ultrahigh-resolution optical coherence tomography

PURPOSE

To test if improving optical coherence tomography (OCT) resolution and scanning speed improves the visualization of glaucomatous structural changes as compared with conventional OCT.

DESIGN

Prospective observational case series.

PARTICIPANTS

Healthy and glaucomatous subjects in various stages of disease.

METHODS

Subjects were scanned at a single visit with commercially available OCT (StratusOCT) and high-speed ultrahigh-resolution (hsUHR) OCT. The prototype hsUHR OCT had an axial resolution of 3.4 mum (3 times higher than StratusOCT), with an A-scan rate of 24 000 hertz (60 times faster than StratusOCT). The fast scanning rate allowed the acquisition of novel scanning patterns such as raster scanning, which provided dense coverage of the retina and optic nerve head.

MAIN OUTCOME MEASURES

Discrimination of retinal tissue layers and detailed visualization of retinal structures.

RESULTS

High-speed UHR OCT provided a marked improvement in tissue visualization as compared with StratusOCT. This allowed the identification of numerous retinal layers, including the ganglion cell layer, which is specifically prone to glaucomatous damage. Fast scanning and the enhanced A-scan registration properties of hsUHR OCT provided maps of the macula and optic nerve head with unprecedented detail, including en face OCT fundus images and retinal nerve fiber layer thickness maps.

CONCLUSION

High-speed UHR OCT improves visualization of the tissues relevant to the detection and management of glaucoma.

Optical coherence tomography: a review of clinical development from bench to bedside

Since its introduction, optical coherence tomography (OCT) technology has advanced from the laboratory bench to the clinic and back again. Arising from the fields of low coherence interferometry and optical time- and frequency-domain reflectometry, OCT was initially demonstrated for retinal imaging and followed a unique path to commercialization for clinical use. Concurrently, significant technological advances were brought about from within the research community, including improved laser sources, beam delivery instruments, and detection schemes. While many of these technologies improved retinal imaging, they also allowed for the application of OCT to many new clinical areas. As a result, OCT has been clinically demonstrated in a diverse set of medical and surgical specialties, including gastroenterology, dermatology, cardiology, and oncology, among others. The lessons learned in the clinic are currently spurring a new set of advances in the laboratory that will again expand the clinical use of OCT by adding molecular sensitivity, improving image quality, and increasing acquisition speeds. This continuous cycle of laboratory development and clinical application has allowed the OCT technology to grow at a rapid rate and represents a unique model for the translation of biomedical optics to the patient bedside. This work presents a brief history of OCT development, reviews current clinical applications, discusses some clinical translation challenges, and reviews laboratory developments poised for future clinical application.

Follow-up of nonarteritic anterior ischemic optic neuropathy with optical coherence tomography

PURPOSE

To study the characteristics of nonarteritic anterior ischemic optic neuropathy (NAION) as measured with optical coherence tomography (OCT) at diagnosis and during the first year after the episode.

DESIGN

Cohort study.

PARTICIPANTS

Twenty-seven patients diagnosed with NAION in our center between April 1, 2004 and March 31, 2006.

METHODS

Patients diagnosed with NAION underwent at the time of diagnosis and 6 weeks and 3, 6, and 12 months after presentation a complete ophthalmologic evaluation, including determination of Snellen visual acuity (VA), visual fields (VFs) (standard automated perimetry, Swedish Interactive Threshold Algorithm strategy 24-2), and optic nerve head (ONH) scanning with StratusOCT.

MAIN OUTCOME MEASURES

Characteristics of the ONH and their relationship with VA and VFs.

RESULTS

Initial mean retinal nerve fiber layer (RNFL) thickness in the affected eye was 200.9 microm (standard deviation [SD], 52.3 microm); this represented a 96.4% increase relative to the fellow eye. Percentages of RNFL loss 3, 6, and 12 months after onset were 38.9%, 42.3%, and 43.9%, respectively. At the 6-month visit, RNFL percentage decreases for the superior, nasal, inferior, and temporal quadrants were 51.5%, 28.5%, 41.2%, and 38.2%, respectively. Reduction in the superior quadrant RNFL thickness was statistically higher. Using regression analysis, it was found that for every micrometer of mean RNFL thickness lost there was a 2-decibel decrease in VF mean deviation (MD) and that there was a 1-line drop in Snellen VA for every 1.6 microm lost. The mean ONH area was 2.6 mm2 (SD, 0.4) in the unaffected eye; there was no correlation with VA, VF MD, or RNFL thickness of the affected eye at the last follow-up visit.

CONCLUSIONS

Optical coherence tomography can diagnose optic disc edema and monitor RNFL loss over time. It is most useful at onset and 6 months after NAION, when RNFL loss has reached a plateau and is correlated with visual function. Future studies that aim to determine if a drug or intervention is useful for treating NAION may include OCT assessment of the RNFL thickness, because it provides an objective outcome measure correlated with visual performance.

Minimum distance mapping using three-dimensional optical coherence tomography for glaucoma diagnosis

Objective imaging of the optic nerve structure has become central to the management of patients with glaucoma. There is an urgent need in diagnosis and staging for reliable objective precursors and markers. Three-dimensional ultrahigh-resolution frequency domain optical coherence tomography (3D UHR OCT) holds particular promise in this respect since it enables volumetric assessment of intraretinal layers including tomographic data for the retinal nerve fiber layer (RNFL) and optic nerve head. The integrated analysis of this information and the resolution advantage has enabled the development of more informative indices of axonal damage in glaucoma compared with measurements of RNFL thickness and cup-to-disc ratio provided by commercial OCT devices. The potential for UHR OCT in enabling the combined analysis of tomographic and volumetric data on retinal structure is explored. A novel parameter was developed; the three-dimensional minimal distance as the optical correlate of true retinal nerve fiber layer thickness around the optic nerve head region. For the purposes of this pilot study, we present data from a normal subject and from two patients with characteristic optic nerve and retinal nerve fiber layer changes secondary to glaucoma.

Evaluation of potential image acquisition pitfalls during optical coherence tomography and their influence on retinal image segmentation

The development of improved segmentation algorithms for more consistently accurate detection of retinal boundaries is a potentially useful solution to the limitations of existing optical coherence tomography (OCT) software. We modeled artifacts related to operator errors that may normally occur during OCT imaging and evaluated their influence on segmentation results using a novel segmentation algorithm. These artifacts included: defocusing, depolarization, decentration, and a combination of defocusing and depolarization. Mean relative reflectance and average thickness of the automatically extracted intraretinal layers was then measured. Our results show that defocusing and depolarization errors together have the greatest altering effect on all measurements and on segmentation accuracy. A marked decrease in mean relative reflectance and average thickness was observed due to depolarization artifact in all intraretinal layers, while defocus resulted in a less-marked decrease. Decentration resulted in a marked but not significant change in average thickness. Our study demonstrates that care must be taken for good-quality imaging when measurements of intraretinal layers using the novel algorithm are planned in future studies. An awareness of these pitfalls and their possible solutions is crucial for obtaining a better quantitative analysis of clinically relevant features of retinal pathology.

Diagnostic capability of optical coherence tomography (Stratus OCT 3) in early glaucoma

PURPOSE

To evaluate the diagnostic capability of optical coherence tomography (Stratus OCT 3) for early glaucoma in Asian Indian eyes.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Two groups of patients (early glaucoma and normal) who satisfied the inclusion and exclusion criteria were included. Early glaucoma was diagnosed in presence of open angles, characteristic glaucomatous optic disc changes correlating with the visual field on automated perimetry (visual field defect fulfilling at least 2 of Anderson and Patella's 3 criteria with mean deviation > or = -6 dB). Normals had visual acuity > or =20/30, intraocular pressure <22 mmHg with normal optic disc and fields and no ocular abnormality.

METHODS

All patients underwent complete ophthalmic evaluation including visual field examination (24-2/30-2 SITA standard program) and imaging with Stratus OCT 3.

MAIN OUTCOME MEASURES

Sensitivity, specificity, positive and negative predictive values, area under the receiving operating characteristic curve and likelihood ratios were calculated for various Stratus OCT 3 parameters.

RESULTS

Seventy-two eyes (72 patients) with early glaucoma and 96 eyes (96 normal subjects) were analyzed. The inferior maximum parameter had the best combination of sensitivity and specificity, 75% (95% confidence interval [CI], 70.2-79.8%) and 89.6% (95% CI, 82.6-96.6%), respectively. The 6-o'clock parameter had a sensitivity of 61.1% (95% CI, 52.3-69.9) and specificity of 99% (95% CI, 95-100); for an assumed prevalence of 5%, the positive and negative predictive values were 75% and 98%, respectively. The positive likelihood ratio for the 6-o'clock parameter (P<5%) in early glaucoma was 61.

CONCLUSIONS

Optical coherence tomography has moderate sensitivity with high specificity for the diagnosis of early glaucoma and may have a potential role in screening.

A Longitudinal Study of LASIK Flap and Stromal Thickness with High-speed Optical Coherence Tomography

OBJECTIVE

To assess corneal anatomic changes after LASIK with a high-speed corneal and anterior segment optical coherence tomography (CAS-OCT) system.

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Fifty-one eyes of 26 healthy persons undergoing LASIK.

METHODS

The CAS-OCT prototype operated at a 1.3-mum wavelength and 2000 axial scans/second. The corneas were scanned with a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each). Both patterns are 8 mm long and are centered on the corneal vertex. LASIK flaps were created using either a mechanical microkeratome (Hansatome; Bausch & Lomb, Inc., Rochester, NY) or a femtosecond laser (Pulsion; IntraLase Corp., Irvine, CA). Intraoperative pachymetry was performed using a 50-MHz ultrasound probe. Three OCT scans were obtained on preoperative and post-LASIK visits up to 6 months. An automated algorithm was developed to process the OCT images and to calculate corneal, flap, and stromal bed thickness profiles and maps. The profiles and maps were divided into central (diameter, <2 mm), pericentral (2-5 mm), and transitional (5-7 mm) zones for analysis.

MAIN OUTCOME MEASURES

Corneal, flap, and stromal bed thicknesses as determined by OCT and ultrasound pachymetry.

RESULTS

The flap interface was best detected in the pericentral zone. One week after surgery, the repeatability of OCT flap and stromal bed thickness measurement was 2 to 7 microm by pooled standard deviation for zones inside a 5-mm diameter. The central flap thickness in 24 Hansatome eyes with a 180-microm setting was 143+/-14 microm by OCT and 131+/-17 microm by ultrasound. In the 8 IntraLase cases with a 120-microm setting, it was 156+/-11 microm by OCT and 160+/-19 microm by ultrasound. Eyes with other settings also were analyzed. There were small systematic changes in flap thickness up to 1 week and bed thickness up to 3 months.

CONCLUSIONS

We have developed a method for using high-speed OCT to measure LASIK flap thickness after surgery. The measurement is noncontact, rapid, and repeatable. Profile and map measurements provide more information than point measurements previously demonstrated. This could be valuable for planning LASIK enhancement and characterizing microkeratome performance.

Reduction of optic nerve fiber layer thickness in CADASIL

Our study aims to assess nerve fiber layer (NFL) thickness in patients affected by cerebral autosomal dominant arteriopathy with subcortical infarcts and leucoencephalopathy (CADASIL). Six CADASIL patients (mean age 42 +/- 16 years, best corrected visual acuity >20/20 with refractive error between +/-3 diopters, intraocular pressure <18 mmHg) were enrolled. They were compared with 16 age-matched controls. In all subjects enrolled, NFL thickness was measured by optical coherence tomography (OCT). Three different measurements were taken in each quadrant (superior, inferior, nasal, and temporal) and averaged. The data from all quadrants (12 values averaged) were identified as NFL overall. In CADASIL eyes there was a reduction of NFL thickness in each quadrant and in the NFL overall evaluation compared with the values observed in control eyes. Our results suggest that in CADASIL patients there is a reduction of NFL thickness evaluated by OCT. This morphological abnormality could be ascribed to an impairment of the retinal vascular supply leading to a global neuroretinal involvement. These anatomical changes may precede the onset of the neurological clinical manifestations.

Abnormal retinal thickness in patients with mild cognitive impairment and Alzheimer's disease

In Alzheimer's disease (AD), brain lesions are marked by severe neuronal loss and retinal degeneration was previously mentioned in affected patients. Mild cognitive impairment (MCI) is a clinical syndrome that could be an early phase of AD. In this study, using optical coherence tomography (OCT), the retinal nerve fiber layer (RNFL) thickness was assessed in patients with mild AD, moderate to severe AD, amnestic MCI and control subjects. The results show that RNFL thickness is statistically reduced in patients with MCI, mild AD or moderate to severe AD compared to controls. In addition, no statistical difference was found between the results in MCI patients and mild AD patients. The RNFL seems to be involved early during the course of amnestic MCI and OCT tests could be carried out in patients with cognitive troubles.

Normal age-related decay of retinal nerve fiber layer thickness

PURPOSE

To determine the normal age-related loss of retinal nerve fiber layer thickness (RNFLT) as measured on Stratus optical coherence tomography (OCT 3; Carl Zeiss Meditec, Dublin, CA) in an Asian Indian population.

DESIGN

Prospective, cross-sectional, observational study.

PARTICIPANTS

One hundred eighty-seven eyes of 187 normal subjects (age range, 5-75 years) who satisfied the inclusion and exclusion criteria were included. The subjects were defined as normal if they had visual acuity of 20/30 or better, intraocular pressure less than 22 mmHg with normal optic disc, and no ocular abnormality.

METHODS

All patients underwent complete ophthalmic evaluation including imaging with Stratus OCT 3. Simple linear regression was performed to study the effect of age on RNFLT, with age as the independent variable and RNFLT as the dependent variable. Spearman's correlation was studied between the age and RNFLT. An analysis of variance was applied to compare RNFLT between the different age groups. The chi-square test also was used to evaluate the relationship between age and RNFLT.

MAIN OUTCOME MEASURE

Effect of age on RNFLT.

RESULTS

One hundred eighty-seven eyes of 187 normal subjects were analyzed. Mean age+/-standard deviation was 33.0+/-19.7 years (range, 5-75). Average RNFLT and RNFLT by quadrant demonstrated the tendency of RNFLT to decrease with increasing age, especially after age 50 years. Average RNFLT demonstrated a negative slope of 0.16 microm/year (95% confidence interval [CI], -0.1 to -0.24). By quadrant, the superior average (negative slope, -0.23 microm/year; 95% CI, -0.1 to -0.34) showed a maximum tendency to decline with age, whereas in the inferior quadrant (negative slope, -0.08 microm/year; 95% CI, 0.05 to -0.24), the age-related decay was minimal. Six clock-hour RNFLT had a least negative slope of -0.022 microm/year (95% CI, -0.08 to -0.1). Chi-square test results showed a significant inverse relationship between age and average RNFLT (P = 0.01).

CONCLUSIONS

Age-related retinal nerve fiber layer (RNFL) loss is not uniform in all the quadrants, with maximum loss in the superior quadrant, and seems to reach a maximum after the age of 50 years. Furthermore, it seems that inferior quadrant RNFL is more resistant to loss.

A Comparison of Structural Measurements Using 2 Stratus Optical Coherence Tomography Instruments

PURPOSE

To compare measures of peripapillary retinal nerve fiber layer (RNFL) thickness, optic disc topography, and central foveal thickness generated using 2 different Stratus optical coherence tomography (OCT) instruments.

METHODS

Ten normal subjects and 10 glaucoma subjects were included. One randomly selected eye per subject was scanned consecutively using a fast RNFL thickness protocol, fast macular thickness map, and fast optic disc protocol by 2 experienced operators on 2 instruments. The order of the machines and operators were randomized. The output power of each machine was measured using an optical power meter. For each OCT measurement 2 factor fixed effects analyses of variance were performed and a restricted maximum likelihood variance component analysis of the proportion of variance due to subject, operator, and machine was calculated.

RESULTS

Significant differences (P<or=0.003) between OCT instruments were observed in average, superior, and inferior RNFL thickness and central foveal thickness values among normal eyes; and average and superior RNFL thickness and cup/disc area ratio values among glaucomatous eyes. Overall, the intermachine variability (0.0% to 16.4%) was larger than the interoperator variability (0.0% to 2.4%) for all OCT measurements. In the glaucoma group the variability due to machine differences was 2.6 microm for the average RNFL thickness, 5.7 microm for the superior RNFL thickness, 0 microm for the inferior RNFL thickness, 0.03 microm for the cup/disc area ratio, 0.01 microm for the cup/disc vertical ratio, and 2.8 microm for the average foveal thickness.

CONCLUSIONS

Measurements of RNFL thickness, optic disc topography, and central foveal thickness significantly differ between OCT instruments.

Effect of lowering intraocular pressure on optical coherence tomography measurement of peripapillary retinal nerve fiber layer thickness

PURPOSE

To assess, with optical coherence tomography (OCT), any changes in peripapillary retinal nerve fiber layer (RNFL) thickness in glaucoma patients after reduction of intraocular pressure (IOP).

DESIGN

Prospective observational case series.

PARTICIPANTS

Twenty-one eyes of 21 glaucoma patients who underwent medical or surgical intervention to lower IOP.

METHODS

Patients with elevated IOP underwent fast peripapillary RNFL measurements with the Stratus OCT (model 3000, software version 4.04; Carl Zeiss Meditec, Dublin, CA) before and after the IOP was lowered by medical or surgical therapy.

MAIN OUTCOME MEASURES

Changes in overall and quadrant RNFL thickness with respect to change in IOP.

RESULTS

The OCT scans were performed before intervention (range, 0-38 days before; mean+/-standard deviation [SD], 9.8+/-9.3 days) and after intervention (range, 32-74 days; mean+/-SD, 46.8+/-11.2 days) to measure peripapillary RNFL thickness. Mean IOP (mean+/-SD) decreased from 31.5+/-8.2 mmHg to 12.8+/-4.6 mmHg with the intervention (P<0.001). Twenty of 21 eyes had an IOP reduction of more than 30%. There was no significant change in the overall RNFL thickness associated with the lowering of IOP (mean+/-SD, 1.02+/-10.3 microm; P = 0.653). Quadrant analysis did not show a significant change in the RNFL thickness of any of the 4 quadrants (superior, -1.71+/-14.5 microm, P = 0.593; inferior, 2.38+/-16.8 microm, P = 0.523; temporal, 2.19+/-9.50 microm, P = 0.303; and nasal, 1.24+/-12.5 microm, P = 0.655). No relationship was found between any of the changes in OCT parameters or percent change in OCT parameters and the extent of IOP reduction, or whether IOP was lowered medically or surgically, with or without adjusting for preintervention OCT measurements as an indication of disease severity (P values ranged from 0.331 to 0.985).

CONCLUSIONS

No significant change in the RNFL thickness was associated with the lowering of IOP by medical or surgical therapy, as measured by OCT.

Retinal nerve fiber layer thickness evaluation using optical coherence tomography in eyes with optic disc hemorrhage

BACKGROUND AND OBJECTIVE

To examine the association between disc hemorrhage and retinal nerve fiber layer (RNFL) thickness.

PATIENTS AND METHODS

RNFL thicknesses were measured using optical coherence tomography at the location of disc hemorrhage, at the superior or inferior symmetrical area of the same eyes, and at equivalent locations in contralateral eyes without disc hemorrhage. RNFL thicknesses were compared using the paired-sample t test. For recurrent hemorrhages, the correlation between the number of recurrences and RNFL thickness was examined using Kendall's tau-b method.

RESULTS

Thirty-two disc hemorrhages were found in 32 eyes with normal tension or primary open-angle glaucoma. RNFL thicknesses in areas with disc hemorrhage were thinner than those in superior or inferior symmetrical locations in the same eyes (81.7 +/- 32.9 vs 100.7 +/- 34.5 microns, P = .005), but were not different from those in equivalent locations in contralateral eyes (92.8 +/- 31.7 microns, P = .092). Number of recurrent hemorrhages and RNFL thicknesses at sites of recurrent hemorrhages tended to show a negative correlation (correlation coefficient = -0.260, P = .079).

CONCLUSIONS

Disc hemorrhage is associated with RNFL thinning measured by optical coherence tomography, and a negative correlation was found between recurrent hemorrhages and RNFL thickness.

Optical coherence tomography in a patient with tobacco-alcohol amblyopia

AIM

To report optical coherence tomography (OCT) finding in a patient with tobacco-alcohol amblyopia.

METHODS

A 45-year-old man presented with a gradual decrease in vision over 4 years. He had smoked a half to one pack of cigarettes per day and had consumed 350 cc of gin per day for 30 years. A detailed ophthalmologic examination was performed.

RESULTS

His corrected visual acuities were 20/800 OD and 20/200 OS. A Goldmann visual field examination showed ceco-central scotomas in both eyes. OCT using a peripapillary Fast RNFL (retinal nerve fiber layer) programme showed a small decrease in the RNFL thickness of the superotemporal quadrant in the normative diagram of the right eye in spite of a markedly increased RNFL thickness in both eyes.

CONCLUSION

During the phase of visual loss in a patient with tobacco-alcohol amblyopia, visual loss may precede optic disc changes as detected by OCT.

Retinal imaging by spectral optical coherence tomography

PURPOSE

To demonstrate applicability of high speed spectral optical coherence tomography (SOCT) method for imaging retinal pathologies in clinical conditions.

METHODS

SOCT was performed in 67 eyes with different macular diseases. Examinations were carried out with the prototype SOCT instrument constructed in the Institute of Physics, Nicolaus Copernicus University, Toruń, Poland. A broadband superluminescent diode was used as a light source.

RESULTS

The disturbances of retinal layer structure concerning mainly outer segments of photoreceptors were observed in case of central serous chorioretinopathy and choroidal neovascularization in age-related macular degeneration. Large drusen were often related to significant changes of outer nuclear layer thickness and reflectivity.

CONCLUSIONS

SOCT detects small disturbances of the retinal structure and helps to precisely determine layers involved in different pathologies.

Evaluating the Optic Nerve and Retinal Nerve Fibre Layer: The Roles of Heidelberg Retina Tomography, Scanning Laser Polarimetry and Optical Coherence Tomography

Introduction: For many years, ophthalmologists have looked at the optic nerve head to evaluate the status of glaucoma. Clinical examination of the optic nerve head and retinal nerve fibre layer (RNFL) is however, subjective and sometimes variable. Recent developments in computer-based imaging technologies have provided a means of obtaining quantitative measurements of the optic nerve head topography and peripapillary retinal nerve fibre layer thickness. Methods: Multiple searches using Medline were carried out. Additional searches were made using reference lists of published papers and book chapters. Results: Studies involving three imaging technologies namely, confocal scanning laser ophthalmoscopy, scanning laser polarimetry and optical coher-ence tomography were reviewed. Overall, these technologies were reproducible and demonstrate good sensitivity and specificity in the range of 70 to 80%. Inclusion of age and ethnicity normative database will make these technologies more effective in screening and diagnosis. Quantitative measurements provide useful parameters for monitoring of patients. Conclusion: There is no consensus on the best technology for assessing structural damage in glaucomatous optic neuropathy. Therefore, as with any investigation, the clinician should exercise clinical correla-tion and judgment before instituting the appropriate treatment. Key words: Glaucoma, Imaging, Ophthalmoscopy, Optic neuropathy, Topography

The relationship between optical coherence tomography and scanning laser polarimetry measurements in glaucoma

PURPOSE

To investigate the relationship between optical coherence tomography (OCT) and scanning laser polarimetry (SLP) in measuring peripapillary retinal nerve fiber layer (RNFL) thickness in glaucomatous eyes.

METHODS

Fifty glaucomatous eyes were evaluated in this study. Evaluations were analyzed two ways. First, parameters of the Stratus OCT (average thickness, superior/inferior average) and GDx VCC (TSNIT average, nerve fiber indicator (NFI), superior/inferior average) were correlated using the Pearson's correlation coefficient (r). Secondly, comparison (r) of these parameters was completed using the mean deviation (MD) of visual field defect.

RESULTS

The following parameters were found to be significantly correlated (P < 0.005). TSNIT average/average thickness (r = 0.673), NFI/average thickness (r = -0.742), superior average (r = 0.841), and inferior average (r = 0.736). In the correlation analysis using the severity of visual field defect, all these parameters had statistically meaningful correlations (P < 0.005).

CONCLUSIONS

GDx VCC and Stratus OCT are highly correlated in glaucomatous eyes. Therefore, peripapillary RNFL thickness measured by Stratus OCT and GDx VCC may be equally helpful in the diagnosis of glaucoma.

Determinants of normal retinal nerve fiber layer thickness measured by Stratus OCT

PURPOSE

To determine the effects of age, optic disc area, ethnicity, eye, gender, and axial length on the retinal nerve fiber layer (RNFL) in the normal human eye as measured by Stratus OCT (optical coherence tomography).

DESIGN

Cross-sectional observational study.

PARTICIPANTS

Three hundred twenty-eight normal subjects 18 to 85 years old.

METHODS

Peripapillary Fast RNFL scans performed by Stratus OCT with a nominal diameter of 3.46 mm centered on the optic disc were performed on one randomly selected eye of each subject.

MAIN OUTCOME MEASURES

Linear regression analysis of the effects of age, ethnicity, gender, eye, axial length, and optic disc area on peripapillary RNFL thickness.

RESULTS

The mean RNFL thickness for the entire population was 100.1 microm (standard deviation, 11.6). Thinner RNFL measurements were associated with older age (P<0.001); being Caucasian, versus being either Hispanic or Asian (P = 0.006); greater axial length (P<0.001); or smaller optic disc area (P = 0.010). For every decade of increased age, mean RNFL thickness measured thinner by approximately 2.0 microm (95% confidence interval [CI], 1.2-2.8). For every 1-mm-greater axial length, mean RNFL thickness measured thinner by approximately 2.2 microm (95% CI, 1.1-3.4). For every increase in square millimeter of optic disc area, mean RNFL thickness increased by approximately 3.3 microm (95% CI, 0.6-5.6). Comparisons between ethnic groups revealed that Caucasians had mean RNFL values (98.1+/-10.9 microm) slightly thinner than those of Hispanics (103.7+/-11.6 microm; P = 0.022) or Asians (105.8+/-9.2 microm; P = 0.043). There was no relationship between RNFL thickness and eye or gender.

CONCLUSIONS

Retinal nerve fiber layer thickness, as measured by Stratus OCT, varies significantly with age, ethnicity, axial length, and optic disc area. These variables may need to be taken into account when evaluating patients for diagnosis and follow-up of glaucoma, particularly at the lower boundary of the normal range. Due to the relatively small numbers of subjects of Asian and African descent in the normative database, conclusions regarding the effect of ethnicity should be interpreted with caution.

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

PURPOSE

To measure total corneal power using optical coherence tomography (OCT).

SETTING

Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA.

METHODS

Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography.

RESULTS

The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001).

CONCLUSIONS

Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.

Recent developments in optical coherence tomography for imaging the retina

Optical coherence tomography (OCT) was introduced in ophthalmology a decade ago. Within a few years in vivo imaging of the healthy retina and optic nerve head and of retinal diseases was a fact. In particular the ease with which these images can be acquired considerably changed the diagnostic strategy used by ophthalmologists. The OCT technique currently available in clinical practice is referred to as time-domain OCT, because the depth information of the retina is acquired as a sequence of samples, over time. This can be done either in longitudinal cross-sections perpendicular to, or in the coronal plane parallel to the retinal surface. Only recently, major advances have been made as to image resolution with the introduction of ultrahigh resolution OCT and in imaging speed, signal-to-noise ratio and sensitivity with the introduction of spectral-domain OCT. Functional OCT is the next frontier in OCT imaging. For example, polarization-sensitive OCT uses the birefringent characteristics of the retinal nerve fibre layer to better assess its thickness. Blood flow information from retinal vessels as well as the oxygenation state of retinal tissue can be extracted from the OCT signal. Very promising are the developments in contrast-enhanced molecular optical imaging, for example with the use of scattering tuneable nanoparticles targeted at specific tissue or cell structures. This review will provide an overview of these most recent developments in the field of OCT imaging focussing on applications for the retina.

High-definition and 3-dimensional imaging of macular pathologies with high-speed ultrahigh-resolution optical coherence tomography

OBJECTIVE

To assess high-speed ultrahigh-resolution optical coherence tomography (OCT) image resolution, acquisition speed, image quality, and retinal coverage for the visualization of macular pathologies.

DESIGN

Retrospective cross-sectional study.

PARTICIPANTS

Five hundred eighty-eight eyes of 327 patients with various macular pathologies.

METHODS

High-speed ultrahigh-resolution OCT images were obtained in 588 eyes of 327 patients with selected macular diseases. Ultrahigh-resolution OCT using Fourier/spectral domain detection achieves approximately 3-mum axial image resolutions, acquisition speeds of approximately 25 000 axial scans per second, and >3 times finer resolution and >50 times higher speed than standard OCT. Three scan protocols were investigated. The first acquires a small number of high-definition images through the fovea. The second acquires a raster series of high-transverse pixel density images. The third acquires 3-dimensional OCT data using a dense raster pattern. Three-dimensional OCT can generate OCT fundus images that enable precise registration of OCT images with the fundus. Using the OCT fundus images, OCT results were correlated with standard ophthalmoscopic examination techniques.

MAIN OUTCOME MEASURES

High-definition macular pathologies.

RESULTS

Macular holes, age-related macular degeneration, epiretinal membranes, diabetic retinopathy, retinal dystrophies, central serous chorioretinopathy, and other pathologies were imaged and correlated with ophthalmic examination, standard OCT, fundus photography, and fluorescein angiography, where applicable. High-speed ultrahigh-resolution OCT generates images of retinal pathologies with improved quality, more comprehensive retinal coverage, and more precise registration than standard OCT. The speed preserves retinal topography, thus enabling the visualization of subtle changes associated with disease. High-definition high-transverse pixel density OCT images improve visualization of photoreceptor and pigment epithelial morphology, as well as thin intraretinal and epiretinal structures. Three-dimensional OCT enables comprehensive retinal coverage, reduces sampling errors, and enables assessment of 3-dimensional pathology.

CONCLUSIONS

High-definition 3-dimensional imaging using high-speed ultrahigh-resolution OCT improves image quality, retinal coverage, and registration. This new technology has the potential to become a useful tool for elucidating disease pathogenesis and improving disease diagnosis and management.

Thicknesses of the fovea and retinal nerve fiber layer in amblyopic and normal eyes in children

Purpose

This study was designed to assess and compare the thicknesses of the fovea and the retinal nerve fiber layer in normal children and children with amblyopia.

Methods

Optical Coherence Tomography (OCT) was performed on 26 children (52 eyes total) with unilateral amblyopia that was due to anisometropia or strabismus. OCT was also performed on 42 normal children (84 eyes), for a total of 136 eyes. Retinal thickness measurements were taken from the fovea, and the retinal nerve fiber layer thickness measurements were taken from the superior, inferior, nasal and temporal quadrants in the peripapillary region.

Results

The average age of the normal children was 8.5 years, and the average age of the children with amblyopia was 8.0 years. The average thickness of the fovea was 157.4 µm in normal eyes and was 158.8 µm in amblyopic eyes. The difference between the two groups was not statistically significant (p=0.551). The thicknesses of the superior, inferior, nasal and temporal quadrants of the retinal nerve fiber layer between the normal children and the children with amblyopia were also not statistically significant (p=0.751, 0.228, 0.696 and 0.228, respectively). However, for the children with anisometropic amblyopia and the children with strabismic amblyopia, the average thicknesses of the fovea were 146.5 µm and 173.1 µm, respectively, and the retinal nerve fiber layer thicknesses were measured to be 112.9 µm and 92.8 µm, respectively, and these were statistically significant differences (p=0.046, 0.034, respectively).

Conclusions

Normal thicknesses of the fovea and the retinal nerve fiber layers were established, and there were no differences in the fovea and the retinal nerve fiber layer thickness found between normal children and children with amblyopia.

The relationship between retinal ganglion cell function and retinal nerve fiber thickness in early glaucoma

PURPOSE

To compare relative reduction of retinal ganglion cell (RGC) function and retinal nerve fiber layer (RNFL) thickness in early glaucoma by means of steady-state pattern electroretinogram (PERG) and optical coherence tomography (OCT), respectively.

METHODS

Eighty-four persons with suspected glaucoma due to disc abnormalities (GS: mean age 56.6 +/- 13.8 years, standard automated perimetry [SAP] mean deviation [MD] -0.58 +/- 1.34 dB) and 34 patients with early manifest glaucoma (EMG, mean age 65.9 +/- 10.7 years, SAP MD -2.7 +/- 4.5 dB) were tested with PERG and OCT. Both GS and EMG patients had small refractive errors, corrected visual acuity > or =20/25, and no systemic or retinal disease other than glaucoma.

RESULTS

MDs from age-predicted normal values were larger for PERG amplitude (GS: -1.113 dB; EMG: -2.352 dB) compared with the PERG-matched RNFL thickness (GS: -0.217 dB; EMG: -0.725 dB). Deviations exceeding the lower 95% tolerance intervals of the normal population were more frequent for PERG amplitude (GS: 26%; EMG: 56%) than PERG-matched RNFL thickness (GS: 6%; EMG: 29%).

CONCLUSIONS

In early glaucoma, reduction in RGC electrical activity exceeds the proportion expected from lost RGC axons, suggesting that a population of viable RGCs in the central retina is dysfunctional. By combining PERG and OCT it is, in principle, possible to obtain unique information on reduced responsiveness of viable RGCs.

Baseline optical coherence tomography predicts the development of glaucomatous change in glaucoma suspects

PURPOSE

To assess whether baseline retinal nerve fiber layer (RNFL) measurements obtained with optical coherence tomography (OCT2; Carl Zeiss Meditec, Dublin, California, USA) are predictive of the development of glaucomatous change.

DESIGN

Cohort study.

METHODS

Participants were recruited from the University of California, San Diego (UCSD) longitudinal Diagnostic Innovations in Glaucoma Study (DIGS). One eye was studied from each of 114 glaucoma suspects with normal standard automated perimetry (SAP) and OCT RNFL imaging at baseline. The cohort was divided into two groups based on the development of glaucomatous change (repeatable abnormal visual fields and/or a change in the stereophotographic appearance of the optic disk). Cox proportional hazards models were used to determine the predictive ability of OCT RNFL thickness measurements.

RESULTS

Over a 4.2-year average follow-up period, 23 eyes (20%) developed glaucomatous changes and 91 (80%) did not. At baseline, thinner RNFL measurements, higher SAP pattern standard deviation (PSD), "glaucoma" stereophotograph assessment, and thinner central corneal thickness (CCT) were associated with the study endpoints in univariate analysis. After adjusting for age, intraocular pressure (IOP), CCT, and PSD in multivariate models, a 10 mum thinner average, superior and inferior RNFL at baseline was predictive of glaucomatous change [hazard ratio (95% CI); 1.51 (1.11 to 2.12), 1.57 (1.17 to 2.18), and 1.49, (1.19 to 1.91), respectively]. Results were consistent when stereophotographic assessment was included in multivariate analysis.

CONCLUSIONS

Thinner OCT RNFL measurements at baseline were associated with development of glaucomatous change in glaucoma suspect eyes. RNFL thinning was an independent predictor of the glaucomatous change, even when adjusting for stereophotograph assessment, age, IOP, CCT, and PSD.