Retinal Threshold Research Articles

A New Understanding of Multiple-Pulsed Laser-Induced Retinal Injury Thresholds

Laser safety standards committees have struggled for years to formulate adequately a sound method for treating repetitive-pulse laser exposures. Safety standards for lamps and LEDs have ignored this issue because averaged irradiance appeared to treat the issue adequately for large retinal image sizes and skin exposures. Several authors have recently questioned the current approach of three test conditions (i.e., limiting single-pulse exposure, average irradiance, and a single-pulse-reduction factor) as still insufficient to treat pulses of unequal energies or certain pulse groupings. Schulmeister et al. employed thermal modeling to show that a total-on-time pulse (TOTP) rule was conservative. Lund further developed the approach of probability summation proposed by Menendez et al. to explain pulse-additivity, whereby additivity is the result of an increasing probability of detecting injury with multiple pulse exposures. This latter argument relates the increase in detection probability to the slope of the probit curve for the threshold studies. Since the uncertainty in the threshold for producing an ophthalmoscopically detectable minimal visible lesion (MVL) is large for retinal exposure to a collimated laser beam, safety committees traditionally applied large risk reduction factors ("safety factors") of one order of magnitude when deriving intrabeam, "point-source" exposure limits. This reduction factor took into account the probability of visually detecting the low-contrast lesion among other factors. The reduction factor is smaller for large spot sizes where these difficulties are quite reduced. Thus the N⁻⁰·²⁵ reduction factor may result from the difficulties in detecting the lesion. Recent studies on repetitive pulse exposures in both animal and in vitro (retinal explant) models support this interpretation of the available data.

Degeneration Modulates Retinal Response to Transient Exogenous Oxidative Injury

PurposeOxidative injury is involved in retinal and macular degeneration. We aim to assess if retinal degeneration associated with genetic defect modulates the retinal threshold for encountering additional oxidative challenges.MethodsRetinal oxidative injury was induced in degenerating retinas (rd10) and in control mice (WT) by intravitreal injections of paraquat (PQ). Retinal function and structure was evaluated by electroretinogram (ERG) and histology, respectively. Oxidative injury was assessed by immunohistochemistry for 4-Hydroxy-2-nonenal (HNE), and by Thiobarbituric Acid Reactive Substances (TBARS) and protein carbonyl content (PCC) assays. Anti-oxidant mechanism was assessed by quantitative real time PCR (QPCR) for mRNA of antioxidant genes and genes related to iron metabolism, and by catalase activity assay.ResultsThree days following PQ injections (1 µl of 0.25, 0.75, and 2 mM) the average ERG amplitudes decreased more in the WT mice compared with the rd10 mice. For example, following 2 mM PQ injection, ERG amplitudes reduced 1.84-fold more in WT compared with rd10 mice (p = 0.02). Injection of 4 mM PQ resulted in retinal destruction. Altered retina morphology associated with PQ was substantially more severe in WT eyes compared with rd10 eyes. Oxidative injury according to HNE staining and TBARS assay increased 1.3-fold and 2.1-fold more, respectively, in WT compared with rd10 mice. At baseline, prior to PQ injection, mRNA levels of antioxidant genes (Superoxide Dismutase1, Glutathione Peroxidase1, Catalase) and of Transferrin measured by quantitative PCR were 2.1–7.8-fold higher in rd10 compared with WT mice (p<0.01 each), and catalase activity was 1.7-fold higher in rd10 (p = 0.0006).ConclusionsThis data suggests that degenerating rd10 retinas encounter a relatively lower degree of damage in response to oxidative injury compared with normal retinas. Constitutive up-regulation of the oxidative defense mechanism in degenerating retinas may confer such relative protection from oxidative injury.

Interphase gap as a means to reduce electrical stimulation thresholds for epiretinal prostheses

Objective. Epiretinal prostheses are designed to restore functional vision to the blind by electrically stimulating surviving retinal neurons. These devices have classically employed symmetric biphasic current pulses in order to maintain a balance of charge. Prior electrophysiological and psychophysical studies in peripheral nerve show that adding an interphase gap (IPG) between the two phases makes stimulation more efficient than pulses with no gap. This led us to investigate the effect of IPG duration on retinal stimulation thresholds. Approach. We measured retinal ganglion cell (RGC) electrical thresholds in salamander retina and phosphene perceptual thresholds in epiretinal prosthesis patients during stimulation with different IPG lengths. We also built Hodgkin–Huxley-type models of RGCs to further study how IPG affects thresholds. Main results. In general, there was a negative exponential correlation between threshold and IPG duration. Durations greater than or equal to ∼0.5 ms reduced salamander RGC thresholds by 20–25%. Psychophysical testing in five retinal prosthesis patients indicated that stimulating with IPGs can decrease perceptual thresholds by 10–15%. Results from computational models of RGCs corroborated the observed behavior. Significance. Incorporating interphase gaps can reduce the power consumption of epiretinal prostheses and increase the available dynamic range of phosphene size and brightness.

Retinal injury thresholds for 532, 578, and 630 nm lasers in connection to photodynamic therapy for choroidal neovascularization

The purpose of this study was to explore the retinal injury thresholds in rabbits and evaluate the influence of retinal pigmentation on threshold irradiance at laser wavelengths of 532, 578, and 630 nm which might be involved in hypocrellin B (HB) and hematoporphyrin monomethyl ether (HMME) photodynamic therapy (PDT) for choroidal neovascularization (CNV). The eyes of pigmented and non-pigmented rabbits were exposed to 532, 578, and 630 nm lasers coupled to a slit lamp biological microscope. The exposure duration was 100 seconds and the retinal spot size was 2 mm throughout the experiment. The minimum visible lesions were detected by funduscopy at 1 and 24 hours post exposure. Bliss probit analysis was performed to determine the ED50 thresholds, fiducial limits and probit slope. In pigmented rabbits, the 24-hour retinal threshold irradiances at 532, 578, and 630 nm were 1,003, 1,475, and 1,720 mW/cm(2) , respectively. In non-pigmented rabbits, the 24-hour threshold irradiances were 1,657, 1,865, and 15,360 mW/cm(2) , respectively. The ED50 for 24-hour observation differed very little from the ED50 for 1-hour observation. The non-pigmented rabbits required a ninefold increase in threshold irradiance at 630 nm comparing to the pigmented rabbits. This study will contribute to the knowledge base for the limits of laser irradiance in application of HB or HMME PDT for CNV.

Functional relationship between retinal sensitivity threshold values assessed by standard automated perimetry in glaucoma

ObjectiveTo study the correlation between the sensitivity threshold values of the different points assessed by the Humphrey Visual Field Analyzer (24-2 Swedish interactive threshold algorithm [SITA] standard strategy) in glaucoma patients. Subjects, material and methodsProspective cross-sectorial study. One-hundred and four eyes of 104 glaucoma patients, defined by the appearance of the optic nerve head, were evaluated. Retinal threshold sensitivity points of standard automated perimetry (SA) with SITA Standard 24-2 program were obtained. The upper and the lower hemifields were studied separately. Pearson correlation coefficients were calculated between the mean threshold sensitivity value at each point of the visual hemifield and the rest of the threshold points in the same hemifield. ResultsPerimetric correlation maps between retinal threshold sensitivity values in the same hemifield were obtained. Most of the points showed moderate to high correlations (r≥0.65. p<0.001) with neighboring points and distant points in the same hemifield. ConclusionsThere is a functional relationship between neighboring and distant points in Humphrey Visual Field Analyzer (SITA Standard 24-2) in glaucoma patients. This correlation is related to the anatomical arrangement of ganglion cell axons. This fact enables perimetric patterns of glaucoma defects to be obtained.

Intervention with vitamins in patients with nonproliferative diabetic retinopathy: a pilot study

BackgroundThe purpose of this study was to determine whether a combination of vitamins B6, B9, and B12 is an effective intervention for reducing the signs and symptoms of nonproliferative diabetic retinopathy.MethodsTen subjects with type 2 diabetes mellitus (n = 20 eyes) with clinically diagnosed mild to moderate nonproliferative diabetic retinopathy were recruited from a private practice ophthalmology clinic for this open-label, uncontrolled, prospective six-month study. Metanx® vitamin tablets (containing 3 mg L-methylfolate calcium, 35 mg pyridoxal-5′-phosphate, and 2 mg methylcobalamin) were administered at a dosage of two tablets daily. Primary outcome indicators were the percent change in mean retinal sensitivity threshold measured by macular microperimetry and the percent change in mean central retinal thickness measured by spectral-domain optical coherence tomography.ResultsThree subjects were lost to follow-up. In the remaining seven subjects, two of 14 eyes had foveal edema that prevented microperimetry measurements due to poor fixation. The remaining 12 eyes showed a nonlinear improvement in mean threshold retinal sensitivity (P < 0.001). Overall change in mean central retinal thickness in 14 eyes was linear (R2 = 0.625; P = 0.034), with a significant reduction between one and six months (P = 0.012).ConclusionIn this pilot study, the Metanx intervention appeared to have some beneficial effects with respect to reducing retinal edema and increasing light sensitivity in subjects with nonproliferative diabetic retinopathy.

Relación funcional entre los puntos de sensibilidad retiniana de la perimetría automatizada en el glaucoma

ObjectiveTo study the correlation between the sensitivity threshold values of the different points assessed by the Humphrey visual field analyzer (24-2 Swedish interactive threshold algorithm [SITA] standard strategy) in glaucoma patients. Subjets, material and methodsProspective cross-sectorial study. One-hundred and four eyes of 104 glaucoma patients, defined by the appearance of the optic nerve head, were evaluated. Retinal threshold sensitivity points of standard automated perimetry (SA) with SITA standard 24-2 program were obtained. The upper and the lower hemifields were studied separately. Pearson correlation coefficients were calculated between the mean threshold sensitivity value at each point of the visual hemifield and the rest of the threshold points in the same hemifield. ResultsPerimetric correlation maps between retinal threshold sensitivity values in the same hemifield were obtained. Most of the points showed moderate to high correlations (r≥0.65. P<0.001) with neighboring points and distant points in the same hemifield. ConclusionsThere is a functional relationship between neighboring and distant points in Humphrey Visual Field Analyzer (SITA Standard 24-2) in glaucoma patients. This correlation is related to the anatomical arrangement of ganglion cell axons. This fact enables perimetric patterns of glaucoma defects to be obtained.

Retinal safety of near-infrared lasers in cataract surgery

Femtosecond lasers have added unprecedented precision and reproducibility to cataract surgery. However, retinal safety limits for the near-infrared lasers employed in surgery are not well quantified. We determined retinal injury thresholds for scanning patterns while considering the effects of reduced blood perfusion from rising intraocular pressure and retinal protection from light scattering on bubbles and tissue fragments produced by laser cutting. We measured retinal damage thresholds of a stationary, 1030-nm, continuous-wave laser with 2.6-mm retinal spot size for 10- and 100-s exposures in rabbits to be 1.35 W (1.26 to 1.42) and 0.78 W (0.73 to 0.83), respectively, and 1.08 W (0.96 to 1.11) and 0.36 W (0.33 to 0.41) when retinal perfusion is blocked. These thresholds were input into a computational model of ocular heating to calculate damage threshold temperatures. By requiring the tissue temperature to remain below the damage threshold temperatures determined in stationary beam experiments, one can calculate conservative damage thresholds for cataract surgery patterns. Light scattering on microbubbles and tissue fragments decreased the transmitted power by 88% within a 12 deg angle, adding a significant margin for retinal safety. These results can be used for assessment of the maximum permissible exposure during laser cataract surgery under various assumptions of blood perfusion, treatment duration, and scanning patterns.

Computational analysis of thresholds for magnetophosphenes

In international guidelines, basic restriction limits on the exposure of humans to low-frequency magnetic and electric fields are set with the objective of preventing the generation of phosphenes, visual sensations of flashing light not caused by light. Measured data on magnetophosphenes, i.e. phosphenes caused by a magnetically induced electric field on the retina, are available from volunteer studies. However, there is no simple way for determining the retinal threshold electric field or current density from the measured threshold magnetic flux density. In this study, the experimental field configuration of a previous study, in which phosphenes were generated in volunteers by exposing their heads to a magnetic field between the poles of an electromagnet, is computationally reproduced. The finite-element method is used for determining the induced electric field and current in five different MRI-based anatomical models of the head. The direction of the induced current density on the retina is dominantly radial to the eyeball, and the maximum induced current density is observed at the superior and inferior sides of the retina, which agrees with literature data on the location of magnetophosphenes at the periphery of the visual field. On the basis of computed data, the macroscopic retinal threshold current density for phosphenes at 20 Hz can be estimated as 10 mA m−2 (−20% to + 30%, depending on the anatomical model); this current density corresponds to an induced eddy current of 14 μA (−20% to + 10%), and about 20% of this eddy current flows through each eye. The ICNIRP basic restriction limit for the induced electric field in the case of occupational exposure is not exceeded until the magnetic flux density is about two to three times the measured threshold for magnetophosphenes, so the basic restriction limit does not seem to be conservative. However, the reasons for the non-conservativeness are purely technical: removal of the highest 1% of electric field values by taking the 99th percentile as recommended by the ICNIRP leads to the underestimation of the induced electric field, and there are difficulties in applying the basic restriction limit for the retinal electric field.

Pilot Study of the Delivery of Microcollimated Pars Plana External Beam Radiation in Porcine Eyes

To investigate the effects of a novel stereotactic radiosurgical system for pars plana delivery of microcollimated x-rays to the retina and determine the retinal radiological dose response and toxicity threshold in a pig model. The x-rays were delivered through the pars plana to the maculae of Yucatan miniswine to verify the targeting and safety of a cornea-scleral, stabilized, office-based delivery system. Twelve eyes were randomized to receive 0, 16, 24, 42, 60, or 90 Gy in a single dose to the retina. Eye examinations, fundus photography, fluorescein angiography, and spectral-domain optical coherence tomography were obtained at days 7, 30, 60, and 90. Indocyanine green angiography was done at day 90. Through day 90 interim analysis, no abnormalities of external structures were noted. A small cortical lens opacity was noted in the 60-Gy group. Fundus evaluation revealed no abnormalities at 16 or 24 Gy. Beginning at day 30, circular pale retinal lesions with sharp margins were noted in the maculae of the eyes that received 42, 60, and 90 Gy. Higher-dose lesions showed late staining on fluorescein angiography, choroidal hypoperfusion on indocyanine green angiography, and defined photoreceptor loss and retinal thinning on spectral-domain optical coherence tomography. Transscleral stereotactic radiation dosing of porcine eyes demonstrates no apparent clinical abnormalities in doses less than 24 Gy. Doses of 42 Gy or higher led to focal choroidal and retinal damage within the target area. Radiation can induce small-blood vessel closure and thereby has therapeutic potential in neovascular diseases such as age-related macular degeneration.

REVIEW OF THRESHOLDS AND RECOMMENDATIONS FOR REVISED EXPOSURE LIMITS FOR LASER AND OPTICAL RADIATION FOR THERMALLY INDUCED RETINAL INJURY

Exposure limits (ELs) for laser and optical broadband radiation that are derived to protect the retina from adverse thermally-induced effects vary as a function of wavelength, exposure duration, and retinal irradiance diameter (spot size) expressed as the angular subtense α. A review of ex vivo injury threshold data shows that, in the ns regime, the microcavitation-induced damage mechanism results in retinal injury thresholds below thermal denaturation-induced thresholds. This appears to be the reason that the injury thresholds for retinal spot sizes of about 80 μm (α = 6 mrad) and pulse durations of about 5 ns in the green wavelength range are very close to current ELs, calling for a reduction of the EL in the ns regime. The ELs, expressed in terms of retinal radiant exposure or radiance dose, currently exhibit a 1/α dependence up to a retinal spot size of 100 mrad, referred to as αmax. For α ≥ αmax, the EL is a constant retinal radiant exposure (no α dependence) for any given exposure duration. Recent ex vivo, computer model, and non-human primate in vivo threshold data provide a more complete assessment of the retinal irradiance diameter dependence for a wide range of exposure durations. The transition of the 1/α dependence to a constant retinal radiant exposure (or constant radiance dose) is not a constant αmax but varies as a function of the exposure duration. The value of αmax of 100 mrad reflects the spot size dependence of the injury thresholds only for longer duration exposures. The injury threshold data suggest that αmax could increase as a function of the exposure duration, starting in the range of 5 mrad in the μs regime, which would increase the EL for pulsed exposure and extended sources by up to a factor of 20, while still assuring an appropriate reduction factor between the injury threshold and the exposure limit.

STRUCTURAL AND FUNCTIONAL CHARACTERISTICS IN CARRIERS OF X-LINKED RETINITIS PIGMENTOSA WITH A TAPETAL-LIKE REFLEX

The purpose of this study was to identify the functional and structural characteristics in three female obligate carriers of X-linked retinitis pigmentosa from the same family by using spectral domain optical coherence tomography, fundus autofluorescence, and microperimetry. Three female obligate carriers with a tapetal-like reflex, 21, 49, and 57 years of age, from a single family of X-linked retinitis pigmentosa that was seen in the ophthalmology department at the University of Illinois at Chicago, were enrolled in the study. All carriers underwent a complete ophthalmic examination. Spectral domain optical coherence tomography measurements, a macular microperimetry examination, and fundus autofluorescence testing were performed. The spectral domain optical coherence tomography examination in all three carriers showed a normal retinal microstructure and thickness. Microperimeter testing showed subnormal retinal sensitivity in the areas of the tapetal-like reflex. Fundus autofluorescence examination showed the presence of speckled areas of enhanced autofluorescence. Our study demonstrates that the carriers of X-linked retinitis pigmentosa with a tapetal-like reflex can show an enhanced reflectance on infrared images, abnormal autofluorescence properties, elevated retinal thresholds, and a normal retinal morphology within the posterior pole on spectral domain optical coherence tomography testing.

Normal Values for Fundus Perimetry with the Microperimeter MP1

To identify age-stratified normal light sensitivity values for microperimetry (fundus perimetry) and to evaluate the short-term repeatability of the MP1 microperimeter in normal volunteers. Multicenter, prospective, observational study. One hundred ninety subjects. One hundred ninety eyes of 190 healthy volunteers (age range, 20-75 years) underwent automatic, full-threshold microperimetry of the central field (20 x 20 degrees, 77 stimulated points) with the MP1 microperimeter (Nidek Technologies, Gamagori, Japan). Fixation was documented simultaneously. A subgroup of 10 subjects was retested after 1 hour and 1 week to determine the repeatability of this technique. By linear regression analyses, light sensitivity values were obtained from 4 fundus areas and were analyzed for differences related to region, age, and, in a subset of subjects, repeat testing over time and right and left eye variability. Short-term repeatability for each area was evaluated by calculating intraclass correlation coefficients. Linear regression analysis showed a significantly greater (P<0.0001) mean macular sensitivity of 19.6+/-0.5 dB in the 20 to 29 years of age group compared with 18.6+/-1.5 dB in the oldest age group of 70 to 75 years. These results were confirmed by the fifth percentile of light sensitivity threshold distribution. Normal and 95% confidence interval age-stratified values were calculated. When results for all 190 subjects were analyzed by region, the superior retinal sector showed significantly lower mean sensitivity values than other sectors (P<0.01, Bonferroni test). In a subset of 10 subjects, repeatability of the test performed at 3 separate visits showed consistent values over time in all areas (P<0.01, intraclass correlation coefficients). Automatic fundus perimetry with the MP1 microperimeter allows for an accurate, repeatable, topographically specific examination of the retinal threshold in selected retinal areas. These findings are the first extensive database of age-related, normal MP1 microperimetry results available to clinicians.

Effect of Pupil Size on Visual Acuity in a Laboratory Model of Pseudophakic Monovision

To investigate the effect of pupil size on visual acuity in pseudophakic monovision. For the simulation, a modified Liou-Brennan model eye was used. The model eye was designed to include a centered optical system, corneal asphericity, an iris pupil, a Stiles-Crawford effect, an intraocular lens, and chromatic aberration. Calculation of the modulation transfer function (MTF) was performed with ZEMAX software. Visual acuity was estimated from the MTF and the retinal threshold curve. The sizes of the entrance pupil were 2.0, 2.5, 3.0, and 4.0 mm. Decreasing pupil diameter and increasing myopia progressively improved near visual acuity. For an entrance pupil size of 2.5 mm and a refractive error of -1.50 diopters, the logMAR value (Snellen; metric) in the non-dominant eye at 40 cm was 0.06 (20/23; 6/6.9). Knowledge of the patient's pupil diameter at near fixation can assist surgeons in determining the optimum degree of myopia for successful monovision.

The risk of retinal injury from Class 2 and visible Class 3R lasers, including medical laser aiming beams

Abstract Experimental retinal injury threshold data from the literature and computer model data were used to quantitatively characterize the risk of retinal thermal injury for visible laser radiation that exceeds the exposure limit defined by IEC 60825-1 and the European Directive on Artificial Optical Radiation (AOR). This discussion is of particular relevance for medical laser aiming beams with powers of up to 5 mW. Exposure to 1 mW (Class 2 emissions) does not appear to be able to cause retinal injury for exposure durations of up to about 5 s. Even though there is uncertainty in the experimental threshold data for collimated beams, experience with laser pointers up to 5 mW shows that there is little risk from accidental momentary exposure to these Class 3R lasers. However, injury threshold data indicate that exposure to some pulsed emission Class 3R lasers as well as extended-source Class 3R lasers with powers above 5 mW could induce retinal injury. Recommendations for amendment of the standards IEC 60825-1 and IEC 60601-2-22 are given that should facilitate an agreement of the involved stakeholders on the necessary user safety precautions of Class 3R laser products and medical aiming beams.

Population-based visual acuity in the presence of defocus well predicted by classical theory

According to classical theory, visual acuity (VA) can be modeled using the intersection of the eye's modulation transfer function with a retinal threshold function. To date, there have been limited attempts to validate this methodology by comparing theory with actual measured data. We use the methodology to predict the visual acuity in the presence of defocus of a population of cataract patients implanted with diffractive multifocal intraocular lenses. For the prediction, we used a set of physiological eye models that include chromatic and higher order aberrations. We found that the simulations correlated strongly to the clinical outcomes (R(2)=0.93). While the simulated VA of the eye models was systematically 0.05 logMAR units lower (better acuity) than the clinical results, this difference was independent of defocus (p=0.98). These results show that when the simple and straightforward classical theory is applied using physiological eye models, accurate predictions of the VA, and through-focus VA of a population can be made. This method may be suited for predicting visual performance of new cataract and refractive treatments.

Functional correlation between retinal sensitivity threshold values of standard automated perimetry in glaucoma

Abstract Purpose To study the interrelationship among the 52 retinal sensitivity threshold values of the Humphrey visual field analyzer with the 24‐2 Swedish interactive threshold algorithm (SITA) standard strategy in glaucoma patients. Methods Prospective cross‐sectorial study. 104 eyes of 104 patients with diagnosed glaucoma by means of optic disc stereophotographs were evaluated. Each 52 threshold points of the SITA Standard 24.2 program in the standard automated perimetry (SAP) were obtained. The upper and the lower hemifield were studied separately. Pearson correlation coefficients were calculated between each of the mean threshold values at each point of the visual hemifield and the rest of the threshold points. Results Perimetric correlation maps between threshold values in the same hemifield were obtained. Most of the points had moderate and high correlations (r ≥ 0,65) with neighboring points and distant points corresponding to the retinotopic distribution of the retinal nerve fibers. Conclusion There is a functional relationship, in glaucoma patients SAP, between neighboring and distant threshold points in corresponding to the anatomical arrangement of ganglion cells axons. This enables to achieve perimetric patterns of glaucoma defects.

RETINAL FIXATION IMPAIRMENT IN DIABETIC MACULAR EDEMA

To evaluate the characteristics of retinal fixation in patients with diabetic macular edema using microperimetry. One hundred seventy nine eyes (98 patients) with untreated diabetic macular edema underwent best corrected visual acuity determination (Early Treatment Diabetic Retinopathy Study charts), digital color stereoscopic fundus photos, fluorescein angiography and Optical Coherence Tomography assessment of macula. Fixation and retinal thresholds were determined with an automatic microperimeter. Best corrected visual acuity (approximate Snellen equivalent) was: 20/25 or better in 90 (52%) eyes, 20/50 to 20/32 in 39 (22.5%) eyes, 20/200 to 20/62.5 in 35 (20.2%) eyes and inferior to 20/200 in 9 (5.2%) eyes. Fixation was central in 128 (71.51%), poor central in 26 (14.53%) and predominantly eccentric in 25 (13.97%) eyes; stable in 133 (74.3%), relatively unstable in 42 (23.46%) and unstable in 4 (2.23%) eyes. Both fixation location and stability were not significantly influenced by edema characteristics (diffuse, focal, cystoid, spongelike, with or without subfoveal neuroretinal detachment), (P > 0.05), whereas they were significantly influenced by the presence of subfoveal hard exudates, (P = 0.004 and P = 0.0046, respectively). Site and stability of fixation were significantly associated, (P < 0.0001). Retinal pseudofovea would have been covered by laser photocoagulation in 24 (47%) eyes with poorly central and predominantly eccentric fixation and in 29 (63%) eyes with relatively unstable and unstable fixation. Microperimetry shows that fixation location and stability in patients with diabetic macular edema are independent of edema characteristics, except when subfoveal hard exudates are present. Location of pseudofovea may influence treatment strategy.

Early morphological changes and functional abnormalities in group 2A idiopathic juxtafoveolar retinal telangiectasis using spectral domain optical coherence tomography and microperimetry

Aims:To report early morphological changes and functional abnormalities in group 2A idiopathic juxtafoveolar retinal telangiectasis (IJRT) using spectral domain optical coherence tomography (SD-OCT) and microperimetry.Methods:Six eyes (three patients; average age,...

New data on the variation of laser induced retinal-damage threshold with retinal image size

In earlier studies, we examined the dependence of the laser induced retinal damage threshold on retinal image size for extended-source ocular exposures. We reported the spot-size dependence of the retinal threshold (based on ophthalmic observations at 24 h postexposure) for two pulsewidth regimes: nanosecond-duration (Q-switched) pulses from a doubled Nd:yttrium–aluminum–garnet laser and microsecond-duration pulses from a flashlamp-pumped dye laser at 590 nm. In either case, the retinal threshold was shown to vary with the area (i.e., diameter squared) for image diameters &amp;gt;5 mrad. More recently, we have collected additional data for the intermediate spot-size range (1.5–10 mrad) and have compared both the absolute values and the spot-size trend of retinal thresholds determined via ophthalmoscopic observation at 1 h postexposure to the analogous threshold data collected with observations at 24 h postexposure. These additional data and analyses reinforce our earlier conclusions regarding the threshold vs. spot-size trend and are compared to and reconciled with results from previously published extended-source ocular damage studies. The experimental spot-size trends are also contrasted to the existing laser safety standard treatments of maximum permissible exposure levels for extended-source ocular exposures.