Harry Zwick

Effects of high peak power microwaves on the retina of the Rhesus monkey

We studied the retinal effects of 1.25 GHz high peak power microwaves in Rhesus monkeys. Preexposure fundus photographs, retinal angiograms, and electroretinograms (ERG) were obtained to screen for normal ocular structure and function and, after exposure, as endpoints of the study. Histopathology of the retina was an additional endpoint. Seventeen monkeys were randomly assigned to receive sham exposure or pulsed microwave exposures. Microwaves were delivered anteriorly to the face at 0, 4.3, 8.4, or 20.2 W/kg spatially and temporally averaged retinal specific absorption rates (R-SAR). The pulse characteristics were 1.04 MW ( approximately 1.30 MW/kg temporal peak R-SAR), 5.59 micros pulse length at 0, 0.59, 1. 18, and 2.79 Hz pulse repetition rates. Exposure was 4 h per day and 3 days per week for 3 weeks, for a total of nine exposures. The preexposure and postexposure fundus pictures and angiograms were all within normal limits. The response of cone photoreceptors to light flash was enhanced in monkeys exposed at 8.4 or 20.2 W/kg R-SAR, but not in monkeys exposed at 4.3 W/kg R-SAR. Scotopic (rod) response, maximum (combined cone and rod) response, and Naka-Rushton R(max) and log K of scotopic b-waves were all within normal range. Retinal histopathology revealed the presence of enhanced glycogen storage in photoreceptors among sham (2/5), 8.4 W/kg (3/3), and 20.2 W/kg (2/5) exposed monkeys, while enhanced glycogen storage was not observed in the 4.3 W/kg (0/4) exposed group. Supranormal cone photoreceptor b-wave was R-SAR dependent and may be an early indicator of mild injury. However no evidence of degenerative changes and ERG depression was seen. We concluded that retinal injury is very unlikely at 4 W/kg. Functional changes that occur at higher R-SAR are probably reversible since we saw no evidence of histopathologic correlation with ERG changes. Bioelectromagnetics 21:439-454, 2000. Published 2000 Wiley-Liss, Inc.

Accidental human laser retinal injuries from military laser systems

The time course of the ophthalmoscopic and functional consequences of eight human laser accident cases from military laser systems is described. All patients reported subjective vision loss with ophthalmoscopic evidence of retinal alteration ranging from vitreous hemorrhage to retinal burn. Five of the cases involved single or multiple exposures to Q-switched neodymium radiation at close range whereas the other three incidents occur over large ranges. Most exposures were within 5 degrees of the foveola, yet none directly in the foveola. High contrast visual activity improved with time except in the cases with progressive retinal fibrosis between lesion sites or retinal hole formation encroaching the fovea. In one patient the visual acuity recovered from 20/60 at one week to 20/25 in four months with minimal central visual field loss. Most cases showed suppression of high and low spatial frequency contrast sensitivity. Visual field measurements were enlarged relative to ophthalmoscopic lesion size observations. Deep retinal scar formation and retinal traction were evident in two of the three cases with vitreous hemorrhage. In one patient, nerve fiber layer damage to the papillo-macular bundle was clearly evident. Visual performance measured with a pursuit tracking task revealed significant performance loss relative to normal tracking observers even in cases where acuity returned to near normal levels. These functional and performance deficits may reflect secondary effects of parafoveal laser injury.

Laser-induced macular holes demonstrate impaired choroidal perfusion.

Purpose To evaluate choroidal perfusion following creation of a laser-induced macular hole in a nonhuman primate model. Methods Six rhesus monkeys underwent macular exposures delivered by a Q-switched Nd:YAG laser. The lesions were evaluated with fluorescein angiography and indocyanine green angiography using scanning laser ophthalmoscopy. Results Each lesion produced vitreous hemorrhage and progressed to a full-thickness macular hole. Indocyanine green angiography revealed no perfusion of the choriocapillaris beneath the lesion centers. Fluorescein angiography demonstrated mild enlargement of the foveal avascular zone due to loss of perifoveal capillaries. Histopathologic evaluation showed replacement of the choriocapillaris with fibroblasts and connective tissue. Conclusions Nd:YAG laser–induced macular holes result in long-term impairment of choroidal perfusion at the base of the hole due to choroidal scarring and obliteration of the choriocapillaris. Evaluation of choroidal perfusion may be useful in assessment of laser-injured patients. Impairment in choroidal perfusion may have functional implications for surviving photoreceptors.

On the use of schematic eye models to estimate retinal image quality

The optical performance of the eyes of nine vertebrate species was evaluated using optical system design techniques and schematic eye models. Essential features of eyes, including the modulation transfer function (MTF) and the MTF cutoff frequency are related to the numerical aperture of the eyes. Superior resolution for in vivo imaging of photoreceptors may be achieved by dilating the iris pupil of an eye, minimizing coherence, and using short wavelength illumination. The difference of lateral and axial resolution between a small and a large eye for imaging photoreceptors in vivo.

Laser induced photoreceptor damage and recovery in the high numerical aperture eye of the garter snake.

The garter snake provides a unique model for in-vivo imaging of photoreceptor damage induced by laser retinal exposure. Laser thermal/mechanical retinal injury induced alterations in photoreceptor structure and leukocyte cellular behavior. Photoreceptors turned white, lost mode structure, and swelled; leukocyte activity was observed in the vicinity of photoreceptor cells. Non-thermal alterations were identified with a bio-tag for oxidative stress. Mechanisms of photoreceptor recovery and replacement were observed and evaluated for active cytoskeletal systems by using an anti-actin tag that could detect the presence of active cytoskeletal systems resident in photoreceptors as well as other retinal systems.

Accidental bilateral Q-switched neodymium laser exposure : Treatment and recovery of visual function

A 21 year old female was accidentally exposed in both eyes when she looked into the 10 cm exit aperture of a military laser designator emitting 1064 nm q-switched (30 ns) pulses at a 10 pulse per second rate. Steroid therapy (methylprednisolone sodium succinate) was initiated within 6 hours post exposure. Initial ophthalmoscopic observation revealed small contained macular hemorrhages in each eye. Fluorescein angiography (FA) showed minimal leakage. Visual acuity was 20/100 and 20/60 in OD and OS respectively. Contrast sensitivity in both eyes was depressed across all spatial frequencies by more than 1.5 log units. At four weeks post exposure, no significant macular scarring was apparent and visual acuity returned to 20/25 in both eyes. Contrast sensitivity had improved to normal levels with a peak at 3 cycles/degree. At one year post exposure, visual acuity was 20/13 in both eyes and measures of contrast sensitivity were within normal limits. During the course of recovery, the patients fixation shifted from a slightly superior temporal site back to the central foveal region. The foveal lesion sites were still evident by ophthalmoscopy and Amsler grid measurements but were deemed functional when the patient placed small targets generated by the scanning laser ophthalmoscope in the lesion site for discrimination. This outcome indicates remarkable recovery of visual function and suggests that early administration of steroids may assist in preserving the natural neural recovery process of the photoreceptor matrix by minimizing intraretinal scar formation.

Efficiently tracking a moving object in two-dimensional image space

A tracking algorithm has been developed to efficiently track a moving object in two-dimensional image space. The algo- rithm employs a limited exhaustive template matching scheme that combines the accuracy of an exhaustive search with the computa- tional efficiency of a coarse-fine template matching scheme. The overall result is an accurate, time-efficient tracking algorithm. After providing a theoretical discussion of the algorithm, three separate biomedical applications of the algorithm are described: (1) stabiliz- ing an irradiating laser on the retinal surface for photocoagulation treatment, (2) measuring target fixation eye movement to construct pattern densities at the retina, and (3) tracking a rat swimming in a Morris Water Maze for psychophysiological studies. Results for each application is provided. The paper concludes with a discussion of the relative merits of the tracking algorithm and recommendations for methods to improve the performance of the algorithm.

Ophthalmoscopic and pathologic description of ocular damage induced by infrared laser radiation

This study compares ocular damage effects induced by infrared (IR) lasers in the “eye-safe” wavelength range to those observed following visible wavlength laser exposures. In addition to routine fundus camera and slit-lamp observations, exposed subjects were examined via confocal scanning laser ophthalmoscopy. Histopathologic evaluation was conducted on eyes with both visible and IR laser induced focal lesions. IR laser exposure parameters which resulted in corneal, lenticular and/or retinal damage will be detailed. Unique aspects of the IR laser–tissue interaction will be discussed, including concurrent lesion formation in several tissues and secondary responses such as delayed inflammatory reactions. This latter effect may involve tissue not directly irradiated during the laser insult. Finally, implications for laser safety standards will be suggested.

A method for producing foveal retinal exposures in an awake, task-oriented, rhesus monkey*

A method for producing retinal exposures from a laser source in an awake, task-oriented S is presented. The technique allows for the measurement of rhesus monkey visual acuity immediately following exposures.

Permanent Visual Change Associated with Punctate Foveal Lesions

The effects of punctate foveal laser damage on the rhesus monkey spatial contrast sensitivity function was investigated. Landolt rings were employed as contrast sensitivity targets. The immediate effects of such exposure yielded transient depressions in contrast sensitivity across the spatial frequency spectrum. The amount of depression was nearly uniform across spatial frequency. While funduscopic retinal examination revealed punctate foveal lesions, no permanent change in contrast sensitivity was immediately measurable. Delayed changes in contrast sensitivity occurred several months after cessation of foveal exposure regimen. Both delayed as well as cumulative foveal punctate lesion effects were obtained.