Steven Chalfin

Millimeter wave absorption in the nonhuman primate eye at 35 GHz and 94 GHz.

The purpose of this study was to evaluate anterior segment bioeffects of pulsed 35 GHz and 94 GHz microwave exposure in the nonhuman primate eye. Five juvenile rhesus monkeys (Macaca mulatta) underwent baseline anterior segment ocular assessment consisting of slit lamp examination, corneal topography, specular microscopy, and pachymetry. These studies were repeated after exposure of one eye to pulsed 35 GHz or 94 GHz microwaves at varied fluences, with the other eye serving as a control. The mean fluence required to produce a threshold corneal lesion (faint epithelial edema and fluorescein staining) was 7.5 J cm−2 at 35 GHz and 5 J cm−2 at 94 GHz. Transient changes in corneal topography and pachymetry were noted at these fluences. Endothelial cell counts remained unchanged. Threshold corneal injury from 35 GHz and 94 GHz microwave exposure is produced at fluences below those previously reported for CO2 laser radiation. These data may help elucidate the mechanism of thermal injury to the cornea, and resolve discrepancies between IEEE C95.1 (1999), NCRP (1986), and ICNIRP (1998) safety standards for exposure to non-ionizing radiation at millimeter wavelengths.

Thermal modeling of millimeter wave damage to the primate cornea at 35 GHz and 94 GHz

Abstract— Recent data on damage to the primate cornea from exposure to millimeter wave radiation are interpreted in terms of a simple thermal model. The measured temperature increases during the exposures (duration 1–5 s, 35 or 94 GHz, 2–7 W cm−2) agree with the model within the variability of the data. The thresholds for damage to the cornea (staining of the corneal epithelium by fluorescein and corneal edema) correspond to temperature increases of about 20°C at both irradiation frequencies. Within the limits of the one-dimensional model, thresholds for thermal damage to the cornea can be predicted for a range of exposure conditions.

A complication of scleral expansion surgery for treatment of presbyopia

PURPOSE To report a patient who developed complications from an experimental technique using scleral expansion to treat presbyopia. METHODS Case report of a 46-year-old woman who underwent scleral expansion surgery on her right eye. Postoperatively, the patient developed chronic pain and swelling that necessitated removal of the scleral expanders. RESULTS After removal of the scleral expanders, the patient demonstrated a -1.4 diopter myopic shift in the right eye relative to her preoperative refraction. Axial length of the right eye was 1.15 mm longer than of the left, whereas preoperative axial lengths were equal. CONCLUSION This patient developed scleral thinning with resultant axial lengthening and myopic shift, akin to the phenomenon observed with scleral buckles used to treat retinal detachments. We believe this was responsible for improved near vision in her right eye, rather than any increased accommodative potential purported to result from this operation.

Choroidal Blood Flow Decreases with Age: An MRI Study

Abstract Purpose: To verify that a visual fixation protocol with cued eye blinks achieves sufficient stability for magnetic resonance imaging (MRI) blood-flow measurements and to determine if choroidal blood flow (ChBF) changes with age in humans. Methods: The visual fixation stability achievable during an MRI scan was measured in five normal subjects using an eye-tracking camera outside the MRI scanner. Subjects were instructed to blink immediately after recorded MRI sound cues but to otherwise maintain stable visual fixation on a small target. Using this fixation protocol, ChBF was measured with MRI using a 3 Tesla clinical scanner in 17 normal subjects (24–68 years old). Arterial and intraocular pressures (IOP) were measured to calculate perfusion pressure in the same subjects. Results: The mean temporal fluctuations (standard deviation) of the horizontal and vertical displacements were 29 ± 9 μm and 38 ± 11 μm within individual fixation periods, and 50 ± 34 μm and 48 ± 19 μm across different fixation periods. The absolute displacements were 67 ± 31 μm and 81 ± 26 μm. ChBF was negatively correlated with age (R = −0.7, p = 0.003), declining 2.7 ml/100 ml/min per year. There were no significant correlations between ChBF versus perfusion pressure, arterial pressure, or IOP. There were also no significant correlations between age versus perfusion pressure, arterial pressure, or IOP. Multiple regression analysis indicated that age was the only measured independent variable that was significantly correlated with ChBF (p = 0.03). Conclusions: The visual fixation protocol with cued eye blinks was effective in achieving sufficient stability for MRI measurements. ChBF had a significant negative correlation with age.

Retinotopic fMRI Reveals Visual Dysfunction and Functional Reorganization in the Visual Cortex of Mild to Moderate Glaucoma Patients

Purpose: To investigate retinotopic functional representation in the visual cortex of mild to moderate primary open-angle glaucoma (POAG) participants and age-matched normal volunteers using high-resolution retinotopic blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI). Methods: fMRI was performed on 9 POAG participants (61±11 y old) and 9 age-matched controls (58±5 y old) were studied. A wide-view visual presentation (±55 degrees) was used to evaluate central and peripheral vision. Cortical magnification factors and BOLD% changes as a function of eccentricity. Correlation analysis between BOLD% changes and visual field scores, and between BOLD% changes and retinal nerve fiber layer thicknesses was performed. Comparison of BOLD% changes for individual visual field quadrants between POAG subgroups and normal group was performed. Results: BOLD% changes of POAG participants in peripheral visual regions were reduced compared to normals but similar in central visual regions, consistent with the notion of peripheral vision being affected first and more compared to central vision. fMRI retinotopic mapping revealed enlarged representation of the parafovea in the visual cortex of POAG participants compared to normals. Cortical magnification of the central, but not peripheral, visual representation in the visual cortex was larger in POAG participants, suggesting functional remapping. BOLD% changes of individual visual field quadrants were significantly correlated with visual field scores and with retinal nerve fiber layer thickness in the corresponding quadrants. Conclusions: These results support the hypothesis that there are functional alteration and remapping in the topographic representation of the visual cortex in POAG participants, and these changes are correlated with disease severity.

MRI Study of the Posterior Visual Pathways in Primary Open Angle Glaucoma

Purpose of the Study: The purpose of the study was to evaluate neurodegeneration along brain visual pathways in primary open angle glaucoma (POAG) using improved analysis methods of volumetric and diffusion tensor magnetic resonance imaging (MRI) data. Methods: Eleven POAG patients (60.0±9.2 y) with primarily mild to moderate POAG and 11 age-matched controls (55.9±7.5 y) were studied using structural and diffusion tensor MRI. Surface-based segmentation was applied to structural MRI to obtain visual cortical area and volume. Fiber tracking was applied to diffusion tensor data to obtain diffusion parameters along the optic tract and optic radiation. MRI parameters in glaucoma patients were compared with the corresponding left and right visual fields and retinal nerve fiber layer thicknesses, instead of with the left and right eye. Results: Area and volume of the primary visual cortex were significantly reduced in POAG patients compared with controls (P<0.05) but did not correlate with visual field loss. Fractional anisotropy was reduced at multiple locations along the optic tracts and optic radiations in POAG patients compared with controls. Axial and radial diffusivity along the fiber tracts showed trends but were not significantly different between POAG patients and controls when averaged over the whole structures. Only fractional anisotropy (P<0.05) of the optic radiations was significantly correlated with visual field loss. No MRI parameters were correlated with retinal nerve fiber layer thickness. Conclusions: Improved analysis techniques of MRI data improves delineation of degeneration in the brain visual pathways and further supports the notion that neurodegeneration is involved with glaucoma pathogenesis.

Absence of corneal endothelium injury in non-human primates treated with and without ophthalmologic drugs and exposed to 2.8 GHz pulsed microwaves

Microwave-induced corneal endothelial damage was reported to have a low threshold (2.6 W/kg), and vasoactive ophthalmologic medications lowered the threshold by a factor of 10-0.26 W/kg. In an attempt to confirm these observations, four adult male Rhesus monkeys (Macaca mulatta) under propofol anesthesia were exposed to pulsed microwaves in the far field of a 2.8 GHz signal (1.43 +/- 0.06 micros pulse width, 34 Hz pulse repetition frequency, 13.0 mW/cm(2) spatial and temporal average, and 464 W/cm(2) spatial and temporal peak (291 W/cm(2) square wave equivalent) power densities). Corneal-specific absorption rate was 5.07 W/kg (0.39 W/kg/mW/cm(2)). The exposure resulted in a 1.0-1.2 degrees C increase in eyelid temperature. In Experiment I, exposures were 4 h/day, 3 days/week for 3 weeks (nine exposures and 36 h total). In Experiment II, these subjects were pretreated with 0.5% Timolol maleate and 0.005% Xalatan(R) followed by 3 or 7 4-h pulsed microwave exposures. Under ketamine-xylazine anesthesia, a non-contact specular microscope was used to obtain corneal endothelium images, corneal endothelial cell density, and pachymetry at the center and four peripheral areas of the cornea. Ophthalmologic measurements were done before and 7, 30, 90, and 180 days after exposures. Pulsed microwave exposure did not cause alterations in corneal endothelial cell density and corneal thickness with or without ophthalmologic drugs. Therefore, previously reported changes in the cornea exposed to pulsed microwaves were not confirmed at exposure levels that are more than an order of magnitude higher.