Longitudinal analysis of precorneal tear film rupture patterns.
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Adult↗
Biomedical subjects
J V Lovasik
Publications and source records attributed to J V Lovasik.
Macular hemodynamic responses to short-term acute exercise in young healthy adults.
We investigated the effects of vigorous exercise on blood flow in the macular vasculature. The velocity and density of entoptically viewed leukocytes in the paramacular retinal capillaries were measured with an Oculix BFS-2000 blue field simulator in 18 healthy adults first at rest, and then after 20 min of exercise. Exercise typically increased the density of leukocytes with more variable effects on their velocity. When leukocyte velocity and density were factored together, macular blood flow increased only marginally after exercise. We conclude that retinal blood flow in the macula is subject to the influence of autoregulatory mechanisms presumably to sustain normal central visual function during increased systemic blood flow.
Adult↗
A novel noninvasive videographic method for quantifying changes in the chromaticity of the optic nerve head with changes in the intraocular pressure, pulsatile choroidal blood flow and visual neural function in humans.
Vision loss in glaucoma may be due to the compressive effects of the intraocular pressure (IOP) on the ganglion cell axons, impaired blood flow to the optic nerve, or some combination of these two factors. While reducing the IOP may preserve vision in patients with elevated IOP, not all patients experience longterm benefits from this therapeutic approach. The survival of ganglion cells may be more dependent upon the degree of vascular perfusion of the optic nerve head (ONH). In this report we present some preliminary data on a new noninvasive videographic technique for elaborating the capacity of the ONH vasculature to maintain perfusion constancy in the presence of transient elevations of the IOP and reductions of the ocular perfusion pressure (OPP). Shortterm, stepwise reductions of the OPP in the test eye of visually normal subjects (n = 5) systematically altered both the chromaticity (hue, saturation, brightness) of the ONH and the pulsatile ocular blood flow (POBF) in the choroid. Similar transient reductions in the OPP were seen to impair normal retinal physiology, as indicated by a significant attenuation of the bilateral pattern-reversal electroretinograms (pERGs) in visually normal subjects (n = 7). This technique was also used to reveal spontaneous rhythmical variations in the ONH chromaticity which were linked to the cardiac pulse rate. This so-called "chromatic pulse" of the ONH offers potential as a useful clinical index for evaluating the vascular perfusion of the ONH. The diagnostic and prognostic potential of dynamic digital imaging for the detection of abnormal hemodynamics in the ONH is discussed.
Blood Flow Velocity↗
Gravity-induced homeostatic reactions in the macular and choroidal vasculature of the human eye.
We quantified the changes in macular and choroidal blood flow during body inversion to elaborate their hemodynamic attributes and relate them to our previously reported effects of inversion on visual neural function. Ten healthy subjects took part in the study. The intraocular pressure (IOP) was altered through graded body tilts. Macular and choroidal pulsatile ocular blood flow (POBF) were measured with an Oculix BFS 2000 and a Langham OBF system, respectively. Body inversion caused a systematic decrease in the POBF for all subjects. Systematic changes in macular leukocyte velocity and density were also found with inversion. Six of ten subjects showed a reciprocal relationship between the leukocyte density and velocity as a function of body orientation, while parallel changes in the leukocyte density and velocity were seen during inversion for the remaining subjects. Our results may reflect individual strategies for autoregulation within the macular circulation and an absence of autoregulation in the pulsatile component of the choroidal circulation.
Adult↗
Unilateral ocular vascular stress in man and retinal responsivity in the contralateral eye.
Autoregulation (AR), the capacity of retinal vessels to maintain a constant flow of blood during alterations in vascular perfusion pressure, has been studied by a variety of techniques. Psychophysical and electrophysiological procedures have been used as indirect measurements of the effectiveness of AR for maintaining normal visual function during altered ocular perfusion pressure (OPP) in a single eye. However, there have been no studies in normal subjects which have investigated the responsivity of the contralateral eye while the fellow eye undergoes transient vascular stress. In the present study, the components of the bilaterally recorded scotopic flash electroretinogram (ERG) were used as objective indices of visual neural function during transiently altered OPP. The principal finding was that a decrease in the OPP for the test eye was associated with a reversible attenuation of the retinal responsivity in that eye, and a supranormal oscillatory potential index in the contralateral eye. The existence of a central control mechanism, triggered at the eye level, may be responsible for this contralateral neural phenomenon.
Adult↗
Differential effects of compression and suction ophthalmodynamometry on the scotopic blue-flash electroretinogram.
Several studies have investigated the effect of elevated intraocular pressure with reduced ocular perfusion pressure on visual neural function by means of compression and suction ophthalmodynamometry. We compared the effects of nominally equivalent reductions in the ocular perfusion pressure induced by compression and suction ophthalmodynamometry retinal function as measured by flash electroretinography. Scotopic blue-flash electroretinograms were recorded in five subjects for baseline conditions; during a 40% reduction in the ocular perfusion pressure effected in a first test session by compression ophthalmodynamometry; and then in a second test session some 4 hours later by suction ophthalmodynamometry. Fifteen consecutive electroretinographic sets were recorded during scleral compression or suction, and also after compression or suction was removed. Compression and suction ophthalmodynamometry decreased the electroretinogram b-wave to different degrees; overall, the electroretinogram was attenuated more by compression than by suction ophthalmodynamometry. In the recovery phase, the group averaged b-wave quickly increased to exceed baseline after both scleral compression and suction. The trends for prolonged implicit times over the duration of the study were similar for compression and suction ophthalmodynamometry.
Adult↗
Comparison of noninvasive methods to derive the mean central retinal artery pressure in man.
PURPOSE: The pressure within the ophthalmic artery can be estimated by several noninvasive procedures based on measurements of the pressure either within the central retinal artery (CRA) or the brachial artery. In this study we compared 5 methods of deriving the mean pressure within the ophthalmic artery in 10 healthy volunteers 21 to 31 years of age. METHODS: The pressure within the ophthalmic artery was calculated from estimates of the systolic and diastolic pressures within the CRA derived by suction ophthalmodynamometry (s-ODM), compression ophthalmodynamometry (c-ODM), interpolation from scleral compression/intraocular pressure (IOP) conversion tables, and measurements of the brachial blood pressure (BP). RESULTS: Group average CRA pressure values varied significantly across techniques, with the largest difference among methods being about 15 mm Hg. CRA pressures derived by s-ODM or c-ODM and direct measurements of the IOP yielded statistically identical values. These latter values were significantly lower than CRA pressures estimated by either scleral compression/IOP conversion tables, or those predicted from brachial BP measurements with the arm held up alongside the head, both of which produced equivalent values. The highest estimates of CRA pressures were obtained when brachial BP values were derived with the arm in its normal anatomical position. CONCLUSION: The patency of the vascular network to the eye and subsequent perfusion of intraocular neural tissue essential to normal visual function can be evaluated by simple clinical procedures. Although all techniques to estimate the pressure in the ophthalmic artery are relatively simple to use, they do not all yield the same absolute values and consequently should be interpreted in this light when used for either clinical or research purposes.
Adult↗
Electroretinographic results and ocular vascular perfusion in type 1 diabetes.
PURPOSE: Although functional changes in vision are known to occur before overt diabetic retinopathy, the relationship between (1) the degree and timing of any visual dysfunction and (2) the duration and control of diabetes remains poorly defined. The authors compared the ability of patients with insulin-dependent diabetes mellitus and nondiabetic subjects to sustain normal retinal function during transient physiologic stress. Such provocative testing may be clinically useful to show subclinical dysfunction. METHODS: Retinal function during altered ocular perfusion pressure (OPP) produced by body inversion and scleral suction was quantified by scotopic white-flash electroretinography (ERG) in 11 young volunteers with insulin-dependent diabetes mellitus but without significant retinopathy and a group of normal subjects. RESULTS: Baseline ERG a- and b-waves did not differ across test groups, but the a-wave implicit time was prolonged for diabetic patients. Furthermore, four of five oscillatory potentials (OP) in the diabetic patients were smaller than those in the nondiabetic subjects. Although the amplitude and implicit time of some OP in the diabetic patients were changed to a larger degree, the overall OP response profile did not differ widely from that for the nondiabetic subjects. Also, the a- and b-waves and the OP index for the test eye in diabetic and normal subjects did not differ, but the control eyes showed some differences in these parameters. CONCLUSIONS: Retinal function and vascular autoregulation in young diabetic patients without visible retinopathy did not differ from those in nondiabetic subjects; they were reduced in both groups by a short-term attenuation of the OPP.
Adolescent↗
Improving the diagnostic power of electroretinography by transient alteration of the ocular perfusion pressure.
In this report we present the results of a series of studies focusing on the effects of transient changes in the ocular perfusion pressure (OPP) on retinal function in normals as assessed by the flash electroretinogram (ERG). A transient increase or decrease in the OPP affected by body inversion and compression/suction ophthalmodynamometry (ODM), respectively, is shown to affect differentially the b-wave of scotopic and photopic ERG's. However, under dark-adapted conditions, the cone component of the red flash ERG b-wave exhibited a vulnerability to decreased OPP which approached that seen for the b-wave of the scotopic blue flash ERG b-wave. Similar test procedures used to investigate the functional response of the inner plexiform layer during altered OPP revealed component-specific changes in white flash scotopic oscillatory potentials (OP's). The results of these provocative tests of retinal function offer new insights into basic retinal physiology and encouraging prospects for practical clinical diagnostic procedures with enhanced sensitivity and specificity for subclinical retinal disorders.
Dark Adaptation↗
Accommodative performance for chromatic targets in diabetes mellitus: a preliminary report.
Our objective was to evaluate the diagnostic potential of a new vision testing procedure for detecting subclinical visual dysfunction in patients with insulin-dependent diabetes mellitus (IDDM) and minimal or absent retinopathy. We used a method which challenged both the ocular focusing and color discrimination systems simultaneously. A computerized laser speckle optometer which measured changes in ocular refraction on a subjective criterion of speckle movement rather than perceived blur was used to measure the accuracy of steady-state accommodative responses. Five volunteers between 15 and 23 years of age who had IDDM and minimal background retinopathy participated in this study. Our results for this small group of diabetics showed that: (1) the overall binocular accommodative response profiles of diabetics for colored targets through increasing negative power ophthalmic lenses did not differ markedly from those of a control population; (2) diabetics exhibited a high sensitivity for perception of optical defocus; (3) the blood glucose level influenced the extent and accuracy of accommodative responses; and (4) the overall accommodative precision, when compared to a group of nondiabetics in the same age range, showed evidence of being more reliant on target colors. The theoretical and clinical implications of these findings are discussed.
Accommodation, Ocular↗
Influence of transiently altered retinal vascular perfusion pressure on rod/cone contributions to scotopic oscillatory potentials.
The oscillatory potentials (OPs) are sensitive to vascular disturbances in the retina. Rods in man are more susceptible than cones to ischaemia. The findings in a recent study showed that the white flash OP-5 had a heightened sensitivity to altered retinal vascular perfusion pressure (RVPP), compared with earlier OPs and the b-wave. In the present study, a comparison was made of the sensitivity of scotopically matched blue and red flash OPs to dim white flash OPs, to transient stepwise changes in the RVPP, in 10 healthy young adults. After 30 minutes of dark adaptation, two consecutive 0.3 Hz flash groups (n = 20) were filtered and averaged to obtain 100 ms flash ERGs and OPs simultaneously via DTL fibre electrodes for white, blue, and red flashes. To minimize any carry-over effects across flash colour, or repeated testing, the retinal responses to each of these stimuli were measured in separate test sessions several hours to days apart. The RVPP was increased or decreased by 20% and 40% non-invasively by body inversion and scleral suction respectively. For the flash intensities used, blue and red flash OPs appeared more variable than white flash OPs. The OP index increased or decreased across all flash colours when the RVPP was increased, with blue OPs showing the greatest spread of data. Retesting in five subjects using white and red flashes against a blue background disclosed the input of both rods and cones to the white flash scotopic OPs. The provocative test conditions described here offer a potentially profitable approach for studies attempting to differentiate the effects of vascular disorders on rod versus cone function.
Action Potentials↗
The effects of altered retinal vascular perfusion pressure on the white flash scotopic ERG and oscillatory potentials in man.
The oscillatory potentials (OPs) of the flash-elicited electroretinogram b-wave have been identified as sensitive indices of abnormalities within the retinal circulation. A recent study by Lovasik and Kothe has identified the rod system as being more vulnerable than the cone system to transient alterations of the retinal vascular perfusion pressure (RVPP). In view of these previous findings, we investigated the susceptibility of the scotopic OPs to altered retinal perfusion, in 10 normotensive paid volunteers between 21 and 31 years of age. A transient increase in RVPP was effected by body inversion, while a transient decrease in RVPP was obtained by suction ophthalmodynamometry. Group averaged data showed that OP-1 to OP-4 decreased in amplitude with either an increase or a decrease in RVPP. In contrast to all other OPs, OP-5 was reduced to a greater degree when RVPP was decreased and showed a pronounced gain in amplitude as RVPP was increased. Our study has isolated a component-specific vulnerability to altered retinal perfusion. This finding may be interpreted as indicating a different retinal site of origin for the generators of OP-5. The heightened sensitivity of OP-5 to alteration of the RVPP may offer diagnostic advantage for investigating chronic diseases causing retinal ischemia.
Adult↗
A parametric evaluation of retinal vascular perfusion pressure and visual neural function in man.
Body inversion and ophthalmodynamometry were used to alter the intraocular pressure/retinal vascular perfusion pressure (IOP/RVPP) relationship in 10 normotensive adults. Using scotopic and photopic flash electroretinograms (fERGs), a heightened susceptibility of the rod system relative to the cone system was identified for transient alterations of the RVPP. The scotopic fERG b-wave decreased by about 40% and the photopic fERG was relatively unchanged when the RVPP was reduced by 51%. When the RVPP was increased by some 90%, the scotopic b-wave decreased by about 11%, and the photopic fERG by 8%. The greater vulnerability of the rod system than the cone system to decreased RVPP was tentatively attributed to structural differences between the photoreceptor types, their retinal distribution, and the effectiveness of vascular autoregulatory mechanisms. The neural generators of the pattern evoked retinal potential (pERG) showed a large impairment in function with both an increase and a decrease in the RVPP. In contrast, pattern evoked cortical potentials (pVEPs) showed a significant reduction in amplitude only when the RVPP was increased. These latter results suggest that the generators of pERGs are dependent on some ideal and narrow range of RVPP for normal activity whereas the generators of pVEPs are more vulnerable to the direct pressure effects of vascular engorgement within the eye and intracranial structures.
Adult↗
Visual and neural function in Leber's optic neuropathy.
This report details the chronology of vision loss for an 18-year-old Caucasian male with Leber's optic neuropathy. Findings of an oculo-visual assessment with auxiliary in-office tests of visual neural function, results of neurophysiological testing, and rehabilitative therapy with low vision aids are presented. The clinical characteristics of this devastating disorder are reviewed.
Adolescent↗
Time course of cycloplegia induced by a new phenylephrine-tropicamide combination drug.
A motorized and computer-interfaced phoropter was used to track the development of cycloplegia and recovery of accommodation over a 60-min period, after the topical application of a phenylephrine 5%-tropicamide 0.8% drug combination (Phenyltrope). Phenyltrope was introduced into the Canadian market about 2 years ago (Compendium of Pharmaceuticals and Specialties, 1987), and advertised as a fast acting cycloplegic and mydriatic drug. Here we report the results of our investigation of the depth of action and the temporal aspects of cycloplegia for this drug combination as a function of iris color. We also compare the action spectrum of Phenyltrope to that of tropicamide 1% under similar test conditions. Our results indicate that the latency for cycloplegia was shorter for tropicamide, the maximum rate of accommodative loss similar for both drugs, and the resultant cycloplegia at 20 min deeper for Phenyltrope. Recovery from induced cycloplegia was greater for tropicamide 60 min after drug administration. For both Phenyltrope and tropicamide, no significant differences in any of the parameters investigated were observed as a function of iris color. We conclude that even though Phenyltrope induced a measurably deeper cycloplegia than did tropicamide, the amount of residual accommodation present at 20 min (about 38%) is insufficient for most refractive purposes.
Accommodation, Ocular↗
Visual neural performance for chromatic displays.
Various subjective procedures have been used in the past to examine the visual disorders and ocular symptoms often associated with prolonged usage of Video Display Units (VDU's). We examined visual neural performance for VDU stimuli which differed in size (14, 21, or 28 min arc), chromaticity (white, red, green, or blue), and retinal clarity, by transient pattern visually evoked potentials (t-p VEP's). Such information could prove useful in the design of electro-optical display systems that optimize visual neural performance and minimize ocular fatigue. Stimuli consisted of "monochromatic" (W, R, G, or Blue on black) and "multichromatic" (Blue/R, Blue/G, and R/G) checkerboards with brightness-matched chromatic elements displayed on a high resolution RGB monitor at 40 and 80 cm. The ambient lighting level was 54 lux. Group-averaged amplitudes and implicit times of t-p VEP's from 20 visually normal subjects indicated a differential neural response across target colors and three experimentally induced levels of blur. For monochromatic stimuli, the Blue/Black targets elicited t-p VEP's with the lowest amplitude, longest implicit time, and greatest sensitivity to optical defocus. Increasing the target element size reduced the VEP sensitivity to defocus across all colors except red. For multichromatic targets, Red/Green targets elicited the most vulnerable t-p VEP's. Several optical and neural explanations are given to explain these results. Implications for VDU designs are presented.
Accommodation, Ocular↗
Influence of target color and vergence of light on ocular accommodation during binocular fixation.
In an earlier study we reported large discrepancies between ocular refraction and perception of blur for black/white letters. In the present study we report on the influence of target color and vergence of light on the human focusing system. Twenty visually normal volunteers between 14 and 25 years of age participated in this experiment. Targets were brightness-matched red, yellow, green, blue or white 21 minarc letters displayed on the black background of a high resolution RGB monitor, at a 40 cm observation distance. Changes in the vergence of light were effected via increasing amounts of positive and negative power spherical lenses placed binocularly in front of the subject's eyes. Our results showed that: 1) the accommodative level varied as a function of the color of a target, 2) the vergence of light was not an infallible cue for accurate accommodation and 3) the inter-subject variability seen in response to the vergence of light was not linked with the chromaticity of test targets. All subjects showed a decreasing ability to fully relax accommodation with increasing plus power lenses. For minus power lenses, accommodative response profiles were divided into 3 categories, category 1 having accurate accommodation for the test target, category 2 showing a low level lag of accommodation and category 3 being totally unresponsive to increasing divergence of light. While subjects across response categories showed differential endpoints in clinical estimates of positive relative accommodation, other clinical measurements related to refractive error, accommodative amplitude and fusional ability failed to predict the accommodative behavior of our subjects as did measurements of the resting focus of accommodation.
Accommodation, Ocular↗
Variability in clinically measured photopic oscillatory potentials.
Oscillatory potentials found on the ascending phase of the electroretinogram b-wave probably originate in some element(s) of the inner plexiform layer. As oscillatory potentials are particularly sensitive to changes in retinal, and possibly choroidal, blood flow, they have been used extensively to provide clinical measures of the degree of retinal ischemia during the progression of diabetic retinopathy. Recent studies in our laboratories have disclosed previously unreported significant variability in the photopic oscillatory potentials on repeated measures even in tightly controlled conditions. The amplitude of five recordable light-adapted wavelets exhibited considerable intra- and inter-subject variability. Until further investigation can determine factors affecting standardization of testing, it appears that changes in oscillatory potential implicit times rather than in amplitudes are a better measurement in clinical neurophysiology.
Adult↗