Transscleral laser thermotherapy of hamster Greene melanoma: inducing tumour necrosis without scleral damage

The feasibility of using transscleral thermotherapy (TSTT) to induce necrosis of choroidal melanoma without causing damage to the sclera was investigated. Fifty-two subcutaneously implanted hamster melanomas covered by human donor sclera were irradiated for 1 min with an 810 nm laser using a 3 mm spot diameter, with and without cooling of the scleral surface. Immediately after irradiation the temperature of the scleral surface was measured with an infrared camera. Irradiation at 2000 mW, without cooling of the sclera, resulted in tumour necrosis to a mean depth of 4.4 mm and a mean diameter of 5.5 mm, without causing damage to the scleral collagen; the surface temperature of the sclera was 44.5 degrees C, and the estimated temperature at the sclera-tumour interface was 60.1 degrees C. There was a sharp demarcation between the viable and the necrotic part of the tumour. TSTT at 2500 mW, without cooling of the sclera, caused maximal tumour necrosis to a mean depth of 5.2 mm and a mean diameter of 5.9 mm; the scleral layers adjacent to the tumour had an estimated temperature of 67.6 degrees C and showed signs of damage in 14% of the experiments. Cooling of the sclera resulted in less thermal damage to the sclera but also less tumour necrosis. Results indicate that TSTT has potential in the treatment of choroidal melanoma.

Temperature dependence of thermal damage to the sclera: exploring the heat tolerance of the sclera for transscleral thermotherapy

Thermal damage to the human sclera in relation to temperature and duration of exposure was studied in order to determine the heat tolerance of the sclera with respect to transscleral thermotherapy of choroidal melanoma. Samples of human sclera were submerged in saline for 10 sec to 10 min at temperatures of 37-100 degrees C. The effects of heat on the shape, weight and size of the samples were studied. Thermal damage of scleral collagen was examined by polarized light microscopy (LM) and electron microscopy (EM). The sclera was embedded in epoxy resin and stained with toluidine blue for LM and with uranyl acetate and lead citrate for EM. Thermal damage of scleral collagen on polarized LM was graded on a five point scale. Scleral damage was visible on macroscopic examination and on LM and EM in sclera heated at 65 degrees C for 20 sec or longer, at 70 degrees C for 10 sec or longer, and at higher temperatures. A sigmoidal function was used to define the relation between temperature and changes in diameter, thickness, and weight of scleral samples. Using fitted functions, the threshold temperature for thermal damage was estimated to be 59-61 degrees C when samples were heated for 10 min, 62-63 degrees C when heated for 1 min, and 66-67 degrees C when heated for 10 sec; the threshold exposure time at 60 degrees C was estimated to be 7-12 min. These results indicate a temperature of 60 degrees C for 1 min is well tolerated by human donor sclera; information of in vivo studies is required to validate whether this setting can be used in transscleral thermotherapy (TSTT) for choroidal melanoma.

Conversion of lens slit lamp photographs into physical light-scattering units

PURPOSE

To derive from lens slit lamp photographs by means of densitometry the physically defined quantity for light scattering (the Rayleigh ratio) and to expand the use of the Lens Opacity Classification System (LOCS III) to include clear lenses and also to calibrate the LOCS III Nuclear Opacity (NO) score in physical terms.

METHODS

Series of slit lamp photographs were taken from 38 eyes from 29 subjects (age range 18 to 84 years old) including cataracts, for 0.1- and 0.2-mm slit width, using 200 ASA and 1600 ASA film speed (Kodak professional; Eastman Kodak, Rochester, NY) and different flash settings with a Topcon SL-6E (12 slit/speed/flash combinations; Paramus, NJ). Additionally 19 eyes were photographed with a Zeiss 40 SL/P (8 slit/speed/flash combinations; Carl Zeiss, Thornwood, NY). A calibrated suspension of latex spheres also was photographed at the same 20 conditions. Densitometry was performed on the nuclear area of all photographs including the LOCS III standards, using a photometrically corrected photocell. Slit width and flash intensity settings were photometrically calibrated. All eyes and the suspension were digitally "photographed" with the EAS-1000 (Nidek, Gamagori, Japan) Scheimpflug system.

RESULTS

For each eye and the suspension, the series of 20 or 12 densities, corresponding to a range of about 1 log unit in the amount of light used, proved to follow closely a course common to all eyes (the two film characteristics), apart from a shift in the amount of light (because of the differences in light back scattering).

CONCLUSIONS

From normal slit lamp photographs, the physical quantity for light (back) scattering can be derived using transformation graphs derived in this study. The LOCS III NO score also can be used for clear lenses and translated into physical units. In this way, slit lamp photography can be used better for more precise studies, provided some minimal calibration of the photograph slit lamp.

Light scattering model for donor lenses as a function of depth

The amount of light scattered by normal donor lenses (n = 15, ages 43-82 years) from a 1 x 0.1 mm white slit beam was measured as a function of depth in the lens for seven angles from 10 to 165 degrees, and for four wavelengths from 400 to 700 nm. Apart from the most superficial layers, the data could be described with a model that consisted of three components. (1) small sized protein particles (alpha-crystallin), (2) large sized protein particles and (3) spectrally neutral rough surface reflectance ('zones of discontinuity'). Component (1) and (3) dominate backward scattering. Component (2) dominates forward scattering, but occupies only around 0.000006 of the lens volume, with the lowest values in the nucleus. Component (3) is important for a small range of backward directions only, being much stronger in extranuclear areas than in the nucleus.

Evaluation of a reference set based grading system for retinal nerve fiber layer photographs in 1941 eyes

PURPOSE

To evaluate a reference set based grading system for retinal nerve fiber layer photographs.

METHODS

A total of 1,941 retinal nerve fiber layer photographs (Canon non-mydriatic camera) were evaluated using a reference grading system. The reference system consists of 25 photographs of the retinal nerve fiber layer (Canon CF60U camera) of normals and patients with glaucoma. Each reference photograph represents a score for the visibility of the nerve fibers. Superior and inferior regions of the 1,941 photographs of the sample of the normal population were matched separately to the references.

RESULTS

The standard deviation of between eyes differences was 2.76. This standard deviation compared reasonably to the standard deviation of 1.0 to 1.9 for real repeated measurements in an earlier study and indicates that the grading system has good accuracy in relation to the total range of 1 to 25. A clear relationship between photograph score and age was established.

CONCLUSION

The photographic grading system demonstrated that it is viable in a different setting. Therefore, evaluation of retinal nerve fiber layer photographs with a photographic reference set might help ophthalmologists and technicians to integrate retinal nerve fiber layer photography in their clinical practice.

Choroidal haemodynamics in glaucoma

AIM

Quantification of haemodynamics of the peripapillary choroid in and the assessment of possible differences between normal subjects (N), ocular hypertensive (OHT), primary open angle (POAG), and normal pressure glaucoma (NPG) patients.

METHODS

Video fluorescein angiograms (Rodenstock SLO 101) were made in 22 N subjects, 12 OHT, 48 POAG, and 46 NPG patients. The angiographically derived dye build up curves were described by means of an exponential model. One of the model parameters is the time constant tau theoretically reflecting local blood refreshment time; the blood refreshment time tau is the time needed to replace the blood volume in the choriocapillaris, inversely proportional to the local choroidal blood flow. Other variables are maximal fluorescence (Fdt) and time of first fluorescence (t0). Mean variable values were calculated for disc area and circular areas around the disc.

RESULTS

Fdt of the disc was significantly lower in the POAG and NPG patients. There was no statistical difference in t0 between the study groups. The choroidal blood refreshment time was significantly longer in NPG patients and to a lesser extent in the POAG patients compared with the normal controls. The slowest choroidal blood refreshment can be found in the NPG group. The median choroidal blood refreshment times (25th-75th percentile) in the controls, OHT, POAG, and NPG patients were 4.1 (3.7-4.5), 4.4 (3.7-6.4), 5.8 (4.3-6.8), and 7.1 (5.5-9.3) seconds respectively.

CONCLUSIONS

With the help of parametrisation of dye curves, using a one compartmental model, choroidal haemodynamics can be quantified. The blood refreshment time of the peripapillary choriocapillaris was found to be significantly prolonged especially in NPG patients; this may indicate slower choroidal haemodynamics in NPG patients.

Light scattering by donor lenses as a function of depth and wavelength

PURPOSE

To determine quantitatively dependence on the wavelength and angle, as a function of depth, of light scattering in the human lens. To compare the result for forward directions with psychophysical data. To derive candidate particle distributions that might be responsible for nuclear light scattering as significant in the psychophysical situation.

METHODS

The amount of light scattered by donor lenses (n = 15, ages 48 to 82 years) from a 1-mm x 0.1-mm white slit beam was measured as a function of depth in the lens for seven angles from 10 degrees to 165 degrees, and for four wavelengths from 400 to 700 nm. Absolute values for light scattering (Rayleigh ratios) were derived.

RESULTS

The light-scattering data are confounded by the short wavelength-absorbing pigments in the lens. After correction, backward light scattering in the nucleus followed wavelength to a power of -4. In the superior layers and for forward directions in the nucleus, light scattering was less dependent on wavelength. The nuclear data could be explained on the basis of a bimodal protein particle distribution: particles much smaller than wavelength, in quantitative accordance with the literature, and particles larger than wavelength, which control forward light scattering.

CONCLUSIONS

The particles of significance for forward light scattering have, on average, a mean radius of 692 nm and constitute only 0.000003 of the volume. The wavelength dependence of retinal stray light is lessened by: the large sizes, the contribution of superficial lenticular layers, the lenticular pigments, and the contribution of other components of the eye.

A comparison of retinal and choroidal hemodynamics in patients with primary open-angle glaucoma and normal-pressure glaucoma

PURPOSE

To quantify, compare, and assess differences between retinal and choroidal hemodynamics in normal control subjects and patients with ocular hypertension, primary open-angle glaucoma, and normal-pressure glaucoma.

METHODS

Video fluorescein angiograms were made in 20 normal subjects, 11 patients with ocular hypertension, 45 patients with primary open-angle glaucoma, and 43 patients with normal-pressure glaucoma. Choroidal dye build-up curves were analyzed using an exponential model. The model time constant tau reflected the local blood refreshment time, the time needed to replace the blood volume in a tissue volume. Retinal arteriovenous passage time was estimated from the time lapse between retinal arterial and venous dye curves.

RESULTS

The retinal arteriovenous passage time was longer in patients with primary open-angle glaucoma compared with normal subjects and patients with normal-pressure glaucoma; the average arteriovenous passage times (+/-SEM) in normal subjects and in patients with ocular hypertension, primary open-angle glaucoma, and normal-pressure glaucoma were, respectively, 2.44 +/- 0.19, 2.90 +/- 0.37, 3.02 +/- 0.17, and 2.55 +/- 0.15 seconds. Choroidal tau was longest in the normal-pressure glaucoma group but not as long in the primary open-angle glaucoma group; tau values in normal subjects and patients with ocular hypertension, primary open-angle glaucoma, and normal-pressure glaucoma were, respectively, 4.6 +/- 0.29, 5.6 +/- 0.69, 6.2 +/- 0.39, and 7.1 +/- 0.33 seconds.

CONCLUSIONS

Whereas choroidal circulation is especially slower in patients with normal-pressure glaucoma, retinal circulation is delayed in patients with primary open-angle glaucoma. The choroidal and retinal vascular systems behave differently in primary open-angle and normal-pressure glaucoma, which may be important in the management of glaucoma.

Analysis of variance of microspheres blood flow measurements in rabbits

As part of a larger study on the interpretation of angiographically derived hemodynamic parameters, blood flow in several ocular tissues was measured using the radioactively labelled microspheres technique. As an unexpected secondary results, it was found that the microspheres data gave quantitative information on hyperaemic effects in the eye. This is the subject of the present paper. The measurements were made in 13 anaesthetized pigmented rabbits. In each animal, three blood flow measurements were performed at three different ocular perfusion pressures (60-15 mmHg). The perfusion pressures of the experimental eye were varied by changing the intra-ocular pressure. The contra-lateral eye served as a control. Labelled microspheres were used as a non-recirculating blood flow indicator, enabling the estimation of regional blood flows, in this case for the iris, ciliary body, peripheral choroid and peripapillary choroid separately. Using analysis of variance with perfusion pressure as covariate and taking into account the blood flow of the control eye, hyperaemia could be quantified in the experimental eye. Apart from a difference amongst animals, hyperaemia depended on tissue type. The amount of hyperaemia proved to be more pronounced in the anterior part of the eye, iris and ciliary body, and to decrease towards the posterior pole. With regard to the causes of this hyperaemia one could speculate about the invasive handling (anterior eye needles) topical administration of tropicamide, in combination with the general anaesthesia.

Near infrared light absorption in the human eye media

Available literature on the spectral transmittance of the eye media in the infrared (IR) is insufficiently accurate to estimate the deposited doses, e.g. in view of light damage risks. Analysis of the data of Boettner and Wolter [Investigative Ophthalmology and Visual Science, 1, 776-783 (1962)] for cornea, anterior chamber, lens and vitreous separately, shows that they can be explained on the basis of the pure water content. The transmittance spectrum of Geeraets et al. [Archives of Ophthalmology, 64, 606-615 (1960)] for all eye media combined is found to be impossible. It is concluded that in the near-IR, light losses in the eye media are best estimated with the absorption coefficient for pure water. A table is given from 700-2500 nm in steps of 10 nm.

Study of choroidal blood flow by comparison of SLO fluorescein angiography and microspheres

Choroidal hemodynamics estimated with parameters describing the dye build-up curves obtained with video fluorescein angiography, were compared with a classical regional blood flow measurement: radioactively labelled microspheres. Video fluorescein angiograms (Rodenstock's SLO 101) and microspheres blood flow measurements were made in 13 anaesthetized pigmented rabbits. Ocular perfusion pressures were varied from 60 to 15 mmHg by changing the intraocular pressure. The angiographically derived dye build-up curves were described by means of an exponential model. One of the model parameters is the time constant tau theoretically reflecting local blood refreshment time. Labelled microspheres act as a non-recirculating blood flow indicator, enabling the estimation of regional blood flows. The relation between choroidal blood flow and perfusion pressure is nearly linear, suggesting the passive nature of choroidal vasculature. There is a significant correlation between tau and microspheres flow (R = 0.67, P < 0.01). According to the rheological model the product of blood flow and tau corresponds to the relevant blood volume. Hence, a function for the volume of the choriocapillaris as a function of perfusion pressure was established. The model parameter tau can be interpreted as the local blood refreshment time. Since the parameter tau, unlike microspheres, can be used clinically, tau may be used to retrieve information on choroidal hemodynamics in clinical practice. Information on the spatial distribution of choroidal hemodynamics is also obtained.

Depth-dependent forward light scattering by donor lenses

PURPOSE

To determine quantitatively forward light scattering in the human lens as a function of depth. To use this to explain the psychophysical result, verified earlier in vitro, showing total light scattering in the human lens to decrease with scattering angle according to an approximate power law (power -2).

METHODS

The amount of light scattered by donor lenses (n = 15; age range, 43 to 82 years) from a 1 mm x 0.1 mm white slit beam was measured as a function of depth in the lens for seven angles from 10 degrees to 165 degrees and for four wavelengths from 400 to 700 nm. Absolute values for light scattering (Rayleigh ratios) were derived.

RESULTS

Light scattering of the total lens corresponded quantitatively to psychophysically determined in vivo stray light data. Powers were approximately -2.2. An important source of forward scattered light is located superficially at the anterior and the posterior poles. Nuclear forward light scattering varied over 2 log units, more or less in line with clinical assessment of nuclear opacity (LOCS III NO score). Nuclear powers were approximately -1.4.

CONCLUSIONS

In vitro forward light scattering of donor lenses as a whole corresponded with in vivo data, but different depths in the lens contributed differently. Studies on functionally relevant light scattering by the human lens proteins should be conducted to explain true (in vivo) lenticular light scattering.

Evaluation of the peripapillary circulation in healthy and glaucoma eyes with scanning laser Doppler flowmetry

In order to study peripapillary perfusion, one randomly selected eye of 34 of healthy volunteers and 40 glaucoma patients (27 suffering from primary open-angle glaucoma (POAG) 10 from normal pressure glaucoma (NPG) nd three from other types of glaucoma) was studied with a Heidelberg Retina Flowmeter. Temporal flow adjacent to the disc edge was significantly higher than the nasal flow (p < 0.01). It was reduced significantly in myopia both in controls (p < 0.05) and in glaucoma patients (p < 0.001). However, there was no difference between either controls and glaucoma patients or between POAG and NPG patients. It was independent of treatment type in glaucoma. Within the temporal peripapillary area extremely high flow values (values higher than the mean + 2 SD of the corresponding individual retinal flow) seemed to represent deep peripapillary vascular rings. They were significantly more frequent in glaucoma (72%) than in healthy volunteers (44%, p < 0.05). Their frequency was 83% in myopic and 23% in non-myopic eyes in the control group (p < 0.001). However, in glaucoma patients they were common both in myopic eyes (71%) and in non-myopic ones (75%). The results suggest that capillary perfusion adjacent to the temporal edge of the disc is significantly reduced in myopia. Deep peripapillary vascular structures can be measured on images focused on the surface of the retina, especially if the retina is thinner than normal (healthy myopic eyes and glaucomatous eyes independently of the refraction). This may mask a deficient function of the retinal capillary bed.

Scanning laser Doppler flowmetry in glaucoma

Scanning laser Doppler flowmetry is a new means for the measurement of capillary perfusion. We studied the retinal and neuroretinal rim capillary perfusion with the Heidelberg Retina Flowmeter in one randomly selected eye of 31 healthy volunteers, 42 primary open angle glaucoma (POAG) patients and 17 normal pressure glaucoma (NPG) patients. The reproducibility of the measurements on the temporal and nasal retina and on the disc rim was 19%, 26% and 28% in the unselected control group, 12% in the POAG and 12%, 13% and 10% in the NPG group, respectively. Optic disc flow was significantly higher than retinal flow (p < 0.000001). Differences in corresponding flow values between healthy volunteers and glaucoma patients as well as between POAG and NPG patients were not statistically significant. There was no correlation between the temporal and nasal flow values of the same eyes. The average variation in flow between adjacent frame positions was 18.5%. Actual intraocular pressure and the type of treatment had no influence on the retinal flow. The results suggest that the position of the test-frame is very important for the final result. We could not find any significant difference in the perfusion between glaucomatous and control eyes with the Heidelberg Retina Flowmeter.

Grading of retinal nerve fiber layer with a photographic reference set

PURPOSE

We developed a new quantitative approach for the evaluation of diffuse atrophy of the retinal nerve fiber layer.

METHODS

A simultaneous, visually supported grading system was designed, which consisted of a set of 25 reference photographs, numbered from 25 (broad, clearly striated nerve fiber bundles) to 1 (no nerve fibers visible). We prospectively evaluated this method by matching 60 retinal nerve fiber layer photographs of patients with glaucoma or ocular hypertension and normal subjects to the reference photographs twice by three observers with varying experience levels in evaluating retinal nerve fiber layer

RESULTS

Intraobserver and interobserver reliability, evaluated by the intraclass correlation coefficient statistics, was excellent (intraclass correlation coefficient > 0.8). Intraclass correlation coefficients within observers were from 0.89 to 0.98 (lower limits 905% confidence interval, 0.84 to 0.97), and intraclass correlation coefficients between observers were from 0.81 to 0.91 (lower limits 95% confidence interval, 0.50 to 0.87). Clinical validity, in which the scores for upper and lower halves of photographs were compared with the mean deviation of the Humphrey 30-2 visual field program by using the Spearman correlation coefficient, was substantial (r = -.68, P < .01 for upper photo score and lower field mean deviation, r = -.53, P < .01 for lower photo score and upper field mean deviation).

CONCLUSIONS

By using a reference set of of photographs of the retinal nerve fiber layer, we defined a method to derive a quantitative measurement of retinal nerve fiber layer with good reliability and to extend evaluation of retinal nerve fiber layer photographs to nonspecialists.

Functional characteristics of blue-on-yellow perimetric thresholds in glaucoma

PURPOSE

Blue-on-yellow (B-on-Y) perimetry assesses the S-cone visual field under yellow adaptation. Glaucomatous field defects have been shown to appear earlier and to be larger in B-on-Y perimetry than in standard perimetry. An upper limit to the use of B-on-Y perimetry is set by the separation of the S-cones from the M- and L-cones. But, because the S-cones may also input to the luminance channel, the actual separation of the color and luminance channels is unknown. Here, the relative sensitivities of the color and luminance channels under B-on-Y test conditions are measured.

METHODS

In 15 eyes with early glaucoma, 19 risk eyes, and 10 normal eyes, B-on-Y thresholds were measured from 0 degrees to 20 degrees eccentric and were compared to pure chromatic (B-in-Y) and achromatic (Y-on-Y) thresholds, obtained under identical yellow adaptation.

RESULTS

In normals, B-on-Y thresholds were found to coincide with B-in-Y thresholds; Y-on-Y values were 0.5 log (at 20 degrees) to 0.9 log (at 0 degrees) higher. In the pathologic groups, the differences between B-in-Y and Y-on-Y thresholds were smaller. Pathologic threshold elevation is on average 1.8 times larger for chromatic than for achromatic stimuli. In some cases, the luminance channel takes over detection of the B-on-Y stimulus.

CONCLUSIONS

In normals, the B-on-Y stimulus is mediated by the color channel. Takeover of detection by the luminance channel might impose limits on following color defects with B-on-Y perimetry. This takeover may occur before the S-cones become less sensitive than the M- and L-cones and might indicate S-cone input to the luminance channel.

Peripheral cone contrast sensitivity in glaucoma

Colour vision tests for detection of glaucomatous damage frequently suffer from two problems: most tests are confined to foveal vision, whereas defects tend to appear first extrafoveally; and the modulation directions in colour space are not optimal. This paper deals with peripheral testing à la Yu, Falcao-Reis, Spileers and Arden [(1991) Investigative Ophthalmology and Visual Science, 32, 2779-2789], and investigates whether there are modulation directions that show preferential sensitivity reduction in glaucoma. In 14 eyes with early glaucoma, 17 risk eyes and 10 normals, 12 deg peripheral colour contrast thresholds were determined for L, M, S, L-M and L+M test directions. Threshold elevations were correlated in all test directions, with S modulation yielding the largest elevations.

Forward Light Scatter at One Month After Photorefractive Keratectomy

BACKGROUND

Although it is known that backward light scatter increases transiently following most excimer laser photorefractive keratectomies (PRKs), it is not clear that there is a significant increase in forward light scatter, which is of primary concern for the patient. The object of this study was to determine if there is a significant change of forward light scatter at 1 month after (PRK) with an ablation zone diameter of 6 mm.

METHODS

Overlapping subsets of 24 normal myopic eyes were tested before (on the day of surgery) and 1 month after PRK, using three instruments: a Stray Light Meter (16 eyes); a Computerized Stray Light Meter (14 eyes); and a mesopic Increment Threshold-Glare Paradigm (six eyes). Differences between the two eyes before PRK were compared with the differences between the same eye before and after PRK, using repeated measured analysis of variance. In addition, increment threshold data obtained from 22 eyes after PRK were compared with those of 60 controls of the same age range and distribution by a t test.

RESULTS

None of the statistical comparisons approached significance at the alpha = 0.05 level. Changes in light scatter as small as a factor of 1.95 (Stray Light Meter) and 1.55 (Increment Threshold-Glare Paradigm) could be detected as significant with a high power (0.8). Changes larger than a factor of 21 could be detected with a power of 0.8 for the Computerized Stray Light Meter.

CONCLUSIONS

In these data, there is no support for the hypothesis that forward light scatter increases significantly 1 month after PRK with an ablation zone of 6 mm. Any increases in forward light scatter are unlikely to be greater than a factor of 1.5 to 2 under daytime or nighttime illumination conditions.

Hemodynamic parameter estimation from ocular fluorescein angiograms

BACKGROUND

A method is proposed for parameterizing choroidal blood flow from fluorescein angiograms.

METHODS

After digitizing and aligning the angiographic sequence, the intensity build-up curves of fluorescence are analysed per pixel (approx. 10 microns in fundo). Two models are compared. A one-compartment model predicts an exponential build-up curve, from which the following parameters are estimated: maximum fluorescence, dye appearance time and local perfusion rate (reciprocal of the time constant of the exponential). To account for the contribution of the systemic circulation to the shape of the build-up curve, a two-compartment model is used which predicts a bi-exponential curve.

RESULTS

Introduction of the second (systemic) compartment resulted in a significant improvement of fit in 37 of 48 patients studied. The rate constants of the systemic compartment found were mainly in the range of 0.30-1.00 s-1.

CONCLUSION

For the individual patient, the local perfusion rates may vary strongly, with lower perfusion rates possibly being of prognostic value for ocular diseases such as glaucoma or diabetic retinopathy.

Relationship between spectral transmittance and slit lamp color of human lenses

PURPOSE

To study the relationship between subjective lens color as observed with slit lamp biomicroscopy and spectral transmittance of the lens. To propose a model for this relationship to derive quantitative information on lens pigmentation from slit lamp observation.

METHODS

Twenty-nine normal lenses, from donors aged 14 to 86 years, were used. The fraction of light transmitted from a narrow beam was measured as function of wavelength. The spectra were fitted with the one-parameter TL model of Pokorny et al. The relationship between this parameter and the color grading from Chylack et al. (lens opacity classification system III nuclear color score) was established.

RESULTS

After slight adaptation of the TL model, the shapes of the transmittance spectra corresponded closely to the TL model (average residual error 0.05 log units). Log transmittance and lens opacity classification system nuclear color score were closely related (r = 0.90, 0.77, and 0.55 for 400, 500, and 602 nm, respectively).

CONCLUSIONS

A mathematical relationship between TL parameter and lens opacity classification system nuclear color score could be established to predict lens transmittance from lens opacity classification system nuclear color score. This relationship was successful in predicting the correction for lens absorption needed in blue-on-yellow perimetry.

Analysis of intraocular straylight, especially in relation to age

PURPOSE

To analyze straylight resulting from different sources (cornea, lens, wall translucency, and fundus reflectance) in combination with different properties (age and pigmentation). To give formulas for straylight calculations.

METHODS

In straylight on the retina multiple scattering can be assumed to be unimportant. Consequently, an additive model can be applied.

RESULTS

An efficient parameter to quantify straylight is the "straylight parameter" s(theta), with theta the scatter angle. The model reads s(theta) = s(cornea)(theta) + s(lens)(theta) + s(wall)(theta) + sfundus(theta), and each of these sources is analyzed as sx(theta) = sx,base(theta) + a(age)*sx,age(theta) + p(pigm.)*sx,pigm.(theta).

CONCLUSIONS

The most important effects, viz., s(lens,age), s(wall + fundus,pigm.) and s(cornea + lens + fundus,base) as well as a(age) and p(pigm.) were estimated. With the straylight parameter different effects of straylight, notably its disabling effect of contrast reduction, follow directly.

Stray light in photorefractive keratectomy for myopia

We performed a study to evaluate the influence on visual function of intraocular straylight after photorefractive keratectomy (PRK). We present 4 eyes of 4 myopic individuals, who had contacted our clinic for keratorefractive surgical treatment. PRK's were performed with a Summit laser, using a 5 mm ablation zone. The straylight meter was used to measure the amount of intraocular scattered light, the physical cause of glare complaints, before and after PRK. This apparatus uses the direct compensation method to assess the amount of intraocular light scatter. The results showed a significant increase in straylight values, in the tested eyes, during the first two weeks after PRK. After the initial rise, straylight values returned to preoperative levels, except for two eyes that clearly developed a haze higher than grade two. Instead of returning to baseline levels, straylight values remained significantly higher in these eyes.

Light transmittance of the human cornea from 320 to 700 nm for different ages

In vivo relative corneal transmittance was estimated using Tan's [(1971) Vision in the ultraviolet, thesis, University of Utrecht, The Netherlands] data on scotopic spectral sensitivity in aphakic eyes. This was combined with in vitro corneal transmittance data and in vivo light scattering data to arrive at absolute data. All data combined, the following function (no age dependence is found) resulted: log(transmittance) = -0.016-c*lambda-4 (lambda = wavelength in nm, lambda > 310 nm). c = 85*10(8) nm4 for direct transmittance (acceptance angle of the order of 1 deg) and c = 21*10(8) nm4 for total transmittance (acceptance angle close to 180 deg).

Quantal and visual efficiency of fluorescence in the lens of the human eye

PURPOSE

To document quantitatively the fluorescence in the human lens relevant to its interference with visual function. To explain quantitatively the experimental findings relative to loss of visual function. To study the relation between the fluorescence and the light transmission of the lens.

METHODS

Three normal lenses, from 22-, 28-, and 69-year-old donors, were used. Fluorescent light was induced by a 4-mm diameter pencil beam of 380, 400, or 420 nm. It was measured as a function of the deflection angle from -10 to 150 degrees for different wavelengths.

RESULTS

The shapes of the emission spectra were comparable to those reported in the literature. Total quantal efficiency of fluorescence was between 5% for 69 years and 380 nm excitation and 0.4% for 22 years and 420 nm excitation. The forward intensity was less than the backward intensity because of secondary absorption.

CONCLUSIONS

Fluorescence of the lens causes light with wavelengths of 420 nm and lower to be much more visually effective. A marked homogeneous veil is added to the point spread function. The total increase in luminous efficiency was a factor 3 to 6 at 400 nm, and a factor 70 to 150 at 380 nm. For other visual effects (glare) the increase can be larger.

Quantitative analysis of iris translucency in Fuchs' heterochromic cyclitis

PURPOSE

To measure intraocular stray light and to quantify translucency of the iris and the surrounding ocular wall in both eyes of patients with Fuchs' heterochromic cyclitis; to investigate whether differences in paired eyes could be measured in patients without heterochromia or with a minimal degree of iris atrophy.

METHODS

Intraocular stray light was measured by means of the direct compensation technique. A modification of this technique was used to quantify translucency of the iris and the surrounding ocular wall.

RESULTS

Intraocular stray light proved to be significantly higher in all patients with Fuchs' heterochromic cyclitis (both eyes) than in normal control subjects. Translucency of the iris and the ocular wall around it were increased in the patients with Fuchs' heterochromic cyclitis, including patients without heterochromia or with a minimal degree of iris atrophy.

CONCLUSIONS

Quantitative analysis of translucency was used to determine iris depigmentation in vivo in patients with Fuchs' heterochromic cyclitis. This technique showed that the process of atrophy and depigmentation in patients with Fuchs' heterochromic cyclitis is probably not restricted to the iris, but also occurs in the surrounding ocular wall. More studies, including other uveitis groups, are necessary to investigate if this technique can be used as a diagnostic tool in Fuchs' heterochromic cyclitis.

Effect of peripheral iridectomy on intraocular pressure in chronic primary angle closure glaucoma

In 67 eyes of 46 patients with chronic primary angle closure glaucoma, the effect of peripheral iridectomy in lowering the intraocular pressure was investigated retrospectively. In the majority of cases the pressure was reduced. For the whole population the mean reduction was 3.9 mmHg (SD 6.7 mmHg). The presence or extent of peripheral anterior synechiae showed no significant correlation with the amount of pressure reduction. A peripheral iridectomy is still recommended as first step in the treatment of primary angle closure glaucoma.

The intraocular straylight function in some hereditary corneal dystrophies

The direct compensation method allows for an accurate determination of intraocular light scattering between 3.5 and 25 degrees of scattering angle and is suitable for untrained subjects. The method was used to study light scattering in four forms of hereditary corneal dystrophies. Light scattering is the basis of glare complaints and was compared with visual acuity loss. The findings corresponded to the complaint patterns: in central crystalline dystrophy light scattering can be much increased with relatively well preserved visual acuity. In posterior polymorphous dystrophy the reverse is true. In macular dystrophy (Groenouw II) and lattice dystrophy the situation is more or less intermediate between these two extremes.

An improved mathematical description of the foveal visual point spread function with parameters for age, pupil size and pigmentation

An analytical description of the point spread function (PSF) for human foveal vision, together with its closed form two-dimensional Fourier transform, the modulation transfer function (MTF), is proposed. It also possesses an analytical line spread function (LSF) counterpart. It fits to both PSF and MTF experimental data and holds over the full angular and spatial frequency domain. Variation in the PSF and MTF descriptions with pupil size, age and iris/skin pigmentation are accounted for by analytical expressions in the parameters.

Stray light in radial keratotomy and the influence of pupil size and straylight angle

Glare is a major sequela of the radial keratotomy procedure. We used the straylight meter to measure intraocular light scatter, which is the cause of glare, in eyes after radial keratotomy. This apparatus uses a direct compensation method to assess the amount of intraocular light scatter. Nineteen patients were tested. Nine individuals served as controls. The mean postsurgery time was 60 months. The influence of the number of radial incisions, the pupil size, and the angle of light entering the eye were evaluated with the straylight meter. The results showed straylight values for normal pupil size (mean, 4 mm) to be statistically significantly higher (P = .0044) only for the smallest angle of light deflection studied (3.5 degrees). For dilated pupils (mean, 8 mm), straylight values were statistically significantly higher (P = .00005) for all three angles of light deflection studied. The number of incisions showed no statistically significant relationship to straylight values. Average stray light values were increased by a factor of 1.4 for 4-mm sized pupils and 2.0 for 8-mm sized pupils. There was an overlap in straylight values between the patient population and the control population.

Oculokinetic perimetry compared with standard perimetric threshold testing

To calibrate oculokinetic perimetry (OKP) as developed by Damato in terms of conventional perimetric threshold values, 33 eyes with either glaucoma or ocular hypertension were tested with a standard Humphrey Field Analyzer using the Central 30-2 test and twice with a 26-point OKP chart. The frequency of seeing of the OKP test spot was plotted against 30-2 thresholds. This showed a weak relation between the two tests. Subsequently eyes that were considered to have had poor fixation were omitted, but false positive and false negative results still occurred in 19 remaining eyes. To check whether Troxler's effect (local adaptation) might have caused false positives, 6 subjects were tested with OKP, controlling the fixation times. Increasing the fixation time from 'very short' to 2 and 5 seconds yielded more 'not seen' responses in OKP. False negative data were found to occur preferentially in border areas of visual defects. The average frequency of seeing curve over all data showed a 50% frequency of seeing for the OKP stimulus at a 14.6 dB equivalent conventional threshold value. The spreading of the curve was 7 dB (95% confidence interval 28 dB).

Clinical assessment of intraocular stray light

A device that measures intraocular stray light for clinical use in, e.g., cataract evaluation by using the psychophysical direct compensation approach is described and evaluated.

Discrepancies between single stimulus and multiple stimulus visual field examinations with the Peritest semi-automated perimeter in glaucoma patients

In a total of 24 randomly selected eyes of glaucoma patients, tested with the automated (single stimulus) and the manual (multiple stimulus) modes of the semi-automated Peritest perimeter, the numbers and depths of defects were compared as estimated with these two different measurement strategies. It was found that with the manual technique considerably less defects were detected than with the automated strategy.

Ocular lubricants and intraocular stray light

The question was raised whether ocular lubricants or artificial tears might have adverse effects on optical clarity of the eye. Eight current commercial products were tested on five young subjects, using the Straylight Meter. No adverse effects were found.

Intraocular light scattering in age-related cataracts

Intraocular light scattering was studied in 34 controls and 65 patients with cortical, nuclear, or posterior subcapsular cataracts by measuring forward scatter and backscatter. Forward scatter was measured by the psychophysical direct compensation method. Backscatter was determined with the Lens Opacity Meter of Interzeag. Contrast sensitivity loss caused by forward scatter was assessed with a glare tester (Vistech MCT 8000). Mean forward scatter was in the upper range for subcapsular cataracts compared to nuclear and cortical cataracts. Experimental results of the glare test (the contrast loss) deviated systematically from expected results based on measured forward scatter. Mean backscatter was largest for nuclear, intermediate for posterior subcapsular, and almost zero for cortical cataracts. Thus, each cataract has a characteristic mean ratio between forward scatter and backscatter. However, this ratio varied considerably among individuals, especially for cortical and posterior subcapsular cataracts. As a rule, forward scatter cannot be derived from backscatter (or the slit-lamp image).

Retinal contrast loss with non-monofocal IOLs

Non-monofocal IOLs are designed to give simultaneously sharp images of distant and near objects. This is achieved by means of different focal distances for various portions of the light reaching the eye. As a result, for any given object, one part of the light will be properly focused, while another part is out of focus. This results in a sharp image superposed on a blurred image, causing contrast loss. The retinal contrast loss as a function of spatial frequency is derived in this paper.

On the relation between glare and straylight

An overview is given of the basic phenomena that may lead to glare complaints in patients. Prominent among them is increased intraocular straylight; this can be measured. Other causes may include: increased sensitivity to normal straylight, the length of (increased) light and dark adaptation times, (small angle) neuronal lateral interaction. Distinction must be made between disability glare and discomfort glare. Tests have been proposed to determine glare-induced loss of various visual functions. Often the test results are thought to be directly related to straylight but this may be untrue.

Dependence of intraocular straylight on pigmentation and light transmission through the ocular wall

The straylight function of the human eye depends on eye color, especially at larger angles of scattering. As a potential cause for this dependence, transmission of light through the ocular wall was measured, using a psychophysical method. For a light-blue eye effective transmission of the iris was 1% for red and 0.2% for green light. Also the eyewall around the iris transmits a significant amount of light. For the dark-brown eyes of pigmented individuals transmission is lower by two orders of magnitude. Although important, transmission proved to be only partly responsible for the pigmentation dependence, the other cause probably being reflection from the fundus.

Blue-yellow perimetry in the detection of early glaucomatous damage

A perimetric method using blue stimuli on a yellow background was compared with perimetry using white stimuli on a white background as a method of detecting glaucomatous damage. Meridian perimetry was used with an adapted Tübinger perimeter. The difference between the blue-on-yellow meridian and the white-on-white meridian was subdivided into two parts: the general blue sensitivity loss (GBSL), probably due to optical factors, and the corrected blue sensitivity loss (CBSL), probably due to glaucoma. Nine normals, fourteen primary open angle glaucoma (POAG) patients and nine ocular hypertensives (OHT) were tested. All POAG patients and some of the OHT group showed higher CBSL values than the controls. The blue-yellow meridian showed broader and deeper defects than the white-white meridian in all of the POAG group; some of the OHT group had defects in the blue-yellow meridian that were not present in the white-white meridian. In conclusion, blue on yellow perimetry shows promise as a method for more sensitive detection of early glaucomatous damage.

Visual field behavior after intra-ocular surgery in glaucoma patients with advanced defects

Fifty advanced glaucoma patients with central visual field islands were evaluated before and after intra-ocular surgery with respect to visual acuity and visual field behavior. Thirteen patients underwent a cataract extraction, and 37 a filtering procedure. Visual acuity generally improved to a certain extent after lens extraction, and deteriorated after filtering surgery. For both groups the visual field island became somewhat larger with respect to the surface area, but the foveal value was usually lower after the operation. There was no clear relationship between visual acuity and foveal values or the surface area of the central island. One patient suffered unexplained loss of central vision.

The intraocular straylight function in 129 healthy volunteers; dependence on angle, age and pigmentation

The direct compensation method allows for an accurate (standard deviation below 0.05 log unit) determination of intraocular light scattering between 3.5 and 25 deg of scattering angle and is suitable for untrained subjects. The method was used to study population behaviour and individual variation in 129 volunteers between 20 and 82 yr of age, visual acuity equal to or better than one and no apparent eye pathology. The results indicate straylight to increase with the 4th power of age, doubling at 70. In addition to the age dependence, there was great variation between individuals. Part of this is due to negative correlation with pigmentation.

Is there general reduction of sensitivity in glaucoma?

Ten normal controls, ten ocular hypertensive patients, ten high tension, 12 medium tension and nine low tension glaucoma patients were studied prospectively by means of automated static perimetry. Multiple double threshold measurements were obtained during at most three years. A computer algorithm estimated for each visual field the Individual General Sensitivity IGS, in which local defects do not contribute. Thus we were able to estimate pure general reduction of sensitivity by comparing the IGS to age-corrected normal reference values. No significant difference in mean general reduction of sensitivity was found between the glaucoma patients and the normal group. Patients with local defects in one field half only also showed IGS values not different from normals. Our findings indicate that the impression of 'general' sensitivity loss may be caused by the presence of multiple local defects, and is not a phenomenon in itself.

Pattern electroretinogram can be more than the sum of local luminance responses

It is generally accepted that the pattern electroretinogram for very large spatial elements is the result of local luminance stimulation. Responses due to the luminance differences between elements may be assumed to be relatively unimportant because in the case of large elements only few retinal units are stimulated by gradients. With decreasing pattern element size one wonders to what extent the electroretinogram continues to be based on the local luminance stimulation. We investigated this question using 8 Hz checkerboard reversal and compared the pattern recordings with the recordings resulting from the same stimulus field modulated homogeneously (focal electroretinogram). A 100% modulated checkerboard at retinal level may be considerably less modulated because of imperfect optics of the eye. So the pattern electroretinogram should be compared with homogeneous field stimulation of correspondingly lower modulation depth. On the basis of the optical transfer properties of the eye we compared by subtracting the proper focal electroretinogram from the pattern electroretinogram. The difference response was virtually zero for check sizes larger than 120'. For checks from 60' down the difference response was of the same order of magnitude as the adjusted focal recording. This difference response for eyes with normal optics is largest around 30'; its wave form was found to be rather invariant with check size.

The point-spread function of the eye from 0 degrees to 100 degrees and the pattern electroretinogram

In studies on the pattern electroretinogram the quality of the retinal image is a major concern. The use of contact lens electrodes was rejected since a good pattern could not be recorded. This is believed to be due to blurring of the retinal image. As indicator of image quality the patient's visual acuity is often used. We wondered whether this is a sufficient criterion. The retinal image is the product of the whole optical point-spread function of the eye whereas visual acuity refers only to the central portion of this function. On the basis of existing reports it can be estimated that for the young normal eye the outer edges of this function (straylight) causes considerable loss of contrast. The strength of the straylight can be much greater in older eyes. We studied the relation between the point-spread function including straylight and the pattern electroretinogram in normal eyes and some pathological cases. The measurements proved to follow the calculated contrasts on the basis of a local luminance model, with the exception of enhancement (tuning) around 60' checksize for the young normal eye. Because of the considerable differences in straylight in an older population one has to take into account that loss of pattern electroretinogram can be suffered in patients with otherwise good visual acuity.

Pattern ERG and glaucomatous visual field defects

In the past five years numerous reports have suggested that ganglion cell function can be tested by means of a specialized form of electroretinography, the so-called pattern electroretinogram (PERG). Because of the important potentials of a ganglion cell test for clinical use this technique has been applied by several investigators to patients with (presumed) ganglion cell dysfunction, especially glaucoma. On grounds of principle we had reason to question whether the reported positive results should be attributed to ganglion cell dysfunction or to other factors such as optical disturbances. We investigated in this study the PERG as a function of visual field loss in glaucoma patients with careful control of optical factors. We did not find changes in PERG as a function of field loss. So either field loss is not related to the mass behaviour of ganglion cells, or ganglion cells are not the prime basis of the PERG. We believe the latter to be true.

Importance of pathological intraocular light scatter for visual disability

For healthy eyes intraocular light scatter was investigated several decades ago. For pathological eyes, however, little is known. As clinical test several techniques have been proposed but none has gained general acceptance. A disadvantage of these tests was that quantities were estimated that related only indirectly to the amount of light scatter. We propose a method that gives a direct estimate of the light scatter. We studied patients with cataract, corneal dystrophy, iris and fundus hypopigmentation, etc. A remarkable finding was that visual acuity correlates rather weakly with the amount of scatter. Since, however, the amount of scatter causes a considerable loss of visual function, the results show that for these patients the visual acuity test gives a rather limited impression of their visual handicap. More attention to the problems associated with intraocular light scatter is needed.

The spectral shape of A2 visual pigments

A mathematical description is proposed for the shape of absorption spectra of A2 visual pigments in which the shape continuously varies with peak wavelength lambdamax. The description consists of two parts: (a) the peak and short wavelength region is described by the sum of three lognormal curves and (b) the long wavelength region is derived from an existing physical model, including temperature dependence. The resulting function can be used as a basis for action spectra, also if a large long wavelength tail is present. The parameters of the functions as a function of lambdamax were assessed by fitting experimental goldfish rod and cone spectra. By interpolation of these parameters spectra can be calculated for lambdamax values all over the visual spectrum.

The fieldmaster 101 PR automatic visual field screener--technical evaluation and clinical results

The perimetric concept and the technical construction of the Fieldmaster 101 PR were evaluated. 64 eyes of patients and 10 normal subjects were examined. We conclude that the Fieldmaster is a well-engineered instrument, using an inefficient test strategy. The presentation of visual field results could be improved. Clinical results of the patient examinations are presented.