Clinical assessment of intraocular stray light

System based on the contrast of Purkinje images to measure corneal and lens scattering

Current methods to measure intraocular scattering provide information on the total scattered light, which consists of the combined contributions originating from different ocular structures. In this work, we propose a technique for the objective and independent assessment of scattering caused by the cornea and the lens based on the analysis of the contrast of the third and fourth Purkinje images. The technique is preliminarily validated first by using artificial eyes with different levels of corneal and lens scattering; second, it is validated in eyes wearing customized contact lenses to simulate corneal scattering and eyes with nuclear cataracts. Finally, it is tested on a larger population of eyes with cataracts and corneal disorders to prove its clinical usefulness.

Double-pass technique and compensation-comparison method in eyes with cataract

PURPOSE

To clinically assess the objective scatter index (OSI) obtained from double-pass images and the log(s) parameter measured with the direct compensation-comparison psychophysical technique in eyes with cataract.

SETTING

Ophthalmology Service, Terrassa Hospital, Barcelona, Spain.

DESIGN

Prospective observational case series.

METHODS

The analysis comprised eyes diagnosed with nuclear, cortical, or posterior subcapsular cataracts and healthy eyes (control group). Patient examinations included assessment of the manifest subjective refraction, corrected distance visual acuity, contrast sensitivity, and cataract grade using the Lens Opacities Classification System III (LOCS III) score. The protocol also included the straylight (log[s]) measured by the C-Quant device, measurement of the objective optical quality (Strehl ratio and modulation transfer function cutoff frequency), and the OSI (HD Analyzer).

RESULTS

Significant correlations with LOCS III classification were found in terms of log(s) and OSI, although they were slightly stronger with OSI for all cataract types, which could be attributable to higher-order aberrations. The OSI and log(s) shared approximately 44% of the scattering estimation and to coincide on the visual function decline with scattering for the 3 cataract types evaluated. Limits to discriminate between healthy and cataractous eyes and sensitivity and specificity values were 1.15 (sensitivity 91%, specificity 100%) for log(s) and 1.18 (sensitivity 89%, specificity 100%) for OSI (P < .05).

CONCLUSIONS

Both instruments provide complementary information to diagnose cataracts and follow patients. Although backscattered light from deeper retinal layers can have an effect on OSI, the double-pass image provides information to grade different types of cataract when assessing cataractous eyes for treatment.

FINANCIAL DISCLOSURE

None of the authors has a financial or proprietary interest in any material or method mentioned.

Calculating Retinal Contrast from Scene Content: A Program

This paper describes a computer program for calculating the contrast image on the human retina from an array of scene luminances. We used achromatic transparency targets and measured test target's luminances with meters. We used the CIE standard Glare Spread Function (GSF) to calculate the array of retinal contrast. This paper describes the CIE standard, the calculation and the analysis techniques comparing the calculated retinal image with observer data. The paper also describes in detail the techniques of accurate measurements of HDR scenes, conversion of measurements to input data arrays, calculation of the retinal image, including open source MATLAB code, pseudocolor visualization of HDR images that exceed the range of standard displays, and comparison of observed sensations with retinal stimuli.

Corneal abnormalities early in the course of Fuchs' endothelial dystrophy

PURPOSE

Corneas with advanced Fuchs' endothelial dystrophy that require endothelial keratoplasty manifest anterior corneal structural and cellular abnormalities that have been associated with visual deficits before and after endothelial keratoplasty. In this study, we determined the onset of these abnormalities in the course of the disease.

DESIGN

Cross-sectional study.

PARTICIPANTS

Sixty-three eyes (39 subjects) with a range of severity of Fuchs' dystrophy and 25 eyes (13 subjects) with normal corneas.

METHODS

All corneas were examined using slit-lamp biomicroscopy, ultrasonic pachymetry, and confocal microscopy. The clinical grade of Fuchs' dystrophy was assessed according to the presence and extent of guttae and clinically evident edema and was categorized as mild (grades 1 and 2), moderate (grades 3 and 4), or advanced (grades 5 and 6). Normal corneas were devoid of any central guttae (grade 0). Corneal backscatter (haze) was measured from the confocal image light intensity profile. Stromal cell density and number and the presence of abnormal subepithelial cells were determined from confocal images. Comparisons between groups were made by using generalized estimating equation models.

MAIN OUTCOME MEASURES

Anterior corneal backscatter, stromal cell density and number, presence of subepithelial cells, and central corneal thickness.

RESULTS

Anterior corneal backscatter was 18% to 67% higher in eyes with moderate and advanced Fuchs' dystrophy compared with normal eyes (P ≤ 0.003); a similar trend was noted in mild Fuchs' dystrophy eyes compared with normal eyes (P = 0.08). Stromal cell density and the absolute number of stromal cells in the anterior 10% of the stroma were approximately 20% and 27% lower, respectively, in Fuchs' dystrophy (regardless of severity) compared with normal (P < 0.001). Abnormal subepithelial cells were visible in 9%, 19%, and 30% of corneas with mild, moderate, and advanced Fuchs' dystrophy, respectively. Only corneas with advanced Fuchs' dystrophy were thicker than normal (P < 0.001).

CONCLUSIONS

Anterior corneal cellular and structural abnormalities begin early in the course of Fuchs' dystrophy, before the onset of clinically evident edema. The chronicity of these changes can explain their incomplete resolution after endothelial keratoplasty, and understanding the onset of these may help to determine the optimal time to intervene to achieve best outcomes.

Assessment of visual performance in the evaluation of new medical products

When evaluating how a medical product affects vision, it is important to assess how that product affects the ability to function in real life, not only the ability to read letters on a vision chart. Nevertheless, the measurement of visual acuity with a vision chart remains the primary test of the effects of medical products on vision. Here, we review efforts to identify reliable, cost-effective clinical tests to serve as surrogate measures of functional visual performance.:

History of ocular straylight measurement: A review

The earliest studies on 'disability glare' date from the early 20(th) century. The condition was defined as the negative effect on visual function of a bright light located at some distance in the visual field. It was found that for larger angles (>1 degree) the functional effect corresponded precisely to the effect of a light with a luminosity equal to that of the light that is perceived spreading around such a bright source. This perceived spreading of light was called straylight and by international standard disability glare was defined as identical to straylight. The phenomenon was recognized in the ophthalmological community as an important aspect of the quality of vision and attempts were made to design instruments to measure it. This must not be confused with instruments that assess light spreading over small distances (<1 degree), as originating from (higher order) aberrations and defocus. In recent years a new instrument has gained acceptance (C-Quant) for objective and controllable assessment of straylight in the clinical setting. This overview provides a sketch of the historical development of straylight measurement, as well as the results of studies on the origins of straylight (or disability glare) in the normal eye, and on findings on cataract (surgery) and corneal conditions.

Comparison between an objective and a psychophysical method for the evaluation of intraocular light scattering

In this study we present the comparison of the performance of two systems to measure intraocular scattering. Measurements were made by using a psychophysical system based on a brightness comparison method that provides a glare index and a physical system based on the double-pass technique, which gives an objective scatter index by measuring the optical quality of the eye. Three external diffuser filters that simulated different grades of intraocular scattering were used in subjects with normal vision. The two measured indexes showed a graded rise with increasing level of scattering. The discrimination ability obtained for both systems showed that they were able to distinguish among conditions ranging from normal to early cataracts.

Prevalence of impairment of visual function in European drivers

PURPOSE

This study aimed to investigate the prevalence of impairment of visual function amongst European drivers.

METHODS

A total of 2422 drivers from five European countries underwent a battery of visual function tests, including tests for visual acuity (VA), visual field (VF), contrast sensitivity, straylight (glare sensitivity) and useful field of view (UFOV). Subjects belonged to the following age categories: 45-54 years; 55-64 years; 65-74 years, and ≥ 75 years. A group of subjects aged 20-30 years served as a reference group. Results were compared with current standards for the visual function of drivers in the EU.

RESULTS

Visual acuity in 0.5% of the younger group and 5.3% of the eldest group was below the standard required for driving. Visual acuity after proper refraction adhered to the standard in the majority of cases. Visual field results were below standard or questionable in 2.7% and 2.4%, respectively, of subjects in the eldest group. In this eldest group, impaired contrast sensitivity was present in 6.3%, increased glare sensitivity in 29.5% and impaired UFOV in 21.6% of subjects, but, as there are no standards for these criteria, the exact level of prevalence depends on the definition of impairment.

CONCLUSIONS

Prevalences of impairment of visual functions that are not included in current standards (particularly contrast sensitivity and glare sensitivity) are much higher than prevalences of functions that are included (VA and VF).

An objective scatter index based on double-pass retinal images of a point source to classify cataracts

Purpose

To propose a new objective scatter index (OSI) based in the analysis of double-pass images of a point source to rank and classify cataract patients. This classification scheme is compared with a current subjective system.

Methods

We selected a population including a group of normal young eyes as control and patients diagnosed with cataract (grades NO2, NO3 and NO4) according to the Lens Opacities Classification System (LOCS III). For each eye, we recorded double-pass retinal images of a point source. In each patient, we determined an objective scatter index (OSI) as the ratio of the intensity at an eccentric location in the image and the central part. This index provides information on the relevant forward scatter affecting vision. Since the double-pass retinal images are affected by both ocular aberrations and intraocular scattering, an analysis was performed to show the ranges of contributions of aberrations to the OSI.

Results

We used the OSI values to classify each eye according to the degree of scatter. The young normal eyes of the control group had OSI values below 1, while the OSI for subjects in LOCS grade II were around 1 to 2. The use of the objective index showed some of the weakness of subjective classification schemes. In particular, several subjects initially classified independently as grade NO2 or NO3 had similar OSI values, and in some cases even higher than subjects classified as grade NO4. A new classification scheme based in OSI is proposed.

Conclusions

We introduced an objective index based in the analysis of double-pass retinal images to classify cataract patients. The method is robust and fully based in objective measurements; i.e., not depending on subjective decisions. This procedure could be used in combination with standard current methods to improve cataract patient surgery scheduling.

The effect of corneal light scatter on vision after penetrating keratoplasty

PURPOSE

To determine the effect of corneal light scatter on vision after penetrating keratoplasty (PK).

DESIGN

Cross-sectional study.

METHODS

setting: Cornea service at Mayo Clinic, Rochester, Minnesota. study population: Sixty-one eyes with clear grafts at 14.6 +/- 10.5 years after PK; 12 eyes with late endothelial failure (LEF) at 19.7 +/- 6.4 years after PK; 41 normal eyes. observation procedures: PK. main outcome measures: Backscatter from different depths of the cornea was measured by a custom scatterometer; intraocular forward light scatter was measured with a stray light meter; high-contrast visual acuity (HCVA) was measured by Early Treatment Diabetic Retinopathy Study (ETDRS) or Snellen chart; mesopic and photopic low-contrast visual acuity (LCVA) were measured with a 10% contrast chart.

RESULTS

Backscatter from the anterior, middle (stroma), and posterior thirds of the cornea was higher in clear grafts compared with normal (P < .001), and higher in grafts with LEF compared with clear grafts (P < .001). Corneal stromal backscatter from clear grafts correlated with time after keratoplasty (r = 0.21; P = .006; n = 61). Forward light scatter was higher in eyes with clear grafts compared with normal (P < or = .006), and higher in eyes with LEF compared with clear grafts (P < or = .01). Corneal stromal backscatter from clear grafts correlated with HCVA (r = 0.34; P = .02; n = 52), mesopic LCVA (r = 0.36; P = .01; n = 28), and photopic LCVA (r = 0.57; P < .001; n = 28). Forward scatter in eyes with clear grafts correlated with photopic LCVA (r = 0.38; P = .04; n = 26).

CONCLUSIONS

Light scattered from the cornea increases with time after PK and is associated with decreased high- and low-contrast vision.

Scattered light and visual function in a randomized trial of deep lamellar endothelial keratoplasty and penetrating keratoplasty

PURPOSE

To compare vision, intraocular forward light scatter and corneal backscatter between deep lamellar endothelial keratoplasty (DLEK) and penetrating keratoplasty (PK) for endothelial dysfunction.

DESIGN

A randomized clinical trial.

METHODS

Thirteen eyes (12 patients) were randomized to DLEK with a 9 mm scleral incision, and 15 eyes (14 patients) were randomized to PK. The primary outcome was high-contrast best spectacle-corrected visual acuity (BSCVA) at 12 months after surgery; intraocular forward light scatter and corneal backscatter were measured at one, three, six, and 12 months after surgery.

RESULTS

BSCVA at 12 months was 0.34 +/- 0.16 logMAR (logarithm of the minimum angle of resolution) for DLEK and 0.25 +/- 0.21 logMAR for PK (P = .23; minimum detectable difference at 12 months was 0.23 logMAR). The change in postoperative forward light scatter after DLEK correlated with the change in BSCVA (r = -0.66; P < .001; n = 11). Corneal backscatter was higher after DLEK than after PK at three and six months in the anterior third (P < or = .005), at one through 12 months in the middle third (P < .001), and at one through six months in the posterior third (P < or = .02) of the cornea. Backscatter after DLEK did not return to normal through 12 months (P < .001).

CONCLUSIONS

BSCVA was similar at one year after DLEK and PK. Improvement in BSCVA after DLEK correlated with decreasing forward light scatter. Increased backscatter after DLEK originated not only from the posterior cornea (interface) but also from the host cornea, which might limit visual outcomes after posterior lamellar keratoplasty.

Rainbow glare as an optical side effect of IntraLASIK

PURPOSE

To report a new optical side effect associated with the IntraLASIK procedure.

DESIGN

Retrospective chart review and questionnaire of all patients treated with IntraLASIK from June 2004 to August 2005.

PARTICIPANTS

A cohort of 585 eyes (312 patients) was divided sequentially into those treated with the older-model IntraLase laser (group 1: 399 eyes of 215 patients) and those treated with the newer model (group 2: 186 eyes of 97 patients).

METHODS

Preoperative and postoperative refraction, visual acuity, pupillometry, pachymetry, intraocular pressure, mean topographic power and cylinder, wavefront aberrations, and answers to a questionnaire of symptoms were recorded. The precise spatial extent of the rainbow spectrum was drawn by select patients for calculation of the diffractive grating size responsible for the light scatter. A glass slide was also irradiated and photographed using the older model to simulate the rainbow spectrum.

MAIN OUTCOME MEASURES

The symptom of rainbow glare was correlated with preoperative and postoperative factors and time between the most recent service call and surgery.

RESULTS

In group 1, 90.2% of patients were successfully contacted, and 37 (69 eyes) reported symptoms of rainbow glare around lights, for an incidence of 19.07%. In group 2, 88.6% of patients were contacted, and 2 (4 eyes) reported similar symptoms, for an incidence of 2.32%. The odds ratio (OR) between the 2 groups is 9.4 (P<0.001). For every 10-mum increase in ablation depth, the OR is 1.2 (P = 0.038), and for every 30 days from the last service call, it is 1.5 (P = 0.007). The spatial extent of the color pattern was used to calculate the spacing array at approximately 9 mum, corresponding to the laser spot and line separation.

CONCLUSIONS

Light scattering from the back surface of the IntraLASIK flap creates a spectral pattern whose visual impact is clinically inconsequential in the majority of patients. The spectral pattern and visual angle correspond to a grating size that matches the raster spot separation of IntraLase pulsing. Eyes treated with newer focusing optics of higher numerical aperture reduced the rainbow symptom. Variance in optical quality and numerical aperture can impact the spot size and uniformity of IntraLase flap creation.

Straylight effects with aging and lens extraction

PURPOSE

To assess possible gains and losses in straylight values among the population to consider straylight as added benefit of lens extraction.

DESIGN

In this cross-sectional design, data from a multicenter study on visual function in automobile drivers were analyzed.

METHODS

On both eyes of 2,422 subjects, visual acuity (logarithm of the minimum angle of resolution [logMAR] in steps of 0.02 log units), straylight on the retina (psychophysical compensation comparison method), and lens opacity (slit-lamp scoring using the Lens Opacities Classification System III [LOCS III] system) were determined. Three groups were defined: 220 pseudophakic eyes, 3,182 noncataractous eyes (average LOCS III score, <1.5), and 134 cataractous eyes (average LOCS III score, >3.0).

RESULTS

Noncataractous straylight values increases strongly with age as: log(s) = constant + log(1 + (age / 65)(4)), doubling by the age of 65 years, and tripling by the age of 77 years. Population standard deviation around this age norm was approximately 0.10 log units. The cataract eyes (in this active driver group) had relatively mild straylight increase. In pseudophakia, straylight values may be very good, better even than in the noncataract group. Visual acuity and straylight were found to vary quite independently.

CONCLUSIONS

Lens extraction holds promise not only to improve on the condition of the cataract eye, but also to improve on the age-normal eye. Lens extraction potentially reverses the strong age increase in straylight value, quite independently from visual acuity.

Effect of aberrations and scatter on image resolution assessed by adaptive optics retinal section imaging

The effect of increased high-order wavefront aberrations on image resolution was investigated, and the performance of adaptive optics (AO) for correcting wavefront error in the presence of increased light scatter was assessed in a model eye. An AO section imaging system provided an oblique view of a model retina and incorporated a wavefront sensor and deformable mirror for measurement and compensation of wavefront aberrations. Image resolution was quantified by the width of a Lorentzian curve fitted to a laser line image. Wavefront aberrations were significantly reduced with AO, resulting in improvement of image resolution. In the model eye, image resolution was degraded with increased high-order wavefront aberrations (horizontal coma and spherical) and improved with AO correction of wavefront error in the presence of increased light scatter. The findings of the current study suggest that AO imaging systems can potentially improve image resolution in aging eyes with increased aberrations and scatter.

Femtosecond laser versus mechanical microkeratome for LASIK: a randomized controlled study

PURPOSE

To compare corneal haze (backscattered light) and visual outcomes between fellow eyes randomized to LASIK with the flap created by a femtosecond laser (bladeless) or with the flap created by a mechanical microkeratome.

DESIGN

Randomized, controlled, paired-eye study.

PARTICIPANTS

Twenty-one patients (42 eyes) received LASIK for myopia or myopic astigmatism.

METHODS

One eye of each patient was randomized to flap creation with a femtosecond laser (IntraLase FS, IntraLase Corp., Irvine, CA) with intended thickness of 120 microm, and the fellow eye to flap creation with a mechanical microkeratome (Hansatome, Bausch & Lomb, Rochester, NY) with intended thickness of 180 microm. Patients were examined before and at 1, 3, and 6 months after LASIK.

MAIN OUTCOME MEASURES

Corneal backscatter, high-contrast visual acuity, manifest refractive error, contrast sensitivity, and intraocular forward light scatter were measured at each examination. Flap thickness was measured by confocal microscopy at 1 month, and patients were asked if they preferred the vision in either eye at 3 months.

RESULTS

Corneal backscatter was 6% higher after bladeless LASIK than after LASIK with the mechanical microkeratome at 1 month (P = 0.007), but not at 3 or 6 months. High-contrast visual acuity, contrast sensitivity, and forward light scatter did not differ between treatments at any examination. Flap thicknesses at 1 month were 143+/-16 microm (bladeless, mean +/- standard deviation) and 138+/-22 microm (mechanical microkeratome), with no statistical difference in variances. At 3 months, 5 patients preferred the bladeless eye, 7 patients preferred the microkeratome eye, and 9 patients had no preference.

CONCLUSIONS

The method of flap creation did not affect visual outcomes during the first 6 months after LASIK. Although corneal backscatter was greater early after bladeless LASIK than LASIK with the mechanical microkeratome, patients did not perceive a difference in vision.

[Optical quality after refractive corneal surgery]

Correction of myopia, hyperopia and astigmatism within its indicated margin by means of refractive corneal surgical procedures such as LASIK and surface ablation (e.g. PRK) is one of the standard procedures in ophthalmology. Now that advances in the fields of surgical techniques and the technical devices employed have further progressed in terms of safety and predictability, research also focuses on optical quality. "Optical quality" is not a clearly defined parameter, but can be captured indirectly by means of directly measured data. One has to start with the anatomical properties of the eye, which determine the optical images on the retinal level. The quality of the retinal image influences the eye's function, i.e. acuity and contrast perception. Finally, there is the subjective perception of the image we receive. "Optical quality" as such is reflected by the patient's evaluation of this image perception. Three phenomena are especially responsible for deterioration of the quality of the retinal image: diffraction, aberrations and dispersion. Some of the methods for measuring optical quality are subjective questionnaires, functional testing procedures for measuring visual acuity and contrast sensitivity, optical measuring procedures for the determination of optical quality, as well as biomicroscopy, aberrometry and corneal topography for assessing anatomical changes.

Influence of Exposure Time and Pupil Size on a Shack-Hartmann Metric of Forward Scatter

PURPOSE

To determine the influence of exposure time and pupil size on a Shack-Hartmann (S/H) derived metric of forward scatter (MAX_SD) using a physical model of nuclear cataract.

METHODS

A physical model eye was developed and mounted to a S/H wavefront sensor. The eye model consisted of a lens, variable pupil, simulated cataract, and retina. Located behind the pupil, a cuvette contained one of five polystyrene microsphere solutions simulating five levels of nuclear cataract severity. Cataract severity was described using a S/H derived metric of forward scatter (MAX SD), which measures aspects of forward scatter contained in the S/H lenslet point spread functions (PSF). To determine the impact of exposure time and pupil size, measurements of MAX_SD were regressed against cataract severity for three different exposure times and three different pupil sizes.

RESULTS

MAX_SD was well correlated to cataract severity. Exposure time had the largest influence, and pupil size had the smallest influence on the forward scatter metric. When pupil size and exposure time were allowed to vary and image saturation was allowed to occur, MAX SD explained 83% of the variance in cataract severity. Excluding images where saturation occurred, holding optimal exposure time constant, and varying pupil size, MAX_SD explained 97% of the variance in cataract severity.

CONCLUSIONS

The ability of the forward scatter metric derived from S/H measurements to predict cataract severity for a longitudinal study is optimized by selecting a patient-specific exposure at the initial cataract assessment to avoid saturation and maximize the dynamic range of the system. This patient-specific exposure should be used in all future visits.

Measurement of stray light and glare: comparison of Nyktotest, Mesotest, stray light meter, and computer implemented stray light meter

AIM

To evaluate the properties of devices for measuring stray light and glare: the Nyktotest, Mesotest, "conventional" stray light meter and a new, computer implemented version of the stray light meter.

METHODS

112 subjects, divided in three groups: (1) young subjects without any eye disease; (2) elderly subjects without any eye disease, and (3) subjects with (early) cataract in at least one eye. All subjects underwent a battery of glare and stray light tests, measurement of visual acuity, contrast sensitivity, refraction, and LOCS III cataract classification. Subjects answered a questionnaire on perceived disability during driving.

RESULTS

Repeatability values were similar for all glare/stray light tests. Validity (correlation with LOCS III and questionnaire scores), discriminative ability (ability to discriminate between the three groups), and added value (to measurement of visual acuity and contrast sensitivity) were all superior for both stray light meters. Results of successive measurements are interrelated for the conventional but not the new stray light meter. This indicates a better resistance to fraud for the latter device.

CONCLUSIONS

The new computer implemented stray light meter is the most promising device for future stray light measurements.

Why HID headlights bother older drivers

Driving requires effective coordination of visual, motor, and cognitive skills. Visual skills are pushed to their limit at night by decreased illumination and by disabling glare from oncoming headlights. High intensity discharge (HID) headlamps project light farther down roads, improving their owner's driving safety by increasing the time available for reaction to potential problems. Glare is proportional to headlamp brightness, however, so increasing headlamp brightness also increases potential glare for oncoming drivers, particularly on curving two lane roads. This problem is worse for older drivers because of their increased intraocular light scattering, glare sensitivity, and photostress recovery time. An analysis of automobile headlights, intraocular stray light, glare, and night driving shows that brightness rather than blueness is the primary reason for the visual problems that HID headlights can cause for older drivers who confront them. The increased light projected by HID headlights is potentially valuable, but serious questions remain regarding how and where it should be projected.

Comparison of intraocular light scatter in carriers of choroideremia and X-linked retinitis pigmentosa

OBJECTIVE

To compare the extent of intraocular straylight in carriers of choroideremia (CHM) and X-linked recessive retinitis pigmentosa (XLRP) to clarify further the relationship between photoreceptor cell degeneration and intraocular light scatter in retinal diseases.

DESIGN

Prospective case-control study.

PARTICIPANTS

Six obligate carriers of CHM, 12 obligate carriers of XLRP, and 30 age-similar control subjects with normal vision. The controls had no posterior subcapsular (PSC) lens opacities, and the carriers had minimal or no PSC lens opacities, as assessed by slit-lamp biomicroscopy.

MAIN OUTCOME MEASURES

Straylight levels were measured using a van den Berg Straylightmeter. Visual acuity and Goldmann visual field area using a II/4e target were assessed for both eyes of each carrier. Electroretinogram (ERG) data were available on four of six carriers of CHM and all carriers of XLRP. The extent of retinal pigment epithelial degenerative changes was evaluated by fundus examination.

RESULTS

All six carriers of CHM had typical fundus abnormalities with normal visual fields. Five of the six carriers of CHM had age-normal levels of intraocular light scatter, and one showed minimally elevated intraocular light scatter. The 12 carriers of XLRP had a spectrum of fundus abnormalities and varying severity of functional impairment as derived from visual field areas and ERG amplitudes. Seven of the 12 carriers of XLRP showed an elevated level of intraocular light scatter in at least one eye. The degree of straylight elevation above the normal mean age value was correlated significantly with both visual field area and amplitude of the maximal-intensity, dark-adapted ERG b-wave.

CONCLUSIONS

The carriers of XLRP who had evidence of photoreceptor cell dysfunction (as determined by visual field loss and reduced ERG amplitudes) had increased levels of intraocular straylight, whereas the carriers of CHM, who showed fundus abnormalities alone, in the absence of demonstrable photoreceptor cell dysfunction, had normal or minimally elevated levels. This finding supports the hypothesis that the increased levels of intraocular light scatter observed in some patients with hereditary retinal degenerations result from subclinical changes in the PSC region of the lens as a consequence of photoreceptor cell degeneration.

Intraocular light scatter in patients with retinitis pigmentosa

We evaluated the level of intraocular light scatter in a group of patients with retinitis pigmentosa (RP) who had minimal or no lens opacities, since such patients not infrequently complain of photoaversion. Intraocular light scatter was measured in 20 patients with RP who were < 60 years of age and who had no more than a trace of posterior subcapsular (PSC) lens opacity by slit-lamp evaluation. Measurements of intraocular straylight were made using a van den Berg Straylightmeter. Results from the patients with RP were compared with those of a control group of 30 subjects with normal vision whose ages were similar to those of the patients with RP. Seventeen of the 20 patients with RP had straylight levels that were above the range of age-similar normal control subjects. In some patients, the straylight parameter was increased by a factor of 2.5 above the normal mean for the patient's age and by as much as four to five times the normal mean for 20-yr-old subjects. There was a statistically significant correlation (r = -0.73, P < 0.01) between the patients' log relative elevation in the straylight parameter and their log visual field areas. Our findings indicate that patients with RP can have increased levels of intraocular light scatter despite minimal or no clinically observable PSC lens opacities. The increased intraocular straylight, which is likely due at least in part to subclinical abnormalities in lens morphology, can accentuate the visual disability of patients with RP in the presence of glare sources.

Preliminary results of photorefractive keratectomy in active-duty United States Navy personnel

PURPOSE

To evaluate the safety, efficacy, and quality of vision after photorefractive keratectomy (PRK) in active-duty military personnel.

METHODS

Photorefractive keratectomy (6.0-mm ablation zone) was performed on 30 navy/marine personnel(-2.00 to -5.50 diopters [D]; mean, -3.35 D). Glare disability was assessed with a patient questionnaire and measurements of intraocular light scatter and near contrast acuity with glare.

RESULTS

At 1 year, all 30 patients had 20/20 or better uncorrected visual acuity with no loss of best-corrected vision. By cycloplegic refraction, 53% (16/30) of patients were within +/- 0.50 D of emmetropia and 87% (26/30) were within +/- 1.00 D. The refraction (mean +/- standard deviation) was +0.45 +/- 0.56 D (range, -1.00 to 1.63 D). Four patients (13%) had an overcorrection of more than 1 D. Glare testing in the early (1 month) postoperative period demonstrated increased intraocular light scatter (P<0.01) and reduced contrast acuity (with and without glare, (P<0.01). These glare measurements statistically returned to preoperative levels by 3 months (undilated) and 12 months (dilated) postoperatively. Two patients reported moderate to severe visual symptoms (glare, halo, night vision) worsened by PRK. One patient had a decrease in the quality of night vision severe enough to decline treatment in the fellow eye. Intraocular light scatter was increased significantly (>2S D) in this patient after the procedure.

CONCLUSIONS

Photorefractive keratectomy reduced myopia and improved the uncorrected vision acuity of all patients in this study. Refinement of the ablation algorithm is needed to decrease the incidence of hyperopia. Glare disability appears to be a transient event after PRK. However, a prolonged reduction in the quality of vision at night was observed in one patient and requires further study.

Excimer laser photorefractive keratectomy

Excimer laser photorefractive keratectomy has been used for the correction of myopia, hyperopia and astigmatism. This laser removes tissue through a process termed photoablative decomposition, in which incident photon energy is sufficient to break molecular bonds. Selective removal of tissue across the anterior corneal surface results in a change in anterior corneal curvature. The surgical outcome may be influenced also by interindividual variability in wound healing and pharmacologic interventions. The nature of the excimer laser-tissue interaction, and clinical outcomes of predictability, stability and complications of surgery for myopia are discussed in detail.

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.

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.

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.