Application of Bayes' to the prediction of referral decisions made by specialist optometrists in relation to chronic open angle glaucoma.

PURPOSE: To determine the accuracy of a Bayesian learning scheme (Bayes') applied to the prediction of clinical decisions made by specialist optometrists in relation to the referral refinement of chronic open angle glaucoma. METHODS: This cross-sectional observational study involved collection of data from the worst affected or right eyes of a consecutive sample of cases (n = 1,006) referred into the West Kent Clinical Commissioning Group Community Ophthalmology Team (COT) by high street optometrists. Multilevel classification of each case was based on race, sex, age, family history of chronic open angle glaucoma, reason for referral, Goldmann Applanation Tonometry (intraocular pressure and interocular asymmetry), optic nerve head assessment (vertical size, cup disc ratio and interocular asymmetry), central corneal thickness and visual field analysis (Hodapp-Parrish-Anderson classification). Randomised stratified tenfold cross-validation was applied to determine the accuracy of Bayes' by comparing its output to the clinical decisions of three COT specialist optometrists; namely, the decision to discharge, follow-up or refer each case. RESULTS: Outcomes of cross-validation, expressed as means and standard deviations, showed that the accuracy of Bayes' was high (95%, 2.0%) but that it falsely discharged (3.4%, 1.6%) or referred (3.1%, 1.5%) some cases. CONCLUSIONS: The results indicate that Bayes' has the potential to augment the decisions of specialist optometrists.

Assessing visual fields for driving in patients with paracentral scotomata.

BACKGROUND: The binocular Esterman visual field test (EVFT) is the current visual field test for driving in the UK. Merging of monocular field tests (Integrated Visual Field, IVF) has been proposed as an alternative for glaucoma patients. AIMS: To examine the level of agreement between the EVFT and IVF for patients with binocular paracentral scotomata, caused by either ophthalmological or neurological conditions, and to compare outcomes with useful field of view (UFOV) performance, a test of visual attention thought to be important in driving. METHODS: 60 patients with binocular paracentral scotomata but normal visual acuity (VA) were recruited prospectively. Subjects completed and were classified as "pass" or "fail" for the EVFT, IVF and UFOV. RESULTS: Good agreement occurred between the EVFT and IVF in classifying subjects as "pass" or "fail" (kappa = 0.84). Classifications disagreed for four subjects with paracentral scotomata of neurological origin (three "passed" IVF yet "failed" EVFT). Mean UFOV scores did not differ between those who "passed" and those who "failed" both visual field tests (p = 0.11). Agreement between the visual field tests and UFOV was limited (EVFT kappa = 0.22, IVF kappa 0.32). CONCLUSIONS: Although the IVF and EVFT agree well in classifying visual fields with regard to legal fitness to drive in the UK, the IVF "passes" some individuals currently classed as unfit to drive due to paracentral scotomata of non-glaucomatous origin. The suitability of the UFOV for assessing crash risk in those with visual field loss is questionable.

Influence of corneal asymmetry on the phakometric measurement of ocular surface alignment.

Phakometric measurements of corneal and crystalline lens surface alignment are influenced by corneal asymmetry in which the corneal apex does not coincide with the limbal centre. The purpose of this study was to determine the horizontal separation (e) between these corneal landmarks. Measurements were made in 60 normal eyes (30 subjects) using the Orbscan IIz corneal analysis workstation. Our results show that both corneal landmarks typically coincide, so that e = 0, but that inter-subject variations of about +/-1 mm can be expected (so that the corneal apex may fall nasal or temporal to the visual axis). This suggests that no correction for corneal asymmetry is required when estimating average amounts of ocular alignment from samples of eyes but that the measurement of e is strongly recommended for measurements in individual eyes.

Correlation between visual function and visual ability in patients with uveitis.

AIM: To investigate the correlation between tests of visual function and perceived visual ability recorded with a quality of life questionnaire for patients with uveitis. METHODS: 132 patients with various types of uveitis were studied. High (monocular and binocular) and low (binocular) contrast logMAR letter acuities were recorded using a Bailey-Lovie chart. Contrast sensitivity (binocular) was determined using a Pelli-Robson chart. Vision related quality of life was assessed using the Vision Specific Quality of Life (VQOL) questionnaire. RESULTS: VQOL declined with reduced performance on the following tests: binocular high contrast visual acuity (p = 0.0011), high contrast visual acuity of the better eye (p = 0.0012), contrast sensitivity (p = 0.005), binocular low contrast visual acuity (p = 0.0065), and high contrast visual acuity of the worse eye (p = 0.015). Stepwise multiple regression analysis revealed binocular high contrast visual acuity (p <0.01) to be the only visual function adequate to predict VQOL. The age of the patient was also significantly associated with perceived visual ability (p <0.001). CONCLUSIONS: Binocular high contrast visual acuity is a good measure of how uveitis patients perform in real life situations. Vision quality of life is worst in younger patients with poor binocular visual acuity.

The influence of high contrast acuity and normalised low contrast acuity upon self-reported situation avoidance and driving crashes.

The aim of this study was to determine the cues used to signal avoidance of difficult driving situations and to test the hypothesis that drivers with relatively poor high contrast visual acuity (HCVA) have fewer crashes than drivers with relatively poor normalised low contrast visual acuity (NLCVA). This is because those with poorer HCVA are well aware of their difficulties and avoid dangerous driving situations while those poorer NLCVA are often unaware of the extent of their problem. Age, self-reported situation avoidance and HCVA were collected during a practice based study of 690 drivers. Screening was also carried out on 7254 drivers at various venues, mainly motorway sites, throughout the UK. Age, self-reported situation avoidance and prior crash involvement were recorded and Titmus vision screeners were used to measure HCVA and NLCVA. Situation avoidance increased in reduced visibility conditions and was influenced by age and HCVA. Only half of the drivers used visual cues to signal situation avoidance and most of these drivers used high rather than low contrast cues. A statistical model designed to remove confounding interrelationships between variables showed, for drivers that did not report situation avoidance, that crash involvement decreased for drivers with below average HCVA and increased for those with below average NLCVA. These relationships accounted for less than 1% of the crash variance, so the hypothesis was not strongly supported.

The effect of pupil dilation with tropicamide on vision and driving simulator performance.

PURPOSE: To assess the effect of pupil dilation on vision and driving ability. METHODS: A series of tests on various parameters of visual function and driving simulator performance were performed on 12 healthy drivers, before and after pupil dilation using guttae tropicamide 1%. A driving simulator (Transport Research Laboratory) was used to measure reaction time (RT), speed maintenance and steering accuracy. Tests of basic visual function included high- and low-contrast visual acuity (HCVA and LCVA), Pelli-Robson contrast threshold (CT) and Goldmann perimetry (FIELDS). Useful Field of View (UFOV--a test of visual attention) was also undertaken. The mean differences in the pre- and post-dilatation measurements were tested for statistical significance at the 95% level using one-tail paired t-tests. RESULTS: Pupillary dilation resulted in a statistically significant deterioration in CT and HCVA only. Five of 12 drivers also exhibited deterioration in LCVA, CT and RT. Little evidence emerged for deterioration in FIELDS and UFOV. Also, 7 of 12 drivers appeared to adjust their driving behaviour by reducing their speed on the driving simulator, leading to improved steering accuracy. CONCLUSIONS: Pupillary dilation may lead to a decrease in vision and daylight driving performance in young people. A larger study, including a broader spectrum of subjects, is warranted before guidelines can be recommended.

Improved computing scheme for measuring eye alignment with Purkinje images I and IV.

This study introduces an improved computing scheme for determining eye rotation from Purkinje images I and IV. The original computing scheme systematically underestimated eye rotation. Paraxial raytracing calculations revealed that this error resulted from failure to account for the fact that Purkinje images I and IV fall at different distances behind the cornea. The error could be overcome with a correction factor derived from paraxial raytracing calculations. A series of experiments were carried out to test the validity of this correlation factor, involving exact raytracing calculations as well as measurements on physical model eyes and human eyes. The influence on the correction factor of ocular surface asphericity, accommodation, age and ocular component variations were examined. The new method was also compared to Hirschberg's technique, which makes use of Purkinje image I alone, as a means of screening for strabismus.

Catoptric properties of eyes with misaligned surfaces studied by exact ray tracing.

PURPOSE: Historically, Purkinje images have been used to calculate ocular surface curvature and misalignment. The purpose of this report is to introduce an exact ray-tracing program that can be used to examine the influence of ocular component variations on the size and position of Purkinje images I, III, and IV. METHODS: Ray tracing was carried out on Le Grand's four-surfaced schematic eye to demonstrate the main features of the program. Any location may be chosen for the point light source and the observer. Ocular component dimensions, eye rotation, and crystalline lens decentering and rotation are fully adjustable. The program computes the coordinates of the Purkinje images in three-dimensional space from the point of view of the observer. It also offers the option of exhibiting Purkinje images seen through standard and telecentric imaging devices. Both options yield Purkinje image positions in relation to the rotating center of the limbus, observed clinically. RESULTS: The resulting program compared favorably with current ray-tracing software and with data from the literature. CONCLUSIONS: The program has many research and teaching applications. For example, our requirement was to calculate a series of linear coefficients that closely approximate Purkinje-image behavior in any given eye. These coefficients may be used to measure misalignment of intraocular components in phakic and pseudophakic eyes.

Determination of the influence of effectivity upon residual astigmatism.

PURPOSE: A computing scheme is described which allows determination of the astigmatic contribution of ocular surface effectivity towards residual astigmatism. METHODS: This involves paraxial raytracing through astigmatic surfaces at random axes and applies the principle of astigmatic decomposition. Calculations are shown for averaged data from 66 normal right eyes. Frequency distribution graphs demonstrate individual variations. RESULTS: The averaged ratio of corneal thickness:anterior chamber:lens thickness cylinder power contributions due to effectivity (1:5:17) did not match the ratio of their respective intraocular distances (1:7:7); a disproportionate amount of astigmatism arose from lens thickness effectivity. Although previous research has revealed that results for individual eyes are prone to accummulative experimental errors, frequency distribution graphs indicate that effectivity predominantly yelds direct astigmatism (axis 180 degrees +/- 22.5 degrees). CONCLUSIONS: This computing scheme offers a means of examining the functional ocular morphology of astigmatic eyes.

Measurement of astigmatism arising from the internal ocular surfaces.

A method is described for measuring internal ocular surface (posterior cornea, anterior and posterior crystalline lens) astigmatism. This involves the use of videokeratography, A-scan ultrasonography, and autorefractometry along with multi-meridional phakometric measurements of Purkinje images I(anterior corneal surface) II(posterior corneal surface) and IV(posterior lens surface). Data was collected from both eyes of 66 subjects. Right and left eyes exhibited similar mean levels of astigmatism from the posterior corneal surface (R + 0.21 DC axis 82 degrees; L + 0.22 DC axis 80 degrees), anterior lens surface (R + 0.52 DC axis 8 degrees; L + 0.49 DC axis 165 degrees) and posterior lens surface (R + 1.48 DC axis 99 degrees; L + 1.16 DC axis 90 degrees). It was generally found that astigmatism arising from the anterior corneal and lens surfaces in conjunction with intraocular distance effectivity are almost completely compensated for by the posterior corneal and lens surface. Repeatability was assessed on 20 subjects. Although the methods is prone to accumulated experimental errors, these are random in nature so that the difference between repeat group averaged data never exceeded +/- 0.27 DC cylindrical component and +/- 6 degrees cylinder axis.

Computation of retinal contour in anisomyopia.

It is well documented that myopia is associated with an increase in axial length of the posterior vitreous chamber. Whether equatorial or transverse dimensions are likewise affected in myopia is relevant to further understanding of the development of ametropia. We have utilised a computing method to determine retinal contour from real eye measurements of keratometry, A-scan ultrasonography and peripheral refraction as a means of assessing the transverse dimensions of the vitreous chamber. This technique has been applied to a 21-year-old female Caucasian anisomyope with a refractive error of R -1.50/-0.50 x 130 and L -4.00/-0.50 x 160. Anisomyopia offers a special opportunity for inter-eye comparison of different degrees of myopia. The repeatability of the technique was assessed by taking 10 separate sets of the aforementioned measurements and thus generating 10 retinal contours for each eye. We conclude that this method is repeatable and is capable of demonstrating differences between anisomyopic eyes although validation against in vivo measurements is required.

A clinical trial of the SUN SK-2000 computer-assisted videokeratoscope.

A clinical trial was performed on 20 optometry students in order to evaluate the suitability of the SUN SK-2000 (SUN contact lens company, Japan) computer-assisted videokeratoscope for use in clinical practice. Suitability was assessed by observing three factors: the accuracy, repeatability and agreement for central mean corneal power (MCP) with a keratometer (Javal Schiotz). Measurements of eight peripheral points and the central radius of curvature were made on 20 optometry students and on an 8 mm calibration surface using the SUN SK-2000. The SUN SK-2000 accurately determined the 8 mm calibration surface (supplied by manufacturers) to be spherical (SD = +/- 0.013, P = 0.776). Careful alignment was found to be an important factor in determining the actual radius of the calibration sphere. Unfortunately, the system required measurements to be taken at a point which was not coincident with the line of sight. A simple improvement to this has been suggested. Readings were most repeatable for central corneal radii. Peripheral radii showed the most repeatability for the horizontal and inferior corneal radii. It has been suggested that the ocular adnexa may be a possible cause for the poor repeatability in the superior area of the cornea. The level of agreement between the Javal Schiotz keratometer and the SUN SK-2000 in terms of mean refractive error (MRE) was not found to be sufficient for the two instruments to be interchangeable (P = 0.011). The C0 and C45 components of corneal astigmatism were found to have little association with the keratometer.

A computing scheme for determination of retinal contour from peripheral refraction, keratometry and A-scan ultrasonography.

In this paper a method is described in which retinal contour is derived from measurements of peripheral refraction, keratometry and A-scan ultrasonography. Theoretical aspects are discussed first, followed by a closer examination of the computing scheme and its evaluation. Evaluation involved investigation of inherent errors arising from the computing scheme's methods of compensating for unmeasured human ocular complexities such as crystalline lens asphericity and gradient index optical structure, finite aperture pupils, ocular surface misalignment and toricity. It is found that computed retinal contours are little affected by these inherent errors.

A study of the axis of orientation of residual astigmatism.

The aim of this study was to determine the axis of orientation of residual astigmatism in a sample of human eyes applying the principle of astigmatic decomposition. Calculations were carried out on keratoscopic and refractive data collected from the right and left eyes of 70 subjects (37 male and 33 female students) of mixed race (including 25 Asians and 43 Caucasians). No statistically significant difference was found for mean levels of residual astigmatism measured in the right (0.46 DC x 98.2 degrees) and left (0.50 DC x 99.4 degrees) eyes. Residual astigmatism was predominantly against-the-rule (83% of right eyes and 66% of left eyes) and was within +/- 20 degrees of being perpendicularly disposed relative to the corneal astigmatic power axis in two thirds of the eyes measured. No statistically significant differences were found for either gender or race.

Model for co-ordination of corneal and crystalline lens power in emmetropic human eyes.

A mathematical model is used to investigate the co-ordination of corneal and crystalline lens power in emmetropia. Modelled depth changes of the anterior chamber, crystalline lens and vitreous chamber were based upon empirical data. Modelled power changes of the corneal and crystalline lens were then purely governed by eyeball shape. Power changes generated by the model closely agreed with empirical data indicating that the co-ordination of both, to maintain emmetropia, can be accounted for by relative axial and equatorial growth of the eyeball.

Peripheral astigmatic asymmetry and angle alpha.

The association between peripheral astigmatic asymmetry and angle alpha was tested in the present study. Measurements were made in 34 eyes. Peripheral astigmatism was measured over the horizontal meridian using a Zeiss (Jena) Hartinger coincidence optometer and a Canon R-1 autorefractometer. Curves were fitted to the measured data of each eye and the minima determined by differentiation. Angle alpha was estimated by alignment of Purkinje images I (anterior cornea) and IV (posterior crystalline lens). Peripheral astigmatism was found to be symmetrical about a point on the nasal retina. This point departed from the visual axis by 8.8 +/- 7.0 degrees (Hartinger) and 9.4 +/- 9.8 degrees (Canon). Both values were found to be significantly higher than angle alpha 5.0 +/- 1.2 degrees. The results indicate that either peripheral astigmatic asymmetry is due to additional factors such as lack of symmetry in the peripheral curvature of individual optical surfaces, or that there is further misalignment of optical surfaces away from an optical axis.

Scheme for the calculation of ocular components in a four-surfaced eye without need for measurement of the anterior crystalline lens surface Purkinje images.

The computing scheme described in this paper allows calculation of the internal radii of the human eye on the assumption that it possesses four (two corneal and two lenticular) axially aligned spherical surfaces separated by homogeneous optical media. It involves measurement of Purkinje images I, II and IV. This approach is novel in that it accounts for the contribution of the posterior corneal surface and does not require measurement of the Purkinje images arising from the anterior crystalline lens surface which are notoriously poor in quality.

Normal variations of the posterior corneal surface.

Measurements of the sphero-cylindrical components of the rear corneal surface were taken from 80 healthy right eyes in order to determine normal variations. Dimensions of this surface were strongly influenced by the front corneal surface with the exception that the rear surface exhibited more toricity. Both surfaces tended to be flatter in males compared to females and in myopes compared to hyperopes. The corneal surfaces were also found to be flatter in younger eyes compared to older eyes, but this finding was most likely due to the preponderance of myopes in the young and hyperopes in the older group. The influence of the ratio of anterior: posterior corneal surface radius upon the estimation of total corneal power as required for intraocular lens implant calculations was also considered.

Effect of iris displacement on oblique astigmatism in aphakic eyes.

Aphakic eyes possess considerably less oblique astigmatism than phakic eyes. The possibility that posterior iris displacement after lens extraction accounts for the observed reduction in oblique astigmatism is tested in this study. Oblique astigmatism was calculated for a phakic and an aphakic schematic eye. With no iris displacement little reduction of oblique astigmatism occurred contrary to experimental findings. However, introduction of varying levels of iris displacement reduced this aberration to values found experimentally. Posterior iris displacement (mean = 1.19 mm) was confirmed in vivo by pre- and postoperative anterior chamber pachometry in eight patients.