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Biomedical subjects

M C Dunne

Publications and source records attributed to M C Dunne.

At least 19 recordsLinked to original sources
PubMed2000

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.

Adult↗
PubMed1997

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.

Adolescent↗
PubMed1997

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.

Humans↗
PubMed1997

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.

Child↗
PubMed1996

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.

Adolescent↗
PubMed1995

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.

Adolescent↗
PubMed1995

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.

Algorithms↗
PubMed1995

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.

Adult↗
PubMed1994

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.

Adolescent↗
PubMed1993

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.

Astigmatism↗
PubMed1993

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.

Anthropometry↗
PubMed1992

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.

Adult↗
PubMed1992

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.

Anthropometry↗
PubMed1991

Posterior corneal surface toricity and total corneal astigmatism.

The influence of posterior corneal surface toricity upon total corneal astigmatism was investigated in 60 young subjects (mean age 22.04 +/- 3.24 years). Both surfaces were found to be flatter horizontally than vertically. Astigmatism arising from the anterior corneal surface was therefore reduced by astigmatism of the opposite sign arising from the posterior surface. Had the toricity of the posterior corneal surface been purely governed by that of the anterior surface this reduction would have amounted to about 5%. However, the posterior surface was found to exhibit additional toricity bringing about a greater reduction of total corneal astigmatism amounting to approximately 14%.

Adult↗
PubMed1991

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.

Adult↗
PubMed1990

Measurement of posterior corneal surface toricity.

We describe a method in which posterior corneal surface toricity can be determined by photographing catoptric (Purkinje) images in three fixed meridians simultaneously using infrared light sources. Keratometry, the comparison of anterior and posterior corneal catoptric (Purkinje) images and pachometric results, are applied to three meridional analysis equations, allowing the posterior corneal surface to be described in spherocylindrical form. The technique is evaluated on five subjects with anterior corneal radii varying from 7.11 to 8.52 mm.

Cornea↗
PubMed1990

Measurement of the posterior corneal radius using slit lamp and Purkinje image techniques.

A method for measuring the radius of the posterior corneal surface using the first and second Purkinje images has recently been developed. Values for the posterior corneal radius in the vertical meridian obtained using this method are compared with those obtained using a more conventional slit lamp method. A good correlation was found between values obtained using the two methods.

Anthropometry↗