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published_or_final_version / Medical Sciences / Master / Master of Medical Sciences

Thesis (M. Med. Sc.)--University of Hong Kong, 2007.

BACKGROUND: Intraocular scatter is an important factor when considering the performance of the human eye as it can negatively affect visual performances (e.g. glare). However, and in contrast to other optical factors that also affect vision such as high order aberrations, there is currently no efficient method to measure accurately and objectively the amount and the angular distribution of forward light scatter in the eye. Various methods and instruments exist to assess forward light scatter (FLS) but the relation between these methods has rarely been quantified. In addition, FLS measurements obtained with existing instruments cannot be related to any physiological factors due to the absence of a valid model. PURPOSE: To investigate the relations between some of the main methods to measure forward light scatter, and to develop an experimental set -up for the objective measurement of forward light scatter that could be ideally related to physiological parameters. METHODS: After a short review of intraocular light scatter, the three main methods used to assess forward light scattering are compared. In this sense, the C-quant (CQ) straylight meter is compared to the van den Berg (VDB) straylight meter and the Hartmann-Shack spot pattern analysis obtained from the Hartmann-Shack aberrometer. The potential of the new Oculus Pentacam functionalities for providing information on backward light scatter (BLS) are also investigated. Finally, an innovative prototype for objective assessment of intraocular light scattering together with a scatter model of the eye is presented. RESULTS and DISCUSSION: Although no significant relationship was found between the different instruments considered (VDB straylight meter, CQ, Pentacam), our results allowed us to clarify some possible confusion introduced by previously published results and to illustrate the fact that existing commercial instruments such as aberrometers and the Pentacam cannot be used to measure FLS without at least some major modifications (hardware or software). Preliminary results with the prototype built in this study suggest that it could be used for the objective measurement of intraocular light scatter. Relating this measurement to physiological parameters stays however elusive, a fact that widens the future scope of this research.

PURPOSE. To evaluate light scatter and stray light in intraocular lenses (IOLs) explanted because of postoperative opacification (13 calcified hydrophilic acrylic, 1 calcified silicone, and 4 polymethylmethacrylate [PMMA] lenses with snowflake degeneration), as well as effect of opacification on other optical quality/performance indicators, in comparison with controls. METHODS. The Complete Angle Scatter Instrument (CASI) scatterometer was used to measure the forward light scattering (FLS) of the IOLs, and the stray light values at various angles were calculated from the measured FLS. Modulation transfer function (MTF) was obtained with an optical bench, and a Badal optometer was used to obtain letter chart images through the lenses. Back light scatter and light transmittance were also measured. RESULTS. Average stray light values (Log (s)) at a scattered angle of 100 were 1.79 +/- 0.37 for hydrophilic acrylic IOLs (controls 0.36 +/- 0.05), 1.53 for the silicone lens (control 0.41), and 1.62 +/- 0.46 for PMMA IOLs (control 0.25). Stray light was significantly higher for explanted opacified lenses (N = 18) in comparison with controls (N = 7; two-tail P < 0.001 at 100). Modulation transfer function and Badal image contrast were drastically reduced in lenses with calcification and snowflake degeneration. CONCLUSIONS. Different studies described the impact of stray light in human vision, with serious hindrance above 1.47 Log (s). Lenses explanted from patients because of clinically significant opacification are associated with a considerable increase in light scatter and stray light, as well as with a decline of other optical quality/performance indicators.

El presente trabajo es una adaptación de la técnica quirúrgica de trabeculectomía para el tratamiento del glaucoma canino. Para tal fin se utilizaron 5 caninos de distintos sexos, razas y edades, con una historia clínica de glaucoma. Ellos fueron intervenidos en la Clínica de Animales Menores de la Facultad de Medicina Veterinaria de la Universidad Nacional Mayor de San Marcos, empleándose la técnica quirúrgica de trabeculectomía, la cual consiste en crear una nueva vía de drenaje del humor acuoso. En cada uno de los casos, se demostró que dicha técnica es una alternativa, rápida, poco costosa y confiable para el tratamiento resolutivo de esta patología. Con el fin de verificar su eficacia se realizaron evaluaciones oftalmológicas post-quirúrgicas. Los hallazgos mostraron en todos los casos una importante disminución de la presión intraocular. Al comparar las presiones intraoculares antes y después de la intervención quirúrgica se encontró una diferencia estadística significativa con un nivel de confiabilidad del 99%. Se concluye que la técnica quirúrgica de trabeculectomía puede ser usada en la resolución del aumento de la presión intraocular en caninos. / The present work in an adjustment of the surgical technique of trabeculectomy for the treatment of the canine glaucoma. To accomplish it, 5 canines of different sex, races and age, with clinical history of glaucoma were used. They were treated in the Clinic of Small Animals of the Faculty of Veterinary Medicine of the National University of San Marcos. The surgical technique of trabeculectomy permitted to find a new way of draining of the aqueous humor. In each of the cases, there was demonstrated that the technique is a quick, cheap and reliable alternative for treatment of this pathology. In order to verify its efficiency, postsurgical evaluations were done. In all the case the findings showed an important decrease of the intraocular pressure. When comparing the intraocular pressures before an after the surgical intervention. There was found a significative statistical difference, with 99% of reliance. It is concluded that the surgical technique of trabeculectomy can be used successfully in the treatment of the increase of he intraocular pressure in canine.

Objetivo: Determinar la diferencias en las mediciones de tonometría de contorno dinámico (TCD) y la tonometría de aplanación (TAG) al relacionarlas con el grosor y la curvatura corneal. Materiales y métodos: Estudio transversal, comparativo. Se obtuvo una muestra por conveniencia, esto incluyó a 116 ojos de pacientes que acudieron al programa social del Instituto de Ojos Oftalmosalud, sin patología ocular demostrada, entre los meses de febrero a Abril del 2009. Se tomaron medidas del grosor, curvatura corneal, presión intraocular por TCD y TAG Resultados: La edad promedio de los participantes fue 55 años, el 52.58% varones. La media de la PIO por contorno dinámico fue 19.3 mm Hg y de la tonometría de aplanación 16.32 mm Hg, existiendo diferencias significativas entre ambas mediciones de PIO (p menor 0,0006). El grosor de la córnea central por paquimetría fue 539.4 en promedio y la media de la curvatura corneal en dioptrías de 43.28. Las comparaciones de las medias de la PIO de la TAG y TCD según el grosor corneal central (GCC) son significativas en las medidas menores a 500 micras (p menor 0.000001). Según la curvatura corneal se mantienen estas diferencias de manera significativa, es mayor con el TCD. Conclusiones: La tonometría de contorno dinámico sobreestima significativamente la PIO comparada con la tonometría de aplanación. La PIO evaluada con ambos instrumentos comparadas con el grosor corneal muestran diferencias significativas, siendo similares en córneas entre 500 a 549 micras. / --- Objective: To determine the differences in the measurements of Dynamic contour tonometry and Goldmann applanation tonometry in relation with the thickness and the corneal curvature. Methodology: Study transverse, a sample was obtained by convenience, included 116 patients’ eyes went to the Oftalmosalud social program, without ocular pathology, showed through February to April, 2009. They took measures of the thickness, corneal curvature, pressure for CDT and AGT. Results: The age average of the participants was of 55 years, 52.58% males. The stocking of the IOP one for contour dinamic was 19.3 mmHg and for applanation tonometry 16.32mmHg, existing significant differences among both mensurations of IOP (p less 0, 0006). The thickness of the central cornea for paquimetry was 539.4 on the average and the stocking of the corneal bend in dioptrias of 43.28. The comparisons of the stockings of the IOP one of AGT and CDT according to the thickness corneal power station (CCT) are significant in the measures smaller to 500 microns (p less 0.000001). According to the corneal curvature stays with significant differences ways being bigger with CDT. Conclusions: The tonometry of contour dinamic overestimates the IOP one compared with the applanation tonometry significantly. The IOP one evaluated with both instruments compared with the corneal thickness shows significant differences, being similar in corneas among 500 to 549 microns.

Purpose: We evaluate solutions for an applanating surface modification to the Goldmann tonometer prism, which substantially negates the errors due to patient variability in biomechanics. Methods: A modified Goldmann or correcting applanation tonometry surface (CATS) prism is presented which was optimized to minimize the intraocular pressure (lOP) error due to corneal thickness, stiffness, curvature, and tear film. Mathematical modeling with finite element analysis (FEA) and manometric lOP referenced cadaver eyes were used to optimize and validate the design. Results: Mathematical modeling of the optimized CATS prism indicates an approximate 50% reduction in each of the corneal biomechanical and tear film errors. Manometric lOP referenced pressure in cadaveric eyes demonstrates substantial equivalence to GAT in nominal eyes with the CATS prism as predicted by modeling theory. Conclusion: A CATS modified Goldmann prism is theoretically able to significantly improve the accuracy of lOP measurement without changing Goldmann measurement technique or interpretation. Clinical validation is needed but the analysis indicates a reduction in CCT error alone to less than +/- 2 mm Hg using the CATS prism in 100% of a standard population compared to only 54% less than +/- 2 mm Hg error with the present Goldmann prism. Translational Relevance: This article presents an easily adopted novel approach and critical design parameters to improve the accuracy of a Goldmann applanating tonometer.

Purpose: To compare the effect of topical prednisolone acetate 1% (PA) used after routine cataract surgery to the effect of difluprednate 0.05% (DFBA) used for the same indication on intraocular pressure (IOP). Methods: An electronic query was created to gather information from all cataract surgeries between January 2010 and January 2015 within the electronic health record database at Barnet Dulaney Perkins, a multicenter, multiphysician private practice in Phoenix, Arizona. Information collected included age, sex, diabetes status, glaucoma history, medication regimen (use of PA or DFBA), and IOP before surgery, 5-10 days postoperatively (TP1) and 3-6 weeks postoperatively (TP2). Postoperative IOP measurements were compared to baseline IOP measurement in each patient. Results: Regardless of steroid used, all patients in this study experienced an increase in IOP within TP1 and returned to baseline IOP (+/- 2.0 mmHg) by TP2. Patients who received DFBA showed a statistically significant increase in IOP at TP1 compared to those on PA (P<0.001) with the mean IOP an average 0.60 mmHg higher (95% CI = 0.3, 0.9). The odds ratio of a clinically significantly increased IOP at TP1 (defined as overall IOP >= 21 mmHg and an increase of >= 10 mmHg) in DFBA-treated patients was 1.84 (95% CI = 1.4, 2.6). In patients treated with PA, 3% reached a significantly increased IOP, compared to 4.4% of patients in the DFBA group (P<0.05). Risk factors for increased IOP were identified, and include advanced age (>75) (P<0.005) and a history of glaucoma (P<0.001). Conclusion: In postoperative cataract patients, use of DFBA increased the risk of a clinically significant IOP increase.

The purpose of this thesis was to investigate the effect of corneal swelling on the accuracy of Goldmann tonometry estimates of intraocular pressure (IOP). In the first experiment, central corneal thickness (CCT, ultrasonic pachymetry), IOP (Goldmann tonometry) and corneal curvature (keratometry) was measured in one eye of 25 subjects every two hours for 24 hours, except for 8 hours overnight (no measurements taken), and for the first two hours after awakening (measurement frequency 20 minutes). CCT (+20.1??10.9 pm) and IOP (+3.1??2.4 mmHg) peaked on eye opening, and then decreased at a similar rate (r=0.967, p<0.001) for the next two hours. Corneal swelling may have influenced the accuracy of Goldmann IOP measurements during this time. In the second and third studies, the CCT, IOP and corneal curvature were measured in both eyes of two groups of 25 subjects before and after the induction of corneal swelling, resulting from two hours of monocular closed eye contact lens wear. The increase in IOP was correlated to the increase in CCT at a rate of 0.33 to 0.48 mmHg per 10 pm, which signified an overestimation error in Goldmann IOP measurement. However, the change in IOP could not be accounted for solely by the change in CCT. In the fourth study, CCT, IOP and corneal curvature were used in conjunction with the Orssengo-Pye algorithm to determine the range of Young's modulus in the normal population, which was 0.29??0.06 MPa. Physiological variations in Young's modulus had a similar effect on Goldmann tonometry to CCT. In the fifth study, the data collected for studies 2 and 3 was used to calculate the Young's modulus changes associated with corneal swelling, again with the assistance of the Orssengo-Pye algorithm. No systematic change in Young's modulus was recorded after contact lens wear, but the model suggested that corneal biomechanical changes were responsible for the remainder of the change in IOP. All experimental results were combined to develop a model to calculate the diurnal variation of Goldmann IOP errors. The likely error in IOP due to overnight corneal swelling was 0.6 to 1.4 mmHg, which may explain as much as 45% (1.4 mmHg) of the 3.1 mmHg diurnal variation of IOP. In summary, small amounts of corneal swelling were shown to have a clinically significant impact on the accuracy of Goldmann tonometry. This may interfere with the measurement of the diurnal variation of IOP, particularly if measurements are taken prior to the resolution of overnight corneal swelling.