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Title: A retrospective survey of orthokeratology on children
Authors: Chan, Lap-kong
Degree: M.Phil.
Issue Date: 2010
Abstract: Orthokeratology (Ortho-k) is a contact lens technique mainly used for temporarily reducing myopia. The Optometry Clinic of The Hong Kong Polytechnic University (PolyU) has been providing an ortho-k service since 1997. Much useful information regarding refractive and ocular changes with ortho-k lens wear awaits collation and analysis. This information will be useful to help ortho-k practitioners improve the practice of ortho-k. The human eye is directly exposed to light and an atmospheric environment containing reactive oxygen species (ROS) which may cause ocular tissue damage and disease. To protect the cornea from ROS, the tear film acts as the first barrier and contains several antioxidants for ocular defense. Ascorbate (vitamin C) is one of the most important antioxidants in human tears. Overnight ortho-k lens wear induces ocular stress which may increase the demand of antioxidants for ocular protection. Contact lens binding, leading to corneal staining, is a commonly observed problem in overnight ortho-k lens wear and artificial tears are commonly prescribed for patients to use before going to sleep with the lenses, and to aid lens removal in the morning. However, artificial tears which do not contain antioxidants would dilute the useful antioxidants in tears, hence altering the ocular defense/protective system conferred by nature tears on the anterior surface of the eye during sleep. Therefore, there is a need to investigate if it is possible to include antioxidants, such as ascorbate, in the formulation of artificial tears to increase their effectiveness for the promotion of safe overnight ortho-k lens wear and corneal health. OBJECTIVES - The objectives of this MPhil study were: 1. collect the demographical characteristics of children undergoing ortho-k at the Optometry Clinic, The Hong Kong Polytechnic University (HKPU), 2. conduct a telephone survey of ortho-k wearers to understand the problems encountered and their satisfaction with treatment, 3. evaluate the validity of the Jessen formula in determining the back optic zone radius (BOZR) of ortho-k lenses, 4. investigate the relationship between corneal topographical changes and refractive changes induced by ortho-k, 5. conduct a pilot study to evaluate the stability of standard ascorbate in commercially-available single dosage formulation artificial tears. METHODS - A clinical survey was conducted by reviewing the records of all patients fitted with ortho-k lenses in the Optometry Clinic, HKPU between April 2000 and November 2003. Only children under the age of 16 years and still continuing with the ortho-k treatment with the same pair of lenses worn for a six month period were included in the analytical study. Demographical and clinical data were retrieved from clinical files, and corneal parameters were retrieved from the corneal topographer. A telephone interview was conducted using a pre-structured list of questions, asking about the visual performance and the satisfaction of the treatment. The validity of the Jessen formula, and the relationship between corneal topographical changes and refractive changes induced by ortho-k was investigated from the same group of patients in the clinical survey. The stability of ascorbate in artificial tears was studied by measuring the degradation rate of different ascorbate concentrations in three commercially available artificial tears using Ferric Reducing/Antioxidant Power assay.
RESULTS - A total of 257 ortho-k patients were fitted with ortho-k lenses in the Optometry Clinic, HKPU during the study period, and most of the wearers (91%) were children. The group of children who met the study criteria had a median age of nine years when they started treatment. A significant myopic reduction, improvement in unaided vision and corneal topographical changes were observed after a single overnight session of lens wear. An optimum ortho-k effect was achieved by one week. However, the spherical ortho-k lens design was ineffective in reducing the refractive cylindrical power. Corneal staining was the most commonly observed complication with overnight ortho-k, and over 80% of patients were advised to apply artificial tears to aid loosening the lenses before removal. Almost all (97%) of the patients whose files were reviewed were using a suction holder to aid lens removal. Ortho-k was mainly undertaken for myopic control, and about 90% of the respondents reported good or very good unaided vision after the procedure and ranked the treatment as satisfactory or very good. Lens binding and ocular discharge were the most frequently reported problems during the treatment. The BOZR of a reverse geometry lens (RGL) determined from the conventional Jessen formula, with a compression factor of 0.75, was found to under-estimate the intended target of myopic reduction. Our study suggests that the original formula (i.e. BOZR = Flat K Target 0.75) should be revised as BOZR = Flat K 1.23Target - 1.27. There is a significant correlation between achieved myopic reduction and the change in apical corneal power (ΔACP), and between achieved myopic reduction and maximum corneal power change within the treatment zone (ΔMCP) after ortho-k. However, both ΔACP (under-estimate) and ΔMCP (over-estimate) were unable to reflect the achieved myopic reduction accurately. We also found that initial Q was not useful as a predictive factor for the outcome in ortho-k. The stability of ascorbate solution in artificial tears, except Bion Tears (Alcon Inc, Fort Worth, TX, USA), was good. A higher concentration of ascorbate added to the artificial tears showed a slower degradation of ascorbate, and the stability was further enhanced when the mixture was stored at a low temperature. Among the three artificial tears tested, Vismed (Lab Chemedica AG, Munich, Germany) showed the greatest potential for the new formulation of artificial tears with ascorbate. CONCLUSIONS - Overnight ortho-k is a fast and effective way for correcting low to moderate levels of myopia. The spherical reverse geometry lens (RGL) design is not effective in the correction of astigmatism. The Jessen formula for deriving an approximate BOZR to correct the refractive error is suspect, if the assumption of 0.75 D over-correction is taken into consideration. The change in apical corneal power determined from the topographical map is not able to reflect the manifest refractive change accurately. However, when compared with retinoscopy and auto-refraction, apical corneal power change is still useful for estimating myopic reduction after the procedure. The initial corneal asphericity Q measured from corneal topography is not a useful predictor for ortho-k success. Our results showed that ascorbate in artificial tears could be stable, especially under higher concentration and low temperature.
Subjects: Orthokeratology.
Myopia -- Treatment.
Hong Kong Polytechnic University -- Dissertations
Pages: xxii, 173 p. : ill. (some col.) ; 30 cm.
Appears in Collections:Thesis

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