Orthokeratology Theory

together result in a change in overall corneal shape from the normal prolate (flattening) asphere towards a more spherical VISUAL OUTCOMES OF OVERNIGHT ORTHOKERATOLOGY

Results in respect of unaided vision reported from various clinical studies of overnight OK have been significant with a large majority of subjects in recent studies achieving 6/6 or better. In clinical practice potential visual problems arises due to under- or over-correction of myopia or significant residual refractive astigmatism. As seen in the cases of refractive surgery, visual disturbances may arise from a small or decentred central treatment zone, resulting in symptoms such as ghosting and flare and decreased contrast sensitivity, particularly with a dilated pupil under

Orthokeratology lenses work particularly on the anterior corneal tissue, however it is not clear whether the effect is mainly on the epithelium, anterior stroma, or both (6). Moreover , there are several theories on forces which are involved in inducing corneal change in orthokeratology. One of such theory proposed that the downward pressure of the central portion of the orthokeratology lens was flattening the underlying corneal epithelial cells (6). Another theory suggested that the orthokeratology lens created an outward pulling pressure in the tear film. This pressure gradient was thought to pull the epithelial cells peripherally, hence resulting in re-distribution of epithelial cells in the cornea (6). Due to this re-distribution, there is a relative flattening in the central cornea compared to the mid-peripheral cornea. There is another theory which proposed that the corneal stroma along with the epithelium was being changed by orthokeratology lenses.(82). Various histological and morphological studies have researched the effect of orthokeratology lenses on the corneal cell layers and the forces that may be involved in these changes. A newer study on

A group of case reports of microbial keratitis in orthokeratology lens wearers was inconclusive regarding the risk involved in orthokeratology. The incidence of microbial keratitis in orthokeratology patients could not be concluded for several reasons as the lack of accurate data regarding the total number of patients wearing the lenses, patient compliance and lens materials (86.). Most of the patients in this study were Asians between the ages of 9 and 15 (86). Orthokeratology lenses are often prescribed for children In these countries, with the hope that they may slow the development of myopia (86). Out of fifty only three cases were from the United States. It is unclear whether there is a higher risk for adverse events in the Asian population or if the higher incidence of keratitis is because of the higher number of patients wearing the lenses in these areas (86). It was found in a small sample pilot study, that the correction of myopia using orthokeratology lenses was safe and effective in children and adolescents (87). In addition no cases of microbial keratitis were reported (87). In total orthokeratology appears to be an effective option for the correction of myopia and has minimal side effects. Orthokeratology patients did not show to have any decrease in