This project aims to investigate/examine high myopic subjects on full correction ortho-k (using Topaz ortho-k lenses for high myopia) compared to subjects undergoing partial reduction ortho-k and their relationship with myopia and myopic progression.

For many years, commercially-available ortho-k lenses aim to reduce low - moderate myopia. Attempts to use these lenses for reduction of high myopia have been shown to give rise to complications such as corneal staining and lens decentration. Thus, practitioners may choose a more conservative way for high myopic children, that is, offering partial reduction ortho-k. Partial reduction ortho-k is target for 4.00-5.00D reduction and the residual refractive errors will be corrected with single vision spectacles to allow good visual acuity in the daytime. Partial reduction ortho-k has been shown to slow axial elongation in high myopic children, however, the main disadvantage is that the children have to wear spectacles in the daytime to correct residual refraction.

Euclid has recently designed a new lens, Euclid's Topaz, for high myopic children. It is currently commercially available to correct myopia for up to 10 D. However, evidence of its effectiveness for visual correction and slowing AL growth is lacking. While numerous studies have shown that orthokeratology is an effective clinical treatment to slow axial eye growth in children, the exact mechanism underlying this reduction in myopia progression associated with orthokeratology remains unclear.

Inclusion Criteria:

• Myopia: at least 5.00D in one eye or in both eyes
• Astigmatism: ≤1.50D; with-the-rule astigmatism (axes 180 ± 30) ≤1.25D; astigmatism of other axes ≤0.50D in both eyes
• Anisometropia: not be more than 1.00D in the former and not more than 2.00D in the latter.
• Best-corrected Monocular Snellen visual acuity 6/7.5 or better

Exclusion Criteria:

• Strabismus at distance or near
• Previous experience in contact lens wear or myopia control treatment (e.g. refractive therapy or progressive spectacles)
• Contraindication for contact lens wear and orthokeratology (e.g. limbus to limbus corneal cylinder and dislocated corneal apex)
• Previous history of ocular surgery, trauma, or chronic ocular disease
• Concurrent use of medications that may affect tear quality or contact lens wear
• Systemic or ocular conditions that may affect tear quality or contact lens wear (e.g allergy and concurrent medication) or that may affect refractive development (e.g Down syndrome, ptosis)
• Poor compliance to tests (e.g poor fixation in noncontact tonometry or intolerance of lens wear)
• Not willing to comply with the allocated treatment, use and care of lenses and follow-up schedule