first patient, Pseudomonas aeruginosa was identified as the causative organism. In the second patient, Haemophilus influenzae was cultured from the ulcer. Both patients were treated with antibiotics, resulting in a rapid resolution of the corneal ulcers and preservation of vision. The writers conclude that "Paragon CRT lenses have been approved for use in patients of all ages. When used in children, these lenses may present unique problems. The absolute incidence of bacterial corneal ulcers in patients with CRT lenses are unknown. Severe caution should be used before prescribing CRT lenses for children and informed consent should include potential sight-threatening corneal ulcers." Although sight threatening corneal ulcers are rare, informed consent should routinely be obtained for all new contact lens wearers, whether worn overnight or not. The Globe and Mail reported two cases in Canada in which Acanthamoeba infections during ortho-k treatment, possibly related to rinsing the lenses in tap water, led to temporary or permanent blindness. Lens types Orthokeratology lenses are made by a number of companies and all use special gas-permeable contact lenses to reshape the cornea. The lens material – and especially its oxygen permeability measured by its 'Dk' rating (the higher the value, the greater the degree of oxygen permeability) – is important for maintaining eye-health during the treatment. Materials and methods vary globally, as does individual country approval. For example, lenses "Menicon Z Night" are generally used overnight only, but in USA they are also FDA-approved for up to 30-day wear, on a regular GP schedule. Some practitioners have also designed their own orthokeratology lenses typically by using various software packages which combine corneal topographical data with the practitioners' own knowledge and experience to determine the appropriate lens parameters for individual patient prescriptions. These practitioners are obviously doing so "off-label" since their designs are not approved for safety by the FDA. Footnotes References Research in Orthokeratology – University of New South Wales (Sydney, Australia). Orthokeratology: part I historical perspective. Journal of the American Optometric Association External links ClinicalTrials.gov Corrective lenses Therapy