procedure being less successful. Counter-indications may include: Inflammations or infection of the anterior segment of the eye or the cornea Disease, injury or abnormality affecting the cornea, conjunctiva or eyelid, or impacting contact lens wear Very dry eyes or low eye moisture Corneal hypoesthesia (low sensitivity) Eye related allergies, including allergies to contact lens solutions Adverse Effects During the first month of lens wear when the treatment zone on the surface of the cornea is in the process of becoming fully formed, some users may experience vision issues such as ghosting, double vision, contrast problems and/or starbursting, especially at night. These issues are generally resolved by the end of the first month of lens wear. If these issues persist beyond this initial period, the cause may be due to lack of centration of the lens on the eye and/or overly large pupil size (in light or dark). Resolution may be possible through redesign, material changes, better eye moisture retention (night eye masks, duct blocking, etc.) or other techniques. Orthokeratology showed few severe side-effects, according to a March 2004 report of a very small sample of cases in China where supply of lenses at that time was not subject to any regulatory regime covering safety and efficacy. More typically, complications can occur due to the patient's failure to follow appropriate hygiene recommendations when handling or cleaning the lenses. One common issue being the use of tap water to rinse (although some systems allow for or suggest the use of "clean" tap water) or store as this may cause unwanted buildup of minerals on the contact as well as other issues. Complications may also be due to relative corneal hypoxia (lack of oxygen) with prolonged or overnight contact lens wear in lenses made from the wrong material. However, the use of high or hyper oxygen-permeable materials as approved by the US Food and Drug Administration significantly reduces hypoxia, and these are the materials that are normally used in orthokeratology. Safety advice applicable to most modes of contact lens wear, also applies to ortho-k night-time lenses. An article in the January 2005 issue of Eye & Contact Lens: Science & Clinical Practice discusses two case reports of children who developed corneal ulcers when fit with Paragon CRT contact lenses, which were worn nightly. Each patient presented with a bacterial corneal ulcer after wearing CRT contact lenses for less than 6 months. In the