The concept of lenses for continuous overnight wear, called extended wear, is not a new one. Over twenty years ago, the Food and Drug Administration (FDA) approved several lens products for use on an overnight basis for up to 30 days. These products, however, were fraught with problems and eventually the FDA and eye doctors reduced their guidelines for wearing time.
Recently, new lenses have been approved for extended wear that provide oxygen levels that are five times greater than most previous products. Combined with a proprietary surface treatment that reduces protein adherence to the surface, this technological advance can provide safe and comfortable contact lens wear for up to 30 days.
When a contact lens is placed on the surface of the eye, called the cornea, several things happen that reduce the ability of the eye to defend itself against infection, the greatest risk of contact lens wear. The cornea relies on direct contact with the air or the rich blood supply that lines the eyelid for its supply of oxygen. This contact is vital because the cornea is one of the few tissues in the body that that lacks its own blood supply. When you think about it, this anatomical oddity is necessary. After all, who could see through a blood vessel full of red blood cells? A rich supply of oxygen is vital for the cornea in maintaining its clarity and for the growth of healthy surface cells. These surface cells, called epithelial cells, form the first line of defense to infection. A layer of bacteria fighting enzymes and chemicals also protects the outer surface.
When a contact lens is placed on the eye, it reduces the amount of oxygen available to the cornea by decreasing the contact of the cornea with its oxygen supply. This reduces the vigor of the surface cells. In addition, the presence of the lens mechanically removes some of the protective enzymes present at the surface of the eye. As the lens is worn, some of the proteins from the tear film begin to accumulate on the lens further reducing the amount of oxygen available to the surface of the eye and increasing the level of mechanical irritation.
Luckily, for patients who remove their lenses at night, this is of little concern since the oxygen available to the cornea while the lens is in place is similar to the levels available when the eye is closed during sleep. Extended wear, on the other hand, changes the equation by reducing the amount of oxygen available to levels that are significantly less than normal during sleeping hours.
Contact lenses called Night and Day™ lenses are now approved for extended wear use up to 30 days. The increased oxygen transmission and cleaner surface provided by this lens can give many wearers a safe and effective alternative for achieving clear vision on a 24 hour basis.
