Gas-Permeable Lenses

Modern corneal Gas Permable (GP) lenses have been used as the gold standard treatment for keratoconus since the 1980s. There have been many well designed clinical studies that have shown the safety and efficacy of GP lenses used for improving vision with keratoconus, such as the Collaborative Longitudinal Evaluation of Keratoconus (CLEK). The CLEK study was a randomized, multi-centered, observational study that established that the safest way to fit a corneal GP lens is by clearing the apex of the cone, thereby reducing mechanical abrasion of the highest portion of the cornea. Poorly fitting GP lenses are correlated with increased risk for central corneal scarring. GP lenses have pros and cons associated with them:

Decentered corneal gas-permeable lens on a keratoconic eye.  A common problem with corneal gas-permeable lenses is that they seek the steepest portion of the cornea, which is often the tip of the cone. The tip of the cone is usually not in the center of the cornea.

When the eye moves a decentered corneal gas-permeable lens can become dislodged.  This is an image taken under blue light with green dye behind the lens.

Pros:

• A high degree of precision can be built into the fit and the optics
• When fit well, are very safe and effective at providing excellent vision, above what glasses or soft contact lenses can provide
• Relatively inexpensive
• Have been around for a long time

Cons:

• Comfort is always a challenge- those who have worn GP lenses for years habituate to the lenses, but never are truly comfortable- the eyelid is always moving over a hard lens edge
• Any debris in the environment that moves underneath the lens becomes intensely uncomfortable- lenses must be removed immediately to clear the debris
• Movement- GP lenses are designed to be picked up by the upper lid during a blink, then drop back down into place. Many patients describe this as being responsible for inconsistent vision
• GP lenses rest on the cornea and have the potential to touch the highest portion of the cornea, the apex of the cone- central touch is associated with increased risk of corneal scarring- there are two scenarios when patients have lenses that touch the apex of the cone- either the lenses were fit improperly to start, or the cornea progressed since the lenses were prescribed- if your contact lens is touching the corneal apex, you may or may not notice this as discomfort.
• Many patients with keratoconus are still using GP lenses for vision correction. For those who are, consistent monitoring is the key to ensuring proper corneal health in the short and long term.